If you try to go on a trip in your automobile, you are not going to get very far if there are no gasoline stations to feed your auto. Or restaurants to feed you. Or auto repair shops. This is what is called infrastructure. In the same way, if you want a rocketpunk future, you are going to need some infrastructure in space or your spacecraft are not going to get very far either.

Having said that, creating these pieces of infrastructure will be very expensive. It will be difficult to fund them. And you can be sure that whoever manages to build them will have iron control over who is allowed to use said infrastructure. And how much will be charged as a fee to use it.

It is possible to use spacecraft without any of this infrastructure, but it will be much more difficult. The owners of the infrastructure will probably adjust the fees so it will be cheaper to use their services, but only barely. The macroeconomics of the solar system will be vital to making all of this work. Especially the various business opportunities.

And of course an entertaining series of future histories can be postulated using various initial conditions. Does one national government have a monopoly? Two or more governments? Does one privately owned corporation have a monopoly? Two or more corporations? Or a several governments and several corporations?


A starship is not an independent entity—no more than a jet plane is independent just because it can leave the ground.

Imagine for a moment, a fully loaded 747 jet airliner flying from Los Angeles to New York. That's several hundred thousand pounds of airplane and three hundred people. First of all, there's the technology to build that airplane. (Not just one factory in the city of Seattle—but aerospace contractors all over the United States supplying components for every part of the plane, from blades for its jet engines to light bulbs over the seats.) Then there's the technology to maintain it; the schools to train the mechanics, the teachers to teach the stewards; the simulators and mockups on which they'll learn; the equipment with which the plane will be serviced, the specialized trucks and tools and devices; and the men and the training to support all of these levels. There's more than the airplane alone. There are airports. Lots of them.

That means the equipment and technology to lay down a perfectly flat runway two miles long. There are ground controllers—that means radar and scopes and computers. And reservation desks and communications systems to service those desks. Luggage-handling systems, porters, allowances for taxis, parking lots, restaurants, rest rooms—and the people to clean, maintain, and work in them.

Then you have to be ready for emergencies—so there will be rescue planes and fire trucks and medical equipment on hand. And detectives to protect the passengers and Federal Marshals to look for hijackers with metal detectors and psychological profile charts.

An airplane burns fuel—a lot of it. A 747 gulps enough petroleum in a single flight to drive an automobile for a year. That requires refineries to crack that oil (men to build and operate those refineries) and trucks to deliver it, tanks to store it. Passengers need meals, that demands another whole service industry. And entertainment—and specialized insurance—and airsickness bags in the back of each seat.

None of these things just happen by chance; they are designed into the system as it grows. The 747 could not exist until most of the support technology has already developed. What did not exist had to be built. All of it was oriented to fit the needs of the passenger as our culture has determined them. The very existence of the 747 as the kind of plane it is, is a direct result of what our culture considers important to the traveler. Imagine an airplane with fourteen bathrooms!

That airplane is a piece of living America. (In fact, I'm told it's the state bird of Hawaii.) It is an active vital symbol of our national technology, but it is no more independent of that technology than is a bird independent of the air in which it flies. The air holds up the bird. Our technology holds up that airplane.

Now. Apply that to a starship.

Extrapolate the needs of the Enterprise. Her fuel requirements, her crew requirements, her maintenance and training needs, her supply needs, her communication and control structures, her relation to the culture that produced her—and why that culture produced her.

Think about it.

Do you think the Enterprise is really an independent entity?

It isn't. It never could be. Her independence is an illusion, just as the independence of the 747 is an illusion. Sooner or later that ship is going to have to return home to have her exhausted energies recharged. Or, if not home, then to a base with an equivalent technology.

From THE WORLD OF STAR TREK by by David Gerrold (1973)

From the invited address of Salter Wherry to the United Nations General Assembly, following establishment of Salter Station in a stable six-hour orbit around the Earth, and shortly before Wherry withdrew from contact with the general public:

Nature abhors a vacuum. If there is an open ecological niche, some organism will move to fill it. That's what evolution is all about. Twenty years ago there was a clear emerging crisis in mineral resource supply. Everybody knew that we were heading for shortages of at least twelve key metals. And almost everybody knew that we wouldn't find them in any easily accessible place on Earth. We would be mining fifteen miles down, or at the ocean bottom. I decided it was more logical to mine five thousand miles up. Some of the asteroids are ninety percent metals; what we needed to do was bring them into Earth orbit.

I approached the U.S. Government first with my proposal for asteroid capture and mining. I had full estimates of costs and probable return on investment, and I would have settled for a five percent contract fee.

I was told that it was too controversial, that I would run into questions of international ownership of mineral rights. Other countries would want to be included in the project.

Very well. I came here to the United Nations, and made full disclosure of all my ideas to this group. But after four years of constant debate, and many thousands of hours of my time preparing and presenting additional data, not one line of useful response had been drafted to my proposal. You formed study committees, and committees to study those committees, and that was all you did. You talked.

Life is short. I happened to have one advantage denied to most people. From the 1950s through the 1990s, my father invested his money in computer stocks. I was already very wealthy, and I was frustrated enough to risk it all. You are beginning to see some of the results, in the shape of PSS-One—what the Press seems to prefer to call Salter Station. It will serve as the home for two hundred people, with ease.

But this is no more than a beginning. Although Nature may abhor a vacuum, modern technology loves one; that, and the microgravity environment. I intend to use them to the full. I will construct a succession of large, permanently occupied space stations using asteroidal materials. If any nation here today desires to rent space or facilities from me, or buy my products manufactured in space, I will be happy to consider this—at commercial rates. I also invite people from all nations on Earth to join me in those facilities. We are ready to take all the steps necessary for the human race to begin its exploration of our Universe.

It was past midnight by the time that Jan de Vries had read the full statement twice, then skipped again to the comment with which Salter Wherry had concluded his address. They were words that had become permanently linked to his name, and they had earned him the impotent enmity of every nation on earth: "The conquest of space is too important an enterprise to be entrusted to governments."

From BETWEEN THE STROKES OF NIGHT by Charles Sheffield (1985)

Surface to Orbit Boost

For most missions, almost half of the delta-V budget is used up in the first 160 kilometers or so, the lift-off from Terra's surface into Low Earth Orbit. This is the reason behind Heinlein's "halfway to anywhere" comment. In dollar terms, the Russian Proton would cost about $5000 per kilogram boosted into LEO while the Space Shuttle would cost about $18,000 per kilogram. Actually if you factored in all the shuttle's design and maintenance costs, the real price was closer to $60,000 per kilogram. NASA was hoping that the shuttle would cost more like $1,400/kg (in 2011 dollars), though part of the over-run was due to the multi-year interruptions in launches following Shuttle failures.

Infrastructure that help reduce the cost include Lofstrom loops, laser launchers, Marshall Savage's Bifrost Bridge, and the famous Space Elevator.

See the section on Laser Launching.

Orbital Propellant Depots

As the delta-V for a mission goes up, the amount of propellant required goes up exponentially (or looking at it another way: the amount of payload shrinks exponentially). Large amounts of propellant are expensive, but the higher the mass-ratio the higher the likelihood that the spacecraft will not be resuable. Propellant expense is bad enough, but that's nothing compared to having to build a new spacecraft for each mission. Increasing the mass ratio means things like making the walls of the propellant tanks thin like foil, and shaving down the support members so they are fragile like soda straws. With such flimsy construction it does not take much normal wear-and-tear to turn the spacecraft into junk.

Having a propellant depot at the mid-point of a round-trip mission cuts the required delta-V in half. Instead of the spacecraft having to lug enough propellant to go to Mars then return to Terra, it only carries enough for half the trip but re-fuels (re-propellants, or re-remasses) at the halfway point. And when you are dealing with exponential growth, cutting the delta-V in half cuts the propellant amount much more than half.

Indeed, Rick Robinson noticed that with access to an orbital propellant depot, most cis-Lunar and Mars missions are well within the delta-V capabilities of a sluggish chemical rocket engine. You do not have to use a nuclear thermal rocket. Hop David noticed this as well. Dr. Takuto Ishimatsu's ISRU optimization algorithm calculated that NASA's Mars Reference Mission was more optimal with no NTR but with ISRU ("optimal" defined as "requiring less mass boosted from Terra into LEO").

This is also an argument for orbital propellant depots in Low Earth Orbit. Remember that once the rocket has traveled from Terra's surface into LEO, you are "halfway to anywhere". This means for a one-way trip, LEO is the mid-point of the mission.

But the general consensus is that the best place for an orbital propellant depot in the Terra-Luna system is at Earth-Moon-Lagrange-1.

However Dr. Takuto Ishimatsu has a dissenting opinion. He makes a case for putting the propellant depot at EML2, for complicated reasons I cannot quite follow. Hop David also makes a case for EML2.

Now to make this work, in addition to the depots you will need sources of propellant and tankers and lighter to keep the depots filled up. Water and hydrogen propellant is available in such places as the lunar poles, asteroids, and perhaps the Martian moons Phobos and Deimos. Dr. Takuto Ishimatsu developed an algorithm to optimize placement and supply of ISRU orbital depots.

Examples of tankers include Kuck Mosquitos, Zuppero Water Ships, and Zuppero Lunar Water Trucks.


For this next depot taxonomy post, we’ll finally be talking about what people usually think about when they hear the term orbital propellant depot — larger, cryogenic refueling facilities, focused on enabling large-scale human spaceflight missions, performed by a diverse variety of users, and going to/from a wide variety of destinations. The idea for such propellant depots for enabling interplanetary human spaceflight dates back to at least 1928 with the writings of Guido von Pirquet1.

This blog post will be focused on what I call “Low-Orbit” human spaceflight depots. These are depots located near the lowest stable orbit around a planetary body. This is the first place at which you can realistically refuel or switch vehicles on your way from a planetary body, and is the last place at which you can refuel or switch vehicles on your way down. As a firm believer in the idea of refueling early and refueling often, low-orbit depots are an important piece of infrastructure for any planetary system humanity wants to travel to/from regularly. A later blog post in the series will talk about “High-Orbit” depots — depots operating in fixed locations2 located out near the edge of a planetary system’s gravitational sphere of influence, and will include a discussion of where those types of depots might make sense.

Before we jump into the weeds about low-orbit human spaceflight depots, I did want to address a recent train of thought I’ve seen that suggests that just using tankers and directly refueling a vehicle is superior to having a depot involved. While this could easily be the topic of a series of its own, I wanted to briefly highlight a few of the biggest advantages I can see of having a depot vs just using direct refueling with tankers:

  • Flexibility: A depot, properly designed, with published, standardized grappling, refueling, and power-data interfaces, can be agnostic about who it gets propellant from and who it sells propellant to. Depots can quickly take advantage of whatever the cheapest source of propellant is at a given time (RLVs, ISRU, propellantless launch, buying leftover capacity from other missions going to destinations near a depot, atmospheric gathering, etc), and can easily service both smaller missions and bigger ones. Tanker-based approaches tend to be a lot less adaptable, typically being optimized for one or two specific vehicles that needs refueling.
  • Robustness: With a fixed installation, that only has to be launched once for a long mission lifetime, you can afford to throw way more resources (dry mass, volume, and power) at making rendezvous, prox-ops, docking (RPOD), and manipulation as safe and reliable as possible. This could include beacons, larger more capable (and/or redundant) relative navigation sensors and comms, longer reach capture robotics that minimize the dry mass requirements on tanker and client vehicle alike, etc.
    • For tankers, on the other hand, you want to minimize parasitic dry mass that has to be launched every time, and for the departing vehicle you want to minimize mass you have to carry through large, high delta-V in-space maneuvers. You could in theory carry a nicer RPOD kit on your departing vehicle that you jettison before leaving LEO, but now you’re amortizing that mass and cost over a much smaller number of missions, or starting to add more complexity than just doing a depot.
    • Another question to ponder is with vehicles that require large numbers of refueling events per mission, is it best to have the client vehicle handle all of those docking maneuvers with its (by definition) less-capable RPOD capabilities? With a depot, each tanker and each client vehicle only has to perform one mission-critical RPOD/refueling operation per mission, whereas with direct refueling via tankers, the client vehicle would typically have to perform a larger number of mission-critical RPOD events. A depot would also have to handle the same larger number of RPOD/refueling events, but as mentioned before, can throw more resources at making these as reliable and safe as possible.
    • Longer refueling cycles from using a direct-tanker refueling approach also increases the MMOD3 risk to the departing vehicle, which by definition can’t afford to throw as many resources at MMOD protection as a depot can.
    • A corollary to this is that depots make the most sense if you use them in a way that offloads as much of the refueling-unique hardware/software as possible from the delivery and client sides of the system to the depot. Ideally a tanker would be a minimally modified upper stage4, and client vehicles would also have similarly minimalistic hardware needed to be grappled and receive propellant. If you’re doing your delivery vehicle or your client vehicle in a way that makes you question the utility of a depot, that may be a hint that you’re doing something wrong.
  • Non-Integerality: Yes I may have made up that word, but the point is that tankers tend to come in integer quantities. Unless you always design your departing vehicles to use only integer quantities of tankers, you’ll almost always end up having wasted propellant. This is especially true given how launch vehicles, and in-space vehicles tend to increase performance and get upgraded over time, and don’t necessarily upgrade at the same rates. If you only ever had a monopoly/monopsony situation where you only had one tanker provider, and only one vehicle needing tankers, you might be able to keep tanker size locked to an integer fraction of the amount of propellant needed, but in reality that isn’t going to happen. So a tanker-based system is always going to end up wasting propellants, and this is even more the case when you have diverse customer vehicle and delivery vehicle sizes. The more diversity you have in your space transportation ecosystem, the more a depot makes sense.

There are probably other good arguments I’m glossing over, but long-story short, unless you’re interested in a boring monoculture world where only one type of in-space transportation system exists, depots make a lot of sense. So, without further ado, let’s jump into some of the taxonomical considerations of human spaceflight low-orbit depots.

Human Spaceflight Low Orbit Depots

Application: Refueling large transfer stages or in-space transports for ferrying people and cargo between LEO, the Moon, Mars, Venus, and other destinations of interest.

Location: As discussed earlier, this blog post is focused on depots located near the lowest practical stable orbit around a planetary body. Details vary for different planetary bodies, as described below5:

  • For Earth, the low-orbit depot location is in LEO, ideally in the lowest inclination that still lets you hit required departure asymptotes and maximize the number of economically useful launch sites to send propellant, people, cargo, and materials to/from the depot6. You probably only need one or two such low-orbit depots, though if you get to high-enough earth departure throughput there may eventually make sense to spread more depots out at different RAANs7. Likely for a first human spaceflight depot, as with the previously discussed smallsat launcher depots, you’ll want to locate it in LEO near other human-occupied facilities like ISS — far enough away to be safe, but close enough to conveniently move between each other, ideally within one work shift8.
  • For a low-orbit depot around the Moon, this would likely be a polar or near-polar LLO9, though due to the very slow rotation and practically zero J2 perturbation10, if you have a lot of non-polar surface sites, you eventually may want multiple smaller depots in equally RAAN-spaced near-polar LLO planes, and maybe one in an equatorial orbit. If you’re trying to do lunar surface missions, having your depot in LLO makes way more sense than in a higher orbit like NRHO, for reasons I should probably go into in another blog post.
  • For Mars this would also likely be a LMO orbit, with an inclination high enough to be able to access any points of interest on the surface, while still being low enough to minimize delta-V penalties11, and keep the nodal precession rate fast enough to minimize phasing orbit time for three-burn departures. You’ll likely also have to put some thought into perturbations from Phobos and Deimos12.
  • For Venus, the extremely low rotation speed and therefor very low J2 pertubation may require you to do multiple smaller depots in similar inclinations but equally RAAN-spaced planes, as you won’t pass over a given point on the surface very frequently, and the very slow nodal precession rate could potentially require very very long phasing orbits for a 3-burn departure. Venus has a deep enough gravity well that you do want to refuel in LVO coming to/from, but it’s not trivial from an orbital dynamics standpoint13.

Size: As big as you can practically get away with — ideally you’d want this depot to be at least 2x the propellant capacity as whatever the largest vehicle you’re refueling. So somewhere in the 100-2000mT range, or even bigger14. Early versions will want to be single-launch if possible, in many cases repurposing at least one of the main propellant tanks from the stage that delivered them to their destination as one of the depot tanks15. Eventually, it may be possible to do multi-tank depots, but if you can do a single launch depot big enough for refueling two missions, you may be better off making more than one depot instead of trying to make the depot super big.

Propellant Types: For low depots, you’re primarily going to be dealing with large transfer stages (Centaur V, Starship, New Glenn Upper stage), which typically use LOX, and either Methane or LH2 for the fuel. Most of these use autogenous pressurization, and use the main propellants for RCS. So most of the depot will be for LOX, LH2, and/or Methane.

  • For Mars or Venus you may eventually also want to store liquid CO for some applications, since it’s an easier ISRU propellant, but that remains TBD.
  • A lunar low-orbit depot may also want to stock storable propellants, depending on what lander propellants end up being most popular16.
  • You may eventually also want to store some secondary fluids (Helium or Neon for active cooling loops, life support consumables like water, air, etc), but you may not explicitly need a depot for that function.

Some Other Considerations for Human Spaceflight Low-Orbit Depots

  • As mentioned before, human spaceflight depots want to be designed in a way to enable offloading as much of the RPO and docking/or berthing from the vehicles they’re servicing. The less parasitic dry mass that tankers and clients have to lug around on every mission, the better.
  • Storing cryogens in low orbits tends to be hard — you have a warm planet blocking half of the sky. So launching the propellant in a subcooled state or even partially frozen (i.e. slushy propellants) can help a lot. Also a lot should be done to minimize heat leaks between the cold part of the depot and any hot sections (habitation, power, etc). If you can’t get to zero boiloff, LH2 is a great thermal sponge, and can be used to chill other propellants, and intercept heat from heat sources before being vented. You may have to vent some hydrogen boiloff, but if you’re smart, you can use that hydrogen boiloff on the way out to eliminate boiloff issues for everything else.
  • These depots are also big debris targets17. Deployable MMOD/MLI18 solutions could be very helpful to avoid a puncture, which would probably be very hard to patch. Since these depots are fixed, and are typically only performing stationkeeping maneuvers, it may be possible to augment their MLI/MMOD protection over time using in-space assembly/manufacturing techniques19.
  • Especially for low-orbit depots around the Moon/Mars/Venus, there may be a benefit to having some temporary habitation/shelter collocated with the depot, especially if you’re supporting multiple sites, as a search and rescue option during exploration phases, and as a stop-over point to act as a buffer between different sizes of transportation between planets and between the depot and the surface.
  • Over time you may want to add in other facilities such as dry docks for assembling, and repairing/maintaining large in-space vehicles/structures, habitation facilities, etc. But they should probably be coorbital, near the depot, not attached (as that will make cryo thermal management all the harder, and the depot is a big hazardous work location, where you should probably minimize the amount of time people spend in close proximity to it). This could be done in two ways — coorbital facilities, spaced where the safe time to travel from one to the other is as short as practical (definitely less than an 8hr work shift if at all possible, and much closer if possible), or by having the two facilities connected by a connecting tether or other structure that includes elevator facilities.
    • If your depot facility starts wanting to have permanent collocated habitation (say for in-space assembly/repair/maintenance of in-space stages), and having the two be coorbital doesn’t work, you’re likely going to want to keep the people separated as far away from propellant tanks as possible, both to minimize heat-leak into the tanks, but also to minimize hazard to the people20.
    • For larger depots, and ones where people will be there more, putting some thought into spatially separating the fuels and oxidizers more could be a good idea. In rockets you often can’t do much to keep the two separated, and many use a common bulkhead, but in a fixed facility it’s more of a possibility. Having fuel and oxidizer that close together for long periods of time is somewhat tempting fate — you kind of have to do it for high efficiency rockets, but there’s something to be said for having your fuel and oxidizer many meters apart for a long-duration facility.
  • One area of disagreement I have with other depot advocates is whether propellants should be shipped to a depot as cryogenic propellants (LOX/LH2/LCH4), or if you should ship them as something more storable like water and CO2, and have the depot itself have large-scale electrolyzing, separation, and propellant refrigeration systems. My concern is that while in theory very large solar arrays could be done in space, combining large flexible structures like multi-MW solar arrays and radiators with a facility that sees a lot of docking, propellant slosh, etc seems like a bad idea from a structural dynamics standpoint. Also depots with very large power generation and heat rejection capabilities are likely to come later in the process, since they’ll almost certainly require multiple launches and in-space construction.

Anyhow, I probably could go on, but as with the previous parts of this series, I am only trying to scratch the surface with considerations and operating details, as I introduce each new type of depot. This definitely isn’t the last you’ll hear from me on the topic.

  1. Sykora, Fritz, “Guido von Pirquet-Austrian Pioneer of Astronautics,” History of Rocketry and Astronautics, R. Cargill Hall, ed., AAS Publications, San Diego, 1986, p. 151. And yes, I’d love to see someone actually fly a depot before we hit the 100th anniversary of von Pirquet first discussing the idea
  2. Or at least orbits/halo orbits where the only maneuvers being performed are for stationkeeping purposes
  3. MicroMeteorite and Orbital Debris
  4. With some lightweight grapple fixtures, refueling modified T-0 umblicals, and maybe an upgraded comms/controls system
  5. Most of these points could easily justify their own blog post, and knowing me, I’ll probably eventually do said blog posts, but I’ll try to keep things in this post brief.
  6. I’m currently noodling doing a paper looking specifically at this optimization — higher inclinations for the depot give you the ability to hit higher departure declinations, and are accessible by more launch sites, but at the cost of slower nodal precession driving longer hang-times for a three-burn departure, and you also have decreased payload mass to higher inclinations. My gut says ISS-like orbit is probably not far from the optimal point, but it would be fun to run the numbers
  7. As a reminder, RAAN stands for Right Ascension of the Ascending Node, which is a measure of where a given orbit plane crosses the equator heading northward. Here’s a decent wikipedia explainer. When you see a multi-plane constellation, like say OneWeb or Starlink, in a lot of cases the satellites will be put into multiple planes with each plane having the same inclination and altitude, but a different RAAN. The different RAANs help with more frequent passes over the same point in the ground, and for depots by increasing the frequency with which a depot passes through the plane of a specific departure asymptote. If that makes any sense.
  8. I could easily see a scenario where the first human spaceflight depot evolves from smallsat launcher depots.
  9. Though how low of a LLO is open to debate — lower orbits have higher stationkeeping requirements, and are harder to do sunshields with because the Moon takes up more of the sky surrounding the depot. I haven’t done an optimization analysis myself, but 250-500km above the Moon might reduce both of these effects noticeably compared to the 100-150km parking orbits used for previous lunar missions
  10. The J2 parameter is a measure of how oblate or “round about the middle” a planet is. This is typically tied to the rotation speed of the planet. The J2 perturbation is what causes orbital planes to precess slowly. If a planet had no J2 or very low J2, the planes would stay in a fixed orientation relative to the stars, which is problematic if you’re trying to get the plane to line up with a departure asymptote for an interplanetary departure… Earth, Mars, and most of the gas giants have high J2s, while the Moon, Venus, and Mercury all have very low J2s.
  11. The equatorial velocity of Mars is ~half that of earth, so it’s not as big of a deal to launch from or into a higher inclination, but it still is some losses
  12. I’m not much of a Mars guy, so am handwaving that part a bit
  13. If someone is looking for a PhD dissertation topic, this feels like an area that could use a lot more skullsweat investigating.
  14. Depending pretty strongly on how successful Starship ends up being, and how open Elon is to using other people’s depots. In a world where Starship either doesn’t pan out, or Elon insists on doing his own thing, the required size for refueling other missions can be a lot more modest initially. Also a world with depots in low-orbits around destination planets, and roving depots and/or high-orbit fixed depots may not need LEO depots to be quite so big.
  15. As shown in Part IV of this series.
  16. For long duration lunar landers there are definitely differences of opinion about whether storables or cryogens make more sense.
  17. Big LEO facilities that don’t want to dodge things all the time is one of those reasons why moving to a leave-no-trace approach to satellite operations is going to be an important part of growing up as a spacefaring civilization.
  18. MLI, or multi-layer insulation is a type of very effective insulation for use in vacuum environments. It is typically made of many layers of thin metalized plastic films separated by nets or spacers so that most of the heat has to transfer via radiation instead of conduction or convection. Having more spacing between layers can help. And in some cases, MLI can be combined with MMOD (which also wants to be multiple thin layers with spacing between them), like what our friends at Quest Thermal have worked on.
  19. Most hypervelocity impact shielding benefits a lot from extra spacing between bumper layers, so an in-space assembled/manufactured MMOD solution could be particularly useful
  20. Though as my wife pointed out, if your depot facility starts wanting anything, it’s probably a sign of an impending robot apocalypse, so the point might be moot.

      For there to be a race, do both contestants have to know the objective? Or even that a race is, in fact, being raced?
     Most Western studies on the future of human (as opposed to automated) solar system exploration have applied technology developed in the twenty-plus years since Apollo designs were frozen to reduce the costs of more or less traditional goals: a Lunar base, human Mars missions, or asteroid mining. Recently, however, three men—a man space historian Tim Kyger refers to “one of America's astronautical pioneers,” a former astronaut, and a planetary scientist—have all begun to advocate a new post-Space Station goal.
     This goal is one they believe will be much easier to accomplish than any of the older projects, and thus more likely to win political approval in this age of limited budgets. It would combine into a single project the traditional aims of the scientific exploration of the solar system with the newer ones of space industrialization, thereby enlarging the constituencies of both. And it would be a significant step toward that ultimate goal, the beginnings of solar system trade.
     This new goal is the human exploration and exploitation of Mars’ two small moons, Phobos and Deimos. In terms of total velocity change, these bodies are essentially the easiest of solar system objects to reach from Earth orbit. The realization of this single fact could allow the early establishment of a solar system economy based on the low-cost delivery of water and fuel from these moons to industries in Earth orbit and eventually on Luna—perhaps as early as the turn of the century.
     Six years of paper studies by these three scientists appear to have been largely ignored by NASA. They may not have been ignored elsewhere.
     At the 16th annual Lunar and Planetary Science Conference in 1985, the Soviet Union chose to dump a decades-old tradition of absolute prelaunch secrecy on automated solar system exploration. At a conference attended by the cream of Western space scientists and management, Soviet scientists described in unprecedented detail a 1988 mission to none other than the Martian moon Phobos.

     Water—for life support, rocket fuel, and industrial processes—is the single most important resource required for the colonization and industrialization of the inner solar system. It is also the heaviest. And worse, down here close to the Sun, it is a very precious commodity, either locked up at the bottom of massive gravity wells (Earth, Mars) or entirely absent (Luna). To find an almost infinite supply, at less delta-V (the sum of all the changes in a spacecraft’s velocity required for a given mission; in this case, a measure of its cost) than any other source regularly available from low Earth orbit, is a miracle which could mean the difference between space industrialization in our lifetimes—or never.
     Yet S. Fred Singer, former astronaut Brian O'Leary, and planetary scientist Bruce M. Cordell believe they have found just that.
     If it were not for their circular, equatorial orbits, it would be easy to conclude that the Martian moons are captured asteroids—and many scientists do anyway. Both are small and very dark; both are saturated with craters blasted into ancient regoliths; both maintain the same face toward Mars, with their major axes aligned toward the center of the primary.
     In other ways, these asteroid-moons are very different. On a small scale (hundreds of meters) Deimos appears rougher than Phobos, while on a larger scale, the opposite is true. Phobos is wracked by what appear to be cracks a few tens of meters deep and hundreds wide, possibly resulting from an ancient impact which blasted a crater nearly a third as wide as Phobos’s longest axis. Crater counts suggest the surfaces of both moons are on the order of three billion years old.
     Phobos's orbit is believed to be decaying at a rate which will result in a Martian impact in a few tens of millions of years; Deimos is very close to Mars's geosynchronous orbit and moves ve slowly through the Martian sky—making it potentially very valuable real estate, as we shall see.
     But the main value of these bodies lies in their estimated composition, which is similar to carbonaceous chondrite meteorites or C-type asteroids; that is, rich in hydrated silicates and carbon, as well as limited amounts of metals.
     Water content could be as high as 20 percent.

     Evidently, the idea was first taken seriously at the first Case For Mars Conference in Boulder, Colo., sponsored by a group of students and held from 29 April through 2 May 1981. (The papers have been published by the American Astronautical Society in Volume 57 of its Science and Technology Series).
     S. Fred Singer, originator of the Minimum Orbital Unmanned Satellite of Earth (MOUSE) proposal of 1953, based on earlier British Interplanetary Society work, presented a paper entitled “The Ph.D. Proposal: A Manned Mission to Phobos and Deimos." Therein he argued that, even for purely scientific missions, it made more sense to send a human crew to Deimos than it did to send either automated or human missions to Mars itself. He felt the high cost of sending a human mission to the planet, involving the development and transportation of landing vehicles, would make near-term approval unlikely, while automated missions are very limited in the science they can obtain—and are not at all that cheap, in any case.
     But what could humans do for science on Deimos? Well, first there's Deimos itself. Thirty years after the beginnings of the space age, Comet Halley is the only one of the solar system’s small bodies humanity is attempting to explore—despite the fact that these bodies could be of the highest long-term importance, both for science and potential industrialization. [As I write, NASA Administrator James Fletcher has declined to request the CRAF Comet Rendezvous and Asteroid. Flyby mission for Fiscal Year 1988, favoring instead international Earth-orbital projects to map yet again (albeit in greater detail) Earth’s radiation belts and ocean surface topography. Both of these are worthy scientific missions, no doubt, but beyond their obvious military utility, I can't help wondering what they will do to advance humanity's long-term future in space.]
     The biggest problem with automated rovers on Mars is the time it takes for a signal to reach Mars and return to Earth, requiring rovers to be highly autonomous and therefore expensive. Singer suggests that a Deimos base would allow essentially real-time control of therefore relatively simple (and therefore inexpensive) rovers and sample return vehicles. Deimos presents a single face to Mars, allowing simple placement of communications antennae, and is in a close-to-geosynchronous orbit, allowing long communication sessions with Mars-based assets. This, in turn, says Singer, allows proliferation of those assets: a Deimos mission would allow “the sequential operation of many rover vehicles at different locations on the surface Mars in a single space mission, thus increasing the probability of discovering scientifically exciting results" at less cost than flying and controlling a similar number of rovers from Earth (emphasis on original). A further advantage would be the effective quarantine of samples and personnel at the Deimos base, reducing the risk of back-contamination of Earth by still barely possible Martian organisms.
     Carl Sagan, meanwhile, has pointed out to Brian O’Leary that telescopes on Phobos and Deimos could easily provide a resolution on Mars ten times superior to that of the Viking cameras, and could define the most interesting locations for sample returns and later human landings. O’Leary also quotes S.J. Adelman and B. Adelman as stating at the second Case For Mars Conference in July 1984 that a Phobos base could also support gravity-wave astronomy, radio astronomy, and astrometry research.
     Singer thus argued that automated Martian landings controlled from Deimos are easier, less costly, safer, and could be done much sooner than a human Mars landing. How costly? “Assumimg that certain development costs can be shared with other programs, the incremental cost … is estimated at less than $10 billion (in 1978 dollars) over a fifteen-year period,” says Singer.
     If Phobos and Deimos can be all that advantageous and inexpensive for purely scientific pursuits, what might they do for the quest to find ever-cheaper resources for proposed Earth-orbital industries? It did not take Brian O’Leary and Bruce Cordell long to find a very favorable answer.
     About this time, aerobraking, the science of using a planet’s atmosphere to take orbital energy from a spacecraft, was coming into vogue as a way of reducing the high fuel costs of Earth-orbit Orbital Transfer Vehicle aperations. Since velocities into the Martian system are always less than those involved in cis-Lunar space, it did not take much imagination to realize that any aerobraking OTV developed for Earth-Lunar or Earth-geosynchronous transportation (it is actually easier to get to Luna than it is to get into geosynchronous orbit) would be applicable to entering Mars orbit from an interplanetary trajectory.
     According to O’Leary, aerobraking at Mars can reduce propulsive delta-V requirements from 1.5 or 2.0 kilometers per second to 590 or 667 meters per second for Phobos and Deimos respectively. However, “even without aeroassist at Mars, the moons of Mars are more accessible to the Earth at biennial opportunities than is the Moon of the Earth. The chief difference is in the requirement to soft land payloads on the Lunar surface… [Further,] the [Phobos/Deimos] missions permit low-impulse propulsion for the entire trip, opening the possibility of using solar electric, mass-drivers, tethers, and solar sails as sources of propulsion. The only advantages the Moon seems to offer are its proximity and launch window frequency: days versus months or years” (National Academy of Sciences/NASA Symposium on Lunar Bases and Space Activities in the 21st Century).
     But Bruce Cordell has gone even further. In a report done for a large U.S. aerospace company, Cordell considers three classes of human missions: a human mission to the Martian surface in the absence of any “significant” Lunar utilization; human missions to Mars after the establishment of a lunar base; and the possibility that “the next major post-Space Station civilian goal in space for the United States (and its collaborators) should be the exploration of the Martian moons. This program should occur prior to, or at least concurrently with, the development of any Lunar industrial capability.”
     Cordell lists many of the same advantages for early Phobos/Deimos development as O’Leary, including regular launch windows, as opposed to asteroids; round-trip delta-V's which “compare favorably with those to any asteroid:” the fact that Phobos and Deimos have “better specified physical and chemical properties than currently known Earth-approaching asteroids;” and potentially of greatest importance, “propellant production plants on Phobos/Deimos … make the manned landings on Mars independent of terrestrial fuel supplies.
     This could be very important because, on Mars, existence of crustal swells, rifting, volcanism, impact cratering, and abundant water … [possible] hydrothermal, dry-magma, and sedimentary mineral concentration processes … [and] tectonic similarities between mineral-rich Africa and portions of Mars, suggest that the potential for mineral wealth on Mars is impressive. ” Any such minerals would reside within Mars's relatively small gravity well, making their use by space-based industries less expensive than Earth-derived materials.
     The main emphasis of Cordell's study was the development of an economy based on the mining and transport of water from these moons. The first human mission of the Martian system, under Cordell’s plan, would take place during the 2001 window, although he can find no reason why that should not happen earlier. In fact, he told me he now considers his entire argument to be far too conservative.
     In order to keep this first interplanetary mission simple (and thus inexpensive), Cordell proposes an unusually small crew of three, one of whom would be a geologist.
     The 2001 mission would use two reusable Orbital Transfer Vehicles stacked one on top of the other into a single spacecraft. All the fuel of one and 30 percent of the fuel of the second would be used to get to Mars, with aerobraking in Mars’ atmosphere used to slow the remaining vehicle. Sixty days would be spent remote sensing, mapping, probing, and sampling both Phobos and Deimos. The spacecraft then would return to Earth via a Venus swingby, and would maneuver to rendezvous with the Space Station. (Altematively, aerobraking in Earth's atmosphere could replace the Venus swingby, resulting in a shorter travel time but at the price of heavier aerobraking equipment.) Total delta-V for this round trip is 7.91 kilometers per second—compared to 9.0 to and from the Lunar surface.
     Using the information obtained on this mission, “intense” research and development on obtaining water and OTV fuel from Phobos/Deimos regolith material would be undertaken. In 2005 a second human mission would continue the exploration begun on the first mission, and would begin initial work on a base on one of the moons, or a rotating orbital station, depending on human zero-gravity endurance. The crew of this OTV would remain in the Mars system, to be relieved at the next Mars window, two years later. Including travel time, each crew would spend approximately four years in space, which, according to James Oberg, would result in sufficient radiation exposure to limit each crew to a single mission.

     Following construction of what Cordell describes as humanity’s first base outside the Earth-Moon system, a new class of automated OTV optimized for water transport would arrive, to be loaded with water mined from Phobos or Deimos, and then head back for Earth orbit or any Lunar base. Phobos/Deimos-Luna transport loops require about half the total delta-V of Earth surface to Lunar surface loops. “This is due to the large ascent requirement for Earth; from Earth to low Earth orbit it is 9.7 kilometers per second." Keep in mind that the Phobos/Deimos loop was 7.91 kilometers per second. It could be less expensive to fuel cis-Lunar OTVs from the Mars system than it would be to deliver the fuel from Earth, and certainly “all necessary propellants—for both the Mars-bound and Earth-bound portions of [water transport flights]—are produced completely from water extracted from the Martian moons.”

From THE PHOBOS RACE by Donald Frederick Robertson (1987)

      Tickets now available: 50 to 107 light-minutes.

     [Heard on live evening newscast 2197-06-01 on Ceres]

     “Twelve kiloseconds ago (~3 hours), Orbital Materials LLC announced their intention to establish an oxyhydrogen propellant depot on Phoebe, a retrograde satellite of Saturn, within the next decade. A spokesman from Orbital said Phoebe was purchased from a private collector. We have colonization analyst Helen Graves here with us on Ceres. Helen?”
     “Right here, Mindy.”
     “Helen, what are your thoughts on the Orbital Materials acquisition?”

     “Well Mindy, as you know, I’ve studied interplanetary colonization for decades, and the Orbital acquisition seems hopelessly long-sighted. Phoebe orbits Saturn, and there are no present plans for colonization that far out. The closest well-frequented base would be Pasiphae Station, in the Jovian system. Frankly, Mindy, they just won’t have any customers.”
     “What do you think is their aim in acquiring such a risky investment, then?”
     “I’m guessing they intend to bootstrap colonization efforts themselves. Phoebe is undeniably well-suited for it. The moon orbits retrograde, which makes it easier to rendezvous with from certain Hohmanns, especially with slingshot capture tethers. It also has the vast wealth in water to make the fuel itself.”

     “Thanks, Helen. Again, if you’re just joining us, Orbital Materials has acquired the moon Phoebe for speculative use as a propellant depot. Construction will start after the first crews arrive; I’m told Orbital will use higher-energy transfers to cut down on the six-year Hohmann from Earth. I’m Mindy Graham, and this is Ceres Evening News.”

From PHOEBE STATION by Ian Mallett (2016)

Part of my series on countering misconceptions in space journalism.

Humans like to reason by analogy. It’s a powerful problem solving technique because so much of our experience generalizes, while first-principles thinking is computationally costly. In space, however, thinking by analogy is almost always wrong, because our terrestrial experience shares so little with the realities of space travel.

One concrete example of this is our intuition around fuel. Humans need to eat to stay alive, and similarly we charge our devices and fuel our cars. All of them can perform their essential duties with a well-contained, discrete battery or fuel tank.

Rockets, on the other hand, are little BUT fuel tank, and it’s important to understand why. Going to orbit involves more than flying beyond the atmosphere, which while difficult is comparatively easy. Going to orbit involves going fast! Roughly 7.8 km/s, or 17,000 mph. These numbers are so huge it’s difficult to imagine how to go that fast.

Rockets work by throwing mass out the back, as fast as possible. A really good rocket engine can eject hypersonic exhaust gas at more than 10 times the speed of sound, which seems fast enough for anything. On the other hand, orbital velocity is more like 25 times the speed of sound. This means that a rocket entering orbit is throwing exhaust products behind it literally as fast as chemistry and physics allows, and yet that gas is still travelling forwards faster than its ejection speed.

The exhaust results from burning fuel, and the fuel that’s ejected has to also be accelerated to these great speeds. This is similar to the effect that fuel mass has on the efficiency of long haul jets, but much much worse. In a jet, the aircraft has to carry passengers, cargo, and the fuel it needs to land for the whole journey. In a rocket, the vehicle also has to carry the oxidizer and the speeds involved are much, much greater.

As a result, the final velocity of the rocket increases only logarithmically with the ratio of fuel mass to everything else, a brain-melting problem often called the “Tyranny of the rocket equation.”

Δv = v_e log(Mi/Mf),

where Δv is the change in velocity, v_e is the exhaust velocity, Mi is the initial mass, and Mf is the final mass. Astronaut Don Pettit has a nice summary of these issues.

In space travel, Δv is everything, and it determines how much fuel is needed to go from place to place, as summarized in this handy chart:

In addition to the absurd 9.3km/s necessary to reach Earth orbit, most other destinations are a reasonable fraction of any achievable exhaust velocity. As a result, mission design is primarily about figuring out how big the fuel tank is and where to put it.

This horrible state of affairs means that it’s basically impossible to get to orbit, let alone deep space, using conventional engineering. In other words, all rocket scientists need to employ at least one crazy idea if they want to get there. The problem with crazy ideas is that it’s hard to tell which ones are almost practical.

As an example, many rocket scientists will reach for hydrogen and oxygen as a high performance fuel, even though hydrogen’s low density and hard cryo temperature mean that the mass fraction of the rocket takes significant penalties. The Space Shuttle is another example of where following wild ideas can lead. SpaceX instead employed lithium/aluminium alloys, materials that are extremely light and almost impossible to weld. There are no easy options – all options involve a great deal of difficulty, and many turn out to be impossible.

At the end of all this, a really good rocket is able to deliver about 4% of its launch mass to orbit. Everything else is structure, engines, and fuel. This is why rockets are really nothing like cars and bicycles and planes.

So it is that, faced with the impossibility of the problem, creative scientists and engineers will face temptation to veer off into wild hypotheticals. Many of the subjects of this blog series deal with the relative impracticality of some of these ideas.

The subject of this blog, after a this fairly sizable preamble, is refueling depots.

The idea behind refueling depots can begin with an analogy to gas stations. Most launch vehicles arrive in LEO with both cargo, such as a satellite, and an empty upper stage. If that stage could be refueled like a car, it would have enough Δv to go almost anywhere in the solar system.

Yes, you read that right. A close examination of the chart above shows that while getting from the surface of the Earth to LEO requires about 9.3km/s of Δv, LEO to the Moon, Mars, or Jupiter requires only about another 3-5km/s of Δv. Since the upper stage of most rockets delivers just over half of the orbital Δv, if refueled they would be able to send missions all over the place.

This fact has been appreciated for a very long time. Indeed, the science fiction author Robert Heinlein said it best when he said “If you can get your ship into orbit, you’re halfway to anywhere.”

Under the status quo, these perfectly good rocket stages are discarded, wasted, and either left to rot in orbit or burned up in Earth’s atmosphere. If a refueling capability existed, existing rockets could also launch larger payloads, or existing deep space payloads could be launched on smaller rockets.

As it happens, I believe that miniaturization and modularity are not sustainable ways to save money on large-scale space projects, a concept embodied in SpaceX’s outsized Starship vehicle. But it turns out that there are bigger weaknesses with the fuel depot concept.

The fundamental problem with refueling from a depot is that, in space, the cost of fuel is not determined by volume or weight, but by location. It’s not possible to economize on fuel by making a hybrid Prius rocket and driving it more slowly, at least not in any conventional sense. Fuel needs to be in the right tank at the right time in the right quantity, and never for very long before it’s used.

The obvious place for a fuel depot is in LEO. Fuel depots further out, such as near or on the moon, have much less benefit. Launch to LEO is the hardest leg, so it makes sense to refuel at each end.

In more detail, when we think of space we think of the night sky overhead, with satellites zooming around. What this disguises, and was ignored in the film Gravity, is that even a set of orbits as restricted as LEO is not one place. In practice, getting from one orbit to another can require more fuel than launching from Earth to that orbit in the first place.

Most orbital vehicles, such as Soyuz, Dragon, or the Shuttle, have 200-500m/s of Δv for orbital maneuvering. Typically, this fuel is used to correct imperfections of the launch, approach and dock with the space station, and then de-orbit.

As players of Kerbal Space Program will know, 500m/s is plenty of Δv to tweak the relative orbital phase, eccentricity, and semi-major axis. It is, however, vastly inadequate to change the inclination or the longitude of the ascending node, which are two other Keplerian orbital elements. Indeed, if we break up all LEO orbits into inaccessible adjacent slots accessible within a 500m/s window, there are about 200 distinct inclinations, and 200 distinct longitudes of ascending node, for about 40,000 discrete orbits.

If orbital refueling was standard practice for every launch, then we’d need to build (and keep supplied) on order 40,000 separate depots. This is obviously impractical, since the cost of building and operating even one is probably more expensive than continuing to manage without them.

In practice, we have to select just a handful of these orbits, just as the ISS needed a discrete orbit. Various considerations can be employed to select these orbits. For example, the ISS, which can be thought of as an orbital depot of people and fatigued aluminium rather than fuel, was placed in an orbit that was accessible for the launch sites and vehicles of the contributing countries.

An ISS-like orbit is a natural choice for a fuel depot, but it would only be useful for a subset of deep space launches. If the depot’s orbital plane doesn’t align with the destination during the relevant launch window, that’s just tough. For the moon, this would mean being able to launch only two or three days per month. For Mars, a dedicated depot would be required for each launch window. There are orbits whose orbital plane precesses in as little as two months, enabling alignment during any launch window, but for only a few days in that time. It is also possible to reduce fuel requirements by changing planes during an extended series of departure burns, but even this strategy requires around 1km/s of Δv.

Additionally, a high inclination orbit like the ISS is designed to be accessible for launches from Baikonur,. This increases the inefficiency of launches from any other launch site, which inflates the operational cost of keeping it fueled.

Some proponents believe that the depot could be topped up by salvaging the dregs of conventional launches. All rockets carry slightly more fuel than they need to to ensure margin for orbital insertion. What little is left over, if it happened to be in the same orbit, could be transferred to a depot for storage and later dispensing. Although such fuel would be “free” as salvage, it’s unclear how enough fuel could be salvaged from a huge variety of launch orbits to meet any kind of demand. If fuel margin on launch is 2%, then the depot would need at least 50 successful salvage operations in close succession to be able to refuel even one additional stage.

Operationally, depot management is complicated by a boom-bust cycle of use. For regular gas stations, usage is relatively steady and predictable, while the cost of unsold fuel lurking in an underground tank for a week or two is relatively negligible. Deep space launches occur infrequently and have differing quantities of use and even propellants. In a future where human flight to LEO or the Moon is ramping up, a depot would have to constantly grow to meet demand, which is a nontrivial requirement. Storing cryogenic fuels for long periods in space is difficult because they heat up and boil off, not least because the Earth is radiating significant heat into the LEO space region.

In practice, an impending deep space launch that required depot refueling would require the depot to be refueled “just in time” by a series of accessory launches, all of which would involve a partly-empty upper stage docking and transferring fuel, only to transfer that fuel back to the probe-carrying upper stage. Given the inefficiencies and losses of doing so, it is much easier to dock the fuel-carrying upper stage(s) with the payload, and perform the requisite injection burn directly. Same outcome, reduced complexity, greatly reduced overhead.

There is a broad exception to the above considerations; a case in which propellant transfer in LEO does make sense. In the SpaceX Starship concept, a fully reusable Starship is refilled by a series of launches while in LEO, before continuing its journey. This is a different model to an orbital fuel depot, though conceivably a Starship could be permanently parked in some orbit as a depot if there was a good enough reason. Orbital refilling is more like the in-flight refueling of a fighter jet than the establishment of a chain of gas stations.

The Starship concept reflects a different line of reasoning. Rather than compensate for a vehicle with a small Δv with a series of gas stations, build a vehicle with a huge Δv and, by refilling in LEO, enable it to fly enormous distances with no further support. Indeed, unlike conventional three stage Lunar lander schemes capable of transporting at most a couple of tonnes of cargo, the Starship is able to refuel in LEO, deliver hundreds of tonnes of cargo to the Moon, and fly all the way back to Earth in a single stage, reusable format. This is more like cargo flights to remote bases and islands that do not refuel after landing.

This graph shows the cargo capacity of Starship to deliver or return cargo from the Moon, depending on where it’s refilled and how low the dry mass ends up being. It can fly similar quantities of cargo to Mars, but must be refueled there by a local propellant factory.

In summary, reasoning by analogy in space almost never works. Be wary of glib assertions that future cis-Lunar industry will employ orbital fuel depots filled by Lunar water and staffed by self-replicating robots. The natural place to fuel rockets is at their launch pad!

From THERE ARE NO GAS STATIONS IN SPACE by Casey Handmer, PhD (2019)

     Introduction: On February 19th 2019 the world entered an era of impending Space-based military asset proliferation with U.S. Space Policy Directive-4 (SPD-4), which codifies legislative efforts in support of the establishment of the U.S. Space Force. There are currently approximately 81 U.S. military assets in Space, based on information that is publicly available. This study provides a preliminary quantitative assessment of potential U.S. military demand for in-Space water-based fuel in support of future U.S. military assets in Earth’s orbit. Our study concludes that if the U.S. military transitioned its future assets to utilization of water-based propulsion, there could be a military market demand for in-Space water-based fuel of 25 metric tons per year (baseline demand). In addition, and more importantly for the evolution of military demand for in-Space water fuel, we determined a parametric relation-ship between the supply of in-Space water on the Moon, Near-Earth Asteroids (NEAs) and the Asteroid Belt and the potentiality of increased U.S. military assets and increased propulsive capability per asset, thus leading to even further increased military demand for in-Space water in the future.

     Note: We make no assertions as to the merit of the establishment of a U.S. Space Force and the militarization of Space. However, increased market demand from civil, commercial and military use cases will all contribute to increased economic viability of Space Resources Utilization (SRU) and investability of associated technological capabilities and SRU value-chain public and private enterprises. The purpose of this preliminary assessment is to provoke holistic thinking on how U.S. military needs for strategic deterrence could drive significant demand for an in-Space water-based fuel supply-chain.

     Results and conclusions: Our estimate of U.S. military baseline demand for in-Space water, assuming today’s number of U.S. military assets in Earth’s orbit, is 25 metric tons, which using ULA’s $3,000/kg price point at LEO, deduces a total market opportunity of approximately $75M USD per year. This market opportunity is not significant and probably does not warrant in itself significant investment in the realization of in-Space water supply-chain for U.S. military needs. However, given the U.S. military’s desire to grow its presence in Earth’s orbit, and the proliferation of low-cost, highly maneuverable, and shorter lifetime military assets in LEO (with higher de-orbiting fuel needs), we foresee the $75M market opportunity growing by at least an order of magnitude in the foreseeable future. In addition, on the supply side, the estimated Lunar, NEA and asteroid belt deposits posit a new paradigm for how the U.S. military could deploy, operate, utilize, refuel, repair and retire or repurpose its assets in Earth’s orbit (and beyond). The U.S.’s notional apportionment of Lunar water deposits alone could, for example, enable the deployment of 33,000(!) equivalent assets (by mass) in Earth’s orbit, with 100X the maneuverability (i.e. station keeping reserves) for 500 years(!). Even though it is not reasonable to assume all of the U.S.’s theoretical Lunar water apportionment would be used for military needs, even a fraction of it would drive significant demand for in-Space water fuel, with a market opportunity in the $Bs/year.

     As noted in the introduction, we make no assertions as to the merit of the establishment of a U.S. Space Force and the militarization of Space. The sole purpose of this preliminary assessment is to provoke holistic thinking on how military needs for strategic deterrence could drive significant demand for an in-Space water-based fuel supply-chain, that enhances the value-chain of SRU for civil and commercial needs, and thus ena-bles increased investability of SRU technologies and enterprises at a global scale.

Water Supply and Demand


MoonNear Earth AsteroidsAsteroid Belt
ProsClosePotential low delta-VEnormous supply
Diverse resources & applicationsSignificant supplyDiverse resources
ConsRelatively high delta-VHighly variableHigh delta-V
Finite supplySignificant unknownsFar away


  • Demand estimates based on current U.S. military assets in orbit
  • Future trends for satellite development are unknown
  • Future assets may be smaller & decentralized, but advancements could dramatically change this

Calculating Demand

Demand Assumptions and Methodology


Deriving water needs:

  • All fuel is replaced with water (Isp~180)
  • Fueling occurs in LEO for deployment
  • Dry mass of satellites does not change
  • CubeSats not considered

Future assets:

  • Follow same orbital distribution
  • Lifetime of 20 years
  • 10X current maneuverability
  • 10% share of all ISRU water


USG Assets Results

Based on ~130 military assets, it was estimated:

  • Current demand is ~45 tons of water per year
  • 333 kg of water per asset per year

A future water propelled U.S. military asset would require:

  • 3,000 kg for deployment
  • 130 kg for disposal
  • 610 kg per year for station keeping

Supply chain demand:

  • ~40% Fuel for LEO
  • ~20% Fuel for MEO
  • ~40% Fuel for GEO

Transportation to destination orbit needs 50% for LH2/LOX and 10% for ion/plasma

Supply Chain Road Map

Calculating Supply 1: The Moon

Supply Chain Road Map

Lunar Assumptions and Methods


  • 1.2 x 1012 kg of water located at the Lunar Poles
  • U.S. Military has 10% share of total
  • About 2/3 of the dry mass of Lunar escape vehicles is payload
  • LH2/LOX fuel for Lunar escape
  • Ion/plasma propulsion used for final fuel delivery


  • 10% multiplier for 10% share of total water
  • ~50% multiplier for water loses due to Lunar escape
  • ~90% multiplier for water loses due to destination orbit insertion

Leaving 5.2 x 1010 kg for military assets

Lunar Results

  • Accessible with conventional technology
  • Relatively high magnitudes of water
  • Important stepping stone to future supply chains
  • Easier to disrupt
  • More Competition
Independent Lunar Supply Chain
Source Water Mass1.2 x 1012 kg
Delivered Water Mass5.2 x 1010 kg
Theoretical Number of Deployable Assets~3.4 Million Assets
Time Until Depletion~510 years

Calculating Supply 2: Near Earth Asteroids

Supply Chain Road Map

Near Term NEA Assumptions and Methods


  • Only 5-30m asteroids are minable
  • C types make up 20% of NEAs
  • Even distribution of asteroid size and type
  • U.S. Military has a 10% share
  • Solar baking method is scalable
  • Ratio of asteroid to spacecraft is 60
  • Min of 50% of water reserved for return
  • 75% of the water is mined out and the remainder of the asteroid is ditched
  • Fuel to despin asteroids is negligible
  • LH2/LOX is used for outbound
  • Derived LH2/LOX fuel for Earth return
  • Ion/plasma propulsion is used for fuel delivery


  • Use assumptions to find delta V max
  • Use delta V in broken plane delta V function to estimate mass in range
  • Adjust based on ratio in size range
  • 20% multiplier for C types
  • Density adjustment multiplier
  • 10% multiplier for percent water
  • 75% multiplier for water taken
  • 50% multiplier for water saved
  • Subtract water inbound for net
  • Adjust using Reiman sums to account for greater recovery at all lower delta Vs
  • 10% share of total water
  • ~90% multiplier for water loses

Leaving 5.3 x 107 kg for military assets

Near Term NEA Model

Near Term NEA Results

  • Comparatively low water mass
  • Extremely efficient for early ISRU cascade all within Lunar delta V
  • Water return ratio here is 5 times the investment
  • Provides value as a redundant supply chain
  • Hard to disrupt
Independent Near Term NEA Supply Chain
Source Water Mass1.6 x 109 kg
Delivered Water Mass5.3 x 107 kg
Theoretical Number of Deployable Assets~3400 Assets
Time Until Depletion~170 years

Long Term NEA Assumptions and Methods


  • All NEAs accessible with minimal in-bound loss
  • C types make up 40% of NEAs
  • Even distribution of asteroid size and type across delta-V ranges
  • All C type asteroids can be mined
  • 100% of the water is mined and the remainder of the asteroid is ditched
  • Fuel to despin asteroids is negligible
  • U.S. Military has 10% share of water
  • Derived LH2/LOX fuel for Earth return
  • Ion/plasma propulsion used for final fuel delivery


  • 20% multiplier for C types
  • Density adjustment multiplier
  • 10% multiplier for percent Water
  • 75% multiplier for water taken
  • ~16% multiplier for loses due to Earth return
  • 10% multiplier for military share
  • ~90% multiplier for water loses due to destination orbit insertion

Leaving 9.8 x 1012 kg for military assets

Long Term NEA Results

  • Tremendous resources available in the NEA population
  • Much of it is currently inaccessible
  • Requires significant R&D but it’s worthwhile
  • Hard to disrupt
Independent Long Term NEA Supply Chain
Source Water Mass6.7 x 1014 kg
Delivered Water Mass9.8 x 1012 kg
Theoretical Number of Deployable Assets1 Million Assets*
Time Until Depletion12,800 years*

*Capped at one million with demand becoming linear thereafter, as # of assets was far too great to be realistic

Calculating Supply 3: The Asteroid Belt

Supply Chain Road Map

Asteroid Belt Assumptions and Methods


  • Asteroid belt mass is 3x1021 kg
  • C type make up 40% of the population
  • C types are 15% water by mass
  • Asteroid intercept requires average delta-V of 8000 km/s
  • Fuel losses due to asteroid escape are negligible
  • Derived LH2/LOX fuel for Earth return
  • About 2/3 of the dry mass of miner is payload
  • U.S. Military has 10% share
  • Ion/plasma propulsion is used for final fuel delivery


  • 40% multiplier for percent C types
  • 15% multiplier for percent water
  • ~13% multiplier loses due to Earth return
  • ~90% multiplier for water loses due to destination orbit insertion
  • 10% multiplier for the 10% share of total water

Leaving 2.1 x 1018 kg for military assets

Asteroid Belt Results

  • Provides inexhaustible resources
  • Much of this water is centralized to Ceres
  • Could support planned Moon like manufacturing
  • Ceres will likely be an important strategic & economic hot spot for humanity
Independent Asteroid Belt Supply Chain
Source Water Mass1.8 x 1020 kg
Delivered Water Mass2.1 x 1018 kg
Theoretical Number of Deployable Assets1 Million Assets*
Time Until Depletion2.8 Billion Years**

*Capped at one million with demand becoming linear thereafter, as # of assets was far too great to be realistic

All Independent Supply Chain Outcomes

Given a 10% share of the total resources and estimated requirements for resource return, based on the average military asset with 10X maneuverability and 2% exponential growth, each supply chain could independently support:

Lunar Supply ChainNear Term NEA Supply ChainLong Term NEA Supply ChainAsteroid Belt Supply Chain
Source Water Mass1.2 x 1012 kg1.6 x 109 kg6.7 x 1014 kg1.8 x 1020 kg
Delivered Water Mass5.2 x 1010 kg5.3 x 107 kg9.8 x 1012 kg2.1 x 1018 kg
Theoretical Number of Deployable Assets3.4 Million3,4001 Million*1 Million*
Time until Depleition510 Years170 Years12,800 Years*Unlimited*

*Capped at one million w/linear demand after for asset replacement, as the exponential # of assets was unrealistic


Controversy and confusion continue to swirl around the issue of a cislunar space base, no matter where it is proposed to be built, what it is named, or what it is supposed to be for. Assuming that one of the primary purposes of such a base should be to supply lunar-derived propellant to vehicles in or departing from cislunar space, there are several immediate problems. First, there is a massive conflict in the potential schedule, since human missions to Mars that could be fueled from the Moon could take place about the time we find out if there is any actually accessible lunar ice available. We know that there are massive ice deposits on Mars, while most of the Moon is more than bone dry and the critical polar water deposit surface characteristics are still hidden from us.

Another problem area is the related but unknown costs of building the lunar mining base, the cislunar base, and the transport system to move the propellant from the lunar surface to the cislunar base, and how much that would add to the cost of lunar propellant. It is very hard to estimate the cost of developing the mining base. Some claim that the cost would be so high as to make the lunar propellant more expensive than propellant brought from the Earth, even though the cost of moving propellant from the Moon is usually quoted at about 15–20 times less that bringing it from the Earth’s surface.

Finally, there is the NASA plan, dormant for a while and now seemingly moving ahead, to create a way to use its obsolete and expendable SLS rocket to support what it still refers to as a lunar “gateway.” This is the project I have referred to in the past as the “Gateway without a Gate.” Very recently, the name and orientation of this project has changed again, to the Lunar Orbital Platform-Gateway, or LOP-G. It may seem as if a station in lunar orbit is more closely associated with actual lunar development, but placing a station in any actual lunar orbit, since it is then in an orbital plane, restricts the number of lunar surface locations that are easily accessible, and some of these so-called “lunar” orbits spend a lot of time far from the Moon.

Since most plans and sources do not mention refueling and logistics facilities as integral and initial parts of this project, such a “gateway” would neither be able to support a lunar base nor support dispensing lunar propellant produced by such a base, and are unlikely to be added later. A NASA request for information (RFI) in late 2017 covered many aspects of gateway science, but not a single transport issue. Most people would agree that an actual gateway provides a pathway to some physical location. So where is the physical path for this gateway? Where does it lead from and what does it lead to? In spite of the acronym, the planners have lopped off the critical gateway (transport-related) features—if they were ever there to begin with.

Some people fear that this project would eat up any and all funds for an actual lunar mining base and thus are now insisting that any lunar base be supported only from low Earth orbit, which is less efficient. If the platform is supported by the SLS, it would eat up even more funds, leaving little for any other human space projects, and could delay the establishment of an actual lunar base by a decade. With time, bureaucrats could decide that it is too dangerous to have propellant depots docked at the “gateway.” When the powers that be finally realize that crews stationed beyond LEO actually do need a significant amount of shielding mass to protect them from cosmic radiation, the whole project could be thrown into redesign disarray and last another decade before even being launched.

Obviously, a vehicle about to depart for Mars will not want to land on the Moon to get its propellant. Even getting into lunar orbit from a cislunar location would waste fuel. Thus, most experts believe that a location like Earth-Moon L1 or L2 is the best place to accumulate a large store of propellant, since it is always in the same position relative to the Moon and Earth, and thus is not subject to orbital plane limitations. Since vehicles ready for either departures to Mars or to cyclers going to Mars would need to be positioned at locations other than L1 (but at about the same distance from Earth), the main propellant depot would probably be at L1, with temporary depots positioned at other locations during Mars transit windows every 26 months.

So why is having a lunar-derived propellant supply in a near-lunar location so favorable? It’s the propellant cost, along with some other good reasons. If you want to go to any location outside the Earth-Moon system, whether it is Mars or an asteroid, a departure location high above Earth is best since that allows a very efficient Oberth maneuver, which uses a departure burn at L1 and another during a close pass of the Earth. This saves more than half of the departure propellant compared to departure from LEO, and for Mars missions, this means the Mars transit propellant weighs less than the mission’s dry mass. (If you are not using any lunar propellant, the advantage of the high departure point is much less.) From L1, lunar propellant can be delivered to LEO for only about 0.85 kilometers per second of velocity change, and even directly from the lunar surface for about 2.74 kilometers per second, making delivery much cheaper there than Earth propellant, which needs about 9.5 kilometers per second for delivery.

Having a base in a location like L1 makes initial support of a new lunar base much easier, since it is possible to reach L1 from anywhere on the Moon’s surface (or the reverse) in about 12 hours without worrying about the orbital planes. It also breaks the trip from LEO to the lunar surface into two smaller steps in terms of velocity change, thus decreasing the dry mass and fuel mass fraction of each vehicle, and allowing each one to carry relatively more cargo or propellant. This also improves the safety factor, since smaller rocket engines can be used, and they do not fire as long.

The big factor, however, is still the vast cost difference in moving the fuel to L1. The difference is primarily caused by the fact that you need a huge amount of propellant to move the Mars transit propellant from Earth, but only a small amount to move it from the Moon. Let us assume that we have vehicles ready for a Mars mission, either three large 85–100 ton dry mass vehicles, similar in size to the SpaceX BFS stage, able to carry a small crew plus a lot of cargo, or a set of ten smaller 30-ton-range dry mass vehicles, some for crew and cargo and some just for cargo. Assume that both fleets have about 4,000 tons of dry mass and need about 2,100 tons of propellant at L1 to depart via an Oberth maneuver. To show the huge numeric difference in cost between Earth-based and lunar based propellant at L1, we do need to do some simple calculations.

Note that in these calculations, I distinguish between the mass of the rocket propellant needed to move the Mars transit propellant to L1, and the mass of the Mars transit propellant (the payload) itself. To avoid confusion, I will refer to the transit propellant as the transit propellant payload, the surface to LEO propellant (a) carrying the transit payload to LEO, and the surface to LEO propellant (b) carrying up the LEO to L1 propellant needed in LEO to move the transit payload from LEO to L1. On the opposite side of the scale, I will refer to fuel for the cislunar tanker which carries the lunar-derived transit propellant payload as the lunar to L1 propellant. The multiple kinds of propellant uses may be confusing, but these distinctions are crucial to understanding the huge cost difference. All named transport propellant loads include the return to base propellant, as all vehicles are reusable.

Let’s assume you have moved your Mars expedition fleet dry mass (less the transit and bootstrapping propellant) to either a position at L1 or into a high orbit that is at a similar distance from Earth as L1 is, ready for its Oberth-style Mars departure maneuver. The transit propellant payload will need to include all of the propellant (a minimum of 400 tons) needed for bootstrapping the initial landings on Mars before the surface propellant plant there can be set up. During transit, this propellant can be kept in vehicles with cryo-coolers or in one or more propellant depots equipped with cryo-coolers. All in-space propellant discussed here is cryogenic liquid oxygen (LOX)-liquid hydrogen with an assumed specific impulse (Isp) of 460 seconds. All loads of the transit and bootstrap propellant to L1 are 150 tons, and thus a single expedition needs 14 such loads.

What is the amount of propellant mass to move the needed approximately 2,100 tons of transit propellant to the fleet at L1 or a similarly high orbit? Propellant created on Earth for use by this fleet would be moved to L1 in two steps, supported by multiple launches from the surface. If the BFR tanker version is used, 150 tons at a time can be delivered by one BFR tanker to a LEO logistics base. Reaching LEO takes about 9.5 kilometers per second of velocity change. There it is transferred to a small reusable tanker with sunshade and cryocoolers, which will take it from LEO to L1. This takes another 3.77 kilometers per second, for a total delta-V of about 13.27 kilometers per second. To provide propellant for the small tanker, almost two more BFR tanker loads of LEO to L1 propellant need to be delivered to the LEO base per 150-ton load of transit propellant payload. All of the BFR tankers would reenter and land back on Earth. The small tanker would offload the cryogenic propellant to shaded and refrigerated depots at the L1 base, and then, minus its payload, would drop back toward Earth, where it would use a single pass aero-capture maneuver to get into LEO again for a tiny amount of fuel.

For transport of Earth-based propellant, I will use current numbers for the SpaceX BFR (tanker version), as the launcher, which uses LOX-methane propellant. At 4,400 tons liftoff mass, we can subtract the dry mass to get the total propellant mass. The payload (wet or dry) is 150 tons, the upper stage is 85 tons, and the first stage is probably at about 125 tons, giving a total dry mass of 360 tons and thus a nominal surface to LEO propellant (a) mass of 4,040 tons, with almost 1,000 tons of propellant in the second stage. The propellant to payload ratio for this description of a BFR would be 4,040/150 or 26.93: 27 tons of propellant is needed for every ton delivered to an LEO base. The payload mass ratio is an impressive 0.0341. The structural mass ratio for the second stage is also an impressively low 6.9 percent (85 tons/1,225 tons). These numbers will probably change some as the extremely efficient BFR designs are refined further by SpaceX.

We will assume that, at LEO, the 150-ton transit propellant payload is then transferred to the smaller tanker of the same cargo capacity but with smaller engines which use only LOX-hydrogen. Note that the launches from the Earth’s surface need to supply the small cislunar tanker with both the LOX-hydrogen LEO to L1 propellant plus the LOX-Hydrogen transit propellant payload itself. The LEO to L1 tanker is already in orbit. Assuming that the smaller tanker is about 25 tons and has its own cryo-coolers and sunshade, its structure and payload would weigh 175 tons. The delta-V from LEO to L1 is 3.77 kilometers per second, and about 1.0 kilometers per second for the return trip where aerocapture and a small orbit trim is all that is needed. This means the small tanker needs to carry only 7 tons of descent propellant with it to L1, with a margin. This then means the mass that reaches L 1 must be 182 tons, so the ascent (LEO to L1) propellant mass, also with a margin, is 250 tons. The total mass departing LEO for L1 is now 432 tons. Note that the structural dry mass for this small tanker at departure from LEO is 5.8 percent.

However, the 257 tons of LEO to L1 propellant needed to move the single load of transit propellant payload propellant to L1 and get the empty tanker back to LEO also needs to be moved up to LEO first, and requires the use of 6,921 tons of LOX-methane surface to LEO propellant (b) on more than one ride to LEO via the BFR. It costs about $200,000 to launch a Falcon 9 with about 500 tons of fuel (LOX and RP1) on board, thus that fuel combination costs about $400 dollars per ton. LOX-methane might cost about the same. So, in order to get each 150-ton batch of Mars departure propellant to L1 from Earth, it takes the following components:

Propellant masses for delivery of 1 & 14 loads of 150 tons of LOX-hydrogen to L1

Payloadpayl massprop massvehiclefromtopropel. cost
Mars Transit prop (payload)1504,0401 BFR tankerEarthLEO$1,616,000
LEO to L1 tanker propellant2576,9212 BFR tankersEarthLEO$2,768,400
Mars transit prop (payload)1502571 cisln tankerLEOL1102,800
Total propellant mass-11,218bothEarthL1$4,487,200
(Multiply by 14 BFR and tanker loads to L1)
Total propellant mass to L12100157,052bothEarthL1$62,800,000

This is a 74.79-to-1 ratio of propellant to payload delivered to L1 (11,218/150). The transport propellant for one load of Earth propellant thus costs $4.49 million at $400 per ton so that just one ton of Earth propellant delivered to L1 would be worth about $30,000, based only on delivery propellant costs. The total propellant mass needed to deliver 2,100 tons, or 14 BFR loads, of transit propellant payload to L1 is 157,052 tons and costs $63 million, making the propellant cost a major part of a single Mars mission cost when conducted with reusable vehicles. If there were 100 civilian passengers on the trip, the fuel would cost each person $628,000.

Now let’s see how much lunar to L1 propellant is needed to get the transit propellant payload propellant from the Moon to L1. In this case, we are using a lunar to L1 tanker with LOX-hydrogen fuel. It takes 2.6 kilometers per second to go from the lunar surface to L1 and the same to return, but fuel use on return is minimal since there is no payload. I assume that this tanker is 30 tons, since it needs landing legs and it leaves the lunar surface with the same 150 tons of cargo, for a dry and payload mass of 180 tons. The empty 30-ton tanker will need just 25 tons of fuel (with a margin) to return to and land at its lunar base without its payload, so the structural mass, payload, and return fuel mass is 205 tons. The vehicle leaving the lunar base with its payload of fuel needs 170 tons of propellant, with a “wet” mass of 375 tons, so for the whole round trip (where the payload is left at L1), the fuel needed is just 195 tons of propellant.

Reusable Lunar surface to L1 & return tanker masses

Trip Component1 load massmass fractionmass for 14 loads
Dry structural mass0 tons0.08420
Transit propellant payload150 tons0.40210
return to base propellant25 tons0.07350
surface to L1 propellant170 tons0.452380
total mass at liftoff3751.00250
total round trip propellant195 tons0.522730

Notice that this means for a well-designed light tanker ferry, for every 1.0 tons of payload propellant it only takes 1.3 tons of fuel propellant to move the payload from the surface base to L1. At lunar takeoff, this tanker would mass 375 tons, but since it is taking off in lunar gravity, it would effectively weigh only about 62 tons. The composite propellant tanks, similar to what will be used for the BFR, would probably mass only 20 tons, leaving 2 tons for the engines, 2 tons for the landing legs, and 6 tons for the rest of the structure, including the cryo-coolers, sunshade and power system. This provides a structural mass fraction of 8.0 percent.

At this point, we do not know the actual cost of the lunar propellants as delivered to the tanker at the lunar base, including the vehicle development costs, but finally we can compare the relative mass of the delivery propellants. Assuming that we have not underestimated the tanker mass, the delivery propellant mass from Earth to LEO to L1 is 57.5 times more (74.8/1.3) than the delivery propellant mass from the Moon to L1 for the same 1 ton of propellant as payload. (Note that this mass difference is about three to four times larger than the typical cost difference of 15–20.) Cost of lunar fuel would include fuel production costs, transport propellant costs, and vehicle operation costs. The wear and tear on lunar-to-L1 vehicles is much less, since no vehicles need to take off from the Earth or re-enter the Earth’s atmosphere. Even the cislunar tanker must undergo an aerocapture to return to LEO from L1, while the lunar-to-L1 tanker would encounter only the stresses of thrust and landings. In addition, propellant at L1 is at least twice as valuable as propellant in LEO, due to the gravitational potential and additional velocity from an Oberth maneuver performed starting at L1.

So if the lunar propellant cost proportionally as much as the delivered Earth propellant before the lunar propellant is delivered to L1, it might cost about $23,012 per ton (57.5 times $400), or ($29,915/1.3) on the lunar surface. If future lunar entrepreneurs can beat that price, and if lunar water ice does exist in minable quantities, the lunar fuel production enterprise seems assured. Sales of large amounts of propellant for Mars expeditions would assure a robust human presence on the Moon. However, to assure that the logistics capability of any cislunar or L1 base will exist, the propellant depot and cargo handling capabilities must be a part of the design of the base from the beginning. Plans for the NASA cislunar base must accept that large propellant depots can be attached to it.

We are getting significant indications of international interest in a lunar base. At the same time, NASA says that there may not be enough room on the LOP-G station to accommodate international science participation. Why does the cislunar base have to be so small if there is support for it? The solution to this problem is a package deal agreement where there is a combined cislunar, lunar, and fully reusable transport development effort taking place simultaneously, with the international partners providing some of the transport and lunar surface infrastructure, with heavy reliance on commercial launches. Development would take place so that the first human landings at a lunar base site would take place within a year of the cislunar base completion. Without such a “package deal,” the NASA cislunar base would probably become the “gateway with no gate” or a “space station junior” copy with very limited utility, as many of us have feared.


Next up: in-space refueling. The first and most obvious customer is NASA; an orbital fuel depot would allow them to launch satellites on smaller LVs or launch larger satellites, allowing a choice between savings on the LV and increased capabilities on the spacecraft. That could mean buying an Atlas 401, Zenit, Soyuz, H-IIA or Falcon 9 instead of an Atlas 551, Ariane 5, H-IIB or Delta 4(5,4). Perhaps less obvious, Russia would see a significant benefit from a LEO depot in the plane of the Baikonur launch site. Vehicles would refuel in order to plane-change to an equatorial orbit for GEO deployment. Further into the future a fuel depot would be essential for the smooth operation of tugs and satellite tenders, serving as a buffer between fuel launches and fuel used in missions.

I think the current leader is Boeing with their in-development ACES vehicle using integrated fluids management. However, they are focused on LOX/LH2 propellant; few customers today have cryogenic upper stages. Hypergolic fuels require a different set of technologies and would most likely require shipping expendable supplies of a pressurant, either nitrogen or helium, but they have a larger potential market right now as hypergolics are typically used for satellite stationkeeping and orbit changes. The third fuel category is inert gases (argon, xenon) for ion engines; these can be stored as compressed gases or cryogenic liquids.

I think a near-term possibility is simply to ship water. It is dense, relatively inert and can be used as a life support consumable or as a propellant after electrolysis. It has a high surface tension and can be wicked out of a bulk tank in microgravity without pressurants or membranes. There are cubesat-scale thrusters available today that separate water over time, accumulating a charge of gaseous O2 and H2 using small amounts of power, then ignite that fuel in a high-efficiency engine. If future satellites were to adopt this technology for RCS and stationkeeping then they could nearly double their Isp while eliminating toxic fuels and cutting down to a single storage tank. Beyond the near-term possibilities, a water depot operator would be able to buy water from any LEO cargo provider as well as any asteroid mining company, relying on the proven launch capabilities today while safely and cheaply allowing for a riskier but cheaper future supply.


     This entry covers cis-lunar space. The topic of lunar mining and fuel supply has a rich field of information available and I cannot claim to know all of it, but hopefully this will show how we can begin to harvest most of our propellant instead of shipping it from Earth.

     First let's establish some basics:

     I assume that we will develop zero boiloff cryogenic storage, reliable cryogenic fluid transfer and reliable cryogenic engine restart. Vehicles will be designed to last 20-30 years, but generally are planned to be replaced every 10 years. Obsolete systems will keep operating until they fail, providing some bonus production capacity.

     The Moon's surface gravity is 1.62m/s², about 16.5% of Earth gravity.
     It takes about 1.87km/s of dV to land or take off from the equator. Only a little more is needed for the poles.
     Another 0.64km/s will take you from low Lunar orbit to EML1 (Earth-Moon Lagrange Point One).
     Low orbits around the Moon are not stable so you should not park anything important there.

     Large amounts of water ice are available at the south pole and most likely at the north pole as well.
     Plenty of metal oxides are available (including iron, aluminum and titanium), but carbon is rare.
     In some locations the mantle interface material (KREEP) is accessible at the surface; this rock is rich in incompatible elements like phosphorus, potassium, rare earths and radioactives.

      Low Earth orbit takes anywhere from 9.4 to 10 km/s of dV to reach.
     Each launch site has a specific, most-efficient inclination. Changing inclinations is very expensive.
     From any low-Earth orbit, EML1 is 3.77km/s away.
     From Kennedy Space Center LEO, geosynchronous orbit is 4.33km/s away. It's only 3.9km/s from an equatorial orbit.
     Earth escape is 3.22km/s away.
     EML1 is the balance point between Earth and the Moon, the place where the gravity of each body cancels out.
     From EML1, a craft with a heatshield can get to any low Earth orbit for 0.77km/s.
     Without a heatshield, 3.77km/s is required.
     A similar maneuver can use the Earth as a slingshot, departing from EML1 with a little nudge into any inclination and then burning at closest approach for best use of the Oberth effect.
     From EML1, geosynchronous orbit is only 1.38km/s away.

     Putting all of that together:
  • The best place for a fuel depot is at Earth-Moon Lagrange Point One (EML1). Fuel can be harvested at the harvester's pace, shipped to the depot at the tanker's pace and accumulated for later use. Fuel is shipped to LEO only as needed and into the correct inclination.

  • The surface to EML1 tanker needs to use chemical engines to overcome the Moon's gravity. It must carry about 4.6km/s of fuel, though the second half of the trip is empty and requires much less fuel. My reference tanker is 5.5 tons, cryogenic with zero boiloff and can deliver 29 tons of propellant from the Lunar surface to EML1, then land at the harvester. Each trip burns just under 34 tons, so the harvester must produce about 2.2kg of propellant for each kg in EML1.

  • The EML1 to LEO tanker can use either chemical or ion engines. The low thrust option requires a very different trajectory and a dV of about 7km/s, plus a lot of onboard power. This can be a competitive option but initially it would be simpler to use the same tanker for each leg. Because the vehicle needs to aerobrake, it requires a reusable heatshield. The same tanker design is used, but it is launched with a dual-use shroud of about 6.3 tons that is kept with the vehicle. This trip delivers 44 tons of propellant at a cost of 19 tons. So, for each kg in LEO the harvester has to produce 3.1 kg. That lines up fairly well with other sources suggesting 75 tons to be harvested for 25 tons in LEO.

     The harvester is an unknown quantity. There are several concepts (including one of mine), but I will use a more generic NASA number of 10kg propellant per kg of harvester per year, with 10% spares. That means if we want to harvest 100 tons of fuel per year we need to send 10 tons of equipment to start and 1 ton of spares for each year. This was for a Mars ISRU system with rover/excavator and integrated power systems. A Lunar system would be much simpler since it would only be melting and filtering water ice then electrolyzing it, so it is possible the equipment will produce far more propellant than this estimate.
     Cheap propellant in LEO is useful for missions to other planets or moons. It is also useful for supplying a LEO station with water and stationkeeping fuel. Satellite servicing tugs could base out of EML1 with a ready supply of fuel and easy access to all inclinations. If commercial satellites were modified to use a water electrolysis thruster system for stationkeeping then they could be launched empty to EML1 for less dV than a direct launch to GEO. A fill of fuel at EML1 and a 1.4km/s nudge would place the satellite in GEO with decades of stationkeeping thrust available. As a bonus, the upper stage of the launch system could be refueled and repurposed. Lastly, water is extremely useful for manned operations as radiation shielding, drinking water and a reserve source of oxygen.

     An example of why the interorbital exchange would be useful is NASA's Mars Design Reference Mission (DRM). Each manned trip requires three flights (one crew and two cargo), each using around 190 tons of fuel. Just the fuel would require 9 Block 1 SLS launches or 6 of the 105-ton launches; being generous we're talking about at least $4.5 billion. This is 570 tons of propellant over 2.1 years, or about 267 tons per year at around $7.9 million per ton. A full campaign of three flights to Mars would cost $13.5 billion in fuel. With lunar ISRU we can save at least $4.7 billion as described below (potentially over $9 billion) and gain a sustainable supply of propellant in EML1/LEO with minimal Earth mass.
     An ISRU plant with this capacity would mass at least 83 tons and consume about 18 tons of spares per Mars opportunity. A modified version of my tanker could land perhaps 17 tons of cargo on the Moon for about 62 tons of fuel; this would require two Falcon Heavy flights (~$300 million) or an SLS block 1 flight with two additional Falcon 9 refueling flights (~$850 million). This initial 15-ton plant (with 1 year of spares and a 500-kg dextrous robot for remote repair operations) would provide the first ISRU fuel as a proof of concept. The cargo lander would be refueled and returned to LEO two months later with plenty of lunar samples. (3 tons of samples, 25 tons of fuel.) At this point the whole architecture can be validated and the final ISRU design can be settled. Until this step happens we have to use some pessimistic numbers (as below), but there is the possibility that the pilot plant will be many times more productive than expected as it will be running a much simpler refining process.
     A standard tanker with dual-use shroud (11.8t empty) would be launched with a Falcon 9. Another standard tanker with 43.5 tons of fuel would be launched on a Falcon Heavy, transfer 27.8 tons of fuel to the shrouded tanker and then head to the Moon and land by the ISRU plant. The shrouded tanker would travel to EML1. At this point fuel can be transferred all the way to LEO; without a proper depot there are some inefficiencies, so only about 40 tons are delivered in each shrouded tanker (requiring two standard tanker flights to fuel up). Fortunately this is enough for the outbound trip of a cargo tug.
     The first shipment of 40 tons would take the rest of the year to produce and would be delivered to the cargo tug in LEO, allowing it to deliver a 22-ton payload to the Lunar surface. (Payload would be a second 15-ton ISRU plant, two years of spares for both plants and 1 ton of other cargo.) This could be a Falcon Heavy carrying two payloads or one of several competing options. After 30 days of surface operation the cargo tug returns to LEO (again, 3 tons of samples and 25 tons of fuel).
     The next 40 tons of fuel take a bit over five months to produce and allow the cargo tug to bring the next 22-ton package. Return fuel for the tug would take three weeks.
     At this point the infrastructure on the moon is producing 1.25 tons of fuel per day. Each cargo trip takes less and less time to refuel (3.4 months, then 2.6 months, then 2 months). A proper depot at EML1 and three more ISRU packages round out the system. The depot is 300-ton capacity, about 20 tons of hardware and delivered to EML1 by a Falcon Heavy. In total the buildup phase takes two years and two months, almost exactly one synodic period. 90 tons of ISRU hardware is on the surface with an annual production of 900 tons and spares requirements of 9 tons. This system can deliver 289 tons of fuel to LEO every year, or 617 tons per Mars synodic period. The target of 570 tons is met with an extra 47 tons of fuel for delivering spares.
     A total of seven Falcon Heavy flights are required, plus one Falcon 9. Let's call this $1.1 billion in launch costs over 26 months. Let's also assume that spares are supplied as 11-ton packages by Falcon 9, two per synodic period. A third assumption is that we will need to replace about 20% of the system every period (at a conservative 10-year lifespan for components), so we budget a Falcon Heavy flight for $150 million. That's an operational cost of about $250 million per period. In the first synodic period the fuel in LEO will cost $2.2 million per ton in launch costs. Each additional period will cost only $405,200 per ton in launch costs.

     The cost of all that hardware is difficult to estimate. Let's look at two very different systems and get a ballpark figure. The BA-330 expandable space station module from Bigelow is estimated to mass about 22 tons and cost about $250 million, or about $12 million per ton. By contrast, the Iridium NEXT constellation of 72 satellites is expected to mass 57.6 tons in total and cost $2.4 billion (not counting launch costs), or about $42 million per ton. The comsats include costs like bandwidth leases and operations, but let's use that figure anyway as a first approximation. The startup phase requires 131 tons of hardware, or about $5.5 billion. Extended operation will replace about 10% of that hardware per year at a cost of about $550 million. This ignores the significant commonalities between systems; all the propulsion units are identical, all tankers identical other than the cargo adapter and all six 15-ton ISRU plants are identical.
     This gives us the second part of fuel costs: capital expense. Each ton of fuel in LEO in the first period costs $9.8 million, with additional periods costing $890,000. These numbers assume we do not spread the capital costs over five periods, but front-load the entire bill into the first launch season. The first Mars caravan would cost $6 billion instead of $4.5 billion in fuel, but the next one would pay only $1.4 billion. A campaign of three flights to Mars would save about $4.7 billion in fuel costs. As a budget line item, the cis-lunar fuel network would cost NASA or a private operator $375 million per year once established.
     What if, instead, a cost-effective hardware program is used that is closer to the BA-330 in terms of cost per kg? Now our 131 tons of hardware only costs about $1.6 billion with annual replacement costs of $160 million (or per-period costs of $342 million). Under this assumption, the startup phase would cost $2.7 billion with operational costs of $592 million per period ($278 million per year). Propellant would cost $5.34 million per ton in the first period and $960,000 per ton thereafter. NASA would save about nine billion dollars on three Mars flights, and would even save about $1.2 billion on the first flight.


The first Earth-Moon Lagrange point, or EML-1, offers a number of key advantages that make it an ideal destination for activities in cislunar space. Over the near-term, however, its utility is constrained by a lack of physical infrastructure. This can change if our approach to space moves away from destinations and towards a strategy of enabling capabilities.

Talk abounds of going beyond Earth orbit, although once beyond low Earth orbit (LEO) what happens next becomes a little fuzzy in most discussions. This need not be the case, as capabilities can be built from the very first test of a trans-LEO vehicle. Some sample test runs:

  1. Out to GEO: Given launch locations of likely US crewed vehicles, the mission could involve a plane change to geostationary (perhaps through a bi-elliptical transfer for extra-credit), close-approach to a “zombiesat”, and perhaps even retrieval of some old hardware for forensic analysis.
  2. Free-return trajectory: A loop around the Moon to give the heat shield a workout on the return. Perhaps some maneuvering out around the Moon. How close can they shave the rear-end of the Moon at perilune?
  3. EML-1 visit: The main purpose would be to establish a halo orbit. Once there, it would make sense to drop off a package of instruments that could serve a number of purposes.

Instrumental to the use of EML-1 is the concept of the halo orbit. It is a technique that allows an object to orbit an empty space, typically a Lagrange point, as with SOHO, WMAP, Genesis, the future JWST, and others. The best, though inapt, way to think about it is as a sort of gyroscopic effect, as with a bicycle wheel, where the hub is on the line connecting the centers of gravity of two objects (like the Earth and Moon), and the satellite is on the wheel. The action of the orbit helps to keep the object in place; some station keeping is required, but at a level orders of magnitude less than the ISS.

The EML-1 neighborhood is good for looking not only outward, but also back in towards Earth. EML-1 is a naturally clutter-free environment, since space junk doesn’t normally have station-keeping capability, as well as a “high ground” that allows observation not only of the Earth but also everything that’s orbiting around the Earth out to GEO. Instruments could be placed there for the specific purpose of keeping an eye on the traffic in cis-GEO space. By the same token, it is also ideal for looking at the Moon, and serves as a natural gateway to the entirety of the Moon’s surface. Bigelow Aerospace has proposed using EML-1 as an aggregation point for modules to be emplaced on the Moon’s surface using Armadillo Aerospace rockets for the descent. As far back as 1986 a Lunar Spaceport was envisioned as a kind of motel/gas station/warehouse/restaurant/garage for space travelers.

The Moon is still a bit in the future, though, as there are still a lot of things to consider on the Earth side of EML-1. One of the key advantages of staging at EML-1 is, as Brad Blair notes, “its ability to fall into various inclinations without a major [delta-V] penalty, thus increasing the number of customers that could be reached by a small set of vehicles and systems elements.”. What this means is that the inclination you end up in in LEO is set by how you depart from EML-1.

If you picture the Earth-Moon system, and the line connecting their centers of gravity, EML-1 is about 85 percent of the way to the Moon. Trace an ellipse from the EML-1 point down around the Earth and back up to EML-1. Put the perigee close enough to Earth and you can even get some aerobraking. This ellipse can be assumed to be in the orbital plane of the Moon. Now, rotate that ellipse around the Earth-Moon axis. This means that every inclination of LEO orbit is accessible, from the 28.5° of Kennedy Space Center, to the 51.6° of ISS, to even polar orbits, although the latter suffer from more delta-V penalty by virtue of the Earth’s oblateness.

If all of the LEO inclinations are available from EML-1, then it is also true that any LEO inclination can get to EML-1. This means that the ISS can serve as a staging point for missions to EML-1 in the nearer term, and later stations in different inclinations can also reach EML-1 as they come online. There is no need to wait to get started. As soon as a crew vehicle comes online it can start staging from ISS to EML-1, first as test-runs, then as missions to emplace as well as service, upgrade, and refuel assets. By the time crewed facilities are emplaced there will already be regular traffic to the location.

One question often raised is “What would a crew do at EML-1?” There are a myriad of answers:

1) In terms of propellant, it is cheaper to go from EML-1 to GEO and back to EML-1 than it is merely to go from LEO to GEO. Over the long term, it makes sense to stage GEO operations from EML-1. What kind of operations? The easiest answer is salvage, given the hundreds of tonnes of scrap circulating in GEO. Crews could fall down to GEO, spend a few days retrieving defunct equipment like failed satellites, and then return to EML-1 to process it. Whatever could be reused in some way is unknown, but the real value is in the forensic analysis of how the satellite weathered in the GEO environment over a known period of time. That kind of information allows for better satellite design.

2) EML-1 is an on-ramp to what are known as the InterPlanetary Superhighways (IPS). These are a network of ridges and ripples in space created by the gravitational effects of the planets and Sun. A satellite pushed onto the IPS will travel very, very slowly along this network to its destination, where it can kick itself into a halo orbit around a Lagrange point and collect data. Locations of interest would include the Sun-Mars L-2 and Sun-Jupiter L-1, to observe the Asteroid Belt; the Sun-Venus equilaterals at L-4 and L-5 to provide communications relay when Mars is on the other side of the Sun from Earth; Sun-Saturn L-2 to look at the Kuiper Belt; Sun-Neptune L-2 to look at the Oort Cloud; Sun-Mars L-1 as a waypoint on the way to Mars and the Asteroid Belt; Sun-Earth L-1 to watch the Sun; Sun-Earth L-2 to watch the stars. The key is that all of these instruments would also be able to return via the IPS to EML-1 for regular maintenance and servicing. As more probes are added to the network, instead of thrown into the void, there will be an increasing stream of probes in need of work.

3) The time lag from EML-1 to the Moon and back is much less than that for Earth-Moon. As a result, it is a better location for safe teleoperation of robots on the lunar surface.

4) As lunar activities ramp up, there will be an increasing need for freight handling of goods destined for the Moon, as well as those from the Moon. Early lunar exports are likely to be low-value-added goods such as oxygen, water, raw regolith, and some metals, but as more capabilities are established the exports will start creeping up the value-added chain: foodstuffs from lunar greenhouses, crafts created by the locals from local materials, increasingly sophisticated entertainment like dance and music, and so forth.

5) EML-1 is an ideal location to aggregate mission components for a trip to an asteroid. Fuel can come from the Moon, while spacecraft come from Earth. Someone’s going to have to put all of that together.

6) A facility at EML-1 can serve as a communications node for lunar operations to overcome the line-of-sight issue. Additionally, with solar sails “pole-sitting” above the north and south lunar poles, communications with the far side can be established.

7) Port services. While probes returning on the IPS will end up in the neighborhood of EML-1, they will need to be picked up. Same thing with freeflyer platforms sent on low-energy trajectories around the Moon for production runs. A space tug would be a good tool to have, and someone has to fly it.

These are just a few ideas, which can easily be expanded. If a crew can fall down to GEO for a servicing run, it can also fall down to HEO, MEO, and LEO for servicing missions. If an asteroid mission can be assembled at EML-1, so too can a Mars mission. If crews are salvaging dead satellites and kick stages from GEO, they may be able to cobble together the parts for other missions, selling the result to whomever wants to buy a space probe.


      Earth Moon Lagrange 2 or EML2 is one of 5 locations where earth's gravity, moon's gravity and so called centrifugal force all cancel out. It lies beyond the far side of the moon at about 7/6 of a lunar distance from earth.

     Infrastructure at any of these 5 locations could be kept in place with a small station keeping expense. Other high earth orbits would be destabilized by the influences of the earth, moon or sun.

     Of these 5 locations, EML2 is the closest to escape. How close?

Specific orbital energy is given by
v2/2 - GM/r

v: velocity with regards to the earth
G: gravitational constant
M: mass earth
r: distance from earth center

     Here's specific orbital energies for a few orbits:

     For EML1 and EML2 I'm looking at resulting earth orbits for payloads nudged away from Luna's Hill Sphere.

     Most the energy is getting from earth's surface to Low Earth Orbit (LEO). Then another huge chunk is getting from LEO to escape.

     EML2 is right next door to escape (aka C3=0). If the goal line is Trans Mars Injection, EML2 is on the 9 yard line.

     EML2's orbital energy is about -180,000 joules per kilogram. How much is that? Well, Kattie is standing next to the small generator which provides electricity for our business during power outages during the summer monsoons. It would take this 20 kilo-watt generator 9 seconds to crank out 180,000 joules.

     An EML2 payload nudged away from Luna would rise to an 1.8 million km apogee. An ordinary earth orbit at 450,000 kilometers from earth's center would move about 0.94 km/s. But since EML2 is moving at the moon's angular velocity, it is traveling 1.19 km/s. Earth's Hill Sphere is about 1.5 million km in radius. So depending on timing, an EML2 nudge could send a payload out of earth's sphere of influence into a heliocentric orbit.

     Another possibility is the sun's influence could send a payload back towards the earth with a lower perigee:

     All of these pellets were nudged from EML2. The sun's influence has wrested most of these from earth's influence. But check out pellet number 3 (orange). The sun's influence has dropped this pellet to a perigee deep in earth's gravity well. For a .1 km/s nudge from EML2 we can get a deep perigee that can give a very healthy Oberth benefit. However, such a route takes about 100 days.

Farquhar Route

     Using an lunar gravity assist along with an Oberth enhanced burn deep in the moon's gravity well, EML2 is 9 days and 3.5 km/s from Low Earth Orbit (LEO):

     This route was found by Robert Farquhar.

Bi-Elliptic Transfers

     If radii of two different orbits differ by a factor of 11.94 or more, a bi-elliptic transfer takes less delta V than Hohmann. EML2 radius / LEO radius is about 67, so LEO to EML2 could definitely be a beneficiary of bi-elliptic.

     From LEO, a 3.1 km/s burn gets us to a hair under a escape. A multitude of elliptical orbits fall under this umbrella!

     As you can see, it takes almost as much to get as high as EML1 as it does to reach a 1.8 million km apogee. I chose 1.8 million as an apogee since a 450,000 x 1,800,000 km ellipse at perigee has the same altitude and speed as EML2. At perigee a payload can slide right into EML2 with little or no parking burn.

     What's needed is an apogee burn to raise perigee to 450,000 km. A 6738x1,800,000 km ellipses moves very slow at apogee, a mere 0.04 km/s. A 450,000 x 1,800,000 km ellipse doesn't move much faster at apogee, about 0.3 km/s So a .26 km/s apogee burn suffices to raise perigee.

     So the total budget is 0.26 + 3.1555 km/s. This 3.42 km/s delta V budget is better than a Hohmann but about the same as Farquhar's 9 day route.

     But recall apogee is beyond earth's Hill Sphere. With good timing, the sun can provide the apogee delta v.

Hop's Route from LEO to EML2

     Here's a route I found with my shotgun orbital sim:

LEO burn is about 3.11 km/s. Payload passes near the moon on the way out, boosting apogee and rotating line of apsides. The sun boosts apogee as well as perigee. Coming back the pellets slide right into EML2 (the circular path alongside the Moon's orbit).

This LEO to EML2 route took 74 days and 3.11 km/s.

EML2 and Reusable Earth Departure Stages.

     Using the Farquhar route, it takes about 0.4 km/s to drop from EML2 to a perigee moving just under escape velocity. At this perigee 0.5 km/s will give Trans Mars Insertion (TMI). After the departure stage separates from the payload it's pushing, it can do a 0.5 km/s braking burn to drop to an ellipse with a near moon apogee. Once at the moon, another 0.4 km/s takes the EDS back to EML2.

     For massive craft moving from between earth's neighborhood and other heliocentric orbits, it makes little sense to climb down to Low Earth Orbit (LEO) and back each trip. It saves time and and delta V to park at EML2 on arrival. If EML2 becomes a stop for interplanetary space craft, a reusable EDS is a good way to depart the earth/moon neighborhood.

     I talk about this in more detail at Reusable Earth Departure Stages.

EML2 and Fast Transits

     Here's a pic of a Non Hohmann Mars transfer:

     Mars and earth orbits are approximated as circular orbits. A Hohmann orbit will have a 1 A.U. perihelion and a 1.52 A.U. aphelion. The transfer orbit above has perihelion 0.7 A.U. and aphelion 1.53 A.U. Semi-major axis of this orbit is (0.7 + 1.53)/2 A.U. or 1.115 A.U. Orbital period is 1.1153/2 years which is about 1.18 years.

     The trip to Mars isn't the entire orbital period though, just the turquoise area swept out from departure to destination. The turquoise area is 31.5% of the ellipse's area. 31.5% of 1.18 years is about 135 days or about 4.4 months.

     Departure and arrival Vinf are indicated by the red arrows. These are the vector differences between the transfer orbit's velocity vector and the planet's velocity vector at flyby. A change in direction accounts for most of the Vinf. I'm assuming Mars and Earth are in circular orbits with a zero flight path angle. Therefore the direction difference between vectors can be described with the flight path angle of the transfer orbit's velocity vector.

     In this case the earth departure Vinf is 11.3 km/s. That's a big Vinf! But if falling from EML2, only a 4.9 km/s perigee burn is needed. This is doable.

     At Mars the Vinf is 5.14 km/s. But a periaerion burn of 2.57 km/s brakes the orbit into an (3697x2345 km ellipse. This orbit could be circularized via periaerion drag passes through the upper atmosphere. Since this ellipse has a period less than a day, orbit could be circularized in a few weeks.

     An upper stage can have a 8 km/s delta V budget. Recall it takes about 0.4 km/s to fall from EML2. Therefore let's try to find a route that takes about 7.6 km/s from perigee burn to periaerion burn.

     Trial and error with my Non Hohmann Transfers spreadsheet gives:

     With chemical rockets departing from EML2, I believe 4 month trips to Mars are doable.

EML2 Proximity to Possible Propellent Sources.

     In terms of delta V, time and distance EML2 is quite close to several possible propellent sources.

     There are thought to be frozen volatiles in the lunar cold traps. Some craters at the lunar poles have floors in permanent shadow. Temperatures can go as low as 30 K. Volatiles that find their way to the cold traps would freeze out and remain. There may be rich deposits of H20, CO2, CH4, NH3 and other compounds of hydrogen, carbon, oxygen and nitrogen. These would be valuable for life support as well as propellent.

     The moon's surface is about 2.5 km/s from EML2.

     Also there are proposals to retrieve asteroids and park them in lunar Deep Retrograde Orbits (DROs). DROs are stable lunar orbits that can remain for centuries without station keeping. Planetary Resources would like to retrieve water rich carbonaceous asteroids. Carbonaceous asteroids can contain up to 20% water by mass in the form of hydrated clays. They can also contain compounds of carbon and oxygen.

     LDROs would be about 0.4 km/s from EML2.


     EML2 would make a great transportation hub. Not only for travel to destinations throughout the solar system but also within our own earth moon neighborhood.

From EML2 by Hollister David (2015)

     This entry covers near-term resources on and near Mars and how they might be transported.
     I assume the propellant network described in part 1 has been built, or at least that lunar propellant is available at Earth-Moon Lagrange Point One (EML1).
     The short version of the below is that using hardware similar (or identical) to the part 1 Lunar infrastructure, cargo to and from Mars becomes relatively cheap.
     A set of three NASA-reference mars missions could have their cargo requirements filled for a total of $11.4 billion (including fuel).
     Nitrogen and argon from Mars could be as cheap as $400 per kg at EML1.

     Again, let's start with the basics.

     Mars has a surface gravity of 3.711m/s² or 37.6% of Earth gravity, and a surface rotation of about 0.24km/s.
     Mars is blessed with a thin atmosphere, mostly carbon dioxide but with useful amounts of nitrogen and argon.
     Martian soils are rich in perchlorates and relatively rich in water.
     The north and south poles have permanent ice caps, mostly water ice under a layer of frozen CO2.
     CO2 slabs violently sublimate in the spring, making the edges of the ice caps fairly dangerous in this season. Otherwise the dust storms are not a serious threat.
     Takeoff and landing requires 4.1km/s; drag can save as much as 2.4km/s of this on landing.
     Mars has two moons, Phobos and Deimos. Both are believed to be captured asteroids, probably C-type.
     From EML1, Mars transfer costs 0.74km/s. Mars capture costs 0.9km/s and the move to low Mars orbit costs 1.4km/s. Trip total is just over 3km/s.
     From low Mars orbit, reaching Phobos costs 1.4km/s and reaching Deimos costs 1.9km/s.
     Launch opportunities to or from Earth (using a Hohmann transfer orbit) occur every 2.135 years, or about 26 months.

     Taking all of this together we want a surface location near the equator and a depot in low orbit. If we find significant amounts of water at either Phobos or Deimos then we want a mining outpost there. It would be nice if we could set up a Phobos-anchored tether for orbit changes.

     Getting cargo to Mars is slow and expensive. The post in part 1 used NASA's Mars Design Reference Mission (DRM) as a baseline to show how in-situ resource utilization (ISRU) could save money on fuel. Let's look at other ways to save.
     For starters, let's use the same cargo tug as described in part 1. This is a 5.5 ton dry vehicle that would deliver 50 tons of payload to EML1 using 77 tons of Lunar fuel and a reusable 15-ton (fueled) drop tank. The tug and payload would refuel with 63 tons of lunar fuel and continue on to Mars with fully propulsive capture to low orbit. These tugs can be used as fuel tankers between Phobos and LMO and can also be used to return cargo to Earth (up to 52 tons to EML1 all propulsive).
     Once cargo is in low Mars orbit, a different vehicle is needed to land it. The cargo tugs are capable, but multiple atmospheric landings in Mars gravity are out of reach. Instead, let's use the Access to Mars SSTO ferry (Strickland, Gopalaswami). This is a 30-ton dry mass, reusable SSTO cargo ferry for moving payloads to and from the Martian surface. The vehicle is 14 meters in diameter, which poses a problem; we would need a larger-diameter booster, a way to assemble the vehicle in orbit or a way to launch such a wide payload on an existing rocket. The design could probably be scaled down, but extrapolating the performance of a smaller version is not trivial. I will assume the problem is solved without specifying how. If reliable fuel is available from Phobos then the lander might be redesigned to refuel both in LMO and on the surface, allowing it to carry more payload each way.
     The cargo lander is intended to fuel up, launch to orbit (with 5 tons of payload or excess fuel), collect a 25-ton payload and land. Repeat as needed. Each 50-ton tug payload would require two lander trips. NASA's reference mission involves 80 tons of cargo to the surface, so two such flights would be required. Note that the ~40-ton DAV would not be required at all; a crew version of the lander can ferry crew to and from the surface. That suggests that a single 50-ton payload could include all of the required surface habitat, science equipment, power, rovers, food, etc. The first mission would require a cargo lander and crew lander as well as an orbital depot, so the first mission would require three 50-ton cargo flights to establish infrastructure along with the surface cargo. Each additional mission would require only one cargo flight.
     Each lander flight requires 95 tons of fuel, and each mission requires two flights (one 25-ton cargo, one 20-ton cargo + crew). This 190 tons requires about 9 tons of ISRU plant and 2 tons of spares. The Phobos ISRU plant should provide 15 tons of fuel for each lander. We might also send a single tug flight back to Earth each period for a cost of 63 tons of propellant from Phobos. That totals about 7.4 tons of ISRU plant and 1.6 tons of spares. A cargo tug can haul fuel from Phobos to the LMO depot, delivering 37.2 tons per trip at a cost of 25.8 tons of fuel.

     So, let's compare this to the baseline NASA plan over the course of three missions to Mars. The first trip requires three cargo flights to establish the orbital depot, Phobos ISRU base, tanker fleet and lander fleet. A habitat, science payload, ISRU facility, food and other supplies are delivered to the surface. Total cost is two landers (60 tons), three tugs (16.5 tons), 420 tons of Lunar fuel, a ~15-ton depot (200-ton capacity) and 16.4 tons of ISRU plant (with 3.6 tons of spares). That leaves 55 tons of payload available for the surface mission, with the understanding that the surface ISRU system will provide sufficient water and buffer gas.
     As a first-mission flight, the fuel alone will cost $2.24 billion. Using a middle of the road estimate of $30 million per ton for hardware (including surface payloads) that's almost exactly $5 billion. Launch costs would be three Falcon Heavy and a Falcon 9, about $500 million. Total mission cost, $7.75 billion. Note that this does not include the crew launch, transit habitat or crew recovery; these will be discussed in a later post.
     Future flights would require a single cargo flight, 55.5 tons of hardware ($1.67b) and 140 tons of lunar fuel ($135m). All other components are reusable and already present. Total pricetag for the cargo end is $1.8 billion for each additional mission. After about the fifth mission some of the infrastructure hardware will need scheduled replacement, so this cycle of one expensive mission and four cheap missions could continue indefinitely or the replacement costs could be spread across multiple missions. Still, $11.4 billion for three missions to Mars is cheap compared to $2.5 billion for MSL/Curiosity (which was money well spent) or the $156 billion for Apollo to put men on the moon.

     Looking past the baseline, the Mars surface base is able to supply nitrogen for breathing gas and argon for electric engine fuel. This can be carried back to EML1 in a reusable cargo tug that paid for itself by delivering its first payload to Mars orbit. The same tug can be reused multiple times. Ultimately the cost of shipping along this route is the cost of spares and replacement of the various ISRU bases, depots and tugs. With Earth launch costs in the $3 million per ton range, any advantage is worth pursuing. Cargo from Mars costs about 126kg of spares to Mars per ton delivered to EML1. Cargo to Mars costs about 76kg of spares to the Moon per ton delivered to LMO. Of course each trip requires the use of a cargo tug costing anywhere from $66-$165 million but in terms of ongoing costs we're looking at 6.78 tons from Earth to support a 50-ton cargo from Mars, a better than 7:1 ratio. Another way to say it is for $20 million in launch costs we can get 50 tons of argon and nitrogen (or Mars core samples or Phobos turnings, etc.) at EML1 for use elsewhere once the system is established, a savings of at least $130 million.


For travel throughout cislunar space, I’ve long been an advocate of having depots on both ends of the journey. The LEO depot provides a refueling stop at the first practical point after leaving the ground, and also a spot for bringing vehicles back from lunar space for refueling for their next trip out. The lunar orbit depot plays a similar role for flights to/from the lunar surface, as well potentially, as being a staging location for departures into interplanetary space. By launching from a lunar facility near the top of earth’s gravity well, it’s both possible to use low-thrust trajectories in and out of cislunar space, as well as to do an earth swingby with a departure burn at apogee for high-thrust departures taking maximum advantage of the Oberth effect.

One important question however has been where to place the lunar orbital facility.

Lunar Orbital Facility Orbit Options
A recent FISO telecon presentation by Ryan Whitley and Roland Martinez of NASA JSC describes and discusses several of these staging orbit options. I’ll be reposting snapshots of a few of their slides to introduce the orbits, but here you can find their full presentation:

They discuss most of the commonly cited options including Low Lunar Orbit, Frozen Orbits, L2 Halo Orbits, Distant Retrograde Orbits, and a more recently discovered option, Near Rectilinear Orbits.

This slide shows some of the smaller lunar orbit options and descriptions (click for larger image):


And this slide shows some of the larger lunar orbit options, with descriptions (click for larger image):


And this slide shows all of the orbits relative to each other to give you a better idea of what they look like (click for larger image):


Comparison of Options

While Whitley and Martinez in their FISO telecon focus on evaluating the various staging orbits from the standpoint of NASA missions using the Orion capsule, they still provide a lot of useful information for evaluating options for the location of a lunar orbital facility/depot. To me, some of the considerations for locating a lunar orbital facility are:

  • How frequently do you have opportunities to travel from a LEO facility to the lunar facility, and how frequently you can travel the other direction?1
  • How much delta-V does it take to go between the facility and LEO and the facility and the lunar surface?
  • How long is the transit between the location and the lunar surface?
  • How useful is the orbit for supporting deep space missions?
  • How hard is it to reach various lunar surface destinations from the lunar orbital facility location?
  • What is the thermal environment like in the orbit?
  • And how much of the lunar surface to destination delta-V can be provided by some sort of propellantless lunar launch scheme2?

Based on these considerations, I’d like to focus the rest of this post on the pros and cons of the two options I consider most interesting–L2 Halo Orbits and Near Rectilinear Orbits.

Pros and Cons of EML-2 Halo Orbits
EML-2 orbits have been my favorite option ever since learning about the low delta-V cost of reaching them via powered lunar swingbys. They have a lot going for them, including:

  • One of the lowest delta-V stopping points in the lunar vicinity, requiring only ~3.43km/s of delta-V from LEO.
  • Easy access to/from a LEO facility on every LEO-lunar or lunar-LEO window.
  • Any-time access to/from anywhere on the lunar surface.
  • Low stationkeeping delta-V3
  • Benign and cold thermal environment4
  • Continuous communications with Earth, and most of the farside of the Moon.
  • Good staging point for both deep-space and lunar missions.
  • Could become a starting location for a lunar space elevator.

But EML-2 does have a few drawbacks:

  • Long LEO-EML2 and EML2-LEO transit times5 for the low delta-V powered-swingby option.
  • Long EML2 to lunar surface (and vice versa) travel times6
  • It wasn’t clear that a propellantless lunar launch option located at either pole could launch easily to EML2. An elliptical orbit from such a launcher would have its line of apsides pass through the launch location, which would be orthogonal to the Moon-EML2 line. You could launch into a polar LLO, and then do multiple burns from there to EML2, but the propellantless launch option would only provide the first leg of the trip (surface to LLO).

The long trip times mean that the vehicles taking people between LEO and EML2 and between EML2 and the Moon will require much more extensive life support and accommodations than would be needed if the trip were shorter. That will drive up the dry mass of those systems, and by extension the propellant and overall cost of moving people to and from EML2.

Pros, Cons, and Questions Regarding Near Rectilinear Orbits
Starting several months ago, some of my astrogator friends started telling me about NASA’s interest in Near Rectilinear Orbits for exploration missions. After all the talk about Distant Retrograde Orbits, this sounded a bit like the “flavor of the week” syndrome, but the FISO presentation helps explain some of the allure of such orbits:

  • Only slightly higher delta-V to/from LEO to NROs compared to LEO to EML27.
  • Because the NRO orbit’s perilune is only 2000km from the Moon’s surface, once per 6-8 day orbit, the orbit lines up so that the travel time between NROs and the lunar surface drops to 0.5 days.
  • Powered swingby trajectories between LEO and NROs take approximately 5 days each direction, instead of 9-11 for EML2.
  • Slight lower delta-V between NROs and the lunar surface compared to EML2.
  • The NRO is close enough to an elliptical polar orbit that it might be possible for a polar base to use propellantless launch techniques to fling payloads nearly into NRO, with possibly only minor adjustments and raising the perilune with a burn near apolune half an orbit later8

The benefits of shorter transit times are pretty important, but there are still a couple of relative drawbacks and open questions:

  • While it’s possible to get from LEO to a given NRO orbit during every lunar injection window, the NRO facility will be at different points in its orbit during each window, which may make a first-orbit rendezvous either infeasible or it might cost additional delta-V. I’d want to get this resolved, because while this isn’t an issue for one-off, ground-launched missions like the NASA folks were thinking of, this would be a real issue for reusable spaceship flights between a LEO and NRO facility.
  • Likewise, departures from the NRO may not be in the optimal part of the orbit for the Earth return maneuver when the timing is right to return to the plane of the LEO facility. This isn’t a problem if you’re doing a direct return, but once again is a big pain in the neck for reuse of space hardware. Once again this is something I’d want to analyze more before settling on an NRO orbit.
  • Additionally, the NRO facility has LOS with one lunar pole about 86% of the time (while heading out and coming back from apolune), but only sees the other facility for a brief period near perilune. If you’re planning on using propellantless launch methods to send stuff from a polar lunar settlement to the NRO facility, it’s going to be in an NRO with apilune on the opposite side of the moon from your lunar settlement, meaning you’ll only be in contact briefly for maybe 1 day out of a week.
  • Because the perilune is only 2000km, the heating environment is going to be warmer than EML2, with slightly higher boiloff, but this is probably only a minor difference–it should still be tons easier to keep cryo boiloff low in an NRO than in LEO.

While NRO orbits have some really interesting characteristics, I’d really want answers to those first two concerns before I’d pick it for the location of a lunar orbital facility. If you can’t get to it on a regular basis from a given LEO depot without having to do complicated trajectories, or paying big penalties in flight duration or delta-V, then that would likely outweigh the benefits. If on the other hand, it’s not a big deal to adjust the trajectory on the way to and from the NRO facility to enable rendezvous with the facility regardless of where it is within its orbit when the LEO to lunar launch window opens, then it could be a really interesting location for a lunar transportation node. I’ll have to see if I can get some of my astrogator friends to weigh in on those questions. Until then I’m probably still more of a fan of EML-2, in spite of the annoyingly long transit times.

[Update 1: After speaking with an astrogator friend who’s been looking at NROs to support lunar missions, he thinks it might be possible to put an NRO facility in an orbit whose period is synchronized with the average time between launch windows from the LEO facility. If that works, that would mean the NRO facility would be in approximately the same part of its orbit during each trip to/from the Earth. There are questions of if you can make an NRO orbit with a long enough period (~9 days) to make that work, and if the NRO facility could be made to line up both for arrivals and departures from/to Earth, but hopefully he’ll have more opportunity to dig into that further later this year.]

  1. Due to the orbital motion of the Moon and nodal regression of a LEO facility, you get optimal lunar departure and/or return options about every 7-9 days IIRC. The choice of lunar orbital facility location may constrain this further.
  2. Lunar Slings, Mass Drivers, Launch Loops, etc. All the stuff I was supposed to write about in my “The Slings and Arrows of Outrageous Lunar Transportation Schemes” series that I still need to finish
  3. <10m/s/yr
  4. About 10x lower LOX/LH2 boiloff rate than LEO. In fact with passive insulation you can completely surpress LOX boiloff and even freeze oxygen at EML-2
  5. 9-11 days
  6. 3-5 days
  7. 3.58km/s vs 3.43km/s
  8. Which you’d also want to be the burn that brings you to rendezvous with the NRO facility

For this next depot taxonomy post, we’ll finally be talking about what people usually think about when they hear the term orbital propellant depot — larger, cryogenic refueling facilities, focused on enabling large-scale human spaceflight missions, performed by a diverse variety of users, and going to/from a wide variety of destinations. The idea for such propellant depots for enabling interplanetary human spaceflight dates back to at least 1928 with the writings of Guido von Pirquet1.

This blog post will be focused on what I call “Low-Orbit” human spaceflight depots. These are depots located near the lowest stable orbit around a planetary body. This is the first place at which you can realistically refuel or switch vehicles on your way from a planetary body, and is the last place at which you can refuel or switch vehicles on your way down. As a firm believer in the idea of refueling early and refueling often, low-orbit depots are an important piece of infrastructure for any planetary system humanity wants to travel to/from regularly. A later blog post in the series will talk about “High-Orbit” depots — depots operating in fixed locations2 located out near the edge of a planetary system’s gravitational sphere of influence, and will include a discussion of where those types of depots might make sense.

Before we jump into the weeds about low-orbit human spaceflight depots, I did want to address a recent train of thought I’ve seen that suggests that just using tankers and directly refueling a vehicle is superior to having a depot involved. While this could easily be the topic of a series of its own, I wanted to briefly highlight a few of the biggest advantages I can see of having a depot vs just using direct refueling with tankers:

  • Flexibility: A depot, properly designed, with published, standardized grappling, refueling, and power-data interfaces, can be agnostic about who it gets propellant from and who it sells propellant to. Depots can quickly take advantage of whatever the cheapest source of propellant is at a given time (RLVs, ISRU, propellantless launch, buying leftover capacity from other missions going to destinations near a depot, atmospheric gathering, etc), and can easily service both smaller missions and bigger ones. Tanker-based approaches tend to be a lot less adaptable, typically being optimized for one or two specific vehicles that needs refueling.
  • Robustness: With a fixed installation, that only has to be launched once for a long mission lifetime, you can afford to throw way more resources (dry mass, volume, and power) at making rendezvous, prox-ops, docking (RPOD), and manipulation as safe and reliable as possible. This could include beacons, larger more capable (and/or redundant) relative navigation sensors and comms, longer reach capture robotics that minimize the dry mass requirements on tanker and client vehicle alike, etc.
    • For tankers, on the other hand, you want to minimize parasitic dry mass that has to be launched every time, and for the departing vehicle you want to minimize mass you have to carry through large, high delta-V in-space maneuvers. You could in theory carry a nicer RPOD kit on your departing vehicle that you jettison before leaving LEO, but now you’re amortizing that mass and cost over a much smaller number of missions, or starting to add more complexity than just doing a depot.
    • Another question to ponder is with vehicles that require large numbers of refueling events per mission, is it best to have the client vehicle handle all of those docking maneuvers with its (by definition) less-capable RPOD capabilities? With a depot, each tanker and each client vehicle only has to perform one mission-critical RPOD/refueling operation per mission, whereas with direct refueling via tankers, the client vehicle would typically have to perform a larger number of mission-critical RPOD events. A depot would also have to handle the same larger number of RPOD/refueling events, but as mentioned before, can throw more resources at making these as reliable and safe as possible.
    • Longer refueling cycles from using a direct-tanker refueling approach also increases the MMOD3 risk to the departing vehicle, which by definition can’t afford to throw as many resources at MMOD protection as a depot can.
    • A corollary to this is that depots make the most sense if you use them in a way that offloads as much of the refueling-unique hardware/software as possible from the delivery and client sides of the system to the depot. Ideally a tanker would be a minimally modified upper stage4, and client vehicles would also have similarly minimalistic hardware needed to be grappled and receive propellant. If you’re doing your delivery vehicle or your client vehicle in a way that makes you question the utility of a depot, that may be a hint that you’re doing something wrong.
  • Non-Integerality: Yes I may have made up that word, but the point is that tankers tend to come in integer quantities. Unless you always design your departing vehicles to use only integer quantities of tankers, you’ll almost always end up having wasted propellant. This is especially true given how launch vehicles, and in-space vehicles tend to increase performance and get upgraded over time, and don’t necessarily upgrade at the same rates. If you only ever had a monopoly/monopsony situation where you only had one tanker provider, and only one vehicle needing tankers, you might be able to keep tanker size locked to an integer fraction of the amount of propellant needed, but in reality that isn’t going to happen. So a tanker-based system is always going to end up wasting propellants, and this is even more the case when you have diverse customer vehicle and delivery vehicle sizes. The more diversity you have in your space transportation ecosystem, the more a depot makes sense.

There are probably other good arguments I’m glossing over, but long-story short, unless you’re interested in a boring monoculture world where only one type of in-space transportation system exists, depots make a lot of sense. So, without further ado, let’s jump into some of the taxonomical considerations of human spaceflight low-orbit depots.

Human Spaceflight Low Orbit Depots

Application: Refueling large transfer stages or in-space transports for ferrying people and cargo between LEO, the Moon, Mars, Venus, and other destinations of interest.

Location: As discussed earlier, this blog post is focused on depots located near the lowest practical stable orbit around a planetary body. Details vary for different planetary bodies, as described below5:

  • For Earth, the low-orbit depot location is in LEO, ideally in the lowest inclination that still lets you hit required departure asymptotes and maximize the number of economically useful launch sites to send propellant, people, cargo, and materials to/from the depot6. You probably only need one or two such low-orbit depots, though if you get to high-enough earth departure throughput there may eventually make sense to spread more depots out at different RAANs7. Likely for a first human spaceflight depot, as with the previously discussed smallsat launcher depots, you’ll want to locate it in LEO near other human-occupied facilities like ISS — far enough away to be safe, but close enough to conveniently move between each other, ideally within one work shift8.
  • For a low-orbit depot around the Moon, this would likely be a polar or near-polar LLO9, though due to the very slow rotation and practically zero J2 perturbation10, if you have a lot of non-polar surface sites, you eventually may want multiple smaller depots in equally RAAN-spaced near-polar LLO planes, and maybe one in an equatorial orbit. If you’re trying to do lunar surface missions, having your depot in LLO makes way more sense than in a higher orbit like NRHO, for reasons I should probably go into in another blog post.
  • For Mars this would also likely be a LMO orbit, with an inclination high enough to be able to access any points of interest on the surface, while still being low enough to minimize delta-V penalties11, and keep the nodal precession rate fast enough to minimize phasing orbit time for three-burn departures. You’ll likely also have to put some thought into perturbations from Phobos and Deimos12.
  • For Venus, the extremely low rotation speed and therefor very low J2 pertubation may require you to do multiple smaller depots in similar inclinations but equally RAAN-spaced planes, as you won’t pass over a given point on the surface very frequently, and the very slow nodal precession rate could potentially require very very long phasing orbits for a 3-burn departure. Venus has a deep enough gravity well that you do want to refuel in LVO coming to/from, but it’s not trivial from an orbital dynamics standpoint13.

Size: As big as you can practically get away with — ideally you’d want this depot to be at least 2x the propellant capacity as whatever the largest vehicle you’re refueling. So somewhere in the 100-2000mT range, or even bigger14. Early versions will want to be single-launch if possible, in many cases repurposing at least one of the main propellant tanks from the stage that delivered them to their destination as one of the depot tanks15. Eventually, it may be possible to do multi-tank depots, but if you can do a single launch depot big enough for refueling two missions, you may be better off making more than one depot instead of trying to make the depot super big.

Propellant Types: For low depots, you’re primarily going to be dealing with large transfer stages (Centaur V, Starship, New Glenn Upper stage), which typically use LOX, and either Methane or LH2 for the fuel. Most of these use autogenous pressurization, and use the main propellants for RCS. So most of the depot will be for LOX, LH2, and/or Methane.

  • For Mars or Venus you may eventually also want to store liquid CO for some applications, since it’s an easier ISRU propellant, but that remains TBD.
  • A lunar low-orbit depot may also want to stock storable propellants, depending on what lander propellants end up being most popular16.
  • You may eventually also want to store some secondary fluids (Helium or Neon for active cooling loops, life support consumables like water, air, etc), but you may not explicitly need a depot for that function.

Some Other Considerations for Human Spaceflight Low-Orbit Depots

  • As mentioned before, human spaceflight depots want to be designed in a way to enable offloading as much of the RPO and docking/or berthing from the vehicles they’re servicing. The less parasitic dry mass that tankers and clients have to lug around on every mission, the better.
  • Storing cryogens in low orbits tends to be hard — you have a warm planet blocking half of the sky. So launching the propellant in a subcooled state or even partially frozen (i.e. slushy propellants) can help a lot. Also a lot should be done to minimize heat leaks between the cold part of the depot and any hot sections (habitation, power, etc). If you can’t get to zero boiloff, LH2 is a great thermal sponge, and can be used to chill other propellants, and intercept heat from heat sources before being vented. You may have to vent some hydrogen boiloff, but if you’re smart, you can use that hydrogen boiloff on the way out to eliminate boiloff issues for everything else.
  • These depots are also big debris targets17. Deployable MMOD/MLI18 solutions could be very helpful to avoid a puncture, which would probably be very hard to patch. Since these depots are fixed, and are typically only performing stationkeeping maneuvers, it may be possible to augment their MLI/MMOD protection over time using in-space assembly/manufacturing techniques19.
  • Especially for low-orbit depots around the Moon/Mars/Venus, there may be a benefit to having some temporary habitation/shelter collocated with the depot, especially if you’re supporting multiple sites, as a search and rescue option during exploration phases, and as a stop-over point to act as a buffer between different sizes of transportation between planets and between the depot and the surface.
  • Over time you may want to add in other facilities such as dry docks for assembling, and repairing/maintaining large in-space vehicles/structures, habitation facilities, etc. But they should probably be coorbital, near the depot, not attached (as that will make cryo thermal management all the harder, and the depot is a big hazardous work location, where you should probably minimize the amount of time people spend in close proximity to it). This could be done in two ways — coorbital facilities, spaced where the safe time to travel from one to the other is as short as practical (definitely less than an 8hr work shift if at all possible, and much closer if possible), or by having the two facilities connected by a connecting tether or other structure that includes elevator facilities.
    • If your depot facility starts wanting to have permanent collocated habitation (say for in-space assembly/repair/maintenance of in-space stages), and having the two be coorbital doesn’t work, you’re likely going to want to keep the people separated as far away from propellant tanks as possible, both to minimize heat-leak into the tanks, but also to minimize hazard to the people20.
    • For larger depots, and ones where people will be there more, putting some thought into spatially separating the fuels and oxidizers more could be a good idea. In rockets you often can’t do much to keep the two separated, and many use a common bulkhead, but in a fixed facility it’s more of a possibility. Having fuel and oxidizer that close together for long periods of time is somewhat tempting fate — you kind of have to do it for high efficiency rockets, but there’s something to be said for having your fuel and oxidizer many meters apart for a long-duration facility.
  • One area of disagreement I have with other depot advocates is whether propellants should be shipped to a depot as cryogenic propellants (LOX/LH2/LCH4), or if you should ship them as something more storable like water and CO2, and have the depot itself have large-scale electrolyzing, separation, and propellant refrigeration systems. My concern is that while in theory very large solar arrays could be done in space, combining large flexible structures like multi-MW solar arrays and radiators with a facility that sees a lot of docking, propellant slosh, etc seems like a bad idea from a structural dynamics standpoint. Also depots with very large power generation and heat rejection capabilities are likely to come later in the process, since they’ll almost certainly require multiple launches and in-space construction.

Anyhow, I probably could go on, but as with the previous parts of this series, I am only trying to scratch the surface with considerations and operating details, as I introduce each new type of depot. This definitely isn’t the last you’ll hear from me on the topic.

  1. Sykora, Fritz, “Guido von Pirquet-Austrian Pioneer of Astronautics,” History of Rocketry and Astronautics, R. Cargill Hall, ed., AAS Publications, San Diego, 1986, p. 151. And yes, I’d love to see someone actually fly a depot before we hit the 100th anniversary of von Pirquet first discussing the idea
  2. Or at least orbits/halo orbits where the only maneuvers being performed are for stationkeeping purposes
  3. MicroMeteorite and Orbital Debris
  4. With some lightweight grapple fixtures, refueling modified T-0 umblicals, and maybe an upgraded comms/controls system
  5. Most of these points could easily justify their own blog post, and knowing me, I’ll probably eventually do said blog posts, but I’ll try to keep things in this post brief.
  6. I’m currently noodling doing a paper looking specifically at this optimization — higher inclinations for the depot give you the ability to hit higher departure declinations, and are accessible by more launch sites, but at the cost of slower nodal precession driving longer hang-times for a three-burn departure, and you also have decreased payload mass to higher inclinations. My gut says ISS-like orbit is probably not far from the optimal point, but it would be fun to run the numbers
  7. As a reminder, RAAN stands for Right Ascension of the Ascending Node, which is a measure of where a given orbit plane crosses the equator heading northward. Here’s a decent wikipedia explainer. When you see a multi-plane constellation, like say OneWeb or Starlink, in a lot of cases the satellites will be put into multiple planes with each plane having the same inclination and altitude, but a different RAAN. The different RAANs help with more frequent passes over the same point in the ground, and for depots by increasing the frequency with which a depot passes through the plane of a specific departure asymptote. If that makes any sense.
  8. I could easily see a scenario where the first human spaceflight depot evolves from smallsat launcher depots.
  9. Though how low of a LLO is open to debate — lower orbits have higher stationkeeping requirements, and are harder to do sunshields with because the Moon takes up more of the sky surrounding the depot. I haven’t done an optimization analysis myself, but 250-500km above the Moon might reduce both of these effects noticeably compared to the 100-150km parking orbits used for previous lunar missions
  10. The J2 parameter is a measure of how oblate or “round about the middle” a planet is. This is typically tied to the rotation speed of the planet. The J2 perturbation is what causes orbital planes to precess slowly. If a planet had no J2 or very low J2, the planes would stay in a fixed orientation relative to the stars, which is problematic if you’re trying to get the plane to line up with a departure asymptote for an interplanetary departure… Earth, Mars, and most of the gas giants have high J2s, while the Moon, Venus, and Mercury all have very low J2s.
  11. The equatorial velocity of Mars is ~half that of earth, so it’s not as big of a deal to launch from or into a higher inclination, but it still is some losses
  12. I’m not much of a Mars guy, so am handwaving that part a bit
  13. If someone is looking for a PhD dissertation topic, this feels like an area that could use a lot more skullsweat investigating.
  14. Depending pretty strongly on how successful Starship ends up being, and how open Elon is to using other people’s depots. In a world where Starship either doesn’t pan out, or Elon insists on doing his own thing, the required size for refueling other missions can be a lot more modest initially. Also a world with depots in low-orbits around destination planets, and roving depots and/or high-orbit fixed depots may not need LEO depots to be quite so big.
  15. As shown in Part IV of this series.
  16. For long duration lunar landers there are definitely differences of opinion about whether storables or cryogens make more sense.
  17. Big LEO facilities that don’t want to dodge things all the time is one of those reasons why moving to a leave-no-trace approach to satellite operations is going to be an important part of growing up as a spacefaring civilization.
  18. MLI, or multi-layer insulation is a type of very effective insulation for use in vacuum environments. It is typically made of many layers of thin metalized plastic films separated by nets or spacers so that most of the heat has to transfer via radiation instead of conduction or convection. Having more spacing between layers can help. And in some cases, MLI can be combined with MMOD (which also wants to be multiple thin layers with spacing between them), like what our friends at Quest Thermal have worked on.
  19. Most hypervelocity impact shielding benefits a lot from extra spacing between bumper layers, so an in-space assembled/manufactured MMOD solution could be particularly useful
  20. Though as my wife pointed out, if your depot facility starts wanting anything, it’s probably a sign of an impending robot apocalypse, so the point might be moot.

Kuck Mosquito

Kuck Mosquitoes were invented by David Kuck. They are robot mining/tanker vehicles designed to mine water propellant from icy dormant comets or D-type asteroids and deliver it to an orbital propellant depot.

Form follows function. So it is unsurprising that the Kuck Mosquito resembles an Enterobacteria phage T4 virus. Only difference is that the virus is injecting, while the Mosquito is sucking out.


Because LEO is drier than the driest desert, there is money to be made by importing water from nearby worlds. The closest hydrated body to LEO, speaking in delta-v terms, is the moonlet Deimos (closer than the Earth, closer than Luna.) See “The Deimos Water Company” by David Kuck of Oracle, Arizona for details.

Kuck mosquito — As icy dormant comets or D-type asteroids are warmed by the sun, they accumulate an outer anhydrous lag layer. An in-situ mining robonaut called the Kuck mosquito is designed to drill through this layer, inject steam, and pump out the water in the core. Some of the water is electrolyzed for fuel for a small H2-O2 chemical engine. To gain a secure foothold, thermal lances melt into the substrate. The targeted bodies must have a cometary matrix of not less than 30% ice. There is a danger of catastrophic fracture of the subsurface mantle layer due to the tensile forces generated by the pressurization. Dave Kuck, “The Exploitation of Space Oases,” Princeton Conference on Space Manufacturing, Space Studies Institute, 1995

(ed note: in the game, Kuck Mosquitos have 220 kilonewtons of thrust and a specific impulse of 460 seconds. The water bag masses 40 metric tons because that is the standard measurement in the game.)

From High Frontier game by Philip Eklund (2010). Atomic Rocket Seal of Approval.

In 2007 it is difficult to maintain a space station in Low Earth Orbit (LEO). Before we can hope to colonize asteroids we need a better way to get out of LEO.

Kuck mosquitoes (designed by David Kuck of Oracle, Arizona) are small, unmanned craft that consist of not more than a drill, a heating element, and a bag house to store extracted volatiles. (ed note: it will also require a source of electricity. Maybe an RTG or solar cell array)

Sending small Kuck mosquitoes to Near Earth Asteroids to retrieve water and fuel is more doable than manned missions.

A few Kuck mosquitoes returning to earth orbit with full bag houses would make LEO much more hospitable.

These mosquitoes could be tankers for LEO fuel depots. LEO fuel would make manned spaceflight to the moon, Mars and asteroids much less difficult.

From KUCK MOSQUITO by Hop David (2007)

Using steam to propel a spacecraft from asteroid to asteroid is now possible, thanks to a collaboration between a private space company and the University of Central Florida.

UCF planetary research scientist Phil Metzger worked with Honeybee Robotics of Pasadena, California, which developed the World Is Not Enough spacecraft prototype that extracts water from asteroids or other planetary bodies to generate steam and propel itself to its next mining target.

UCF provided the simulated asteroid material and Metzger did the computer modeling and simulation necessary before Honeybee created the prototype and tried out the idea in its facility Dec. 31. The team also partnered with Embry-Riddle Aeronautical University in Daytona Beach, Florida, to develop initial prototypes of steam-based rocket thrusters

“It’s awesome,” Metzger says of the demonstration. “WINE successfully mined the soil, made rocket propellant, and launched itself on a jet of steam extracted from the simulant. We could potentially use this technology to hop on the Moon, Ceres, Europa, Titan, Pluto, the poles of Mercury, asteroids — anywhere there is water and sufficiently low gravity.”

WINE, which is the size of a microwave oven, mines the water from the surface then makes it into steam to fly to a new location and repeat. Therefore, it is a rocket that never runs out of fuel and can theoretically explore “forever.”

(ed note: well, to be more precise, it never runs out of water propellant. It still needs fuel in the form of solar panels or RTGs. The former suffers a power drop-off once you get further from the sun, such as at Mars or the asteroid belt. The latter currently have a useful lifespan of about 30 years)

The process works in a variety of scenarios depending on the gravity of each object, Metzger says. The spacecraft uses deployable solar panels to get enough energy for mining and making steam, or it could use small radiosotopic decay units to extend the potential reach of these planetary hoppers to Pluto and other locations far from the sun.

Metzger spent three years developing technology necessary to turn the idea into reality. He developed new equations and a new method to do computer modeling of steam propulsion to come up with the novel approach and to verify that it would actually work beyond a computer screen.

The development of this type of spacecraft could have a profound impact on future exploration. Currently, interplanetary missions stop exploring once the spacecraft runs out of propellant.

“Each time we lose our tremendous investment in time and money that we spent building and sending the spacecraft to its target,” Metzger says. “WINE was designed to never run out of propellant so exploration will be less expensive. It also allows us to explore in a shorter amount of time, since we don’t have to wait for years as a new spacecraft travels from Earth each time.”

The project is a result of the NASA Small Business Technology Transfer program. The program is designed to encourage universities to partner with small businesses, injecting new scientific progress into marketable commercial products.

“The WINE-like spacecrafts have the potential to change how we explore the universe.” – Kris Zacny, vice president of Honeybee Robotics

“The project has been a collaborative effort between NASA, academia and industry; and it has been a tremendous success,” says Kris Zacny, vice president of Honeybee Robotics. “The WINE-like spacecrafts have the potential to change how we explore the universe.”

The team is now seeking partners to continue developing small spacecraft.

Metzger is an associate in planetary science research at UCF’s Florida Space Institute. Before joining UCF, he worked at NASA’s Kennedy Space Center from 1985 to 2014. He earned both his master’s (2000) and doctorate (2005) in physics from UCF. Metzger’s work covers some of the most exciting and cutting-edge areas of space research and engineering. He has participated in developing a range of technologies advancing our understanding of how to explore the solar system. The technologies include: methods to extract water from lunar soil; 3D printing methods for structures built from asteroid and Martian clay, and lunar soil mechanic testers for use by gloved astronauts.

Honeybee Robotics, a subsidiary of Ensign Bickford Industries, focuses on developing drilling tools and systems for finding life as well as for space mining for resources. Honeybee has previously deployed and operated Rock Abrasion Tool (RAT) on Mars Exploration Rovers (MER), Icy Soil Acquisition Device (ISAD) on Mars Phoenix, and Sample Manipulation System (SMS) for the Sample Analysis at Mars (SAM) instrument on the Mars Science Laboratory (MSL). The MSL also has Honeybee’s Dust Removal Tool. Current flight and R&D projects include systems for Mars, the Moon, Europa, Phobos, Titan, and others.

Kuck Mosquito
PropulsionH2-O2 Chemical
Specific Impulse450 s
Exhaust Velocity4,400 m/s
Wet Mass350,000 kg
Dry Mass100,000 kg
Mass Ratio3.5
ΔV5,600 m/s
Mass Flow49 kg/s
Thrust220,000 newtons
Initial Acceleration0.06 g
Payload100,000 kg
Length12.4 m
Diameter12.4 m

Deimos, the outer moon of Mars, is possibly the most accessible source of water to LEO. Lewis has shown the delta-V to go from LEO to Deimos is less than that needed to land on Earth's Moon. Partial loss of velocity at Mars might be obtained by a shallow dip into the Martian atmosphere. The delta-V to return from Deimos to HEEO (Highly eccentric Earth orbit) is very small. The travel time is roughly two years. The Moon may be used as an aid to accelerate and decelerate a vehicle as it leaves LEO and arrives at HEEO. Shallow penetration of the Earth's atmosphere may be used to loose velocity and aid in capture into HEEO.

Bodydelta-V Surface to LEO (m/sec)time of flight (d)delta-V LEO to Surface (m/sec)time of flight (d)

(ed note: the important part is LEO to Deimos Surface is deltaV=1800 m/s and 270 days transit, Deimos Surface to LEO is deltaV=5600 m/s and 270 days transit.)

A disadvantage of Deimos is the 26 month delay between launch opportunities.

Fanale calculates that ice should exist at a depth of 100 meters at the equator and at a depth of 20 meters at the poles of Deimos. Thus, the drilling equipment proposed in 1995 by Kuck should be able to reach ice at or near the poles, but not near the equator.

To move 100 tonnes of water ice from Deimos to LEO will require 250 tonnes of water ice for propellant (Z). Thus, in order to leave Deimos 350 tonnes must be propelled from the surface. A 1,000 cubic meter collection bag should be large enough to contain the 350 tonnes of ice, cuttings & other precipitates.

(ed note: 100 metric tons payload + 250 metric tons propellant implies mass ratio of about 3.5, since engine and structural mass are a small fraction of this (e.g, by the table below, the drilling equipment is 0.3 metric tons). If it electrolyzes the propellant into O2 and H2 and burns it as chemical fuel with a specific impulse of 450 seconds, this would give a delta-V of around 5,500 m/s or so.)

Table 1. Mass of Drill and equipment for the Deimos version of the drill presented in "Exploitation of Space Oases" presented at Princeton May 1995. The total mass is in grams. The drill pipe is titanium for lightness and chemical resistance to corrosion.
Down The Hole Hammer Drill, Titanium drill pipe & accessories
L (mm)OD (mm)ID (mm)Weight (grams)NumberWeight (grams)Ti
Hammer DTH210162333699No
Under-reamer Guide782049.43148.2Yes
Casing Shoe21241610160No
Collar Pipes1000434013741013740Yes
From THE DEIMOS WATER COMPANY by David Kuck (1997)

The images below are details of the "Spider" water harvester carried by the Robot Asteroid Prospector. It performs much like the business end of the Kuck Mosquito.

Optimizing Depot Placement

Takuto Ishimatsu's Ph.D. thesis is titled Generalized multi-commodity network flows : case studies in space logistics and complex infrastructure systems (abstract here). Basically: manually chosing where to place in-situ resource allocation depots so as to get the maximum benefit is a very very hard problem. Wouldn't it be nice to create a computer program that could automatically find the optimum solution?

This is very relevant to our interests.

For the Apollo missions NASA used a "carry-along" strategy, where all vehicles and resources traveled with the crew at all times. Along with the horrific propellant cost to boost all of this from Terra into LEO. For the International Space Station NASA adopted a "resupply" strategy. This also has horrific boost cost plus it requires a close resupply source (Terra).

The resupply strategy ain't a gonna work for a Mars mission (as Terra gets further and further away), so the conventional view was to use a carry-along strategy. Dr. Ishimatsu examined NASA's Mars Design Reference Architecture 5.0 (plus addendum 1 and 2).

As you know from reading this section the way to avoid the horrific boost costs is in-situ resource utilization: travel light and live off the land. The problem is figuring out what is the best placement of in-situ mining, refining, and orbital depot assets.

Dr. Ishimatsu's software determined that using lunar propellant mines and tankers would cut the cost of the conventional NASA Mars mission by a whopping 68 percent!

It is very similar to the military. The old bromide is that amateurs talk about battle tactics while professionals talk about logistics. Well, deep space exploration is going to require a well-planned logistics strategy as well.

Dr. Ishimatsu examined several prior solutions, but all either were not scaleable as the mission complexity increased, required the user to pre-define the logistics network (i.e., solve the problem manually), or were not capable of doing optimization with no human input.

Dr. Ishimatsu used Dale Arney and Alan Wilhite's technique of modeling space system architectures using graph theory. The nodes are physical locations in space wihle the arcs (connections between the nodes) are possible movements or transports between nodes. Note that arcs are one-way, an arc going from node A to node B is totally different from an arc going from node B to node A. This is because one can, for instance, use aerobraking to traverse an arc going from LEO to Terra's Surface, but one cannot use aerobraking to go from Terra's surface into LEO.

To allow for the optimal solution, it is best to include as many nodes and arcs as possible. The optimizer obviously cannot use arcs and nodes that are not present. If the optimal solution requires use of a missing node or arc, it will not be found.

One peculiarity is that you use an arc that starts and ends on the same node to model a node that is a resource processing facility. This is required in order to allow the optimizing mathematics to work. These are called a "graph-loop", "self-loop" or a "buckle".

Another peculiarity is having several arcs between a given pair of nodes. For instance, if the mission could move items between node A and node B by either chemical rockets or nuclear thermal rockes, each rocket type would have its own arc between node A and node B. This is because the two rocket types have different specific impulse and thus different propellant consumption. Additional arcs will be required for the same rocket type if it has different delta V usage choices. For instance, a nuclear thermal rocket can do either an economical burn with a long time of flight or an expensive burn were more propellant was expended in order to reduce the time of flight. There will also be an additional arc where aerocapture is possible.

Basically the multiple arcs allow the optimization to explore multiple mission choices. One choice per arc.

These multiple arcs between a given pair of nodes are called "parallel arcs."

For logistics calculation, you state the mission as a set of demands at certain nodes in the network. A demand for "plantISRU" at the LSP node corresponds to a lunar mission to transport an in-situ resource allocation industrial plant to the lunar south pole.

Dr. Arney modeled the propellant required in a mission as costs on a given arc. But Dr. Ishimatsu found it more useful to model the propellant required for all subsequent stages of the mission as payload on a given arc. In addition, since in-situ resource allocation (ISRU) allowed propellant and other resources to be generated at other nodes besides Terra, it made sense to model propellant as commodities included in the flow variables rather than as costs like Dr. Arney did. This allows formulating the problem as a multi-commodity network flow, with some commodities coming from Terra and others from ISRU sites.

The optimization problem becomes finding the best routes in the network that satisfies the mission demands while also meeting certain constraints (i.e., figuring out which nodes and arcs to use). The result will tell you "where to deploy what."

The program is trying to optimize TLMLEO, which is Total Launch Mass from Terra to Low Earth Orbit (LEO) required to set up the entire logistics network. The program is trying to find the solution with the lowest TLMLEO.

Note that there are lots of other things that could be optimized for, but this system only optimizes TLMLEO. Other things that might be optimized include:

  • Development, Test, and Evaluation cost of the various components (ISRU and orbital propellant depots will require lots of expensive R&D)
  • Number of rendezvous and refueling events (the more, the higher the chance of a malfunction or accident)
  • Complexity (the more complicated, the more potential points of failure)

For the nitty-gritty mathematical details of the optimization, please refer to Dr. Ishimatsu's thesis. It contains lots of calculus and matric algebra which makes my head hurt. There are matrix multiplications for flow equilibrium, flow transformation, and flow concurrency.

As a case study, Dr. Ishimatsu ran NASA's Mars Design Reference Architecture 5.0 through his software.

In the model, everything that travels from node to node is a "commodity", even the crew. The 20 commodities are listed in the table below. Each commodity has a flow and demand all measured in kilograms.

  • Crew
    • crew (traveling to Mars)
    • crewRe (returning to Terra)
  • Resources
    • hydrogen
    • oxygen
    • water
    • methane
    • carbonDioxide
    • food
    • waste
  • Infrastructure
    • habitat
    • plantISRU
    • sparesISRU
  • Transportation
    • vehicle
    • inertLOXLH2
    • inertLOXLCH4
    • inertNTR
    • tankLOX
    • tankLH2
    • tankLCH4
    • aeroshell

Commodity "crew" represents the crew traveling from Terra to Mars while "crewRe" represents the crew returning to Terra. A self loop on Mars transforms crew into crewRe, enforcing the rule that the mission is a round trip. This is a mathematical trick that allows the optimization math to work.

The Resources catagory includes the rocket propellants, crew provisions, and crew wastes.

The Infrastructure catagory includes habitation facilities, ISRU industrial plants, and ISRU spares.

The Transporation catagory includes vehicles, propulsive elements, and non-propulsive elements. "InertX" means "rocket engine utilizing propellant X" while "TankX" means "tank full of propellant X. The three engines are: chemical liquid oxygen + liquid hydrogen, chemical liquid oxygen + liquid methane, and nuclear thermal rocket. Note that NTR can use any of the three tanks as propellant, the others require tanks of each of their named propellants. For NASA reasons, the NTR is not allowed for lift-off or landing on a planet, and aerocapture is allowed for unmanned cargo missions but not allowed for manned missions.

Solar electric rockets were not included because they require a different way of defining the arc parameters.

For the Mars mission it requires a demand for "habitat" at GC and a demand for "crewRE" at PAC. This translates into a mission to send a crew of six and a surface habitat to Mars Gale Crater, the crew becomes crewRE (crew ready to return to Terra) on Mars after a 540 day stay, which forces a mission to send the crewRE from Mars to Terra Pacific Ocean Splashdown.

Dr. Ishimatsu used the graph below, which does show the self-loops but only shows a single arc even when parallel arcs are present. Otherwise the diagram would be an unreadable mess. The full graph has 16 nodes and 598 arcs. There are self-loops at LSP (Lunar south pole), DEIM (Deimos), PHOB (Phobos), and GC (Mars Gale Crater).

ISRU availability/technology have the folloiwng assumptions:

  • Lunar ISRU can produce O2 from regolith or H2O from water ice at a rate of 10 kilograms per year per unit plant mass while requiring spares of 10% of plant mass per year.
  • Mars ISRU can acquire CO2 from the atmosphere or H2O from water ice with the same production rate and spares requirement as those for lunar ISRU.
  • Mars CO2 can be converted into CH4 and H2O via the Sabatier reaction or can be converted into O2 via solid oxide electrolysis.
  • Electrolysis of H2O and pyrolysis of CH4 are assumed to be available along with lunar/Mars ISRU

All these chemical reactions are modeled as an optional self-loop.

First, a "baseline" problem is defined and sent through the program for a solution. This is a simplified problem whose solution will be used to measure the results of altering various parameters. Among other things the baseline problem has the propulsion system modeling simplified. For the details about the baseline problem, please refer to Dr. Ishimatsu's thesis

Dr. Ishimatsu rubs salt in the wound by cheerfully telling us "Using MATLAB 8.3 (R2014a) with CPLEX 12.6 on an Intel R CoreTM i7-2640M CPU at 2.80 GHz, one run of the optimization model takes approximately 12 seconds for preprocessing and 1.2 seconds for optimization (TLMLEO minimization)." He did a test validation by constraining the model to NASA's Mars Reference Mission, the results were practically identical.

The baseline solution has a TLMLEO of only 271.8 metric tons, a 68% savings from the NASA Mars Reference Mission NTR scenario, and a 78.3% savings from NASA's chemical/aerocapture scenario.

Then the user can alter various propulsion parameters and measure the results against the baseline solution. The other parameters and assumptions remain the same. The surprise here is that NASA's Mars Reference Mission's reliance on nuclear thermal rockets is sub-optimal. LOX/LH2 chemical engines are superior, if you include ISRU (which NASA did not). The massive amounts of oxygen and hydrogen produced by the Lunar ISRU more than makes up for the relatively low specific impulse of the chemical rocket.

The arc from Kennedy Space Center (KSC) to Low Earth Orbit (LEO) has by far highest delta V cost: 9.8 km/s. This is the mathematical way to model Heinlein's "Halfway to Anywhere" observation. The emergent property produced by optimization is the need for in-situ resource utilization.

Now the user can alter various ISRU availability scenarios and measure the results against the baseline solution. The other parameters and assumptions remain the same.

Example: Depot into Story Plot

This section has been moved here.

Drydocks and Wetdocks

Spacecraft will need maintenance, and some will occasionally need major repairs due to damage (or gunfire). Obviously repairs will be eaiser if the engineers can perform them while wearing shirt-sleeve clothing instead of encumbering space suits. Most spacesuits raise the energy expenditure to do a task by about 400%.

Surrounding a spacecraft with an atmosphere can be easily done if:

  • the spacecraft is near a planet with a ground repair dock
  • the planet with the dock also has a breathable atmosphere
  • the spacecraft is designed to land on a planet with an atmosphere, that is, the ship is not an orbit-to-orbit type or can only land on airless planets
  • the damage to the spacecraft is mild enough that it is capable of landing

If any of these are not true, the ship will need an orbital drydock.

This is a space structure big enough to hold the spacecraft, capable of pressurizing the interior to shirt-sleeve conditions, and full of repair-crew and their tools. Probably inside or near a space station.

Locations too impoverished to afford such structures will just have to make do with space suited crews or remote drones with waldoes. Such facilities are called orbital wetdocks.

The non-offical term for both is "spacedock."


Space Drydocks

One of the most unique applications of inflatables is being proposed by General Electric: emergency drydocks for and maintenance of orbiting vehicles and spacecraft, some of which by necessity will always be rigid metallic types.

“The drydocks”, reports E. J. Merrick, project engineer in GE's MSVD, “could be as simple as a plastic sausage-skin drawn over and around the entire craft and then inflated.” Once pumped full of breathable air, space repairmen would not need to wear spacesuits as they scrambled over the metallic vehicle to be fixed.

Their job done, the sealed drydock “sausage” is again deflated, folded and packed away in a ferry rocket for use and again. Safety and comfort for the spacemen, Merrick adds, plus reliability and efficiency of the inflatable technique, will make such convenient once shelters ideal for space drydock missions.

For lesser maintenance and jobs that do not require an immense drydock area around the entire vehicle, GE has designed the smaller “space hog.” Based on the earthly sand-hog technique of providing caissons for men to work in, GE's concept is a pressure-tight, non-rigid tube or cylinder with stiffening rings down its length. In different sizes for one spaceman or several, these “space hog” units could easily be inflated outside of a spacecraft, providing a safe temporary environment for specific at any desired spot.

From VICTORY IN SPACE by Otto Binder (1962)

      Engineer-Captain Mikhail Borisovich Andreev sat strapped behind his oversize desk and worked to peel off a few more items from his overflowing day list before his VIP visitor showed up.  Extending beyond his office in every direction were the slate gray bulkheads and oversize machinery of Orbital Shipyard Delta Seven, recently departed from orbit around Halcyon IV, and now in orbit about Eulysta II, known to its former owners as Corlis.  That, at least, was the human transliteration of the unpronounceable Ryall phonemes that made up the true name of the planet.

     At its most basic, an orbital dockyard performed the same functions as its groundside counterparts.  It just did so in micro gravity.  Delta VII had the ability to build anything up to a light cruiser from scratch, and with some monkeying of the cradles with which it enveloped its wounded patients, could perform major surgery even on one of the big blastships.
     At the moment, the big dock’s restorative hangars were empty, and its first customer was to be a mere light cruiser, which didn’t seem to justify the epic journey through the nebula that Andreev and his men had just completed.
     A space dock is not quantitatively different in function from any other spaceship.  To be effective, it had to have compartments conditioned to shirtsleeve environments in which its crew lived.  Unlike a planet-based dock, one of the big spherical dockyards had to be mobile so that it could be moved to where it would be most useful, which meant it required power reactors and both normal space and jump engines.
     In a war that extended across dozens of star systems and hundreds of light-years, it was unreasonable to expect a wounded ship to return to the place of its birth.  It was more efficient for them to jump one or at most two systems back from the front lines and be repaired close to the scene of battle, the better to return as quickly as possible to the fray.

     Philip explained the damage that his ship had taken in a few carefully composed sentences.  The repair officer listened, then nodded slowly.
     “Standard Illustrious-class light cruiser, isn’t she?”
     “Yes, sir.  Built last year at Sandar from Terrestrial Space Navy specifications.”
     “Good, then you use all standard modules.  That means that we will be able to work quick and dirty.  Rip out everything that doesn’t work, weld on a new bow section, and then stuff the hull with new equipment still in the packing boxes from the factory.  We won’t even try to repair your old equipment, just ship what seems salvageable back to human space for a depot to handle.  When we get through with Queen Julia, she’ll be better than new.”
     “How long?” Philip asked.
     “A month, six weeks at the max.  That assumes that something with higher priority doesn’t materialize in the foldpoint and have you kicked out of the bay before we have time to finish the job.”
     “Can my crew help?”
     “Sure.  We can always use some trained hands and that way, they will be up to speed on the new stuff when we send you back to space with a shiny new coat of paint inside and out.”
     “Thank you, Captain.  You don’t know how frustrating it has been to turn our backs on the action and limp here when our mates are getting the hell kicked out of them.”
     “Captain Walkirk,” Andreev replied with a wistful tone, “I have been a repair officer for sixteen years and seen ships and men head out into the deep black to fight the enemy, never to return.  I know precisely how frustrating it is...”

     Despite Philip’s impatience, repairs on Queen Julia had progressed with surprising speed after the tugs maneuvered the crippled cruiser into the all-encompassing embrace of the big space dock.  As Captain Andreev, the dock commander, had pointed out, repairs were greatly facilitated by the cruiser’s design, which was based on the Terrestrial Space Navy’s Illustrious III class of warships.  Like her Illustrious sisters, Queen Julia used standard modules throughout her hull.

     The damage to Julia was sufficiently extensive that had the cruiser been one of the older ships of the Royal Sandarian Navy, or of the Altan Space Navy for that matter, she would probably have been scrapped.  Repairing those 150-year-old designs would have taken too much time and too many scarce resources.  For modern ships, with their interchangeable parts, repairs were the equivalent of a child’s game of building sticks.
     The space dock technicians had begun the repair by slicing away the cruiser’s smashed-in bow with a laser as powerful as any carried by a blastship.  It had been disconcerting to look at his ship and see it in cross-section, with compartments, passageways, and utility conduits all open to space.  It had been even more disconcerting to watch the minor surgery that had followed the amputation of the bow.  For more than a week, dockyard technicians had swarmed over the ship, cutting out partially melted sections of hull and interior structure, stripping away kilometers of optical cabling that had been clouded by radiation exposure, and emptying equipment racks of components that triggered fault messages when queried by diagnostic routines.  At first, Philip and his crew acted as unskilled helpers in this systematic vandalism, taking direction from the dock’s skilled cadre of ship wreckers.
     As 16- and 20-hour days began to blur together, however, the cruiser’s crew began to take on more of the repair tasks themselves.  Not only were they becoming more skilled, but also the dock’s personnel were increasingly diverted to service other cripples.

     Altogether, Queen Julia spent 22 days surrounded by space dock scaffolding and movable work centers.  At the end of that time, when the ship was once again vacuum tight, Captain Andreev ordered his dock cleared so that he could begin repairs on another victim of the continuing contest over who would control Spica.  Philip had watched from an inter-orbit scooter as the dock’s massive clamshell doors opened and his ship was again exposed to Eulysta’s warming yellow rays.
     Interior work on the ship proceeded apace even while tugs gently shifted the recuperating cruiser to a parking orbit aft of the repair dock.  Repairs continued for four more weeks as Julia’s crew slowly put their ship back together.  The list of things needing fixing seemed endless.  There were networks to synchronize, interface nodes to reconnect, missile launchers to align.  Most of these tasks required the attention of skilled technicians, all were time consuming, and Philip never seemed to have enough labor of the right sort to satisfy even half of the demands for immediately attention.

     Yet, despite workdays that were much too long and infrequent sleep periods, looking back on it, he could not remember a time when he had been happier.

     Orbital Shipyard Delta Seven was more than just an oversize body and fender shop.  In addition to its primary function, it sported a completely equipped orbital hospital where doctors worked to heal injured crewmembers while the dock technicians attempted to repair their ships.


(ed note: the planet Canaan has an asteroid-moon named TerVeen. The latter has been converted into a shirt-sleeve repair dock. "Climbers" are small cloaked starships delivered to their patrol routes by motherships.)

      Westhause continues to explain. “What they did was drill the tunnels parallel to TerVeen’s long axis. They were cutting the third one when the war started. They were supposed to mine outward from the middle when that was finished. The living quarters were tapped in back then, too. For the miners. It was all big news when I was a kid. Eventually they would’ve mined the thing hollow and put some spin on for gravity. They didn’t make it. This tunnel became a wetdock. A Climber returns from patrol, they bring her inside for inspections and repairs. They build the new ones in the other tunnel. Some regular ships too. It has a bigger diameter.”
     In Navy parlance a wetdock is any place where a ship can be taken out of vacuum and surrounded by atmosphere so repair people don’t have to work in suits. A wetdock allows faster, more efficient, and more reliable repairwork. (so this author has Docks and Wetdocks, instead of Wetdocks and Drydocks)
     “Takes a month to run a Climber through the inspections and preventive maintenance. These guys do a right job.”

     The bus surges forward. I try to watch the work going on out in the big tunnel. So many ships! Most of them are not Climbers at all. Half the defense force seems to be in for repairs. A hundred workers on tethers float around every vessel. No lie-in-the-comer refugees up here. Everybody works. And the Pits keep firing away, sending up the supplies.
     Sparks fly in mayfly swarms as people cut and weld and rivet. Machines pound out a thunderous industrial symphony. Several vessels are so far dismantled that they scarcely resemble ships. One has its belly laid open and half its skin gone. A carcass about ready for the retail butcher. What sort of creature feeds on roasts off the flanks of attack destroyers?
     Gnatlike clouds of little gas-jet tugs nudge machinery and hull sections here and there. How the devil do they keep track of what they’re doing? Why don’t they get mixed up and start shoving destroyer parts into Climbers?

     Our mother ship is one of several floating in a vast bay. The others have only a few Climbers suckered on. Each is kept stationary by a spiderweb of common rope. The ropes are the only access to the vessel. “They don’t waste much on fancy hardware.” Tractors and pressors would stabilize a vessel in wetdock anywhere else in the Fleet. Vast mechanical brows would provide access.
     “Don’t have the resources,” Westhause says. “‘Task-effective technological focus,’” he says, and I can hear the quotes. “They’d put oars on these damned hulks if they could figure out how to make them work. Make the scows more fuel-effective.”

     I want to hang back and look at the mother, to work out a nice inventory of poetic images. I’ve seen holoportrayals, but there’s never anything like the real thing. I want to catch the flavors of watching hundreds of upright apes hand-over-handing it along with their duffel bags neatly tucked between their legs, as if they were riding very small, limp, limbless ponies. I want to capture the lack of color. Spacers in black uniform. Ships anodized black. The surface of the tunnel itself mostly a dark black-brown, with streaks of rust. The ropes are a sandy tan. Against all mat darkness, in the low-level lighting, without gravity, those lines take on a flat two-dimensionality, so all of them seem equally near or far away.

     The bearing and tilt on the camera tell me nothing. Forward. It should be staring at the wall of the wetdock. Instead, the screen shows me an arc of darkness and only a small amount of wall. The lighting seems brilliant by contrast with the darkness.
     High on the wall, at the edge of the black arc, a tiny figure in EVA gear is semaphoring its arms. I wonder what the hell he or she is up to. I’ll probably never know. One of the mysteries of TerVeen.
     Damn! How imperceptive can one man be? We’re moving out. We’re under way already. Must have been for quite a while. That creeping arc of darkness is naked space. The mother is crawling out of TerVeen’s backassward alimentary canal.

     I close my eyes and try to imagine our departure as it would appear to an observer stationed on the wall of the great tunnel. The Climber people come hustling in, hours after the mothercrew has begun its preparations. They swarm. Soon the mother reports all Climbers manned and all hatches sealed and tested. Her people scamper over her body, releasing the holding stays, being careful not to snap them. Winches on the tunnel walls reel them in.
     Small space tugs drift out from pockets in the walls and grapple magnetically to pushing spars extending beyond the mother’s clinging children.
     Behind them, way behind them, a massive set of doors grinds closed. From the observer’s viewpoint they’re coming together like teeth in Brobdingnagian jaws. They meet with a subaudible thud that shakes the asteroid.

     Now another set of doors closes over the first. They snuggle right up tight against the others, but they’re coming in from left and right. Very little tunnel atmosphere will leak past them. Redundancy in all things is an axiom of military technology.
     There are several vessels caught in the bay with the departing mother. They have to cease outside work and button up. Their crews are cursing the departing ship for interrupting their routine. In a few days others will be cursing them.

     Now the great chamber fills with groans and whines. Huge vacuum pumps are sucking the atmosphere from the tunnel. A lot will be lost anyway, but every tonne saved is a tonne that won’t have to be lifted from Canaan.
     The noise of the compressors changes and dwindles as the gas pressure falls. Out in the middle of the tunnel, the tugs slow the evacuation process by using little puffs of compressed gas to move the mother up to final departure position.
     Now a pair of big doors in front of the mother begins sliding away into the rock of the asteroid. These are the inner doors, the redundant doors, and they are much thicker that those that have closed behind her. Great titanium slabs, they’re fifty meters thick. The doors they back up are even thicker. They’re supposed to withstand the worst that can be thrown against them during a surprise attack. If they were breached, the air pressure in the 280 klicks of tunnel would blow ships and people out like pellets out of a scattergun.
     The inner doors are open. The outer jaws follow. The observer can peer down a kilometer of tunnel at a round black disk in which diamonds sparkle. Some seem to be winking and moving around, like fireflies. The tugs puff in earnest. The mother’s motion becomes perceptible.

     A great long beast with donuts stuck to her flanks, moving slowly, slowly, while “Outward Bound” rings in the observer’s ears. Great stuff. Dramatic stuff. The opening shots for a holo-show about the deathless heroes of Climber Fleet One. The mother’s norm-thrusters begin to glow. Just warming up. She won’t light off till there’s no chance her nasty wake will blast back at her tunnelmates.
     The tugs are puffing furiously now. If the observer were to step aboard one, he would hear a constant roar, feel the rumble coming right up through the deckplates into his body. Mother ship’s velocity is up to thirty centimeters per second.
     Thirty cps? Why, that’s hardly a kilometer per hour. This ship can race from star to star in a few hundred thousand blinks of an eye.

     The tugs stop thrusting except when the mother’s main astrogational computers signal that she’s drifting off the cen-terline of the tunnel. A little puff here, a little one there, and she keeps sliding along, very, very slowly. They’ll play “Outward Bound” a dozen times before her nose breaks the final ragged circle and peeps cautiously into her native element. Groundhog coming up for a look around.
     The tugs let go. They have thrusters on both ends. They simply throw it into reverse and scamper back up the tunnel like a pack of fugitive mice. The big doors begin to close.
     The mother slides on into the night, like an infant entering the world. She hasn’t actually put weigh on but has taken it off. She’s coming out the rear end of TerVeen, relative to the asteroid’s orbit around Canaan. The difference in orbital velocities is small, but soon she’ll drift off the line of TerVeen’s orbit.
     Before she does, word will come from Control telling her the great doors are sealed. Her thrusters will come to life, burning against the night, blazing off the dull, knobby surface of TerVeen. She’ll gain velocity. And up along her flanks will gather the lean black shapes of her friends, the attack destroyers.

From PASSAGE AT ARMS by Glen Cook (1985)

Space Logistics Base

James Snead has written a few paper about space infrastructure. Most interesting is Architecting Rapid Growth in Space Logistics Capabilities. On page 23 he gives an example of an orbiting space logistics base, including a space dock. Refer to that document for larger versions of the images below.

...the space logistics base’s functions are: (1) housing for travelers and operating crews; (2) emergency care; (3) in-space assembly, maintenance, and repair; and (4) materiel handling and storage.

The example space logistics base consists of four elements. At the top in Fig. 10 is the mission module providing the primary base control facility, emergency medical support, and crew and visitor quarters. The personnel quarters are located inside core propellant tanks that are retained from the SHS used to launch the mission module. The overall length of the mission module and propellant tanks is approximately 76 m (250 ft). Solar arrays and waste heat radiators (shown cut-away in Fig. 10) are mounted on a framework surrounding the mission module to provide additional radiation and micrometeoroid protection.

The second element consists of twin space hangars. These serve as airlocks for receiving spaceplanes and provide a pressurized work bay for conducting on-orbit maintenance of satellites and space platforms.

As shown in Fig. 11, the space hangar consists of a structural cylindrical shell 10 m (33 ft) in diameter, a forward pressure bulkhead containing the primary pressure doors, and an aft spherical work bay. These elements, which define the primary structure, would be manufactured as a single unit and launched as the payload of an SHS. The large, nonpressurized, space debris protection doors would be temporarily mounted inside the hangar for launch and then demounted and installed during the final assembly of the hangar at the LEO construction site. All of the other hangar components would be sized for transport to orbit in the cargo module of the RLVs and then taken through the hangar’s primary pressure doors for installation.

Future logistics supportability is a key feature of this hangar design. The size, weight, location, and access of the internal hangar components enables them to be inspected, repaired, and replaced without affecting the primary structural / pressure integrity of the hangar. With the exception of the space debris protection doors, this would be done inside the hangar when it is pressurized. The ISS-type airlock and space debris protection doors, although mounted externally, would be demounted and brought into the hangar for inspection, maintenance, and repair. For the repair of the primary pressure doors, they would be demounted and taken into the spherical work bay or the other hangar for servicing.

The hangar’s design enables both pressurized and unpressurized hangar operations to be undertaken simultaneously. When the main hangar deck is depressurized to receive cargo or spaceplanes, for example, pressurized maintenance operations would continue inside the 9.8 m (32 ft) diameter spherical work bay and the 2.8 m (9 ft ) diameter x 4.3 m (14 ft ) work compartments arranged along the top of the hangar.

Hangar operations in support of the passenger spaceplanes, as shown in Fig. 12, highlight the improvement in on-orbit logistics support enabled by the large hangars. After entry into and repressurization of the hangar, the passengers would disembark from the spaceplane. Support technicians, working in the hangar’s shirtsleeve environment, would inspect the spaceplane and, in particular, the thermal protection system for any damage to ensure that it is ready for its return to the Earth. While at the space base, the spaceplane would remain in the hangar to protect it from micrometeoroid or space debris damage. Minor repairs to the spaceplane could also be undertaken to ensure flight safety.

The third element is the air storage system. The prominent parts of this system are the large air storage tanks that are the reused core propellant tanks from the two SHS used to launch the twin space hangars. Besides storing air from the hangars, this system also: manages the oxygen, carbon dioxide, and moisture levels; removes toxic gases, vapors, and particulates; and, controls the temperature and circulation of the air within the hangar and its compartments.

The fourth and final element is the space dock. It would be constructed from structural truss segments assembled within the space hangars using components transported to orbit in the RLVs. The space dock would provide the ability to assembly and support large space logistics facilities, such as the space hotels and large manned spacecraft described in the following. It could also used to store materiel and as a mount for additional solar arrays.

The space hangars and space dock would enable traditional logistics operations of maintenance, assembly, and resupply to be routinely conducted in Earth orbit. This is an enabling capability necessary to become spacefaring and achieve mastery of operations in space.

The space logistics base would have approximately 20 personnel assigned. The tour of duty would be 90 days with half of the crew rotating every 45 days. Crew rotation and base resupply would require approximately 32 RLV missions per year per base with 8 spaceplane missions and 24 cargo missions. This would provide approximately 12,000 kg (26,000 lb) of expendables and spares per person per year. At $37M per mission, a ROM estimate of the annual transportation support cost per base would be approximately $1.2B.

While the LEO space logistics base would have sufficient housing capacity to support the 20 assigned personnel and a modest number of transient visitors, it would not be a primary housing facility. Since people cannot simply pitch a tent and “camp out” in space, establishing early permanent housing facilities is an important and enabling element of opening the space frontier to expanded human operations. The architecture of the Shuttle-derived heavy spacelifter and the LEO space logistics base was selected so that the first large space housing complexes, referred to as space hotels, could be constructed using the same space logistics base modules.

A composite illustration of the design, assembly, and deployment of the example space hotel is shown in Fig. 13. This hotel design is configured as a hub and spoke design with a long central hub and opposing sets of spokes attached to the central hub module. This configuration makes it possible to use variants of the space base’s mission modules and space hangars as the primary elements of the space hotel’s design.

Element 1, in Fig. 13, shows the start of the hotel assembly sequence. The central hub module, shown with the SHS’s core propellant tanks still attached, is being positioned at the space logistics base’s space dock. The central hub module would be a version of the mission module used in the space logistics base. Its design would include 12 docking ports around its circumference for attaching the spokes.

Element 2 shows the completed hub and one attached spoke. Two space hangars are located at the ends of the hub and the first spoke is shown attached to the central hub module. In assembling the hub, the core propellant tanks from the two SHS missions used to launch the hangars would be incorporated into the hub to provide additional pressurized volume. This approach would be also used for the spokes. Each spoke would consist of a generalpurpose mission module with the SHS’s core propellant tanks reused for additional pressurized volume. As with the mission module on the space logistics base, the spokes would be surrounded by solar arrays and waste heat radiators. This is what provides their “boxy” appearance.

Element 3 shows the completed 100-person space hotel with two pairs of spokes on opposing sides of the hub. This is the baseline space hotel configuration. Seven SHS missions would be required to launch the hub and spoke modules for the baseline hotel. One additional SHS cargo mission would be used for the solar arrays and waste heat radiators.

This design enables the hotel to be expanded to 6, 8, 10, or 12 spokes. Each spoke would require one additional SHS mission. The 12-spoke configuration would accommodate up to approximately 300 people. Each additional spoke would be tailored to provide a specific capability, such as research and development facilities, tourist quarters, office space, retail space, etc.

Element 4 shows the completed space hotel after being released from the space dock. It also shows how the hotel would rotate about the long axis of the hub to produce modest levels of artificial gravity in the spokes. At about two revolutions per minute, a Mars gravity level is achieved at the ends of the spokes. This use of artificial gravity enables the spokes to be organized into floors (Element 5 in Fig. 13). Each spoke would contain 18 floors with 14 of these available for general use and the remaining 4 floors used for storage and equipment. The spokes would be 8.4 m (27.5 ft) in diameter. This would provide a useful floor area of approximately 42 m2 (450 ft2) per floor. The total available floor area in the baseline configuration would be 2,340 m2 (25,200 ft2). The 12-spoke configuration, having 192 floors total, would have 3 times this floor area—7,026 m2 (75,600 ft2) or about 23 m2 (250 ft2) per person.

An estimate can be made of the number of guests visiting the hotel each year. Assuming a 3:1 ratio of guests to staff, approximately 76 guests would be staying each night in the baseline configuration and 228 guests in the full configuration. With one third of the useful floors configured as guest cabins, two cabins to a floor, each cabin would have a useful area of approximately 21 m2 (225 ft2).* With an average stay of one week, approximately 4,000 guests and 12,000 guests would visit the 4- and 12-spoke hotels each year, respectively.

If each passenger spaceplane carries 10 guests, approximately 400 and 1,200 RLV flights would be required each year. With an additional 25% required for staff transport and resupply, the 4-spoke hotel would require about 10 flights per week and the 12-spoke hotel would require about 30 flights per week. If the RLVs could achieve a one-week turnaround time, and allowing for one in five RLVs being in depot for maintenance, 12 RLVs would be required to support the 4-spoke hotel and 36 RLVs for the 12-spoke hotel.†

At the $37M per flight cost discussed previously for first generation RLVs, the per passenger transportation cost would be approximately $3.7M. With this transportation cost structure, a sustainable space tourism or space business market may not be possible. However, if a second generation RLV could reduce this cost by a factor of 10 to $0.37M per passenger, as an example, then an initial market demand for the baseline hotel may develop and be sustainable. In such case, the annual transportation revenue for the baseline hotel would be $3.7M x 500 = $1.9B and the 12-spoke hotel would be $5.6B.‡ This improvement in transportation costs would also yield a savings of 90%—approximately $1B per year—in the transportation costs to support the LEO space logistics bases. Human space exploration missions would also realize a significant cost reduction.

While developing a conceptual design of a space hotel would appear premature at this early stage of considering the architecture of an initial space logistics infrastructure, several important conclusions emerge that indicate otherwise:

1) Careful selection of the initial space logistics architecture can also establish the industrial capability to build the first space hotels necessary to enable the expansion of human enterprises in space.

2) A commercially successful space hotel will require second generation RLVs to lower further the cost of transportation to orbit.

3) In order for these second generation RLVs to be ready when the first space hotel is completed, the technology research investment would need to begin concurrently with the start of the detailed design of the initial space logistics systems. Conversely, for private investment to seriously consider building the first hotels, significant science and technology progress in developing the second generation RLVs must be demonstrated by the time the initial hotel construction contracts are made.

4) The benefits of reduced space transportation costs will also substantially lower the cost of operation of the initial elements of the space logistics infrastructure, leading to a likely increase in demand for more in-space logistics services.

5) Space hotels and second-generation RLVs may become an important new aerospace product for the American aerospace industry, establishing American leadership in this new and growing field of human astronautical technologies.

6) It is not unrealistic to expect, with the building of an integrated space logistics infrastructure, that hundreds of people could be living and working in space by 2020, growing to thousands of people by 2040 with many of these living in the first permanent orbiting space settlements.

* A standard cabin on the new Queen Mary 2 cruise ship has an area of 18 m2 (194 ft2). A premium cabin has an area of 23 m2 (248 ft2).

† Launch sites for these RLVs would be distributed around the world. This would allow operations at the space hotel to run 24 hours per day since there is no day and night in LEO.

‡ This further reduction could come about through the introduction of a spiral version of the first-generation RLVs where improvements to the high maintenance cost subsystems, e.g., engines, could substantially reduce the recurring costs. Another approach would be development of entirely new RLV configurations—perhaps a single-stage configuration—that would also result in a substantial reduction in recurring costs per passenger through subsystem design improvements and the ability to carry more passengers per trip. A key issue in both approaches is the amortization of the development and production costs. High flight rates, probably dependent on space tourism, would be required to yield an overall transportation cost sufficiently low to enable profitable commercial operations.

Solar Power Stations

Solar Power
PlanetSol Dist
☿ Mercury0.3876.6779,121
♀ Venus0.7231.9132,613
⊕ Terra1.0001.0001,366
♂ Mars1.5200.433591
⚶ Vesta2.3620.179245
⚵ Juno2.6700.140192
⚳ Ceres2.7680.131178
⚴ Pallas2.7720.130178
♃ Jupiter5.2000.03751
♄ Saturn9.5800.01115
♅ Uranus19.2000.0034
♆ Neptune30.0500.0012

Space-based solar power (aka "Powersat") is one of those concepts that make one think about idealistic hippy futurists in the 1970's drunk on the idea of MacGuffinite that is also ecological and green. It is solar energy on steroids. By placing the solar collectors in orbit you get all the solar energy since ground based solar collectors can only gather the frequencies that our atmosphere is transparent to, and are hampered by rain clouds and/or the fact that it is nighttime.

You can get almost unlimited amounts of green energy: no nasty coal, oil, natural gas, or uranium is required. Groovy, man!

The fact that none of these exist today tells you that the difficulties are overwhelming.

At Terra's orbital distance from Sol the solar power flux is abotu 1,366 watts per square meter. Due to the inverse square law the power increases the closer you get to Sol (see table). And vice versa as well.

But we do not care about such stations, since other than being a species of MacGuffinite, it has nothing to do with spacecraft, right?

Au contraire! Read on to see how solar power stations can be a boon to spacecraft.

Beamed Power

Let me take a minute to talk about solar moth rockets.

Remember the fundamental rule of rocket design: Every Gram Counts. The motivation behind the solar moth is "just imagine how much mass we could save if we eliminated the rocket engine from the design! Using the "magnifying glass incinerating an ant" principle, the solar moth utilizes a large mirror to focus the heat from the sun on the propellant, energizing it so it rushes out the exhaust bell, resulting in thrust.

It is a pity that solar energy is so diffuse around Terra's orbit. To really get worth-while amounts of heat, the solar moth will need huge mirrors. Which sort of eliminates the mass advantage of removing the engine.

That's where the powersat comes in. Have a powersat send power in a beam of microwaves and give the solar moth a microwave rectenna to receive the electrical energy! You will be using Beam-powered propulsion. The electricity can be used to heat the propellant. Suddenly your pathetically weak solar moth will be a super-powered muscle machine.


NASA’s Dr. Ernst Stuhlinger, a leading authority on electric (ion) propulsion, has often said that such a rocket system would be ideal for a manned journey to Mars.

“Yeah,” a wag once cracked, “if you can just find an extension cord long enough."


The joke is saying that electromagnetic and electrostatic propulsion systems (e.g., ion drives, VASIMR) are power hogs. Solar power arrays will have to be huge due to the low power concentration in sunlight (low as compared to the propulsion power demands). Nuclear reactors can easily supply the power but have ugly mass penalties. But the joke is on the cracking wag, beamed power is pretty much the same as an extension cord long enough.

Microwaves are difficult to focus, and the conversion from electricity to thermal energy has unavoidable inefficiencies. It would be nice to beam thermal energy instead of electrical energy. Can do: replace the microwave with a laser! Now you can use the same lightweight mirror on a solar moth, but with the much more intense radiant energy of a laser. It will be a laser thermal rocket. You can also use it on a solar sail craft and make it into a high-powered laser or photon sail. The advantage is that your delta-V capacity will be incredibly large. The disadvantage is that you are at the mercy of whoever owns the powersat.

If your laser thermal rocket is renting laser time from Beams-Я-Us, you better make sure that your bill is paid up. Otherwise they will pull the plug and your rocket will suddenly be powerless, and on a one-way ticket to nowhere. You might be able to limp along using solar power instead of the laser from Beams-Я-Us, but I would not bet your life on it.

And make sure you stick closely to the flight plan you filed with Beams-Я-Us, or they might have a problem keeping the beam aimed at you.

Beams-Я-Us might purchase their own laser thermal or laser sail ships. They will then be like a rail-road company, owning both the trains and the rails they run on.

As a matter of fact, the solar collector on the powersat will be much more effective if it was closer to the sun than Terra's orbit. Say: the orbit of Mercury. Now we're cooking!

And if we start beaming power to interstellar spacecraft, a stray beam might give some alien civilization a Wow! signal.


The report Fusion Energy for Space Missions in the 21st Century had an informative analysis on a proposed beam-powered spacecraft system, tucked away in Appendix B: An Alternative Strategy for Low Specific Power Reactors Powering Interplanetary Spacecraft, Based on Exploiting Lasers and Lunar Resources.

If you are in a hurry, skip to the end of this segment to see the chart showing laser power and range.

They started by examining a standard mission to Mars, and how chemical propulsion resulted in travel times that would exceed the astronaut's career limits on radiation exposure. Cyclers (with their more massive levels of radiation shielding) were rejected as a solution because they force the round trip periods to be inconveniently long four-year cycles.

A better solution would be a more powerful rocket engine. One that was strong enough to reduce the trip time so that the radiation dosage would be acceptably low (even with the spacecraft's lower level of radiation shielding).

The analysis assumed a radiation dose limit of chronic 5 rem/year (0.05 sievert) and acute 25 rem/incident (0.25 sievert). It assumed that spacecraft radiation shielding would still result in an exposure of 0.1 rem per day (0.001 sievert). This means that the astronauts would receive 25 rems in only 0.7 year (250 days).

So the spacecraft would have to be capable of making a round trip to Mars in only 0.7 year. Of course this means the astronauts will have to retire at the end of the mission.

The report then starts looking at a 0.33 year (4 month) one-way mission. This would only result in a radiation exposure of 12 rem (0.12 sievert). It would require the engine to have a minium specific power (αp) of 10 kWe/kg (ten kilowatts of electrical power per kilogram of engine). How they arrived at that figure is a bit complicated, see the report for details.

For a 1,000 metric ton initial vehicle mass the engine would need about 129 MWe output (engine mass of 12.9 metric tons at 10 kWe/kg)

The little NASA spacecraft in the center of the diagram receives beam power from an inflatable foil mirror focused on a photocell receiver.

The inflatable foil mirror has a diameter of about 1,000 m (one kilometer!) and an areal mass of 10-2 kg/m2. A=πr2 so the mirror has a surface area of about 785,000 m2. Times the areal mass means the foil mirror has a mass of about 7,850 kilograms (7.9 metric tons).

The photocell receiver has a diameter of about 100 m and an areal mass of 1 kg/m2. 7,850 m2 and a mass of 7,850 kilograms. In case you are interested, this represents a diamond film semiconductor 100 μ thin film voltalic array along with support structure. The dark non-laser side has heat radiators to get rid of the 10 kWth/m2 waste heat (see below).

We want the mirror+photocell combo to have a specific power of 10 kWe/kg for our 4 month Mars trip.

Each square meter of photocell is fed by 100 square meters of mirror. 1 square meter of photocell has a mass of 1 kg. 100 square meters of mirror has a mass of 1 kg. So each square meter of photocell represents 1 + 1 = 2 kg worth of mirror+photocell.

So in order for the mirror+photocell to have αp=10 kWe/kg, each square meter of photocell needs to be fed 20 kW of laser light. 20 kW divided by 2 kg equals the desired 10 kW/kg.

But that is only if the photocells were 100% efficient at converting laser light into electricity. The report assumes they are actually only 70% efficient (with an ultraviolet laser beam with a wavelength between 100 and 200 nanometers). Therefore they will need 30 kW/m2 of laser light (20 × 0.70) in order for αp=10 kWe/kg. In case of emergency, the diamond film semiconductor photocell can use solar energy at about 2% efficiency. This would probably be about 0.2 kWe/m2 at Terra orbit, raw solar energy is about 1.366 kW/m2.

The inflatable foil mirror probably has an efficiency of about 90%.

Since the 30 kW of laser power fed to each square meter of photocell comes from 100 square meters of foil mirror, each square meter of foil mirror needs to feed 30,000 W / 100 = 300 W of laser light to the photocell array.

The point of all this is that the specified NASA spacecraft needs the incoming laser beam to have a power density of 300 watts per square meter and have a spot size of at least one kilometer in diameter.

Unlike the diagram above, the report is of the opinion that it makes more sense to base the laser station on Luna instead of Terra. Lack of atmosphere (so you can use laser beams in the vacuum frequencies, like ultraviolet c), and access to Lunar Helium-3 for fusion plants (though that is more or less a Chimera). And it would also work on a Solar Power Station.

Desired characteristics:

  • 100 megawatt-level high average power
  • High conversion efficiency (20% to 40%)
  • High specific power (≥1 kWe/kg)
  • Tunability to any desired wavelength (Free Electron Laser)
  • Laser intensity at emitter 100 kW/m2

You want to use a free-electron laser so the beam frequency can be tuned. This allows laser to be adjusted to wavelength optimal for the photocell receiver of that particular spacecraft.

Laser intensity at emitter 100 kW/m2 in order to balance beam losses using optical coatings with radiative cooling.

The emitter would be large, to limit diffraction losses and to allow adequate cooling (otherwise intensity becomes too many kilowatts per square meter and the emitter melts). Probably an array of thin hexagonal wafer mirrors, each supported by three computer controlled actuators. It would have an areal mass of about 40 kg/m2.

Note that these specs are for a single laser emitter that might be dedicated to a Mars mission for four months. Beams-Я-Us will probably have an antenna farm full of these lasers. The more lasers, the more clients they can service simultaneously. More clients can be added by, say, having a given laser alternate boost periods between two clients. Scheduling might become tricky.

As previously mentioned, the astromilitary will be alarmed at the prospect of civilian ownership of huge batteries of ship-killing laser death-ray turrets. There might be a move to put Beams-Я-Us under military control.

Given the lunar-base transmitter intensity of 100 kW/m2, and the spacecraft photocell receiver needing 30 kW/m2 of laser light, it is possible to make an equation relating the average laser power PL and the range between transmitter and receiver R. If you are going to change either of those variables the equations will need tinkering with.

Well, if you were running Beams-Я-Us, you'd want to know how much energy each laser will require, and the maximum range a customer can be at, wouldn't you?

The other main variable is on the spacecraft, the ratio of the diameter of the inflatable foil mirror Dr to the diameter of the photocell receiver Df. The ratio is Dr/Df.

Dr = diameter of inflatable foil mirror
Df = diameter of photocell receiver
Dr/Df = ratio of mirror diameter to photocell diameter (10.0 in our sample ship)
R = range
λ = laser wavelength
PL = average laser power. Note in first two equations this is in watts, in third it is megawatts (200 MW in our sample ship)
3×104 W/m2 = 30 kW/m2, needed laser energy density at photovoltaic array
0.9 = efficiency of inflatable foil mirror (90%)
x1/2 = square root of x

So for the first equation:

Dr = (Dr/Df) * sqrt((4/π) * ((0.9 * PL)/(3×104)))

Dr = 10.0 * sqrt((4/π) * ((0.9 * 2×108)/(3×104)))

Dr = 870 m ≈ 1000 m diameter of inflatable foil mirror

Second equation:

Df = sqrt((4/π) * ((0.9 * PL)/(3×104)))

Df = sqrt((4/π) * ((0.9 * 2×108)/(3×104)))

Df = 87 m ≈ 100 m diameter of photocell receiver

Third equation is used to draw the chart below. Each diagonal λ line is plotted by choosing a constant value for λ then stepping along the X-axis with various values for R, using R and λ to calculate the Y-value PL

Given the requirement of a 1,000 metric ton initial vehicle mass on a Mars 4 month mission needing an engine with 129 MWe output, an inflatable foil collector with an efficiency of 90% and a photocell receiver efficiency of 70%, the required UV laser beam power is 200 MW. 129 / (0.9 * 0.7) ≈ 200 MW.

As indicated on the chart by dotted lines, the intersection of the Mars trip line and the λ=0.16 μM (UV) line is at the 200 MW average laser power point. Longer wavelengths would require a higher laser power, or several laser stations enroute to decrease the range requirement. A laser station on, say, Phobos would cut the range requirement in half. The ship would use a laser station based on Luna up to the half-way point, then switch to Phobos station.


Power beaming is clearly central to space-based solar power concepts. Here I will provide a quick overview of my understanding of power beaming, the various equations involved, typical example calculations.

If power beaming were efficient and cheap, I believe space-based solar power would be quite viable even for grid power. However it’s not, and that largely has to do with the distances involved AND the fact that you need to convert energy multiple times, with losses along the way. The distances involved aren’t a complete show-stopper, since you can solve that problem just by operating at a large enough scale. However, the conversion inefficiencies (and the need to dump waste heat, etc) is not going to go away simply by operating at greater scale (although it helps).

The first equation we need is the diffraction limit. Roughly speaking, the spot size of a transmitted beam (microwave or laser) is:

Spot size = distance-to-spot * wavelength / (aperture diameter)

This is close enough for an order-of-magnitude estimate. More detailed work to follow.

But if we have a satellite out in Geosynchronous orbit (36,000 km altitude) transmitting power at roughly 10GHz (3cm wavelength, the shortest wavelength that still penetrates readily through the atmosphere) with an antenna 300m in diameter (NRO SIGINT/ELINT satellites are rumored to be that big, but maybe only around 100m in diameter), you’d have a spot size on the order of:

3.6×107m * 3×10-2m / (3×102m) = 3.6×103m or 3.6 km in diameter

…turns out that not all the energy of your beam is contained in this diameter (“Where’s that factor of 1.22,” you cry), but that’s a halfway decent start (and you’d need an infinitely wide aperture to collect all the energy in the beam…). 3.6km is obviously huge. The biggest full-aperture dish ever built is the half-way finished Chinese Arecibo clone at 500m. Still, there are ways to tweak this.

With a laser operating at 1micron, in medium-Earth-orbit (10,000 km) with 1 meter diameter optics needs only a:

1×107m * 1×10-6m / 1m = 10m diameter receiver to receive the vast majority of the beam’s energy. This is much, much better, obviously. You could put a 10m diameter receiver on top of a tethered airship or drone or something that allows you to transmit it to the ground without interference from clouds.

Or heck, use it to power high-altitude aircraft… but that’s a whole ‘nother blog post! (And suffice it to say, there are lots of caveats about laser transmission of energy, too.)

From POWER BEAMING by Chris Stelter (2015)

8.25 Solar power satellites

     While in skeptical mode, let me say a few words about another concept of initial high appeal, the Solar Power Satellite. This was proposed in the 1960s by Peter Glaser, and like the L-5 colonies it had its heyday in the 1970s and early 1980s. Proponents of the idea believed (and believe) that it can help to solve Earth's energy problems.

     A solar power satellite, usually written as SPS, has three main components. First, a large array of photoreceptors, kilometers across, in space. Each receptor captures sunlight and turns it to electricity. The most usual proposed location is in geosynchronous orbit, though some writers prefer the Earth-Moon L-4 location. The second component is a device that converts electricity to a beam of microwave radiation and directs it toward Earth. The third component is a large array on the surface of the Earth, usually known as a rectenna, that receives the microwave radiation and turns it into electricity for distribution nationally or nternationally.

     The SPS has some great virtues. It can be placed where the Sun is almost always visible, unlike a ground-based solar power collector. It taps a power source that will continue to be steadily available for billions of years. It contributes no pollution on Earth, nor does it generate the waste heat of other power production systems. It does not depend on the availability of fossil or nuclear fuels.

     Of course, the SPS cannot be built without a powerful in-space manufacturing capability, something that is lacking today. We are having trouble putting modest structures, such as the International Space Station, into low orbit. It is likely that we will not be able to build an object as large as the proposed SPS for another century or more.

     But when a century has passed, we are likely to have much better energy-raising methods, such as controlled fusion. Admittedly, progress on fusion has been slow—we have been promised it for fifty years—but it, or some other superior method, will surely come along. A fusion plant (or, for that matter, a fission plant) in orbit would have all the advantages of SPS, and none of the disadvantages. Sunlight is a highly diffuse energy source unless you get very close to the Sun. As we pointed out in Chapter 5, the history of energy use shows a move in the direction of more compact power sources—oil is more intense and compact than water or wind, nuclear is more compact and intense than chemical. The other problem is that the Sun, unlike our future fusion reactors, was not designed to fit in with human energy uses and needs. I put the question the other way round: Why build a kilometers-wide array, delicate and cumbersome and vulnerable to micrometeor damage, when you can put the same power generating capacity into something as small as a school bus? Admittedly, we don't have controlled fusion yet—but we also can't build an SPS yet.

     However, the real killer argument is not technological, but economic. Suppose you launch SPS to serve, say, the continent of Africa. You still have the problem, who will pay for the energy? Economists distinguish two kinds of demand: real demand: the need for food of starving people with money to buy it; and other demand: the need for food of starving people without money. Regrettably, much demand for energy is in nations with no resources to pay for it.

     In spite of this economic disconnect, many people have suggested that an SPS would be great for providing energy to Africa, where energy costs are high. Suppose that you put SPS is geostationary orbit and beam down, say, 5 gigawatts. That's the power delivered by a pretty substantial fossil fuel station. Now, you could also generate that much energy by building a dam on the Congo River, where it drops sharply from Kinshasa to the Atlantic. So ask yourself which you would prefer if you were an African. Would you like SPS, providing power from a source over which you had no control at all—you couldn't even get to visit it. Or would you prefer a dam, which in spite of all its defects, sits on African soil and is at least in some sense under your control? SPS has to compete not only from an economic point of view, but from a social and political point of view.

     I think it fails on all those counts. Like the L-5 colony, SPS is part of a false future. It is not surprising to find Gerard O'Neill arguing that the sale of electricity generated by an SPS at L-5 would pay for the colony in the breathtakingly short period of twenty-four years. When we want to do something, all our assumptions are optimistic.

     There are still SPS advocates. A recent NASA study suggested that a 400 megawatt SPS could be built and launched for five billion dollars. Do I believe that number? Not in this world. We all know that paper studies often diverge widely from reality. NASA's original estimated cost to build the International Space Station was eight billion dollars. Over the years, the station has shrunk in size and the costs have risen to more than 30 billion dollars. Projects look a lot easier before you get down to doing them. Recall the euphoria for nuclear power plants in the 1940s, "electricity too cheap to meter." And that was for something we had a lot more experience with than the construction of monster space structures.

     Certainly, we hope and expect that the cost of sending material to space will go down drastically in the next few generations. We also will become increasingly unwilling to pollute the Earth with our power generation. But frequent space launches have their own effects on the environment of the upper atmosphere. If there is ever an SPS, which I doubt, it will more likely make little use of Earth materials and depend on the prior existence of a large space infrastructure.

     I feel sure that will come—eventually. By that time the idea of power generation plants near population centers will be as unacceptable as the Middle Ages habit of allowing the privy to drain into the well. However, I want to emphasize that our solutions to the problems of the future can be expected to work no better than two-hundred-year-old solutions to the problems of today. We can propose for our distant descendants our primitive technology as fixes for their problems. But I don't believe that they will listen.

From BORDERLANDS OF SCIENCE by Charles Sheffield (1999)

Power beaming stations might well be dual purpose, the space age equivalent of the military frontier posts of the American west.

The military purpose would be to protect Earth from infalling asteroids or whatever military threat develops in deep space, but they pay for themselves by beaming power to cooperative targets like friendly shipping or energy receivers mounted on NEOs. Unless there is a red alert, shipping takes priority and even if the beam is interrupted, the ships continue to coast on predictable orbits and can be picked up after the interruption is resolved (repairs made, asteroid vapourized etc.)

Life in Fort Heinlein revolves around maintaining the solar energy arrays and maintaining the tracking systems, and life will be pretty tedious. Daily routine includes system checks and battle drills, and screw-ups get to go out and polish the mirrors under the first sergeant's unforgiving gaze. A secondary economy of service providers (saloons and whorehouses) will grow around the "fort" to service the crew, and other business might set up shop as well, everything from contractor repair depots to futures traders monitoring ship traffic and energy consumption.

Lightweight ships tapping into this system have torch like performance, economy traffic might go by cycler (although the "taxis" might need torch like performance to match the cycler or slow down to orbital velocity after dropping off) and bulk traffic will still go by low cost transfer orbits.


This and similar proposals on power and propulsion generated a great deal of speculation and study in the 1970s. These activities, although generally incomplete and sometimes contradictory, identified several themes:

  • Lower cost power and propulsion is key to the development of near-Earth space.
  • Solar- and nuclear-powered lasers have the characteristics for high payoff in space applications.
  • Expensive transportation applications show high potential for cost reduction through the use of remote laser power.
  • Economical power beaming in space requires multiple customers who cannot use available (solar photovoltaic) power sources.
  • High laser conversion efficiency is a key power-beaming challenge.
  • NASA laser power requirements are very different from those of DOD and DOE, but NASA can benefit from the breadth of basic research generated by the programs of other agencies.

From the studies, then, a general set of requirements are emerging for beaming power by laser to currently envisioned space missions. First, the laser must be capable of long-term continuous operation without significant maintenance or resupply. For this reason, solar- and nuclear-powered lasers are favored. Second, the laser must supply high average power, on the order of 100 kW or greater for applications studied so far. For this reason, continuous wave or rapidly pulsed lasers are required.

Since solar energy is the most available and reliable power source in space, recent research designed to explore the feasibility of laser power transmission between spacecraft in space has focused on solar-pumped lasers. Three general laser mechanisms have been identified:

  • Photodissociation lasing driven directly by sunlight
  • Photoexcitation lasing driven directly by sunlight
  • Photoexcitation lasing driven by thermal radiation

Solar-Pumped Photodissociation Lasers

Several direct solar lasers based on photodissociation have been identified, including six organic iodide lasants that have been successfully solar pumped and emit at the iodine laser wavelength of 1.3 micrometers. (See figure 40 for a possible application of such a laser.) Another lasant, IBr, has been pumped with a flashlamp and lased at 2.7 μm with a pulsed power of hundreds of watts. One organic iodide, C3F7I, and IBr have been investigated intensively to characterize their operation. Several reports on experimental results and modeling have been published. An important characteristic of the photodissociation lasers under consideration is that they spontaneously recombine to form the lasant molecule again. Both C3F7I and IBr do this to a high degree, permitting continuous operation without resupplying lasant, as is generally required for chemical lasers. In addition, C3F7I absorbs almost no visible light and thus remains so cool that it may require no thermal radiator except the pipe that recirculates the lasant. A variety of other lasants offering increased efficiency are under study.

Solar-Pumped Photoexcitation Lasers

Another group of direct solar-pumped lasers rely on the electronic-vibrational excitation produced by sunlight to power the laser action. Two systems are being actively studied. The first is a liquid neodymium (Nd) ion laser, which absorbs throughout the visible spectrum and emits in the near-infrared at 1.06 μm. This lasant has lased with flashlamp pumping and is currently being tried with solar pumping, since calculations indicate feasibility. A second candidate of this sort is a dye laser, which absorbs in the blue-green range and emits in the red, near 0.6 μm. These lasers offer good quantum efficiency and emission that is both of short wavelength and tunable. However, the lasers require extremely high excitation to overcome their high threshold for lasing, and the feasibility of achieving this with concentrated sunlight is still a question for further research.

Indirect Photoexcitation Lasers

Photoexcitation lasers driven by thermal radiation produced by the Sun are termed indirect solar-pumped lasers. The lower pumping energy implies longer wavelength emission than with photodissociation lasers. Two lasers, the first blackbody-cavity-pumped laser and a blackbody-pumped transfer laser, work on this principle. Molecules such as CO2 and N2O have lased with emission wavelengths between 9 μm and 11 μm. These lasers are inherently continuous wave and have generated powers approaching 1 watt in initial laboratory versions, with blackbody temperatures between 1000 K and 1500 K. While such lasers, powered by solar energy, may be used in space, they also offer great potential for converting to laser energy the thermal energy generated by chemical reactions, by nuclear power, by electrical power, or by other high- temperature sources.

From LASER POWER TRANSMISSION by Edmund J. Conway. Collected in Space Resources NASA SP-509 vol 2

The laser power needed to accelerate the 82,000 ton interstellar vehicle at one percent of earth gravity was just over 1300 terawatts. As is shown in Figure 5, this was obtained from an array of 1000 laser generators orbiting around Mercury (which has 6.7 times the solar flux compared to Terra's orbit). Each laser generator used a thirty kilometer diameter lightweight reflector that collected 6.5 terawatts of sunlight and reflected into its solar-pumped laser the 1.5 terawatts of sunlight that was at the right wavelength for the laser to use.

When fed the right pumping light, the lasers were very efficient and produced 1.3 terawatts of laser light at an infrared wavelength of 1.5 microns. The output aperture of the lasers was 100 meters in diameter, so the flux that the laser mirrors had to handle was only about 12 suns. The lasers and their collectors were in sun-synchronous orbit around Mercury to keep them from being moved about by the light pressure from the intercepted sunlight and the transmitted laser beam.

The 1000 beams from the laser generators were transmitted out to the L-2 point of Mercury where they were collected, phase shifted until they were all in phase, then combined into a single coherent beam about 3.5 kilometers across. This beam was deflected from a final mirror that was tilted at 4.5 degrees above the ecliptic to match Barnard's elevation, and rotated so as to always face the direction to Barnard.

The crew to construct and maintain the laser generators were housed in the Mercury Laser Propulsion Construction, Command, and Control Center. The station was not in orbit about Mercury, but hung below the "sunhook," a large ring sail that straddled the shadow cone of Mercury about halfway up the cone.

The final transmitter lens for the laser propulsion system was a thin film of plastic net, with alternating circular zones that either were empty or covered with a thin film of plastic that caused a half-wavelength phase delay in the 1.5-micron laser light. (During the deceleration phase, when the laser frequency was tripled to produce 0.5-micron green laser light, the phase delay was three half-wavelengths.) This huge Fresnel zone plate acted as a final lens for the beam coming from Mercury. Since the focal length of the Fresnel zone plate was very long, the changes in shape or position of the billowing plastic net lens had almost no effect on the transmitted beam. The zone plate was rotated slowly to keep it stretched and an array of controllable mirrors around the periphery used the small amount of laser light that missed the lens to counteract the gravity pull of the distant Sun and keep the huge sail fixed in space along the Sun-Barnard axis.

From ROCHEWORLD by Dr. Robert E. Forward

(ed note: this is a laser thermal rocket energized by powerful lasers on an L5 colony)

     The next item on the agenda was the laser-powered high-acceleration tug, otherwise referred to as the ultra-fast optical system, UFOS having more dash and elan than LPHAT. Corporate Susan made the presentation it had worked up with Skaskash and Lady Dark.

     "The basic idea isn't bad,” said Cantrell. “How would you keep the lens oriented normal to the laser when you start to move the engine to a different orientation?"
     "We have a pair of pipes at the equator of the sphere, pumping water in opposite directions,” said Corporate Susan. “Also, inside the sphere, under the photovoltaic surface, are two pairs of circular loops, set flush with the surface and at right angles to each other. Each pair pumps water in a counterrotary direction. The pumps are all controlled, so the UFOS is gyroscopically stabilized in three planes."

     "I see,” Dornbrock said. “How do you move the engine around on the surface of the geodesic sphere?"
     "The sphere rests on this little egg cup here,” said Corporate Susan. “The egg cup is a plastic perforated surface. When we want to move, we pressurize the surface, and the geodesic sphere floats on an air cushion. Then the mechanical hands around the perimeter of the egg cup orient the engine while the sphere stays put, or the engine stays put and the hands reorient the sphere, depending on how you work the gyroscopic pumps."
     "Wouldn't you lose a lot of air pressurizing the perforated surface?” Corporate Forziati asked.
     "No, actually,” Corporate Susan replied. “We have built a little valve into each perforation which only operates when the surface is depressed by the weight of the element of the sphere in contact with it.” A diagram flashed on her telecon screen for a moment.
     "Thank you,” Forziati said. “And when you are not under thrust, weight is no problem and you don't pressurize. Very good."

     "On the other end of the egg cup,” Bogdanovitch said, “where you have the engines and the tanks for the reaction mass, you have a long cable supporting the warship. Couldn't you have the ship on an egg cup, too?"
     "No,” Corporate Susan replied. “The engines are thrusting against the geodesic sphere, which rests on top of the egg cup. The warship must keep its center of mass in line with the axis of thrust. Put it on the sphere with its own egg cup, and it would have to stay lined up with the engines—on the other side—which means the sphere would have to be built stronger, and heavier."
     "And it would get in the way of the big laser beam,” Cantrell added.

     "Then how does the warship stay out of the jet of ions?” asked Bogdanovitch.
     "It rotates at the end of its cable,” said Corporate Susan, “and makes a little circle around the jet of uranium ions which provide the main thrust. The jet of boron and hydrogen is flared off, simply to provide electrical neutrality, but it also provides a tiny bit of thrust, which can be used to offset the wobble the ship would otherwise cause by swinging around the main jet."

     "I don't understand,” Marian said.
     Corporate Susan dissolved into a diagram. “Consider the vector diagram of the force exerted by the cable supporting the ship,” said the computer. “Most of it runs through the axis of thrust, but there is a small component going at right angles to that thrust. The boron and hydrogen, flared off with the excess electrons from the decaply ionized uranium, can be adjusted to exactly balance that small component. The flare—a very soft jet—would be in the same plane as the ship, and pointing in the same direction, to push where the ship is pulling."
     "The jet—the flare, I mean, turns with the cable?” asked Marian.
     "Of course,” said Corporate Susan.

     "Orange and green,” said Marian. “Very pretty. What color is the uranium jet?"
     "Hard X-ray,” Skaskash said. “It would probably be dangerous for two or three hundred kilometers."
     "That might be an idea whose time has come,” Cantrell said at last. “Any more questions? No? Shall we build it? ... It seems to be unanimous."

     "I have a model at the shop you can use. I'll have the changes you wanted put on, and you can use that, if you want."
     "How big is it?"
     "Not big—” Ilgen stretched his arms. “Maybe a meter and a half. Did you decide what ship you wanted with it?"
     "The Alamo. We need to impress people, and the Alamo is the biggest thing we've got."
     "Right, Charlie. I'll throw in a model of the Alamo to the same scale. The UFOS plus the Alamo figures to go seven to eight times as fast as any cruiser."
     Cantrell whistled softly.
     "I'll tell the Navy,” he said. “That ought to make them very happy."

From THE PIRATES OF ROSINANTE by Alexis Gilliland (1982)

Last on my list is beamed power, which arguably does not belong in an 'early days' roundup. The usual example is a solar power satellite network beaming power down to the surface. Other uses include long-range power (probably laser) to a vehicle or satellite for propulsion and short-range power (probably RF) between a carrier satellite and payload cubesats or other small craft.

The SPS concept has been thoroughly explored over the decades. All necessary technologies exist and have been demonstrated. Environmental impact studies have been performed. The main barrier now is launch costs, which can be overcome by low-cost reusable LVs and / or the use of material harvested in space. As long as human civilization continues to use electricity there will be a market for SPS power on the surface. As the impact of human-induced climate change grows, the demand for power that does not threaten our species will continue to grow.

This kind of baseload power is further into the future but there are near-term applications. In particular, electric space tugs would benefit from a constellation of modest-sized SPS craft. Instead of carrying large solar panel arrays, a tug could carry just the rectenna and power conditioning equipment necessary to receive beamed power. This hardware would be lighter and much more resistant to radiation, allowing for a longer service life for LEO-GEO tugs. The reduced mass would make the tug more fuel-efficient, while a proper network of satellites would allow full-time operation of the tug's ion engine without requiring large battery packs. This same network of satellites could provide peak power to other assets with intermittent high power demand, particularly to a low-orbit space station that periodically does energy-intensive materials processing or uses electric engines for reboost / CAM. A further set of customers might include satellites intended only for short missions; formation flights of cubesats for example would benefit from requiring a smaller mass (and lower price) of rectenna than they would have required in solar panels.

A 'retrofit' option would be an SPS network that beams power using IR or visible lasers rather than RF. The specific frequency would be one that solar cells can efficiently convert. The SPS would simply lase the solar panels of the client craft, providing power when the sun is not available or increasing power while the craft is lit. This is significantly less efficient than RF but it would work on existing satellites and at longer ranges.


The spacecoach is a design pattern for a reusable solar electric spacecraft, previously featured on Centauri Dreams here and developed in A Design for a Reusable Water-Based Spacecraft Known as the Spacecoach (Springer Verlag), which I wrote with Alex Tolley. It primarily uses water as its propellant. This design has numerous benefits, chief among them the ability to turn consumables, ordinarily deadweight, into working mass.

The recent announcement of the Breakthrough Starshot project, which aims to use beamed power to drive ultra lightweight lightsail probes on interstellar trajectories, is of note. This same infrastructure could be used to augment the capabilities and range of spacecoaches (or any solar electric spacecraft), while providing a near-term use for beamed power infrastructure as it is developed and scaled up.

The spacecoach design pattern combines a medium sized solar array (sized to generate between 500 kilowatts and 2 megawatts of peak power at 1AU) with electric propulsion units that use water as propellant (and possibly also waste streams such as carbon dioxide, ammonia, etc). We found that, even when constrained to these power levels, they could fly approximately Hohmann trajectories to and from destinations in the inner solar system. Because consumables are converted into propellant, this reduces mass budgets by an order of magnitude, and effectively eliminates the need for an external interplanetary stage, all while greatly simplifying the logistics of supporting a sizeable crew for long duration missions (more consumables = more propellant).


The primary constraint for space coaches, especially if you want to travel to the outer solar system, is available power. This is an issue for two reasons. First, solar flux drops off by 1/r2, so at Jupiter, a solar array will generate roughly 1/25th the power as it does at Earth distance. Second, trips to more distant locations will typically require a greater delta V (and thus higher exhaust velocity to achieve this with a given amount of propellant). The amount of energy required to generate a unit of impulse scales linearly with exhaust velocity, so the net result is the ship’s power requirements are increased, all while the powerplant’s power density (watts per kilogram of solar array) is decreased.

Testing Beamed Power

Beamed power infrastructure would enable space coaches and solar electric spacecraft in general to operate at higher power levels for a given array size, which would enable them to operate at higher thrust levels, and to utilize higher exhaust velocities to maximize delta V and propellant efficiency. This means they would be able to accelerate faster, achieve higher delta-v, while using less propellant. In effect beamed power to SEP spacecraft will give their operators the equivalent of a nuclear electric power plant (without the nukes).

A spacecoach built for solar only operation would be able to serve as a testbed for beamed power. For example, a space coach departing Earth orbit could be illuminated with a beam that increases its power output by a small amount, say 10% (large enough to make a measurable difference in performance, yet small enough that major modifications are not required to the ship as it just experiences slightly brighter illumination while in beam). At higher light levels, this technique could also be used to simulate lighting and heat loading conditions expected at the inner planets while remaining in near Earth space. Note also that lasers can be tuned to the absorption wavelength(s) of the photovoltaic material, greatly improving conversion efficiency (and reducing heat gain per unit of power delivered). An even cheaper way to build out and test power beaming infrastructure will be with satellites and probes that utilize solar electric propulsion.

The pathway to a system based primarily on beamed power then becomes one based on incremental improvements, both for the ground based facilities and for the ships. This would result in near term applications for the beamed power facilities while the much more technically challenging aspects of the starshot project are sorted out. Meanwhile, satellite and space coach operators could test ships with ever higher levels of beamed power until they hit a limit (heat rejection is probably the main limit to how much power can be concentrated per unit of sail area, as this is similar to concentrated photovoltaics).

The chart below illustrates the power/performance curve by showing the amount of impulse that can theoretically be generated per megawatt hour using electric propulsion, as a function of exhaust velocity. Real world performance will be somewhat lower due to efficiency losses, but this shows the relationship between thrust, ve and power. We see that impulse per MWh varies from 72,000 kg-m/s (ion drive, ve ~ 100,000 m/s) to 1,400,000 kg-m/s (RF arcjet, ve ~ 5000 m/s). A Hall Effect thruster, a flight proven technology, would yield about 300,000 kg-m/s per MWh. Compare this to pure photonic propulsion, which would yield only 12 to 24 kg-m/s per MWh. Clearly photonic propulsion will be necessary to achieve a delta v of 0.2c, but for more pedestrian applications such as satellite orbit raising, launching interplanetary probes or cargo ships from LEO to BEO (beyond earth orbit), electric propulsion will work well at power levels many orders of magnitude lower than what’s required for a starshot.


Driver for an Interplanetary Infrastructure?

Closer to home there could be lots of opportunities to sell beamed power to space operators. It’s costly to launch large payloads beyond low earth orbit (which isn’t cheap in the first place). Meanwhile, payload fairings limit the size of self-deploying solar arrays, which limits the use of electric propulsion for satellites and probes. If one could launch spacecraft with small solar arrays to LEO, and then use beamed power to amplify their power budget they could use electric propulsion to boost themselves to their desired orbits or interplanetary trajectories within a reasonable time frame. The beamed power infrastructure can also be built up incrementally. Early systems would beam 100 kilowatts to 10 megawatts of power to targets measuring meters to tens of meters in diameter. This should be readily achievable, and can be scaled up from there in terms of power output, beam precision, etc. The result: lower costs per kilogram to deliver a payload to its destination or desired orbit compared to all chemical propulsion.

This could make electric propulsion for transit from LEO to GEO and beyond an attractive option. Meanwhile, the power beaming operator would accrue lots of operational experience with beam shaping, tracking objects in orbit, etc, all things that will need to be mastered for the starshot project, while providing an economic foundation for the power beaming facilities during the buildup to their intended purpose.

In fact, one can imagine the starshot project becoming a profitable LEO to BEO (beyond earth orbit) launch operator in its own right. The terrestrial power beaming infrastructure is one component. A standardized “power sail” that can be fitted to many different payloads, from geostationary satellites to interplanetary probes, is another. The power sail would consist of a self-deploying solar array that is sized to work well with beamed power, heat rejection gear, and electric propulsion units. It would use beamed power during its boost phase to rapidly accrue velocity for its planned trajectory, and then as it leaves near Earth space, would transition to use ambient light as its power source from there. Meanwhile these power sails would provide an evolutionary path from conventional spacecraft to solar electric propulsion to the nanocraft envisioned for purely photonic propulsion.

As a starting point, it would be interesting to conduct ground based vacuum chamber tests to see how a variety of PV materials respond to being illuminated with concentrated laser light tuned to their peak absorption wavelengths. What do the conversion efficiencies look like? How much waste heat is generated? How do the materials perform at high temperatures in simulated in-beam conditions? Building on that one can imagine experiments involving cubesats to validate the data from those experiments in real world conditions, and if that all works out, one could scale up from there to build out beamed power infrastructure for use by many types of solar electric vehicles.

Ambitious R&D projects have a way of generating unintended side benefits. It’s possible that the starshot initiative, in addition to being our first step toward the stars, will also make great contributions to travel and exploration within the solar system.

From SPACECOACHES AND BEAMED POWER by Brian McConnell (2016)

Powersat Weapons

About this point all but the hopelessly dull are thinking "wait just a darned minute, what are the military applications?" Pretty good, actually. Have you ever heard of a Laser Combat Mirror? The laser-propulsion mirror eliminates most of the mass of the engine, the laser combat mirror eliminates most of the mass of a laser cannon. This will free up payload mass in the space warship so it can carry more of other kinds of weapons.

As the range increases the powersat beam rapidly becomes too diffuse to do damage due to diffraction. But a warship sporting a laser combat mirror can focus the seemingly harmless diffuse beam into an eye-searing ship-destroying pin-point. Again much in the same way that sunlight is too diffuse to harm ants, unless a naughty boy uses a magnifying glass to focus it into an ant-destroying death ray.

Powersat's weapon potential is so effective that they will probably be nationalized, removed from civilian hands, and turned over to the military.

And even without a fleet of warships with laser combat mirrors, a powersat all alone is a pretty fair orbital laser weapon. Without laser combat mirrors the range is limited, but within that range, whoo boy can they vaporize the heck out of enemy spacecraft, space assets, and even torch ground targets. Their huge solar panels make them fragile, but they can do plenty of damage before they are neutralized.

Not quite green ecological hippy anymore, is it?


G. Harry Stine's (writing as Lee Correy) wrote a rocketpunk novel called Manna. In the novel, the military branches of the space-faring nations would like to put five gigawatt High Energy Laser (HEL) satellites in orbit. Using fancy techniques they are powerful enough to get their weapon laser beam through Terra's atmosphere and incinerate targets on the ground.

The trouble is the militaries want the HEL beamer satellites to be stealthy. The root of the trouble is that a five gigawatt HEL beamer containing a +five gigawatt power source is about as stealthy as a New York 4th of July fireworks display.

If only the power source could be at some distance from the HEL beamer, sending the energy by electromagnetic waves. You know, the same way a powersat sends microwave energy to ground power stations... hmmmmmmm.

That would work, the HEL beamers could be stealthy little dastards with no nuclear power plant, but rapidly unfurling a powersat reception antenna when it came time to zap something.

Now comes a bigger problem. Nobody can build any powerstats.

Why? Well, no corporation is going to embark upon a multi-billion dollar project like a powersat without insurance. And no insurance company is going to underwrite a multi-billion dollar installation which becomes a military target the instant it redirects its power beam from a power station in order to energize a HEL beamer. Especially a military target so huge, easy to hit, and incredibly fragile as a powersat.


How to solve the problem? Well, since it is an insurance problem, there should be an insurance solution.

Through a series of international agreements, the Resident Inspection Organization (RIO) was formed. This international group regularly inspected all powersats, and insured that they stayed pointed at ground power stations. In exchange, the insurance companies would underwrite the powerstats. If any powersat started to energize something that might be a stealthed HEL beamer, RIO would sound the alarm to all the astromilitaries, presumable giving the military units enough time to blow the living snot out of the powersat.

Naturally the astromilitary of Nation Alfa would be angry at RIO squealing when astromilitary Alfa tried to energize one of their HEL beamers. But astromilitary Alfa would be vary grateful if RIO squealed about astromilitary Bravo, Charlie, Delta or Echo doing the same thing.

     "I'm worried about RIO's reaction," Captain Kevin Graham remarked from the space port. "Our captains are concerned that PowerSat, InPowSat, and InSolSat powersats could have their power beams diverted to the American beam weapon stations on orbit . . . and we know where every one of them is stationed even though the Aerospace Force tried to hide them in inclined Clarke orbits."
     That was Top Secret information! How had the League of Free Traders found these battle stations, shrouded as they were with hard stealth technology?
     Ursila Peri reported from L-5, "I don't know if the powersat crews would carry out an order to redirect power beams to military battle stations. Whether the Aerospace Force has plans for a military takeover of the powersats is another matter, but such an attempt would put them in confrontation with the RIO teams on the powersats."...
     ...Vaivan went on, "Sandy, energy war isn't difficult to understand. Most low-tech countries will continue to do business with us in spite of any embargo or boycott. We provide value received and take very little off the top. The Tripartite may try to invoke sanctions against our customers by pulling their powersat plugs, but we'll be there with another plug. And we have a space port, space lift capability, primary metals and plastics industries, and the lunar mine and smelter at Criswell Center. You haven't see that yet, but it's just a lunar mine and smelter. Commonwealth Glaser's capable of supplying powersat electricity to anyone the Tripartite cuts off because they're now building powersats with lunar materials at a much faster rate than the Tripartite companies."
     "They'll react," I warned.
     "They'll go after your powersats."
     "In the face of international law and the Resident Inspection Organization? The insurance trusts won't stand for it," Wahak maintained. "Those trusts are controlled by the Tripartite, but not even a consortium of all the Tripartite banks could possibly cover the insurance losses. And there won't be any because the insurance trusts will place a rather strong damper on any military powersat takeovers. Then RIO will drive in the bung."
     "RIO teams are un-armed," I reminded him.
     "We'll see what happens when everybody shows their cards. RIO will have to become the first Space Patrol whether they want to or not because circumstances will force it ... and so will we."...
     ..."How much capacity has been dropped off the powersat net?" Ali tried to get back on track.
     "Fourteen gigawatts," Shaiko reported. "The cut-offs in­volved split beams, so no powersat is totally off-line, but One-Zero-Five-East and Six-Zero-East have near-zero loads."
     I didn't like that. "Which powersats will have near-zero if they pull the plug on Annom, Nireg, and Sorat?" I asked.
     Shaiko consulted a nearby display before replying, "Two-Zero-East and One-Zero-Five-East."
     "That drops One-Zero-Five-East down to zilch, doesn't it?" I observed.
     "Any load left on One-Zero-Five-East if Annom and Sorat go off?"
     "What are you worried about, Sandy?" It was Vaivan who caught my concern.
     "A ten gigawatt powersat can pump a big laser, Vaivan," I explained. "A high-energy laser—they're called hell beamers from their acronym, H-E-L—is limited in beam power density and range only by its energy source. If it's a self-contained unit, the space facility is large and vulnerable. But if a hell-beamer's energized remotely, it's small and hard to identify. Powersat One-Zero-Five-East could put its ten gigawatts into a hydrogen-fluoride hell-beam station to punch a beam right down to surface from GEO!"
     This was obviously news to them. Rayo Vamori broke the silence, "Is there a battle station over us?"
     "The Aerospace Force has them over all parts of the world in sixty-degree inclined geosynchronous orbits. Kevin Graham's captains have spotted them."
     Ali said slowly, "I'd better pay Peter Rutledge a visit."
     I went with Ali to the Resident Inspection Organization's headquarters, GEO Base Zero. Ali needed a pilot, and he wanted me to meet those upon whom the delicate stability of space power depended.
     I'd never known any RIO people. They kept to themselves as an anational paramilitary organization with a tradition of non-involvement. They had to be aloof. Thanks to RIO, there hadn't been a conflict in space since the Sino-Soviet Incident.
     Ali wanted to make certain that RIO knew what was happening with the powersats. He was also covering his anatomy by insuring that Powersat One-Zero-Five-East or any other powersat didn't get its power beam redirected to a hell-beamer.
     The approach to RIO Headquarters was a two-man job. The first challenge from RIO came at a thousand kilometers. We answered with the proper transponder code. Then we had to close at no more than ten meters per second, matching orbits and station-keeping ten klicks behind at zero closure rate. There we were thoroughly scanned. Once we proved we were sweet, pure, and unrefined as well as incapable of swatting a bee in revenge for being stung, they put a RIO pilot aboard. She strapped into the jump seat between Ali and me and flew the ship. It was rather disturbing to sit next to someone wearing about twenty kilos of Comp-X around her waist. From her accent as she reported on her comm set to RIO Approach, she was Japanese. I knew she wouldn't hesitate to self-destruct and take the ship and the two of us with her if we tried to ram GEO Base Zero...
     ..."He had to be. How much do you know about RIO and how it's run, Sandy?"
     "Only what I've read, which was reasonably extensive be­cause the Academy wanted future officers to understand RIO not as an adversary, but as a potential obstacle."
     The Resident Inspection Organization had been the factor which permitted the powersat network. Without non-national or international inspection, who was to know whether or not a powersat also contained a hell-beamer? Who could have ascer­tained whether or not an attack satellite was hidden in the structures of the photovoltaic panels? And who'd be sure that the power beam wouldn't be diverted—as Ali and I now feared— from the ground rectenna to an otherwise passive and silent hell-beamer satellite? Could someone really pirate the pilot beam that kept the power beam phased on the rectenna and then concentrate several power beams on an Earth or space target, even though the power density of a single powersat beam is only one-fifteenth that of a microwave oven?
     These questions left unanswered posed a military threat which in turn made a powersat a military target because nobody could take chances if an armed conflict appeared imminent.
     A powersat is a terribly vulnerable thing—square kilometers of solar panels and bus bars carrying megawatts of power. No businessman, entrepreneur, financier, banker, or investor would have risked a worn penny on a powersat that was a certain target in the opening moments of any future war. Neither Lloyd's nor Macao's would or could have underwritten the insurance re­quired for the long-term financing.
     Obviously, a non-political international inspection organization was required. But how could it be organized, financed, and operated to insure that it remained non-national? That had been an enormous problem.
     But technology always creates the new social organizations necessary to finance, manage, and control it.
     People hacked away at the problem until RIO was organized at the Hartford Convention. RIO was formed with the funding from the groups who'd lose the most if a powersat were attacked as a military target, whether it was an actual threat or not. The damage or destruction of a multi-billion dollar powersat would be an expensive loss to the insurance underwriters.
     The world needed space power and the insurance consortiums were the critical bottleneck. Whether or not there were economic pressures applied is a moot point today because the fraction of a percent that was tagged onto the kilowatt-hour consumer electric bill amounted to billions of dollars in insurance premiums which in turn more than paid for the 2,000 RIO inspectors and specialists with their independent communications and transportation systems.
     Rutledge had been accurate in using the sentry as the analogy for RIO.
     A lot of people didn't understand that an unarmed RIO was considered to be very effective. If a resident team or one of the ubiquitous spot inspection teams under the command of Rutledge found something unusual, there were two options open to the team leader: (a) report it covertly to RIO Headquarters for evaluation there; or (b) in a real emergency communicate the military activity to everybody. In the latter case, it was then important for RIO to get out of the line of fire.
     Because of its unique anational character and novel operational methods, RIO often acted in strange and unfathomable ways. Unarmed as they were, they posed no military threat to |anyone. But the threat of their capability to saturate the comm/info network with the danger cry of the watch dog was a sure and certain restraint on military space activities. I suspected—and knew in some cases—that RIO had intelligence operations which penetrated deeply into nearly every military organization in the world. It wouldn't have surprised me, either, if their intelligence activities also embraced the world of commerce.
     A lot of military planners had spent a lot of time and effort drafting plans and programs for circumventing RIO. The Aerospace Force—whose job was ostensibly to keep and guard the peace, too—had a continual highly-classified think-tank activity going on "should it be necessary to activate such plans and programs." But the job of any military service is to ensure the security of its nation...

From MANNA by G. Harry Stine (1983)

Tether Propulsion


A momentum exchange tether is a kind of space tether that can be used as a launch system, or to change spacecraft orbits. Momentum exchange tethers create a controlled force on the end-masses of the system due to centrifugal acceleration. While the tether system rotates, the objects on either end of the tether will experience continuous acceleration; the magnitude of the acceleration depends on the length of the tether and the rotation rate. Momentum exchange occurs when an end body is released during the rotation. The transfer of momentum to the released object will cause the rotating tether to lose energy, and thus lose velocity and altitude. However, using electrodynamic tether thrusting, or ion propulsion the system can then re-boost itself with little or no expenditure of consumable reaction mass.

A non-rotating tether is a rotating tether that rotates exactly once per orbit so that it always has a vertical orientation relative to the parent body. A spacecraft arriving at the lower end of this tether, or departing from the upper end, will take momentum from the tether, while a spacecraft departing from the lower end of the tether, or arriving at the upper end, will add momentum to the tether.

From the Wikipedia entry for MOMENTUM EXCHANGE TETHER

A good low-mass way to prevent cables from failing catastrophically is to use Hoytethers (cables that are elongated Hoytubes). Strengthening a cable by increasing its diameter quickly becomes too expensive in terms of mass. A Hoytether on the other hand is a low mass network of redundant cables that fails gracefully.

Momentum exchange Hoytethers were featured in the novel Saturn Rukh by Robert L. Forward.


A momentum exchange tether is a long thin cable used to couple two objects in space together so that one transfers momentum and energy to the other. A tether is deployed by pushing one object up or down from the other. Once the two objects are separated by enough distance, the difference in the gravitational force at the two locations will cause the objects to be "pulled" apart. This is called the "gravity gradient force". The tether can then be let out at a controlled rate, pulled by the tension caused by the gravity gradient force. Once the tether is deployed, if there are no other forces on the tether it will have an equilibrium orientation that is aligned vertically. There are a number of different concepts for momentum exchange using tethers. Some general categories are:

A bolo is a long rotating cable anywhere in space that is used as a "momentum-energy bank". It could be used to "catch" a payload coming from any given direction (in its plane of rotation) at any given speed (less than its maximum tip speed), and then some time later, "launch" the payload off in some other direction at some other speed. A gravity gradient stabilized bolo orbiting some planet has the property that if the tether is cut, then one-half an orbit later, the separation distance between the two masses is seven times larger than the initial separation. This can be used to deorbit the lower mass, or throw the upper mass to a rendezvous or to escape.

(ed note: momentum-energy bank is just like a financial bank, only using momentum instead of money. You can put momentum in by catching a spacecraft and take momentum out by launching a spacecraft, but the important point is you cannot take out more momentum than the blasted bank currently contains. This means you need a strategy to balance deposits and withdrawals of momentum.

The most common strategy is to have two bolos, for example one at Terra orbit and one at Mars orbit. Each bolo lobs payload modules at the other, and catches modules aimed at them. This keeps the momenum balanced.)

Tip Velocity and Material Strength

The maximum tip speed of all these systems is a function of the "launcher to payload mass ratio" of the tether system and the "characteristic velocity" of the material used. The characteristic velocity of the material in a tether is given by the square root of the ratio of the design tensile strength T of the tether to the density D of the tether material. u = (T_d/D)^1/2. In practice, the design tensile strength is usually chosen to be 50% of the measured strength for metals and 25% of the measured short-term individual fiber strength for other materials. Thus, using imperfect materials with reasonable safety margins, the characteristic velocity of most metals and fibers is around 1 km/s, with optimistic predictions for graphite and improved polymers reaching 3 km/s. With the development of a design for a high strength-to-weight tapered Hoytether, the design tensile strength can be safely chosen to be 60% of the measured strength of the individual fibers, allowing commercially available fibers to have characteristic velocities up to 4 km/s.


     "Just the general idea. It starts with the Spider again. Now it's spinning a different kind of web. Rockets are wrong. That's sitting there in your desk as we talk, but I didn't follow it far enough. I should have known you wouldn't stop with the beanstalk, that just gets us up and down from Earth. You wanted a way of moving materials around the whole System without using drives. And the Spider could give you that."...
     ..."Spin another cable," he went on. "Make it like the beanstalk, with superconducting cables and drive train attached to the load cable. This time, put the powersat at the center of the cable, with an equal length on each side of it. Fabricate it in space, but don't ever plan to fly it in and tether it. Leave it out near the orbit of Mars, or in the Belt, or in near Earth—key places in the System. Then start it rotating about its center, like a couple of spokes on a wheel. I assume that you began with just a couple of them, one in the Belt and one near Earth?"
     Regulo nodded calmly. He had finished fiddling with the control panel and now seemed oddly relaxed. "We started with two. That's just the beginning. The more you have, the better the efficiency of the whole operation. I've been thinking we'd build about five thousand of them through the Earth-Belt region."
     "You could handle that many?"
     "With Sycorax? Easily. We can track that number, and more—there are millions of orbits in the data banks already. This is just a few extra ones." Regulo's tone was that of a patient teacher. "I've told you before, Rob," he went on. "Think big. The System's a big place. You have to scale your thinking to match it."...
     ..."Just as you did. You have a rotating cable out in a free orbit—thousands of kilometers of it." He leaned forward, at the same time as Regulo moved his chair farther away from the desk.
     "Now suppose you want to move a space pod from the Belt to the Moon," Rob went on. "You make it rendezvous with the center of the cable, where the powersat sits. The center of mass of the cable would be moving in a free-fall orbit, travelling about the same speed as the pod, so you use hardly any reaction mass to make the rendezvous. You don't need much acceleration from the pod's drives, either, just a fraction of a gee will be enough. Once you have the pod at the middle of the cable, you let it move out along the drive train. As the pod moves from the center it feels a centripetal acceleration. You need to use the drive train on the cable to restrain it. When it reaches the end of the cable, you release it to move in free fall. You've given it a big velocity boost. But the trouble from the point of view of a human on the pod is the acceleration. Out at the end of the cable, it's huge. I looked at a couple of examples. A cable four thousand kilometers long, with end velocity of twenty-four kilometers a second, would give thirty gees at each end. That's what killed the Goblins."...
     ...He put the space pod to a cable rendezvous with a cargo Slingshot—one with high accelerations, never intended for people."
     "Do you have Slingshots for passengers?" Rob moved forward right up to the desk.
     "We built the first two, just a month ago. I could find out which cable your Goblins used easily enough, by checking the angular momentum of all of them. Each time we use a Slingshot we naturally increase or decrease its angular momentum." Regulo stood up, his back to the wall. "We lose angular momentum when we throw a cargo in toward the Sun, and pick it up when we catch something thrown in from Mars or the Belt. Provided we move the same mass of materials in and out, the whole system balances—just like the beanstalk back on Earth.

From THE WEB BETWEEN THE WORLDS by Charles Sheffield (1979)
     My post Orbital Momentum as a Commodity describes how a tether with a healthy anchor mass can catch and throw payloads. I tried to think of ways a tether might restore orbital momentum lost during a catch or throw. Two way traffic is one way to pay back borrowed momentum.

     Well, Mars' moon Phobos masses 1.066×1016 kg. With this huge momentum bank, catching and throwing payloads would have less effect than a gnat hitching a ride on a Mack truck. A Phobos anchored tether could catch and throw for millennia with little effect on Phobos' orbit.

     The tether illustrated above doesn't suffer the enormous stress of a full blown earth elevator or even a Mars elevator. It could be made from Kevlar with a taper ratio of about 11.

Access to Mars

     The tether foot pictured above moves about 0.6 km/s with regard to Mars surface. This is about 1/10 of the ~6 km/s the typical lander from earth needs to shed. Mars Entry Descent and Landing (EDL) would be vastly less difficult.

     Some have suggested Phobos 1.88 g/cm3 density indicates volatile ices. If so, the moon could also be used as a source of propellent. A Phobos propellent source would make EDL even less of a problem. However Phobos' low density might also be due to voids within a rubble pile.

     On page 2 of the Acceleration of the Human Exploration of The Solar System with Space Elevators Asteroid Initiatives founder Marshall Eubanks takes a look at how the foot of Phobos-Anchored Martian Space Elevator (PAMSE) might interact with Mars' atmosphere:
     The orbital eccentricity of Phobos amounts to 283 km, which is by coincidence comparable to the effective depth of the Martian atmosphere for satellite drag (typically ~ 170 km, but subject to variations due to atmospheric events such as dust storms). The average relative velocity between the lower tip and the surface of Mars is only 534 m/sec, roughly Mach 2 in the cold Martian atmosphere, and slow enough that it should not cause significant heating of the tip. This raises the interesting possibility that the PASME tip could dip down deep into the atmosphere to leave or recover payloads or perform reconnaissance, acting as a supersonic airplane for the period near periapse when it is near the surface.
     Eubanks' 534 m/sec is a little slower than the 0.6 km/s of my tether tip. This might be because I had placed my tether tip 300 km/s above Mars' surface thinking atmospheric friction would destroy a lower tether foot. Eubanks' analysis has changed my view.

     In the Facebook Asteroid Mining Group, Eubanks noted:
     The orbit of Phobos is equatorial, and there is a big mountain in the way, Pavonis Mons, the middle of the Tharsis volcanoes, straddling the equator and by far the highest obstacle in the path of the elevator tip. Maybe a railroad on top of the volcano could match speeds with the elevator tip, once every 3 days or so (when the orbit and volcano aligned). If so, you would have up to 3 minutes to shift cargo on and off. 
     as well as
     …the cool thing is that the tip can be something like a tethered airplane (with wings and flaps, etc.) and you should be able to use that to control oscillations. I was hoping to get money to begin actually "testing" this (i. e. in simulation), but, alas, not so far. 
     Remember, too, with the PAMSE the counterweight has ~ infinite mass, and so any oscillations have to end there. (of course, anchoring a PAMSE in Phobos is left as an exercise for the reader.)
     If Phobos is indeed a loose rubble pile, anchoring the elevator would be difficult. So while Eubanks eased my anxieties on oscillations and atmospheric friction, he calls my attention to a problem I hadn't thought of.

Access to Earth

     6155 km above Phobos the tether is moving faster than escape velocity with a Vinf of 2.65 km/s. This is sufficient to toss a payload down to a 1 A.U. perihelion. This could provide most of the delta V for Trans Earth Insertion.

     A ship coming from Earth would have a Vinf of 2.65 km/s and so rendezvous with this part of the tether might be accomplished with little propellent.

Access to the Main Belt

     7980 km above Phobos the elevator is moving with a Vinf of 3.27 km/s, enough to hurl payloads to a 2.77 A.U. aphelion. This part of the tether might send/receive payloads to/from the Main Belt. There are a lot of asteroids with healthy inclination, though. So there would be substantial plane change expense at times.

Possible Mars exports to the main belt

     One thing about the Main Belt, the pace is much more leisurely. Ceres moves about 1º every 5 days. In contrast earth moves about 1° a day and a satellite in low earth orbit moves about 4° a minute.

     So a month-long, low-thrust ion burn over there looks a lot more like an impulsive burn than it does in our neck of the woods. I believe high ISP ion engines are well suited for travel about the Main Belt.

     The inert gas argon can be used as reaction mass for ion thrusters. Mars' atmosphere is about 2% argon. It is also about 2% nitrogen and 96% carbon dioxide with traces of oxygen and water. Mars also has respectable slabs of water ice at the poles.

     Mars would be a good source of propellent for the entire belt as well as CHON for the volatile poor asteroids in the inner main belt.

     Ion engines don't have the thrust to weight ratio to soft land on the larger asteroids. But asteroids often have high angular velocity (in other words, they spin fast). High angular velocity combined with shallow gravity wells make asteroids amenable to elevators.

     For example the balance point for a Ceres elevator would only be 706 km above Ceres surface, that is the altitude of a Ceres-synchronous orbit. To provide enough tension to remain erect, the elevator would need to extend to an altitude of 2000 km. At 2000 km, the tether tip is moving about 0.46 km/s, a good fraction of the 2.82 km/s needed fro Trans Mars insertion. If this Ceres elevator is Kevlar, taper ratio would be about 1.02.

     If extended to an altitude of 14,500 km, the Ceres elevator top would be moving fast enough for Trans Mars insertion. This would require a taper ratio of around 5 for a Kevlar tether.

Incremental Development

     The tether pictured at the top of this post is ~14,000 km long with a taper ratio of 11 for Kevlar. While much smaller than a full blown Mars elevator, this elevator would still be a massive undertaking. But the whole thing doesn't need to be built overnight. Early stages of the elevator would still be useful.

     Pictured above a Deimos tether drops a payload to a Phobos tether.

     At apoapsis of the large ellipse, payload velocity matches the Deimos tether foot. At periapsis, the velocity matches the speed of the Phobos tether top. Thus payloads can be exchanged between these Martian moons using practically zero reaction mass.

     After descending the Phobos tether, the payload can be dropped to a Mars atmosphere grazing orbit.

     These tethers are a lot shorter than 14,000 km tether we were talking about and taper ratio is close to 1.

No Moons to Dodge

     A full blown Mars elevator capable of throwing payloads to the Main Belt or even earthward would have to dodge Deimos as well as Phobos.

     A Phobos elevator for flinging payloads to Ceres ends well below Deimos' orbit. And of course a Phobos anchored tether doesn't need to dodge Phobos.


     Tsiolkovsky's rocket equation and big delta V budgets are touted as show stoppers for routine travel to Mars' surface or the Main Belt.

     With judicious use of tethers and orbital momentum, rhinoceros sized delta V budgets are shrunk to hamster sized delta V budgets. No bucky tubes needed, ordinary materials like Kevlar can do the job.

Inert Cargo Vessels

If you were shipping asteroid ore (actually "ore" is not quite the right word but there isn't a good one) from Ceres to Terra (or manufactured goods going the other way), well, the cargo is going to take a bit more than 15 months for the trip. Which is a long time for the cargo spacecraft to be idle, doing nothing but surrounding the cargo. It is hard to amortize the cost of the spacecraft and spacecraft maintenance when it is only doing billable work at the start and end of the trip while the engines are burning.

A few innovative thinkers had the bright idea that since the expensive engines (specifically the propulsion bus) are only needed at the start and end of the journey, why not jettison them so they can be reused? Have the cargo in cannisters or tied to a frame, and the propulsion bus is only present at the start and the destination. The cargo cannisters will probably be sized to be compatible with standard cargo cannister form factors.

I've found three techniques using temporary engines:

Space Tug Catapult

In this Inert Cargo Vessels scheme the propulsion bus becomes a space tug. This latches onto a cargo container, pushes (or pulls) the container into the desired trajectory, detaches to let the cargo go on its merry way, then the tug flies back to the cargo staging area for a propellant re-fill and to grab the next scheduled cargo cannister.

The cargo cannister flies in its trajectory, with no engine but no need for an engine either.

15 months later the launched cannister approaches Terra where it is intercepted by a Terran space tug. It then decelerates into the cargo storage orbit, parks the cargo, refills, and heads out to catch the next incoming cargo cannister.

Since the tugs are constantly working they can amortize their little hearts out.

A distantly related concept is the Flyaway Engine.


Note to science fiction authors: coasting alongside in a manned rocket to keep tabs on a cluster of inert payloads on months long flights (to prevent, say, pirates from snatching pods) is almost exactly a space cowboy.

by SSailor67 (2020)

My notion of a reusable Earth Departure Stage (EDS) assumes a staging platform at Earth Moon Lagrange 2 (EML2). Propellent, water and air at EML2 might come from an carbonaceous asteroid parked in lunar orbit and/or volatiles in the moon's polar cold traps.

A .15 km/s burn at EML2 will drop a spacecraft to a perilune 111 km from the moon's surface. At this perilune the spacecraft is traveling nearly lunar escape velocity with regard to the moon and so enjoys an Oberth benefit. A .19 km/s perilune burn suffices to send the spacecraft earthward to a perigee deep in earth's gravity.

At perigee the spacecraft is traveling about 10.8 km/s, just a hair under earth escape. A burn at this perigee enjoys a huge Oberth benefit. A .6 km/s burn would suffice for Trans Mars Injection (TMI). So delta V from EML2 to TMI is (.15 + .19 + .6) km/s. I will round .94 km/s up to 1 km/s to give a little margin and also 1 is an easier number to type.

After TMI the EDS as well as it's payload is moving 11.5 km/s. To reuse the EDS we would need to return it to EML2.

Farquhar notes the trip from perilune to perigee takes about 140 hours. In that time the moon will advance 76º and the space craft 180º. So in my shotgun orbit simulator I set the perigee 104º ahead of the moon. My first try had pellets ranging from 10.7 to 10.9 km/s and then I'd narrow the blast to the pellets coming closest to the moon. After a few iterations I arrived at a perigee velocity of about 10.85 km/s. This gives an apogee of about 396,000 km and a period close to 2/5 that of the moon. After 50 days, the pellets return to a near moon fly by:

Thus braking about .6 km/s drop the EDS hyperbolic path to a trajectory where it will do a near moon fly-by after 50 days. At the near moon fly by it can do a .14 km/s burn for lunar capture. Then when it reaches an apolune near EML2, a .19 km/s burn to park at EML2.

Thus the EDS' delta V for returning to EML2 will be about 1 km/s.

This page still a work in progress, I'm getting good comments and info from a NASA spaceflight thread. Cryogenic boil off was an issue raised in that thread. A sixty day round trip goes well beyond what present hydrogen/oxygen upper stages do.

The United Launch Alliance has done work on hydrogen/oxygen upper stages that could do longer missions. See Advanced Cryogenic Evolved Stage (ACES). Cyrogenic boil off might be mitigated by Multi Layer Insulation (MLI). Another cooling device is a Thermodynamic Vent System (TVS). Those who live in the southwest are familiar with "swamp coolers" where water soaked pads cool by evaporation. In a similar fashion hydrogen boil-off can be used to cool the cryogens. The hydrogen boil off can be vented in a specific directions and used for station keeping or attitude control.

Besides these passive thermal control systems the ACES might also utilize a two stage turboBrayton cryocooler.

"This design was based on the Creare NICMOS cooler that has been flying on the Hubble Space Telescope for the last ~4 years. The turboBrayton cycle uses GHe as the working fluid and this cooled gas can be easily distributed to the loads (i.e. the 22K and 95K shields). The ACES cryocooler configuration, shown in Figure 3-1, has 3 compressors in series and 2 expansion turbines in parallel, one for the 22K load, and one for the 95K load".

In this ULA pdf an ACES 41 propellant tanker has 5 tonnes dry mass and 41 tonnes propellent. I will be much more conservative in my hypothetical reusable EDS. A Centaur has 2.25 tonnes dry mass, 21 tonnes propellent and 99.2 kilo newtons. I will use the same but boost the dry mass to 5 tonnes for MLI, cryocooling, solar arrays, etc.

Specs for Hop's EDS
  • 5 tonnes dry mass
  • 21 tonnes hydrogen/oxygen
  • 99.2 kilo newtons thrust

After the EDS sends the payload on its way, it will need 1 km/s of propellent of delta V to return to EML2. Exhaust velocity of hydrogen and oxygen is about 4.4 km/s. Exp(1/4.4) - 1 is about .255. To get back the EDS' 5 tonnes of dry mass we'd need 1.3 tonnes of propellent.

So for the first leg of the trip we have (21 - 1.3) tonnes of propellent or 19.7 tonnes. The first leg is also a 1 km/s delta V budget. With a 1 km/s delta V budget, 19.7 tonnes of propellent can do 19.7tonnes/.255. That's about 77 tonnes. But recall 6.3 tonnes is EDS dry mass plus propellent for the return trip. That's (77 - 6.3) tonnes of propellent available for payload. Let's call that 70 tonnes.

This little EDS could impart Trans Mars Insertion (TMI) to 70 tonne payload. Two of these EDS stages could send a 140 tonne payload on its way to Mars. Wilson and Clarke imagine a Mars Transfer Vehicle (MTV) of 130 tonnes.

Of the MTVs 130 tonnes, about 60 tonnes is propellent and consumables. If propellent, water and air are available from an asteroid or lunar volatiles, it would only be necessary to send the MTV's 70 tonne dry mass to EML2.

Wilson and Clarke also call for two EDS stages (they call them TMS — Trans Mars Stages). Their stages are 110 tonnes and not reusable.

130 + 2*110 = 350. 350 tonnes to LEO for each (non reusable) conventional MTV. Vs 70 tonnes to LEO for an MTV that relies on extra terrestrial propellent and consumables. And an MTV departing from and returning to EML2 would have a much lower delta V budget. Making the MTV reusable would be much more doable.

The EDS would zoom through the perigee neighborhood very quickly. Would it have enough time to do the burns and enjoy an Oberth benefit?

The EDS and payload would spend about 54 minutes in the shaded region above.

A 70 tonne payload plus a 26 tonne EDS total 96 tonnes. The thrust of the engine is 99.2 kilonewtons. Acceleration is newtons/kilograms. 96/99.2 is ~.96. .96 meters/second^2 is about a tenth of a g.

Delta V imparted is acceleration * time of burn. Recall the perigee burn is about .6 km/s or 600 meters/second. We solve for t.

a * t = v
.96 m/s^2 * t = 600 m/s
t = 600/.96 seconds = ~620 seconds, a little over 10 minutes. The ten minute neighborhood just preceding perigee is all close to 10.8 km/s.

After separating from payload, the EDS and it's return propellent mass 6.3 tonnes. 99.2 kilonewtons divided by 6.3 tonnes is 15.75 meters/second^2 or nearly two g's. The deceleration burn to brake the hyperbolic orbit to an elliptical capture orbit would take about 40 seconds.

Near Earth Asteroid Retrieval

The Near Earth Asteroid retrieval described in the Keck Report uses xenon as a propellent. The exhaust velocity would be 30 km/s. What possible use could an EDS with a measly 4.4 km/s exhaust velocity be for such a vehicle?

Along with xenon's high exhaust velocity comes very low thrust. It would take nearly two years to spiral from Low Earth Orbit (LEO) to escape velocity. A good part of that long spiral would be spent in the Van Allen Belts. Low Earth Orbit also has a relatively high debris density.

Low thrust rockets don't enjoy any Oberth benefit. So the spiral from LEO to C3=0 would take about 7 km/s. Recall the exhaust velocity of the xenon rockets is around 30 km/s. Exp(7/30) - 1 is .26. Using an EDS would leave the asteroid fetcher with about 33% more xenon.

Many NEAs are much closer than Mars in terms of delta V. So perigee burn would be much less than .6 km/s for TMI, probably more often in the neighborhood of .2 or .3 km/s.

From REUSABLE EARTH DEPARTURE STAGE by Hollister David (2014)

(ed note: So Fast Forwarding Uninc. moves their high-powered space tug catapult and spider "catcher" to new covert point X. Their bootleg miner clients use low-powered mass drivers to launch crates of illegal ore to point X, thus avoiding the Fed's taxes and tariffs. Fast Forwarding uses the spider to catch the incoming crates, sorts and consolidates them into the appropriate framework, one framework for each destination. A fusion drive space tug catapult boosts each fully loaded framework into a high-energy transfer orbit to the intended client at Earth-Luna. The catapult detaches and goes to the next loaded framework, as the launched frame goes on its merry way. Fast Forwarding Uninc. then sends the boost bill to the bootleg miner clients, which is considerably less than the Fed's taxes and tarriffs.)

      With Terran federal authorities controlling all Lunar extraction and regulating the authorized industries operating from the Belt, big profits were to be had from bootlegging (transporting illegal goods) primary asteroid materials direct into the Inner System. A lot of independent operators (illegal or pirate miners) got themselves organized to go after a share. Many of these were small-scale affairs—a breakaway cult, minicorp, even a family group—who had pooled their assets to set up a minimum habitat and mining-extraction facility, typically equipped with a low-performance mass launcher. Powered by solar units operating at extreme range (barely 10% of the solar power available at Terra), such a launcher would be capable of sending payloads to nearby orbits in the Belt, but not of imparting the velocities needed to reach the Earth-Luna vicinity.

     This was where ventures like Fast Forwarding Uninc. came into the picture. Equipped with high-capacity fusion-driven launchers (Space Tug Catapult with fusion drive), they consolidated incoming consignments from several small independents into a single payload and sent it inward on a fast-transit trajectory to a rendezvous agreed upon with the customer.

     Consolidators moved around a lot and carried defenses. The federal agencies put a lot of effort into protecting their monopolies. As is generally the case when fabulous profits stand to be made, the game could get very nasty and rough. Risk is always proportional to the possible gain.

     Fully loaded, the Maddox’s cargo cage combined the consignments from over fifty independents, averaging a thousand tons of asteroid material each, and stretched the length of an old-time naval cruiser. The loads included concentrations of iron, nickel, magnesium, manganese, and other metals for which there would never be a shortage of customers eager to avoid federal taxes and tariffs. A good month’s work for a team of ten working one of the nickel-iron asteroids would earn them a quarter million dollars. True, the costs tended to be high, too, but the offworld banks offered generous extended credit with the rock pledged as collateral. This was another source of friction with the federal authorities, who claimed to own everything and didn’t recognize titles that they hadn’t issued themselves. But ten billion asteroids, each over a hundred meters in diameter, was a lot to try to police. And the torroidal volume formed by the Belt contained two trillion times more space than the sphere bounded by the Moon’s orbit.

     Better money still could be made for hydrogen, nitrogen, carbon, and other light elements essential for biological processes and the manufacture of such things as plastics, which are not found on the Moon but occur in the carbonaceous chondrites. This type of asteroid contains typically up to five percent kerogen, a tarry hydrocarbon found in terrestrial oil shales, “condensed primordial soup"—a virtually perfect mix of all the basic substances necessary to support life. At near-Earth market rates, kerogen was practically priceless. And there was over a hundred million billion tons of it out there, even at five percent.

     The driver (space tug catapult), consisting of a triple-chamber fusion rocket and its fuel tanks, attached at the tail end when the cage was ready to go. Now flight-readied, the assembled launcher hung fifty miles off the Turner Maddox’s beam. The search radars were sweeping long range, and the defenses standing to at full alert. There’s no way to hide the flash when a two-hundred-gigawatt fusion thruster fires—the perfect beacon to invite attention from a prowling federal strike force (who will punish violators of Terra's tax and tarriff laws with a barrage of hunter-killer missiles).

     “We’re clean,” Fuigerado reported from his position on one side of the bridge. He didn’t mean just within their own approach perimeter. The Maddox’s warning system was networked with other defense grids in surrounding localities of the Belt. Against common threats, the independents worked together.

     Cassell checked his screens to verify that the Maddox’s complement of spiders, shuttles, maintenance pods, and other mobiles were all docked and accounted for, out of the blast zone. “Uprange clear,” he confirmed.

     (Captain) Liam Doyle tipped his cap to the back of a head of red, tousled Irish hair and ran a final eye over the field- and ignition-status indicators. A lot more was at stake here than with just the routine retrieval of an incoming crate. The skipper liked to supervise outbound launches in person.

     “Sequencing on-count at minus ten seconds,” the controller’s voice said from the operations deck below.
     “Send her off,” Doyle pronounced.
     “Slaving to auto…Guidance on…Plasma ignition.”

     White starfire lanced across twenty miles of space. The launcher kicked forward at five gs, moved ahead, its speed seeming deceptively slow for a few moments; then it pulled away and shrunk rapidly among the stars. On the bridge’s main screen, the image jumped as the tracking camera upped magnification, showing the plume already foreshortened under the fearsome buildup of velocity. Nineteen minutes later and twenty thousand miles downrange, the driver would detach and fire a retro burn, separating the two modules. The cage would remain on course for the Inner System, while the driver turned in a decelerating curve that would eventually bring it back to rendezvous with the Maddox.

     “We’ve got a good one,” the controller’s voice informed everybody. Hoots and applause sounded through the open door from the communications room behind.

From MADAM BUTTERFLY by James Hogan (1997)

      The observation bubble on the side of the Cay Habitat had a televiewer, Leo discovered to his delight, and furthermore it was unoccupied at the moment. His own quarters lacked a viewport. He slipped within. His schedule allowed this one free day to recover from trip fatigue and jump lag before his course was to begin.

     The curve of Rodeo’s horizon bisected the view from the bubble, and beyond it the vast sweep of stars. Just now one of Rodeo’s little mice moons crept across the panorama. A glint above the horizon caught Leo’s eye.

     He adjusted the televiewer for a close-up. A GalacTech shuttle was bringing up one of the giant cargo pods, refined petrochemicals or bulk plastics bound for petroleum-depleted Earth perhaps. A collection of similar pods floated in orbit. Leo counted. One, two, three … six, and the one arriving made seven. Two or three little manned pushers were already starting to bundle the pods, to be locked together and attached to one of the big orbit-breaking thruster units.

     Once grouped and attached to their thruster, the pods would be aimed toward the distant wormhole exit point that gave access to Rodeo local space. Velocity and direction imparted, the thruster would detach and return to Rodeo orbit for the next load. The unmanned pod bundle would continue on its slow, cheap way to its target, one of a long train stretching from Rodeo to the anomaly in space that was the jump point.

     Once there, the cargo pods would be captured and decelerated by a similar thruster, and positioned for the jump. Then the superjumpers would take over, cargo carriers as specially designed as the thrusters for their task. The monster cargo jumpers were hardly more than a pair of Necklin field generator rods in their protective housings so positioned as to be fitted around a constellation of pod bundles, a bracketing pair of normal space thruster arms, and a small control chamber for the jump pilot and his neurological headset. Without their balancing pod bundles attached the superjumpers reminded Leo of some exceptionally weird and attenuated long-legged insects.

     Each jump pilot, neurologically wired to his ship to navigate the wavering realities of wormhole space, made two hops a day, inbound to Rodeo with empty pod bundles and back out again with cargo, followed by a day off; two months on duty followed by a month’s unpaid but compulsory gravity leave, usually financially augmented with shuttle duties. Jumps were more wearing on pilots than null-gee was. The pilots of the fast passenger ships like the one Leo had ridden in on yesterday called the superjumper pilots puddle-jumpers and merry-go-round riders. The cargo pilots just called the passenger pilots snobs.

     Leo grinned, and considered that train of wealth gliding through space. No doubt about it, the Cay Habitat, fascinating as it was, was just the tail of the dog to the whole of GalacTech’s Rodeo operation. That single thruster-load of pods being bundled now could maintain a whole town full of stockholding widows and orphans in style for a year, and it was just one of an apparently endless string. Base production was like an inverted pyramid, those at the bottom apex supporting a broadening mountain of ten-percenters, a fact which usually gave Leo more secret pride than irritation.

From FALLING FREE by Lois McMaster Bujold (1988)

One of the incredible mentions in the Elon Mars concept was a thousand spacecraft in orbit ready for the Mars launch window to open. I’m not sure how many launch windows down the road this would be, I assume several decades. Whether it is next decade or next century though, an expensive asset like a spacecraft that is only capable of being used once every other launch window is a massive investment that is mostly idle.

I suggest an alternate concept for having a thousand vehicles heading toward Mars during one launch window. Each vehicle is an inert barge with a homing beacon and barely enough structure to house the cargo during thrust and coast. No engines, electricity, shielding, or other frills. These barges carry only items that store well. Machinery, provisions, propellant, clothing, etc.

The orbital gathering place for these barges is a high Earth orbit above the radiation belts but below Lunar orbit. The storage orbit keeps however many barges are heading out during each launch window in parking lot adjacent to a refueling facility that is stocked up between launch windows. The third item is skeletal booster tugs with no frills like ability to reenter or handle gravity.

There is a certain limited amount of time in a Mars launch window when the Hohman transfer orbit uses minimum propellant. There are periods of time on both sides of the ideal window that still get you to Mars, just at the expense of additional propellant. Total available time in the window can be a few months depending on available propulsion.

At the first opportunity, a tug with a dry mass of perhaps ten tons, a propellant load of two hundred tons, and a hundred ton barge, does a short burn to drop its’ perigee to just outside noticeable atmospheric drag. At perigee it is at nearly escape velocity when it does a strong (~4km/sec) Oberth effect burn to place the whole assembly on a Mars trajectory. Immediately after reaching the required velocity, the tug separates and a short retro burn to place the light tug back into an eccentric orbit with an apogee equal to the barge parking lot. The orbital equivalent to the SpaceX Falcon 9 boostback.

Back at the parking lot, the tug does a short burn to match velocities and goes for docking. Refuel, clamp onto another barge, and go again on intervals of one to four days. Depending on assumptions, each tug could send as many as a hundred barges per window to be caught on the other end.

So I can see the possibility of a hundred thousand tons of vessels heading to Mars during one launch window. The main hardware investments being launch vehicles, depots, and tugs that are kept employed at other tasks in the meantime between windows. People launch separately in vehicles suitable.

The main strength I would see in a scenario such as this is that the expensive hardware would be constantly available for use for other tasks. This is important for those of us that don’t see that much value in Mars as the next step out. The same equipment would be useful for asteroid missions or sending a Pluto lander. A heavyweight to Europa or a close solar corona investigation. Or more immediately useful support for Lunar and NEO missions.

From MARS BARGES by John Hare (2017)

FAR, FAR OUT on Pluto, where the sun is only a very bright star and a frozen, airless globe circles in emptiness; far out on Pluto, there was motion. The perpetual faint starlight was abruptly broken. Yellow lights shone suddenly in a circle, and men in spacesuits waddled to a space tug—absurdly marked Betsy-Anne in huge white letters. They climbed up its side and went in the air lock. Presently a faint, jetting glow appeared below its drivetubes. It flared suddenly and the tug lifted, to hover expertly a brief distance above what seemed an unmarred field of frozen atmosphere. But that field heaved and broke. The nose of a Pipeline carrier appeared in the center of a cruciform opening. It thrust through. It stood half its length above the surface of the dead and lifeless planet. The tug drifted above it. Its grapnel dropped down, jetted minute flames, and engaged in the monster tow ring at the carrier’s bow.

The tug’s drivetubes flared luridly. The carrier heaved abruptly up out of its hidingplace and plunged for the heavens behind the tug. It had a huge classmark andsnumber painted on its side, which was barely visible as it whisked out of sight. It went on up at four gravities acceleration, while the spacetug lined out on the most precise of courses and drove fiercely for emptiness.

A long, long time later, when Pluto was barely a pallid disk ‘behind, the tug cast off. The carrier went on, sunward. Its ringed nose pointed unwaveringly to the sun; toward which it would drift for years. It was one of along, long line of carriers drifting through space, a day apart in time but millions of miles apart in distance. They would go on until a tug from Earth came out and grappled them and towed them in to their actual home planet.

But the Betsy-Anne, of Pluto, did not pause for contemplation of the two-billion-mile-long line of ore carriers taking the metal of Pluto back to Earth. It darted off from the line its late tow now followed. Its radiolocator beam flickered invisibly in emptiness. Presently its course changed. It turned about. It braked violently, going up to six gravities deceleration for as long as half a minute at a time. Presently it came to rest and there floated toward it an object from Earth, a carrier with great white numerals on its sides. It had been hauled ofl Earth and flung into an orbit which would fetch it out to Pluto. The Betsy-Anne’s grapnel floated toward it and jetted tiny sparks until the tow ring was engaged. Then the tug and its new tow from Earth started back to Pluto.

There were two long lines of white-numbered carriers floating sedately through space. One line drifted tranquilly in to Earth. One drifted no less tranquilly out past the orbits of six planets to reach the closed-in, underground colony of the mines on Pluto. Together they made up the Pipeline.

Carriers drifted on through space. They were motor-less save for the tiny drives for the gyros in their noses. They were a hundred feet long, and twenty feet thick, and some of them contained foodstuffs in air-sealed containers—because everything will freeze, in space, but even ice will evaporate in a vacuum. Some carried drums of rocket fuel for the tugs and heaters and the generators for the mines on Pluto. Some contained tools and books and visiphone records and caviar and explosives and glue and cosmetics for the women on Pluto. But ail of them drifted slowly, leisurely, unhurriedly, upon their two-billion-mile journey.

They were the Pipeline. You put a carrier into the line at Earth, headed out to Pluto. The same day you took a carrier out of space at the end of the line, at Pluto. You put one into the Earth-bound line, on Pluto. You took one out of space the same day, on Earth. There was continuous tratfic between. the two planets, with daily arrivals and departures from each. But passenger-trafiic between Earth and Pluto went by space liners, at a fare of fifty thousand credits for the trip. Because even the liners took six months for the journey, and the Pipeline carriers—well, there were over twelve hundred of them in each line going each way, a day apart in time and millions of miles apart in space. They were very lonely, those long cylinders with their white-painted numbers on their sides. The stars were the only eyes to look upon them while they traveled, and it took three years to drift from one end of the pipeline to the other.

But nevertheless there were daily arrivals and departures on the Pipeline, and there was continuous traffic between the two planets.

The Pipeline was actually a two-billion-mile arrangement of specks in infinity. Each of the specks was a carrier. Each of the carriers was motorless and inert. Each was unlighted. Each was lifeless. But—some of them had contained life when they started.

The last carrier out from Earth, to be sure, contained nothing but its proper cargo of novelties, rocket fuel, canned goods, and plastic base. But in the one beyond that, there was what had been a hopeful stowaway. A man, with his possessions neatly piled about him. He’d been placed up in the nose of the carrier, and he’d waited, mousy-still, until the spacetug connected with the tow ring and heaved the carrier out to the beginning of the Pipeline. As a stowaway, he hadn’t wanted to be discovered. The carrier ahead of that—many millions of miles farther out—contained two girls, who had heard that stenographers were highly paid on Pluto, and that there were so few women that a girl might take her pick of husbands. The one just before that had a man and woman in it. There were four men in the carrier beyond them.

The hundred-foot cylinders drifting out and out and out toward Pluto contained many stowaways. The newest of them still looked quite human. They looked tranquil. After all, when a carrier is hauled aloft at four gravities acceleration the air flows out of the bilge-valves very quickly, but the cold comes in more quickly still. None of the stowaways had actually suffocated. They’d frozen so suddenly they probably did not realize what was happening. At sixty thousand feet the temperature is around seventy degrees below zero. At a hundred and twenty thousand feet it’s so cold that figures simply haven’t any meaning. And at four gravities acceleration you reach a hundred and twenty thousand feet before you’ve really grasped the fact that you paid all your money to be flung unprotected into space. So you never quite realize that you’re going on out into a vacuum which will gradually draw every atom of moisture from every tissue of your body.

But, though there were many stowaways, not one had yet reached Pluto. They would do so in time, of course. But the practice of smuggling stowaways to Pluto had only been in operation for a year and a half. The first of the deluded ones had not quite passed the halfway mark. So the stowaway business should be safe and profitable for at least a year and a half more. Then it would be true that a passenger entered the Pipeline from Earth and a passenger reached Pluto on the same day. But it would not be the same passenger, and there would be other differences. Even then, though, the racket would simply stop being profitable, because there was no extradition either to or from Pluto.

So the carriers drifting out through emptiness with their stowaways were rather ironic, in a way. There were tragedies within them, and nothing could be done about them. It was ironic that the carriers gave no sign of the freight they bore. They moved quite sedately, quite placidly, with a vast leisure among the stars.

From PIPELINE TO PLUTO by Murray Leinster (1945)

3 March 2094, 0250 hours Flight Deck of the Catapult Hercules

His finger tips sweated in the close-fitting control caps. Only eighteen k-k's from Vesta and still no Company. What had they done—written the station off? The entire ship reached into his heightened awareness. The awesome engines designed to hurl inert cargo on multi-million-kilometer tracks through space. The heavy mining laser converted into a terrifying main weapon now slung in the cargo grapples. The thousands of bits of information from the ship‘: computers and sensing radars. Where the hell were they‘? “Come on, you Company fish, swim out into the pan."

Violently the ship executed a maximum burn maneuver with her nine and twelve o'clock engines. Some of the datastream elements were now glowing red. “Damage report: two mike hit on plates 1023/24 negative critical. Integrity 80-80."

“Beautiful, Dee, You saved our jewels with that cut."

Ulans tapped his foot reflexively. On the blue cross hair showing on the main screen, a yellow dot bloomed. Six thousand kilometers distant, several people died.

Catapults: Catapults are the means by which materials are transported within the Solar System. Catapults latch on to a container of ore extract and accelerate to a high speed before releasing the load in the direction of the destination. Another catapult at the destination intercepts the container and latches on to decelerate it. The ore is delivered to orbital factories or delivered by shuttle to the surface (None of the interplanetary space ships is designed to land on the surface of a Planet.)


Unlike Miners and the so-called Transports (which were more like Ares Patrol ships), the Catapults were not normally equipped with lasers. Consequently, at the start of the game, they have no combat capacity. However, it would not be difficult to mount a laser in a Catapult, as the computer would be quite capable of aiming it, and plenty of spares were around. Hence, Catapults may easily gain combat capacity.

[9.91] At the start of the game, until serviced (see Case 9.92), all Catapults are treated as if they had major damage,

[932] During any Logistics and Maintenance Phase, Catapults may be fitted with lasers (“serviced”) at any Friendly Asteroid or Planet (see Case 15.35).


Cargo Tether

This Inert Cargo Vessels scheme can be used if the delta-V requirement for the trajectory is not too excessive. The pair of space tugs can be replaced by a pair of momentum exchange tethers ping-ponging momentum energy back and forth between each other as they launch and catch cargo cannisters. With this scheme it is important that each tether in the pair launches the same amount of mass that they catch. Otherwise one or the other tether will start running out of energy and will have to be spun up again with solar power or something.

Again these are are constantly working and constantly amortizing.

NEO Stalk 1

I briefly got into another discussion about moving asteroids recently. It involved parking an eleven ton spacecraft next to the asteroid and letting the gravitational attraction between the two shift the asteroids orbit. Then when the spacecraft gets too close, use the thrusters to open the gap again. When I said that a piece of thread would have just as much pull, and that just as much propellant would be used in either case, it was suggested that I learn something about conservation of momentum. Without further explanation from the other guy, I just assume that it is another of those concepts over the intellectual head of this redneck. If the rock is dangerous though, we need to do something about it.

Thinking about the subject, with the eleven ton vehicle and fifteen year time frame in the article, I think much more efficient use of mass, time and money can be done. They suggest in the article that a half kilogram or so of force would be applied by this gravitational tractor concept. I’m going to try newtons this time to see if I can get them right. Fortunately there are several people here to straighten me out if I screw it up. 5 newtons force for an hour is 18,000 newton seconds applied. In a day that is 432,000, and a year is 157,680,000, and 15 years is 2,365,200,000 newton seconds. That’s a real big number, maybe. The F1 Saturn engine averaged about 7,000,000 newtons, so in about 338 seconds, one of them could apply as much total force. The problem is propellant of course.

During the discussion the ideas of painting part of the NEO and vaporizing the surface for reaction came up. In other discussions nuclear bombs, electric engines and fancy flyby trajectories are mentioned. I think most people miss the point that most asteroids have enough internal energy to move themselves if properly persuaded. The rotational energy alone is more than enough to shift an orbit to safety if we are clever enough to tap into it for propulsion purposes. The ideas here are not new, just my interpretation.

An asteroid is a natural for a beanstalk. A tiny rock with 10 m/s escape velocity and a four hour day would have a NEOsync orbit at about 23 km. If the composition contains enough steel, iron, or other tensile useful materials in attainable form, then an in situ material beanstalk is feasible. Here I am assuming that something can be extruded with material properties half as good as terrestrial rebar. A 100 m/s stalk tip velocity would be at 230 km which would be the total length of the system. With a taper ratio of two, a mass ratio of one results for tether to tip payload.

If a thousand tons of NEO material can be extracted for beanstalk use, then thousand ton payloads become feasible from the tether tip at 100m/s. It would seem that throwing a thousand tons at 100m/s would deliver a 100,000,000 newton impulse to the NEO. 24 such payloads would exceed the impulse delivered by the gravity tractor in the other article. One a beanstalk is set up though, sending dozens or thousands of payloads is just a matter of extracting and bagging NEO material.

The neat thing about a beanstalk much longer than NEOsync is that the energy needed to lift and throw the payloads is supplied by the rotational energy of the NEO itself. Once past NEOsync, the payloads fall out to the end point, and they are quite capable of lifting the next payload off the ground with a light spectra tether that masses a fraction of a percent of the material lifted. This spectra tether is separate from the beanstalk and just used for propulsion purposes, returning to the ground after each lift. There is no need for energy delivery to the vehicles, which are basically bags with tether brakes.

One the beanstalk is built and reaction mass collected, the operation waits until the proper orbit to throw the payloads/propulsion sacks to Earth or Lunar orbit. It seems likely that this window will be during perihelion and last a month or so. A thousand ton payload every four hours for a month puts 180,000 tons of material on trans Earth/Luna trajectory. At the same time, it delivers an 18,000,000,000 newton impulse to the NEO, which should move it out of the danger zone during the flyby that happens in a dozen years or so. This impulse, delivered earlier and more concentrated than the gravity tractor idea, should be proportionately more effective, even discounting the nearly eight times more power.

If harvesting is not wanted for some reason, and mission mass is the critical restriction, then one ton payloads could be done with a hundred kg spectra tether every four hours for the fifteen years suggested in the article. The thousand ton units seem more worth chasing and catching to me for a space faring economy.

From MOVING ASTEROIDS by John Hare (2009)
NEO Stalk 2

This is a sketch from a post I did in 2009 about moving asteroids using their own rotational energy to sling ISRU mass to change their orbit.

With Jons’ last post, I got to speculating about other ways of getting mass from an asteroid back to Earth. What if the exploring spacecraft carried a tether to the asteroid that was long enough to serve as a beanstalk. Instead of carting the selected boulder(s) all the way home, the beanstalk is used to sling a number of samples to Cislunar space to be caught by some TBD craft.

Instead of the thousand ton boulders slung in the original post, ten or so tons per throw would send a sizable mass to explore and exploit close to home while leaving the spacecraft in the field for a continuing mission. While as in the previous it would be necessary to wait for launch windows to use the tether, the time between arrival and window could be spent exploring the body in question and organizing multiple throws. When the window opens, the ten (or one or thirty) ton samples could be sent Earthward every time the asteroid rotates. A four hour asteroid rotational day would give six launches per Earth day. A window a week long could have forty or so samples heading home at once.

If the asteroid is considered explored by that time to the limits of the available craft, it is time to head to the next target. One way of doing that would be for the spacecraft to climb the tether to well past astrosync orbit to a point calculated to sling it to the next body to be explored. At the right time, the tether is cut loose at the asteroid end to send the vehicle and its’ tether to a new little unexplored world. Once there, the cycle is repeated. It would seem that the craft could explore and exploit indefinitely without running out of propellant.

For a second phase of exploitation, tethers are left attached to the asteroids for use by future visitors. Eventually, spacecraft could visit dozens of rocks during an operational lifetime to prospect for different substances or to test new techniques in a variety of locations.

Certain asteroids would be exceptionally useful in a third phase if they proved exceptionally well suited for transportation hubs. Instead of slinging small robotic prospectors to other rocks, long beanstalks could relay humans and cargo to Mars and other points of interest throughout the inner solar system.


Cargo Mass Driver

This Inert Cargo Vessels scheme relies upon a concept envisioned by visionary Gerard K. O'Neill

Back in 1976 O'Neill had a problem which was preventing the construction of his O'Neill cylinder L5 space colonies. Their isn't enough money in the entire world to boost the required million metric tons or so of construction into orbit using rockets. Therefore O'Neill designed a lunar mining base which would dig up the required materials, cheaply catapult them into cis-Lunar space with a huge Mass drivers, and when a given load approached the L2 point it would be intercepted by a huge net-like construct picturesquely named a "catcher." No cargo spacecraft required. The power requirements of the mass driver were reduced by having the source of the materials on Luna instead of Terra, since Luna has a much milder gravity well.

Some mass driver designs have the masses of ore encased in ferromagnetic cannisters to give the mass driver's magnetic field something to grab. Others need no cannisters, instead they use ferromagnetic buckets which are halted and returned to be reused at the end of the mass driver. The ore goes flying into space toward the catcher. This saves on cargo cannisters cost.

Landing Grid

This is very fringe science. I'm no expert, but the Richmond concept is probably more impractical than it is actually forbidden by the laws of science.

Nikola Tesla

Back in the early 1900's noted genius and mad scientist Nikola Tesla figured he could tap a conductive layer in Terra's upper atmosphere and used it to wirelessly broadcast electricity. The electricity would be held in standing waves around the entire globe, and could be tapped by machines in remote locations for electrical power. It would also make the entire upper atmosphere glow, making cities and shipping lanes happy while infuriating astronomers. Oh, and it would also work as a wireless telegraph.

While many of Tesla's devices were brilliant, this one was a total crack-pot idea. Telsa was suspicious of these new-fangled ideas about air-borne electromagnetic waves. Not to mention there was no way to send an electricity bill to the people using it.

Telsa managed to talk a bunch of investors into funding a pilot project, the Wardenclyffe Tower. The project was a disaster for various reasons and Telsa had a nervous breakdown.

Murray Leinster

About fifty years later science fiction writer Murray Leinster wrote a series of short stories featuring a huge device called a "landing grid." I have been unable to discover the source of Leinster's inspiration, but I suspect Telsa's Wardenclyffe Tower. As far as I have been able to determine the first of these stories was Sand Doom (1955), first of the Colonial Survey series.

Anyway a landing grid is a circular arrangement of steel girders and copper cables about half a mile high and one mile in diameter. It is set firmly into the planet's bedrock.

For a planetary colony, it supplies electrical power by tapping the electrical potential difference between the ground and the planet's ionosphere. The planet acts like a huge capacitor. One plate is the ground, the other plate is the ionosphere, and the insulating dielectric is the atmosphere in between.

Since the ionosphere is basically energized by the planet's sun it will supply electricity for as long as the sun shines. As to how much energy is available, the best I can say is "lots and lots." A certain Dr. Elizabeth Rauscher estimated that the ionosphere and magnetosphere had a potential energy of about 3 terawatts. No idea of how rapidly the energy would be replenished by the sun.

The second vital function a landing grid supplies a planetary colony is landing services. It can use technobabble tractor beams to grab a spacecraft at a range of tens of thousands of miles and gently lower it to land in the center of landing grid. Or gently lift a spacecraft from the grid up into space, releasing it several thousand miles altitude. The spacecraft does not have to spend horrific amounts of delta V to get halfway to anywhere. The inexhaustible supply of ionospheric electricity will do it for you.

The framework of girders requires about one foot of diameter for every ten miles of tractor beam range. They are typically one mile in diameter, giving the tractor beam a range of about 53,000 miles (about 6.7 Terran diameters).

When a new planetary colony is founded, the first construction crew lands in rocket-propelled vehicles (since there is no existing landing grid). Their priority is to quickly build a grid to get the colony started.

In theory, interplanetary and interstellar war was not possible in Leinster's novels. Naturally a planet would not be foolish enough to use their grid to land a hostile invasion force. And the grid was perfectly capable of attacking an enemy orbiting fleet with tractor-beam launched missiles, or even rocks for that matter. Without grid support, an invasion force trying to land troops would need lots of rockets with ugly mass ratios. The invading fleet can launch missiles and bombs, but they have limited supplies (limited to what they brought with them). The planet ain't going to run out of rocks.

And if the invaders destroy the landing grid, they will lose easy access to the surface. Worse, any invading forces actually on the planet will be stranded until a new grid can be constructed. So the invaders do not want to nuke the grid, but the grid can decimate their fleet with hypervelocity rocks.

The theory was exploded in Leinster's 1957 story The Grandfathers' War. Basically they built a space-going landing grid.

Conventional grids grab objects in space with a tractor beam and pulls it to the ground. This monster grabs the ground with a tractor beam and pushes the grid into space. Conveniently the FTL drive can operate the instant a ship (or space-going landing grid) is several planetary diameters away from the planet, so the grid does not even need any rockets. Directly into FTL drive it goes. The warlike grid travels under FTL drive then emerges into real space in orbit around the target planet. There it uses its tractor beam to land itself, instantly creating an invader-controlled grid on the surface of the hapless planet. The grid then lowers the hordes of invading troop carrier starships gently to the surface and the attack begins. The only question I have is can the space-going grid tap the target planet's ionosphere while in orbit?

Lucky for the peace of the galaxy, in Leinster's universe nobody ever copied the grid-ship idea, and it was forgotten. The idea was not used in subsequent novels.

Walt and Leigh Richmond

In 1962 Walter Richmond was doing research into atmospheric electricity and invented what he called the Solar Tap. It was a way to access the potential energy difference between the ionosphere and the ground, but it was rather hair-raising.

You build an insulator, a pyramid shaped pile of rock about 150 meters tall. Be sure you locate the insulator well away from the magnetic poles of the planet. From the peak is shot a powerful laser beam pulse to create a conducting ionized trail all the way to the ionosphere. A titanic bolt of lightning travels down the trail to hit the insulator. There equipment does its best to harvest as much of the lightning as it can, without destroying the equipment or too much of the surrounding landscape.

As an encore, distribute the energy world-wide by using some sort of technobabble Tesla style energy broadcasting technology.

Why is it so important to site this far away from the magnetic poles? Well, the lightning bolt will create a magnetic field cross-wise to the planet's natural magnetic field. The result is to pinch the bolt and stop it after a few microseconds. Then you shoot another laser blast to created the next lightning bolt. All nice and controlled.

If the insulator is at a magnetic pole, the lightning bolt's magnetic field will be parallel to the planet's field. The bold will not be pinched. It will be permanent until the ionosphere is depleted after a week or so (an "avalanche"). In other words about 3 terawatts of power will start evaporating the continent around the magnetic pole, split the tectonic plates and start the continents moving around, create nuclear winter, destroy all civilization and cause a global extinction event.

That would be bad.

In 1967 Walt and Leigh Richmond wrote The Lost Millennium aka Shiva. The idea behind the novel was that solar taps were not only possible, they had been invented about eight thousand years ago. The reason we were unaware of this is because the idiots back then had sited the main tap at the magnetic pole in the name of maximum power harvesting, and they resolved to be very very careful not to let an avalanche start. With predictable results. Pretty much erased their entire civilization, it did.

The reason the insulator for a solar tap is about the same size and shape as the Great Pyramid of Cheops is because the latter is an insulator for a solar tap. Apparently some survivors from Atlantis built Giza a couple of thousand years after the avalanche (the pyramid that caused the avalanche was pretty much obliterated). Well away from the magnetic pole you will note. The laser firing makes a noise that sounds like "SHEEEEE!" and the returning lightning bolt makes a sound like "OPS!". So the solar tap in operation sounds like SHEEE-Ops!, SHEEE-Ops!, SHEEE-Ops!. Which is where the Cheops pyramid got its name. Cute.

The novel includes all sorts of historical anomalies harvested from tales of Atlantis, ancient astronauts, and Chariots of the Gods? The reason archaeologists are not constantly stumbling over eight thousand year old automobiles and skyscraper girders is because the broadcast power system made large metal objects a dangerous idea.

Anyway the other item relevant to our interests is that the solar tap could also be used to boost and land spacecraft. The Richmonds are vague in the details but they maintain that a network of smaller pyramids can create a pattern of laser beams to craft a titanic Jacob's Ladder. The high-voltage traveling arc boosts or land spacecraft by electromagnetic induction. Somehow (the details are left as an exercise for the reader). In the novel, during boost mode the solar tap sounds like ANGOR-WATT! ANGOR-WATT! which is also cute.

In their later novel Gallagher's Glacier the Richmonds take up planetary liberation by solar tap. In the novel, all the poor planetary colonies are controlled by an evil corporation. The colonies are not allowed to have solar taps, because the corporation do not want the colonies to be anywhere near being self-sufficient.

Gallagher takes a tip from Leinster and mounts the solar tap on a spaceship. It is impossible for a colony to covertly build a solar tap over a couple of decades without the evil corporation goons noticing. But once Gallagher's space ship shows up, the colony instantly has a solar tap, and can use its energy to defeat the goons and kickstart building their own permanent solar tap. Corporation Revolutionary War soon follows.


     The landing system was from what Gallagher called their solar tap. They were tapping the electrical potential that exists between a planet and its orbiting proton and electron belts—the belts of ionized particles caught in the planet's magnetic field.
     The landing system was part of a power system that produced, from this one site, enough electricity to power the entire continent on a broadcast basis.
     Broadcast power. It had been known on Earth since the days of Nicola Tesla—the system for putting power on the airwaves the way radio and TV are broadcast. Electric power that you could tune into, the way you tune in a radio.
     With broadcast power, you didn't have to have wires strung around the continent to plug in motors and appliances and furnaces and the like. You didn't have to carry your own fuel in! your ground car. You tuned in your motor to the power frequency, the way you tune in a radio to the frequency of the station you want to hear.
     Earth hadn't had broadcast power, though she'd known how to broadcast it, because the production of power was geared to installations that didn't have sufficient potential that you could waste it on the airwaves. But the power potential in the solar tap was so great you could throw it away on an inverse-square: basis and still be able to tune in at the coast lines, two thousand miles distant, and run anything you wanted to run, from a manufacturing complex to a skimmer. (I'm not sure that is such a good idea. The thrown-away power has to go somewhere, probably turning into waste heat. And you thought greenhouse gases were bad for climate change...)
     The power that exists between the ground potential on any planet and the orbiting proton and electron belts trapped in the magnetic field of any planet, is fed by the solar wind of the sun around which the planet orbits, and it is a practically limitless potential. Electrons from the solar wind make their way in through the magnetic poles of the planet, distribute themselves at its crust, and seep through the insulating atmosphere towards the strong positive potential of the inner proton belt.
     If you make a "short circuit" through the atmosphere by creating an ionized pathway with a laser beam that reaches to the ionosphere, the top of the insulating atmospheric layers, the electrons will jump across the short circuit, changing the groundside potential. When the groundside potential lowers, it makes it possible for more electrons to pour in from the solar wind to equalize the potential. The planet is effectively recharged, and you can short-circuit again.
     It's done in milliseconds, and it's done on a pulse-basis. You turn the laser-beam short circuit off and on in an alternating-current effect, and it's most efficient at a low sonic frequency, although it has radio-frequency overtones.
     There was a group of huge pyramidal structures that were the bases for the solar tap and landing system. A huge, central pyramid was the -tap itself; built of granite with a marble overlay, and of sufficient size to insulate the tremendous bursts of power flow from the ground. The laser installation was on a small platform at the peak of the pyramid, and the control systems were centered well inside where the X rays and other radiation from the flow would not harm the technicians.
     From this, central pyramid, the pulsed power was broadcast across the continent, and even from inside the canteen and at this distance you could hear the deep-throated roar of that power, pulsing through at a frequency within the audible range. Chee-ops, chee-ops, chee-ops, it seemed to say as it shorted in, was cut off, and pulsed in again.
     It was the landing system that used the smaller, satellite pyramids around the big one and that used other factors of the huge central pyramid as well.
     The landing system was a gigantic web of laser beams, angled upward and focused to create a huge electrical discharge spiral that used magnetic induction and repulsion to bring the meteors in. They could bring in any metallic ship as well on that huge spiral, even though the interstellar ships were comparatively fragile; for the gentle cradle of the magnetic induction-propulsion system could raise or lower the gigaton masses as evenly as a freight elevator might bring down a crate of delicate electronic equipment.
     The landing system. From the huge central pyramid, and from each of the smaller ones, two great alternating laser beams angled upward, aimed through tunnels internal to the pyramids and geometrically accurate to a hair. Those beams discharged their alternating spirals into a crisscrossed web of induction-repulsion that caught a ship and either stepped it up from rung to rung of the magnetic induction-propulsion ladder, or cradled it gently downward.

     "But where's the meteor we came down in, and what's your plan for it?" I asked. "That's a big plenty of steel."
     "Over there." He pointed to where I could see a glow in the sky at the center of the port complex. "We can use steel. We just use it with know-how."
     I stared at the glow in the sky. "Surely it couldn't have been that hot?"
     "That glow? That's not the meteor. That's the melting tap in operation. But the meteor did come in at a red heat, at least in its surface layers. You see, we land them direct into the furnace, "and as soon as everybody's clear, they change the frequency of the induction current and start melting them down. Saves quite a bit of time, and time's our most precious commodity. The energy we're not worried about—that we've got in plenty. But it takes time to reheat, and if the ejection mechanism doesn't work, it's a couple or three hours' setback to lose the heat that was built up during descent so that we can get the people out. And then we have to reheat the darned thing so we can melt it down."
     "Isn't that a rather expensive way to get steel?" I asked.
     He grinned. "You're just not used to the idea of really planetary power," he said. "Those meteors—asteroids, really—can be brought in, melted down, and ready to use for tool steel at a cost per ton of, say, a hundredth of a solar credit."

     The ridiculousness of shooting a planet with the ineffective sting of a fine-focused laser began to creep up on me. The focus on this device was so fine that it would probably make no more than a centimeter-diameter hole in whatever target it hit, and though that's plenty big to play utter havoc with a space vehicle, it would be less than the sting of a mosquito so far as a planet was concerned.
     "Set power pulse to three seconds."
     Gallagher's voice was slightly edged, but Cricket's came back in a singsong that showed no overtones of emotion.
     "Power pulse on three seconds by off point five seconds."
     "Initiate pulse."
     Cricket didn't have to respond to that one because the power machinery did it for her. There was a slow, rhythmic, mmm-pop, mum-pop from the power supply that went on and on and combined with the cheeee to form a now-familiar repetitive pattern; the song of power that I had heard on Betsy Ann: cheee-ops, cheee-ops.
     "I'll be damned," I yelled. "This thing's an upside-down solar tap!"
     There was a choke behind Gallagher's laugh, and his voice had a sweep and flow that spoke of tensions releasing.
     "We're way above the radiation belts," he said, "but the oscillating lens of our zoom focus makes an ionized path from Durango's ionosphere to its ground, and that's all you need for a tap. We didn't have time to build a pyramid down there, so we turned the tap upside-down."

From GALLAGHER'S GLACIER by Walt and Leigh Richmond (1970)


3-D Printing

3-D printing is also known as "additive manufacturing". This is because the object is created by adding blobs of new material, instead of the conventional method of starting with a block of material and carving away the unwanted bits (for example, as done by a CNC router).

This was a mind-blowing concept when Keith Laumer used it in his 1981 novel Star Colony, but with the advent of hobbyist 3-D printers it is now considered trendy but not impossibly futuristic.

Corporations will be angered by 3D printers: if you thought the RIAA went ballistic about digital music piracy and the MPAA was freaking out about movie file sharing, you ain't seen nuthin' yet. Manufacturers are going to start foaming at the mouth about digital object piracy. I predict even more draconian Digital rights management laws.

There is already in the real world people who are stirring up trouble by making blueprints that will 3D print plastic "ghost" firearms with no serial numbers. They are trying to strike a blow for Libertarianism, but they might just wind up making 3D printers illegal. Angry corporations are one thing, angry governments are even worse.

And if you take a super-duper 3-D printer and hook it up to a super-duper 3-D scanner, you have a Star Trek Replicator. Which will create unintended consequences.

But I digress.

NASA is interested in 3-D printing because Every Gram Counts. It would be a valuable savings in mass if a spacecraft did not have to carry spare parts for every conceivable thing that might break, but could instead only carry a 3-D printer and the raw material. You do not have to waste payload on spare parts you might never need. And the computer blueprints have zero mass.

Most currently available 3-D printers only print with one material (generally some kind of plastic). Innovators are frantically working on printers that can handle multiple materials. This is vital for printing, say, an electric motor or an electronic circuit. Currently available printers deposit blobs of material, in the future they will deposit on an atom-by-atom basis.

In September 2014, SpaceX delivered the first zero-gravity 3-D printer to the International Space Station (ISS). On December 19, 2014, NASA emailed CAD drawings for a socket wrench to astronauts aboard the ISS, who then printed the tool using its 3-D printer.

As a proof-of-concept, Markus Kayser created the Solar Sinter. He noted that in the deserts of Terra, there is a lack of useful artifacts but unlimited amounts of sunlight and sand. The Solar Sinter is a computer controlled magnifying glass that 3-D prints by melting layers of sand. There are many planets and moons where such a tool would be incredibly useful.

Architecture Et Cetera (A-ETC) is working on Project SinterHab. This will use microwaves to fuse Lunar dust in order to 3-D print habitat modules for a Lunar base.

Foster + Partners is working with the ESA to make a 3-D printed lunar base. Lunar soil is mixed with magnesium oxide to produce the material. Layers are bound by being sprayed with a binding salt in a controlled pattern. The binding salt turns the material into a stone-like solid.

A 3-D printer can also be used as the "assembler" component of a Santa Claus Machine.


      Archinaut, a NASA Technology Demonstration Mission (TDM) project developing cutting-edge technology to build and assemble complex hardware and supersized structures on demand in space, achieved an unprecedented milestone this summer.
     "To our knowledge, this is the first time additive manufacturing has been successfully tested on such a large scale in the vacuum and temperature conditions of space," said Eric Joyce, Archinaut project manager for Made In Space Inc. of Mountain View, California, which spearheads the project for NASA.
     The Archinaut test series, using Made In Space's innovative Extended Structure Additive Manufacturing Machine, was conducted in a vacuum chamber in the Engineering Evaluation Laboratory at NASA's Ames Research Center in Moffett Field, California.
     The team conducted hundreds of hours of tests to complete the series. Working around the clock for much of June, they printed large beam segments — similar to those used to construct a variety of space structures — and subjected printing equipment and printed hardware alike to the pressures, temperatures and other rigors of deep space.
     "This was a big step for us," Joyce said. "It advances the technology — and gives us real confidence the hardware will do the job in space that it does here on the ground, enabling us to print sturdy, reliable structures of unlimited size. It was a history-making test."
     Archinaut is one of three "tipping point" projects NASA is funding in pursuit of groundbreaking new solutions under the umbrella of TDM's In-space Robotic Manufacturing and Assembly (IRMA) project, sponsored by NASA's Space Technology Mission Directorate. These projects help NASA determine whether the technology has been sufficiently matured to pursue flight demonstrations or for infusion into future exploration missions.
     "We couldn't be more pleased about Archinaut's successful demonstration," said Trudy Kortes, TDM program executive at NASA Headquarters in Washington. "In-space robotic manufacturing and assembly technologies are destined to be key building blocks for a thriving space infrastructure, and will enable robust future exploration missions across the solar system. Milestones such as this one are crucial steps toward that future."

Building in space to curtail cargo launches

     Better known as 3-D printing, additive manufacturing could offer solutions for quickly and cheaply mounting new space infrastructure missions to Earth orbit and beyond. Combined with robotic manufacturing and assembly, the technology could help NASA and its commercial partners remotely construct new habitats and hardware in space — without the costs or risks associated with flying heavy materials or structures via rocket from Earth to space.
     Just as crucially, building to order in space frees future missions from the limitations of conventional spaceflight. "Until now, everything we have sent to space has been constrained by the volume available on various launch vehicles," Joyce said. "That fundamentally limits the size and geometry of anything we send up."
     Additive manufacturing would nullify that obstacle. "Instead of launching a rocket with a complete vehicle crammed on board, what if we just launch feedstock — raw material — and do all manufacturing and assembly in space?" he added. "All the constraints go away, and rockets become more efficient at delivering cargo to space."
     The logical next step — following another test series in early 2018 to further hone the capabilities of the ESAMM prototype and refine Archinaut's robotic manipulator — is full-scale, in-space flight demonstration. The team is already pondering its ideal project for that potential future mission: a massive communications satellite dish, or perhaps a supersized truss designed to robotically deploy solar panels? Time will tell, Joyce said.
     Ultimately, Archinaut could evolve into a build-to-order space platform. Vehicles or satellites could dock to enable construction, assembly and integration of whatever space-optimized hardware or systems they require, Joyce suggested — permanently rewriting the way humans travel to space.
     "This technology is absolutely transformative," Joyce said. "Archinaut has the potential to dramatically advance discovery in space, reducing the time and money spent launching hardware and equipment and putting the focus on the human explorers who will use that made-in-space equipment to explore the cosmos."
     The Archinaut team includes lead subcontractor Northrop Grumman Corp. of Falls Church, Virginia; Oceaneering Space Systems of Houston, Texas; and Ames Research Center. TDM projects such as Archinaut mature groundbreaking technologies for infusion into government and commercial programs, dramatically extending human capabilities and opportunities in space. NASA's Marshall Space Flight Center in Huntsville, Alabama, leads the TDM program for the agency.

For more information about Archinaut, visit:

To learn more about NASA's Space Technology Mission Directorate, visit:

At the heart of Archinaut technology is Made In Space's Extended Structure Additive Manufacturing Machine (ESAMM). ESAMM is a manufacturing method which incorporates Archinaut’s additive manufacturing system with a robotic manipulator to create objects in free-space and install both additively manufactured and pre-fabricated components.

Created by MIS and validated in microgravity research flights through the NASA Flight Opportunities Program, ESAMM produces arbitrarily large and complex structures. By using its robotic manipulators to position the part being created, ESAMM can constantly reorient the part with a high degree of freedom, allowing the part to be created as large as desired in any dimension. This allows ESAMM to create structures as small as an oxidizer tank fueling cap, as large as a Mars cruise vehicle back bone, or any size in between.

Archinaut is capable of additively joining previously manufactured or pre-fabricated elements together, enabling assembly of spacecraft. Different mission profiles require different materials and prefabricated components, thus the variety of feedstocks available for use by Archinaut is broad and includes multiple spaceflight-proven materials such as space-grade polymers and composites.


      Men, unlike plants, cannot thrive on pure energy and a few simple chemical compounds. Ever since the gates of Eden clanged shut with such depressing finality, the human race has been engaged in a ceaseless struggle for food, shelter, and the material necessities of life. More than two million, million man-years have been expended in this agelong battle with nature, and only in the last four or five of the fifty thousand generations of mankind has the burden shown signs of lifting.

     The rise of modem science, and in particular the advent of mass production and automation, is of course responsible for this; but even these techniques are only pointers toward far more revolutionary methods of manufacture. The time may come when the twin problems of production and distribution are solved so completely that every man can, almost literally, possess anything he pleases.

     To see how this may be achieved, we must forget all about our present ideas of manufacturing processes and go back to fundamentals. Any object in the physical world is completely specified or described by two factors: its composition, and its shape or pattem. This is quite obvious in a simple case; such as a one-inch cube of pure iron. Here, the two phrases “pure iron” and “one-inch cube” provide a complete definition of the object, and there is no more to be said. (To the first approximation, at least: an engineer would like to know the dimensional tolerances, a chemist the precise degree of purity, a physicist the isotopic composition.) From this brief description, containing only five essential words, anyone with the correct equipment and skills could make a perfect copy of the object specified.

     This is true, in principle, for much more complicated objects, such as radio sets, automobiles, or houses. In such cases it is necessary to have not only verbal descriptions but plans or blueprints—or their modern equivalent, pulses stored on magnetic tape. The tape which controls an automated production line carries, in suitably coded form, a complete physical description of the object being manufactured. Once the master tape has been made, the act of creation is finished. What follows is a mechanical process of replication, like printing a sheet of letterpress when the type has been set up.

     During the last few years, more and more complicated artifacts have been produced in this wholly automatic manner, though the initial cost of equipment (and skill) is so high that the process is worthwhile only where there is a demand for enormous numbers of copies. It requires a specialized machine to manufacture one particular type of object; a bottle-making machine cannot switch to cylinder heads. A completely general-purpose production line, able to produce anything merely from a change of instructions, is inconceivable in terms of today’s techniques.

     It may seem inconceivable in terms of any technique, because many (perhaps most) of the artifacts we employ and the materials we consume in everyday life are so complicated that it is impossible to specify them in explicit detail. Anyone who doubts this should try to write out the complete description of a suit of clothes, a pint of milk, or an egg, so that an omnipotent entity who had never seen any of these things could reproduce them perfectly.

     Perhaps a specification for a suit might be just possible today, if it were made of synthetic fabric; but not if it were made of organic materials like wool or silk. The pint of milk is a challenge that the biochemists of the future may be able to meet, but I shall be very surprised if, in this century, we have a complete analysis of all the fats, proteins, salts, vitamins and heaven knows what else that goes into this most comprehensive of foods. As for an egg—this represents an even higher order of complexity, both in chemistry and structure; most people would deny that there is the slightest possibility of ever creating such an object, except by the traditional methods.

     Yet let us not be discouraged. In Chapter 7, when discussing the possibility of instantaneous transportation, we considered a device that would scan solid objects atom by atom to make a “recording” that could ultimately be played back, either at the same spot or at a distance. Though such a device cannot be realized, or even remotely envisaged, in terms of today’s science, no philosophical contradictions or absurdities are raised if we suppose its operations limited to fairly simple, inanimate objects. It is worth remembering that an ordinary camera can, in a thousandth of a second, make a “copy” of a picture containing millions of details. This would indeed have been a miracle to an artist of the Middle Ages. The camera is a general-purpose machine for reproducing, with a considerable, though not complete, degree of accuracy, any pattern of light, shade, and color.

     Today we have devices which can do very much more than this, though even the names of most of them are not known to the general public. Neutron activation analyzers, infrared and X-ray spectrometers, gas chromatographs can perform, in a matter of seconds, detailed analyses of complex materials over which the chemists of a generation ago could have labored in vain for weeks. The scientists of the future will have far more sophisticated tools, that can lay bare all the secrets of any object presented to them and automatically record all its characteristics. Even a highly complex object could be completely specified on a modest amount of recording medium; you can put the Ninth Symphony on a few, hundred feet of tape, and this involves much more information or detail than, say, a watch.

     It is the “playback,” from recording to physical reality, which is rather difficult to visualize, but it may surprise many people to learn that this has already been achieved for certain small-scale operations. In the new technique of microelectronics, solidcircuits are built up by controlled sprays of atoms, literally layer by layer. The resulting components are often too tiny to be seen by the naked eye (some are even invisible under high-powered microscopes) and the manufacturing process is of course automatimlly controlled. I would like to suggest that this represents one of the first primitive breakthroughs toward the type of production we have been trying to imagine. As the punched-tape of the Jacquard loom controls the weaving of the most complex fabrics (and has done so for two hundred years) so we may one day have machines that can lay a three-dimensional warp and woof, organizing solid matter in space from the atoms upward. But for us to attempt the design of those machines now would be rather like the imagined efforts of Leonardo da Vinci to make a TV system.

     Leaping lightly across some centuries of intensive development and discovery, let us consider how the replicator would operate. It would consist of three basic parts —which we might call store, memory and organizer. The store would contain, or would have access to, all the necessary raw materials. The memory would contain the recorded instructions specifying the manufacture (a word which would then be even more misleading. than it is today!) of all the objects within the size, mass, and complexity limitations of the machine. Within these limits, it could make anything—just as a phonograph can play any conceivable piece of music that is presented to it. The physical size of the memory could be quite small, even if it had a large built-in library of instrucfions for the most commonly needed artifacts. One can envision a sort of directory, like a Sears Roebuck catalogue, with each item indicated by a code number which could be dialed as required.

     The organizer would apply the instructions to the raw material, presenting the finished product to the outside world—or signaling its distress if it had run out of some essential ingredient. Even this might never happen, if the transmutation of matter ever becomes possible as a safe, small-scale operation, for then the replicator might operate on nothing but water or air. Starting with the simple elements, hydrogen, nitrogen, and oxygen, the machine would first synthesize higher ones, then organize these as requested. A rather delicate and fail-safe mass-balancing procedure would be necessary; otherwise the replicator would produce, as a highly unwanted byproduct, rather more energy than an H-bomb. This could be absorbed in the production of some easily disposable “ash” such as lead or gold.

     Despite what has been said earlier about the appalling difliculty of synthesizing higher organic structures, it is absurd to suppose that machines cannot eventually create any material made by living cells. Any last-ditch vitalists who still doubt this are referred to Chapter 18, where they will discover why inanimate devices can be fundamentally more efficient and more versatile than living ones—though they are very far from being so at the present stage of our technology. There is no reason to suppose, therefore, that the ultimate replicator would not be able to produce any food that men have ever desired or imagined. The creation of an impeccably prepared filet mignon might take a few seconds longer, and require a little more material, than that of a thumbtack, but the principle is the same. If this seems astonishing, no one today is surprised that a hi-fi set can reproduce a Stravinsky climax as easily as the twang of a tuning fork.

     The advent of the replicator would mean the end of all factories, and perhaps all transportation of raw materials and all farming. The entire structure of industry and commerce, as it is now organized, would cease to exist. Every family would produce all that it needed on the spot—as, indeed, it has had to do throughout most of human history. The present machine era of mass production would then be seen as a brief interregnum between two far longer periods of self-sufliciency, and the only valuable items of exchange would be the matrices, or recordings, which had to be inserted in the replicator to control its creations.

     No one who has read thus far will, I hope, argue that the replicator would itself be so expensive that nobody could possibly afford it. The prototype, it is true, is hardly likely to cost less than $1,000,000,000,000, spread over a few centuries of time. The second model would cost nothing, because the replicator’s first job would be to produce other replicators. It is perhaps relevant to point out that in 1951 the great mathematician John von Neumann established the important principle that a machine could always be designed to build any describable machine—including itself. The human race has squalling proof of this more than a hundred thousand times a day.

     A society based on the replicator would be so completely diiferent from, ours that the present debate between capitalism and communism would become quite meaningless. All material possessions would be literally as cheap as dirt. Soiled handkerchiefs, diamond tiaras, Mona Lisas totally indistinguishable from the original, once-worn mink stoles, half-consumed bottles of the most superb champagnes—all would go back into the hopper when they were no longer required. Even the furniture in the house of the future might cease to exist when it was not actually in use.

     At first sight, it might seem that nothing could be of any real value in this utopia of infinite riches—this world beyond the wildest dreams of Aladdin. This is a superficial reaction, such as might be expected from a tenth century monk if you told him that one day every man could possess all the books he could possibly read. The invention of the printing press has not made books less valuable, or less appreciated, because they are now among the commonest instead of the rarest of objects. Nor has music lost its charms, now that any amount can be obtained at the turn of a switch.

     When material objects are all intrinsically worthless, perhaps only then will a real sense of values arise. Works of art would be cherished because they were beautiful, not because they were rare. Nothing—no “things”—would be as priceless as craftsmanship, personal skills, professional services. One of the charges often made against our culture is that it is materialistic. How ironic it will be, therefore, if science give us such total and absolute control over the material universe that its products no longer tempt us, because they can be too easily obtained.

     It is certainly fortunate that the replicator, if it can ever be built at all, lies far in the future, at the end of many social revolutions. Confronted by it, our own culture would collapse speedily into sybaritic hedonism, followed immediately by the boredom of absolute satiety. Some cynics may doubt if any society of human beings could adjust itself to unlimited abundance and the "lifting of the curse of Adam—a curse which may be a blessing in disguise.

     Yet in every age, a few men have known such freedom, and not all of them have been corrupted by it. Indeed, I would define a civilized man as one who can be happily occupied for a lifetime even if he has no need to work for a living. This means that the greatest problem of the future is civilizing the human race; but we know that already.

     So we may hope, therefore, that one day our age of roaring factories and bulging warehouses will pass away, as the spinning wheel and the home loom and the butter churn passed before them. And then our descendants, no longer cluttered up with possessions, will remember what many of us have forgotten—that the only things in the world that really matter are such imponderables as beauty and wisdom, laughter and love.

From ALADDIN'S LAMP by Arthur C. Clarke (1960)

      Players may be amused or annoyed by my view of future technology. Radiators doing yeoman duty on a laser thermal engine suddenly transform into mining and smelting equipment. A large rocket with several motors reassembles itself into several missiles with nuclear warheads. A rocket gets a new weapon even though that technology was not available when it left orbit.

     Versatility is a key factor in space, as elsewhere. Energy is abundant in space, but materials are scarce. Thus, rockets that are able to rearrange themselves for every occasion. The game provides the basic raw materials: metals (represented in the game by space wood (foamed titanium)), hydrogen, water, and carbon. Materials have to be recycled, only being used as propellent as efficiently as possible. Hydrogen and carbon are the basis of hydrocarbons, the backbone of the versatile organic molecules. Hydrogen and oxygen can power fuel cells, forming water. Hydrogen is also the universal propellent.

     For instance, imagine a peaceful freighter that realizes that an attack is imminent (in space this means the ship has only a couple of weeks to prepare). Bioengineered spiders spin a framework of advanced organic material, while the onboard vapbot (vapor depositing robot) sprays on a layer of space wood. Within days the fighting mirror is finished, and the freighter is now a fighter.

From ROCKET FLIGHT rulebook by Phil Eklund (1999)

"I am a physicist. I specialize in the field of molecular structures. As you know, practically all of the raw materials used in industry today are synthesized from artificially transmuted elements—using techniques originally perfected on Mars." The Assassin nodded, keeping his eyes fixed on the professor. Brozlan went on: "The synthetic compounds used today are amorphous in nature—they do not possess any highly organized internal structure. Essentially the processes just turn out vast quantities of some particular kind of molecule, without assembling the molecules together into any higher level of organization." He took a long breath and then said: "An area of research that I was involved in some time ago had to do with taking the idea one step further."

"To produce a full range of materials needed on Mars, it was not sufficient to just synthesize or import unstructured molecules in bulk," Brozlan resumed. "We needed to be able to duplicate, say, the crystal lattice structures of many metal-base compounds, or the polymer chains of organic substances—things that are abundant on Earth but totally lacking back there."

"I'd have thought that that's where you'd use traditional processing methods," the Assassin muttered. He didn't mind talking as long as it was he who was asking the questions. It could only be to his ultimate advantage to know more about what was going on.

"On Earth, yes," Brozlan replied. "Primary raw materials are cheaper than they've ever been, because they're now synthetic. From those primary materials, things like steel, rolled alloys, fabricated goods, and so on must still be produced in much the same way as they've always been. Hence the costs are much the same as they've always been, and by the time they get to Mars that means expensive."

"If you ship it all up from Earth," the Assassin agreed. "But why bother? Why not just set your own processing plants up right there?"

"We could have done that." Brozlan nodded. His face creased into a frown. "But somehow we were not satisfied with that idea. We had a virgin planet with no set ways or traditions to uphold. It seemed unsatisfactory simply to follow slavishly the methods that had evolved on Earth. We could have spent fortunes copying all of Earth's industrial complexes on Mars only to find them obsolete before they went into production. You see, we were convinced that there had to be a better way."

The Assassin thought for a moment and looked puzzled. "How?" he asked at last. Brozlan's eyes glinted. He replied:

"Consider any form of component that is used in the construction of a larger assembly . . . the parts of a machine, for example. How is the component made? Answer—we take a lump of whatever material we need and cut away from it all the excess to leave the shape that we require. That forms the basis of just about every machining process that is used traditionally. Cut away what you don't want to leave behind what you do want."

"Okay." The Assassin shrugged. "What other way is there?"

"Deposition!" Brozlan peered at him intently as if expecting some violent reaction. The Assassin looked back at him blankly. Brozlan explained: "Instead of cutting material away to leave the part, we deposited material to build the part up!"

"You mean like electrolytic forming? That's not new."

"The idea isn't," Brozlan agreed. "But the way we were doing it was. You see, electrolytic forming works only with certain metals. We were working with every kind of molecule."

"You mean you could build up something out of anything—any substance at all?" The Assassin looked astounded.

"Exactly! And it didn't have to be all from the same kind of molecule, either. We could mix them together any way we chose.' For instance, we could produce a solid block that was phophor-bronze at one end and polythene at the other, with a smooth transition from one to the other in between. It opened up a whole new dimension in engineering design possibilities. The whole process was computer-controlled. A designer could develop a program to create any part he wanted out of any material he chose or any combination of materials—molecule by molecule if he really wanted to go down to that level of detail and if he had the patience and the processor power to handle it."

"Molecule by molecule .. ." The Assassin's face registered undisguised disbelief. "That's incredible . . ."

"Nevertheless, it worked," Brozlan told him. "There have been experimental plants on Mars operating for years now, turning out goods that are higher in quality and far cheaper to produce than anything' that could ever come out of the factories of Earth—even things normally processed from organically derived substances, such as paper, oils, fats, sugars . . . you name it."

"Oil . . . food . . . paper ... all synthesized from transmuted elements?" The Assassin gaped as his mind struggled to take it all in. "Why have we never heard of such things?"

"Politics." Brozlan sighed. "By that time there was a different brand of thinking among the higher echelons of the Federation government. Ambitious and unscrupulous men were taking over. They did not see these discoveries as potential benefits.for all mankind, but only as a means toward furthering their own designs by securing full economic autonomy. They began to see themselves as undisputed rulers over a thriving and self-sufficient world. Those purposes would be served better if Earth were allowed to lag behind, with its industries unable to compete against the newer Martian ones. The Federation authorities assumed tight control over our work and placed a strict security blanket over everything. That was why few people knew about what we were doing. That was also where the movement for Martian independence had its origins. Only a handful of individuals stand to gain, and not in the ways that are popularly believed."

"Interesting, isn't it, Hadley?" the colonel came in, spinning suddenly on his heel to face the bed. "But if you think that's hard to swallow, wait until you hear the next bit." He nodded at Brozlan, who continued:

"That was just one aspect of the research work going on at that tune. Another aspect was Dr. Franz Scheeman's work on structural scanning with neutrino beams. You see, Scheeman developed a method for scanning a material object, inside and out, and for extracting from the transmitted beams a complete encoding of the arrangement of atoms and molecules within the object. It was analogous to the way in which an old TV camera encoded the information contained in a visual scene." Brozlan took a deep breath. "The real breakthrough came when we combined Scheeman's technique with the molecular-deposition process that we have just been talking about!"

Silence reigned for a long tune while the Assassin digested the professor's words. Then his eyes widened slowly and transfixed Brozlan with a dumbfounded, unblinking stare.

"You're joking . . ." the Assassin breathed at last.

"A solid-object camera!" the colonel confirmed for him. "Yes, Hadley, you've got it. They could scan an object and derive a complete structural code for it. From that code they could generate a computer program to control the deposition process. Result—a perfect analog, a molecule-by-molecule copy of the original. And, of course, if they could make one they could just as easily make as many as they liked. Think of it, Hadley . . ."

The Assassin thought about it. Raw materials in abundance at negligible cost and the ability to transform them into any object for which an original already existed—it would be the Golden Age come true. Something in his expression must have betrayed what was going through his mind.

The colonel nodded and continued. "But think of some of the deeper implications, too. What would happen if somebody suddenly introduced that kind of technology into a complex and established economy like Earth's? Suppose that once you'd built the prototype of, say, a domestic infonet terminal"—he pointed to the bedside console—"you could churn out a million of them, all for peanuts. What would happen to the conventional electronics industry then? What about the components industry that supplies it? What would happen to the industries that supply all the parts—the plugs, sockets, metalwork, moldings, and all that kind of thing? And then what about the service industries that depend on all those in turn ... office equipment, furnishings, data processing, real estate, and so on through the list? How could they survive if half their customers and half their business went to the wall?" The colonel spread his arms wide in the air. "All finished, Hadley. Total collapse. How could you cope with ninety-five percent of a planet's population being suddenly redundant? How could a global economy, with its roots buried in centuries of steady evolution, survive an upheaval like that?"

"You see," Brozlan added, "That is exactly what the Federation government wanted to do. They wanted to rush full-speed into setting up a huge Martian industrial conglomerate based on the new technology, flooding Earth's markets with goods at giveaway prices."

"Earth would have been ruined," Barling interjected. "Or at best would have faced the prospect of existing as a very second-rate entity, dependent on a new rising star."

From ASSASSIN by James Hogan (1978 )

(ed note: the intruders have entered the solar system and are systematically committing genocide on the species known as Man. For asteroid colonies, an alien laser broadcast subtly reprogrammes the 3D printer to create a deadly infective substance that destroys technology. This would not kill on the habitable planet Terra, but sabotaging the technology of an asteroid colony will destroy all life support)

SOMETHING LIKE A LIGHTHOUSE BEACON WAS SWEEPING the solar system. On Ceres, the first cluster of gamma-ray telescopes picked it up and passed the signal to the station's computer net with a red tag for instant analysis. Within a microsecond, one of the most highly advanced brains ever built by the hand of man had undergone a subtle change in programming and, against all previous instructions, sent an ominously less subtle program change on a hairline beam of laser light to one of the gamma-ray telescope robots.

And still within that part of a second, the robot began to respond. Its multiple brains had been designed to predict the consequences of any action— from lifting a rock to constructing a new furnace— and then to decide the appropriate action at lightning speed by committee voting. Even without a human presence, the machine was capable of manufacturing a gamma-ray telescope, or anything else it was instructed to supply.

Antlike in appearance, it was more like a colony of machines than an entity of its own—just as its creator had intended. It had been built from and by a hive of smaller, simpler machines, which still circulated inside it like motile cells through a dense matrix of connective pathways. There were workers of many shapes and sizes, and what could even be called "drones" and "egg-laying queens." Each was so intensely social, and so intimately connected to the robot's circuits, that if it were isolated from the rest of the hive, it could no more function on its own than an ant cast out of its hill or a human marooned on an island, and it quickly "died." Like an anthill, or the Ceran colony itself, every robot hive met all the essential criteria of an organism.

One half second after the laser flashed into its eyes, the hive sensed a loss of some essential material it had been programmed to produce—a loss that, left unaccounted for, would have violated its first directive and allowed the human colony inside Ceres to come to harm.

The material in question had never existed before on Ceres, or anywhere else in the solar system; but the hive did not know that it was being deceived, that it was not acting on orders given it by human beings. Egoless, incapable of friendships or emotional ties, free of conscious sympathies and antipathies, without overt motives or concealed ones, the hive's brains and workers responded so fast that it was already fashioning the right tools and had already located the right chemicals before Ed Bishop could even begin to notice that a problem was developing.

By the time the maglev had shuttled him from Isak's home to the north pole, a pressurized tent had already sealed off his stricken, half-eaten robot from the rest of Ceres, a phone line had already connected the tent to the colony, and a system of laboratory modules was already being trucked to the site.

The tent itself enclosed a fully equipped "clean room." Bishop had entered through an airtight; glove box that fit him like a space suit, and in the ; center of the room one of the robots was melting in the warming air. It occurred to him then—too late—that no one should have been allowed to pressurize the chamber or heat the air. A second robot had stood on the far side of the room, ready to assist him. Now both machines lay in melting heaps, and Bishop found himself alone and afraid, abandoned by his crew, as he retreated to a module whose hull was twisting out of shape and leaking air. He tightened his suit helmet, hoping to buy himself a few more minutes of life, and began his final report to Isak.

There was very little he could say that was new to the president. One of the machines had been tricked into producing a substance that, for lack of a better name, Bishop had come to call a "molecular virus" or "template"—with which it had promptly infected itself.

Isak had replayed the images over and over on his pad: Bishop's robot on the floor of the tent, being eaten as if by a mighty cancer, breaking open like a poorly constructed beehive, thousands of microrobots spilling out of the rent across its abdomen, writhing and melting; Bishop's robot assistant placing samples under the scopes; and then, after an hour or two, the scopes themselves seizing up and flaking apart. .. and then the robot assistant.

The scopes had lasted long enough to show, with brutally realistic computer animation, what was happening. Most chemical reactions released their excess energy as heat: random, chaotic molecular motion. Every schoolgirl knew that every chemical compound had a specific melting point, at which the motion became so distorted that the crystalline structure broke down and individual molecules drifted off in different directions. The template molecule, when it came into contact with a ceramic composite, a carbon-metallic alloy, or with almost any substance likely to be manufactured by civilized beings, rearranged its crystal structure in such a way that the old melting points were drastically altered. Superconductors failed to function. Mechanical parts either became brittle or liquefied at room temperature. On the face of each crystal in the "tissues" of the stricken robot molecules were rearranging identically and transmitting a pattern of molecular recoil to every neighboring crystal— which vibrated and softened in turn.

"To get the ball rolling, the robot need only have innocently manufactured a few milligrams of this stuff," Bishop said. He was breathing hard inside his helmet, speaking quickly now, and Isak detected a touch of panic in his voice. "When all this got started, that's all there was—less than a gram. Then we had the whole robot and the scopes and then another robot crumbling to pieces—tons of the stuff lying around. How do you contain something like that? Sooner or later it was bound to get out. Sooner or later it should even get to the Intruders."

"But that seems an unlikely hope," Sargenti added. "They would have built-in protection, some sort of half-life. Otherwise they'd have to worry about meteorite impacts flicking bits of contaminated dust off Ceres and one of their ships coming in contact with it."

"If this had happened on Earth," Bishop said, "they could have counted on the winds to spread template dust to every continent. All the towers and skylines would have vanished. Obviously that wasn't enough. They wouldn't have been happy just knocking Earth back to the Stone Age and leaving the Acropolis, Renaissance cathedrals, Mayan temples, and the forests still standing. But here on Ceres, where we depend on high-tech for everything—even the air we breathe—the template is the perfect Final Solution." He stopped and took a deep breath. "One other thing. I'm afraid it's gotten through the material in my suit and into me. From the feel of it, I'd say even the iron and calcium in our bodies is vulnerable."

There was a popping sound behind Bishop, and the room fogged as the air pressure dropped.

From THE KILLING STAR by Charles Pellegrino and George Zebrowski

Santa Claus Machine

The Santa Claus Machine is sort of a technological version of Aladdin's Lamp or a Cornucopia. Basically it is a Star Trek Replicator that uses in-situ resources as feedstocks. More crudely it is a mass spectrometer feeding a 3D printer.

If you are trying to set up a base or colony on a desolate moon or planet, a Santa Claus Machine could be the difference between success and failure. The less equipment and prefab base you have to bring and the more stuff you can manufacture with local resources, the better.

As with any such thing, it has two parts: a disassembler and an assembler. This is because there are two basic operations possible in the universe, analysis and synthesis. That is, breaking one large object into smaller parts, or assembly smaller parts into one larger object. The ancients called this "solve et coagula" (e.g., written on the arms of the Sabbatic Goat in the famous illustration by Eliphas Levi).


The disassembler breaks down the input material into atoms, then sorts the atoms by element and isotope. This provides the raw materials needed by the assembler.

In most Santa Machine designs, the disassembler is a fusion torch attached to a mass spectrometer (in this context the fusion torch has nothing to do with the similarly named "torchship").

You shovel rocks, dirt, and other regolith into the hopper of the fusion torch. The input matter is flash heated to a temperature of about 15,000 K by the awesome power of thermonuclear fusion, disassembling all the compounds into individual atoms and ionized atoms at that. You now have all the atoms separated in a plume of ultra-high temperature plasma.

There are many proposed ways of sorting the atoms into bins for each individual element and isotope. The most commonly mention method is using a mass spectrometer.

Atoms have inertia, like anything else that is matter. And like all other matter the more mass an atom has, the more inertia is has. So if the atoms are moving in one direction in a atomic beam, if you give each atom a shove to the right with a given strength push the atoms with less inertia will be nudged off course more than the atoms with more inertia. Without the push all the atoms in the beam will strike the target point. The shove with smear the target point to the right. If you nudge enough, the target will smear into a row of points, one for each element. Nudge it more and the points will separate further into points for each isotope of each element.

All you have to do is put a collection bin at each target point and they will fill up with pure isotopes. But do be careful about the bins for fissionable isotopes. Allowing a critical mass to accumulate will have unfortunate consequences.

Mass spectrometers generally use a magnetic or electrostatic field to give atomic beam a shove.

Keep in mind that what you get out depends upon what you load into the input hopper. If the asteroidal regolith you are shoveling in contains no uranium, none is going to show up in the collection bins. You might have to import isotopes that are absent in your location.

Just imagine how useful the fusion torch+mass spectrometer combo would be for recycling the mountains of trash filling up our real life land-fills. The entire blasted world is impatiently waiting for somebody to tame fusion power.

Also note that this technology makes it easy to refine uranium ore into weapons grade uranium, which will make the astromilitary and the authorities extremely nervous. Current enrichment techniques such as gas centrifuges require the resources of an entire nation the size of Iran. A fusion torch could do in your garage.


(ed note: in this space opera Aarn needs some super-duper chemical elements to create his new super-science weapons. He figures the giant star Torka has all sort of chemical elements beyond atomic number 100 which have not been discovered yet but presumably exists. He uses handwaving tractor beams to grab a blob of star-stuff and uses handwaving force fields to create a crude mass spectrometer.)

      Three and a half hours later Aarn stopped the Sunbeam. The gravito-magnetic sheath had been relaxed, but (the giant star) Torka was still a great disc, and their strange cargo glowed angrily orange, at a temperature of thousands of degrees. Aarn got to work immediately, setting up new fields he had carefully plotted.
     "Well—here goes, and Carlisle, cast an eye over this one. It's got your scheme beaten, I suspect. I have about five hundred million tons of matter there—and every ounce of it ionized. Now watch—"
     Suddenly a vast gout of flame spurted out of the compressed, tightly bound mass of incandescent matter. For hours Aarn had been holding it in check only by his tremendous forces. Now it shot out through a gap he had made, thrusting out under the incalculable power of released pressure, pressure generated in Torka where trillions of tons of matter had weighed down on it, pressed by the terrific gravitational force of the sun. (Aarn is holding the high-pressure ionized star-stuff inside a handwaving force field. By opening a small hole in the field, the hot gas spurts out)
     The stream bent abruptly, fanning out strangely, and part seemed to wrap about itself, forming a new center, smaller and colder. Most of it curved, some circling half way round, but it escaped, fanned, and spread in space, beginning a long, long fall back to Torka. "It works," Aarn exulted. "It works, Carlisle! I'm getting it—an atomic spectro-separator on a gigantic scale, and I'm collecting no atoms lighter than atomic number one hundred!" (like a mass spectrometer Aarn bends the stream with a magnetic field, to sort the atoms by mass. Atoms with an atomic mass of 99 are allowed to escape, the higher mass atoms are captured in a second force field.)
     Carlisle started, and stared at the swiftly growing dark center. "One hundred! There isn't any known, let alone heavier." (Fermium was discovered after this novel was written, discovered in the debris of the first hydrogen bomb explosion in 1952)
     "Not on a planet—but inside a nuclear reactor that big and that violent, a lot of improbable but possible events have a chance to occur. These super-atoms are synthesized. And I pulled that out from about one thousand miles down—just as deep as I could reach before the forces simply tore my beam to fragments. But now that stuff is cooling by expansion—"
     The expansion as the ions escaped was cooling it rapidly, and Aarn drove a heating transpon beam into it. Resting almost motionless, his own ship was using little power, and the excess he turned into the seething mass swiftly drove the temperature up, and increased the ionization.
     It took over four hours to finish the operation, but at last a great cold ball of matter rested in the beams, while a vast cascade of flaming atoms was falling, falling, falling the fifty millions of miles back to Torka. (the cold ball has all the fascinating new undiscovered elements, the cascade of falling atoms are all the old boring known elements)
     "I wish I could work on that stuff right now," exclaimed Aarn as, with full coils, he drew the mass of the gleaming metallic sphere inside the walls of his forces, and set himself for the homeward trip. Torka grew hazy, space changed, and even the stars moved slowly, while three flashing, curved lines represented the three planets as the Sunbeam shot toward swiftly expanding (planet) Cornal.
     In an hour the Sunbeam was back in her berth. In half an hour more, workmen under Aarn's directions had cut off a fifty pound mass of the cooled stuff, and put it in a great lead case. The rest, Aarn took out to a deserted island, far from any city. The rays from this mass of super-heavy elements were potent, and deadly. In that mass, carefully separated as it had been, perhaps a tenth of one percent of lighter, known elements had been included. Radium was there, more than the Tornans had ever had before.
     "I'm going to get to work," said Aarn decisively, as they landed again. "I want to see what I've got. Carlisle, you can have about five pounds of that stuff if you want it—you might determine all the chemical elements present, and all the properties thereof. You'll probably find some of them are still missing—gaseous substances that escaped—but you'll get them as other, heavier ones break down to form them. And—here's something to watch out for: there will be numerous elements of the same chemical structure but having different atomic weights. Pick 'em out carefully, will you? I'll have some of these physicists here rig up a very high-power spectro-separator for you that ought to catch an atomic weight difference of one in any element less than 350."

From THE INTERSTELLAR SEARCH by John W. Campbell, Jr. (1949)


The assembler takes atoms from the disassember's output, and puts them together according to the user selected blueprint.

This will basically be advanced versions of the 3D printers and rapid prototyping machines available now. Instead of just handling one material (typically plastic) they will be capable of printing in multiple materials. They will accept as feedstock the elements and isotopes from the disassembler, and either chemically create the required compounds or just print the compounds by alternating the atoms.

Early crude versions will print blobs of paste composed of compounds created from the atom feedstocks, much as a commercial 3D printer makes objects out of molten plastic. Later advanced versions will assemble the object atom by atom.

The limits will be

  1. the chemical elements required from the disassembler for object currently being printed (does the local regolith have all that is necessary?)
  2. the availability of blueprint files for the desired object (are the blueprints illegal?)
  3. the speed of printing the object (if it takes ten years to print, forget it)
  4. the supply of fusion fuel to power the fusion torch (though with fusion you are talking gigawatts per centigrams/seconds of fuel)

Faster printers will be more expensive, because that's the way it always is.

Some blueprints will be illegal (e.g., DIY nuclear warhead) and of course will be readily available anyway from data smugglers and on the dark web. There might be illegal blueprints which on the surface look innocent, but combining part 23 of the dust precipitator plan with part 17 of the air conditioner plan creates a working submachine gun.


The human consequences of the singularity reverberated endlessly, too. The exiles hadn’t simply been dumped on any available world; in almost all cases, they’d been planted in terrain that was not too hostile, showing crude signs of recent terraforming.

And the Eschaton had given them gifts: cornucopias, robot factories able to produce any designated goods to order, given enough time, energy, and raw materials. Stocked with a library of standard designs, a cornucopia was a general-purpose tool for planetary colonization.

Used wisely, they enabled many of the scattered worlds to achieve a highly automated postindustrial economy within years. Used unwisely, they enabled others to destroy themselves. A civilization that used its cornucopia to produce nuclear missiles instead of nuclear reactors—and more cornucopias—wasn’t likely to outlast the first famine, let alone the collapse of civilization that was bound to follow when one faction or another saw the cornucopia as a source of military power and targeted it. But the end result was that, a couple of hundred years after the event, most worlds that had not retreated to barbarism had achieved their own spacegoing capabilities.

Newpeace had been settled by (or, it was more accurate to say, the Eschaton had dumped on the planet) four different groups in dispersed areas—confused Brazilian urbanites from Rio; ferocious, insular, and ill-educated hill villagers from Borneo; yet more confused middle-class urban stay-at-homes from Hamburg, Germany; and the contents of a sleepy little seaside town in California.

Each colony had been plonked down in a different corner of the planet’s one major continent—a long, narrow, skinny thing the shape of Cuba but nearly six thousand kilometers long—along with a bunch of self-replicating robot colony factories, manuals and design libraries sufficient to build and maintain a roughly late-twentieth-century tech level McCivilization, and at ten-meter-tall diamond slab with the Three Commandments of the Eschaton engraved on it in ruby letters that caught the light of the rising sun.

From IRON SUNRISE by Charles Stross (2004)

Needless to say the invention of a Santa Claus Machine will have a drastic effect on the economy of your civilization.

And other things too. I've already mentioned how the powers-that-be will be concerned with giving rock-rats the ability to manufacture weapons of mass destruction and refine kilogram lots of weapons grade fissionables. And I'm sure the futuristic equivalent of the MPAA and RIAA will be furious with Joe Asteroid wallpapering their habitat dome with atom-level perfect copies of the Mona Lisa. Not to mention how angry the banks will be with a device that can crank out undetectable counterfeits of coins, bills, cheques, and other legal documents.

Of course things get astronomically worse if a Santa Claus Machine can produce copies of itself. Now you've got a freaking Von Neumann self-replicating machine on your hands.

I have a feeling that Santa Claus Machines will always be under military guard, much like the beam propulsion lasers controlled by the Laser Guard. The Santa Guard will place the machine at the site of a future base/colony, and watch what is manufactured like a hawk. If a colony builder submits a blueprint for something questionable, they are liable to be apprehended by the Santa Guard and questioned.

In the far future Santa Claus Machines might be equipped with a law-abiding artificial intelligence. If the user asks it to make a nuclear warhead, the machine will refuse and call the cops.


Matter Replicator: The cornucopia machine or autofac, which can build matter into pretty much any object that you want and have – or can write – a recipe for.

Sadly, they are required to obey the laws of thermodynamics and conservation of mass-energy. They also tend to incorporate – especially in larger models designed to build larger objects – arrays of specialized nanofactories and macro-scale tools, and require plenty of energy and specialized appropriate feedstocks for whatever it is you want them to build (so mining, refining, recycling, bactries, and the rest of the industrial supply chain haven’t gone away quite yet). You can make them increasingly general-purpose in these areas at the cost of greater inefficiency – field autofacs are a lot less elegant and more energy-hungry and expensive to run than standard household models.

Living things generally have to be grown in a medical vat instead; simply because most of them tend to die when only half-printed. Yes, this is exactly as gross as it sounds. (Also, some dead organic matter – well, let me put it this way. While you can print up a steak in an autofac, steak is still made in carniculture vats, because first, self-replicating steak is cheaper, and second, it gets boring eating the exact same steak hundreds or thousands of times. Similar although aesthetic considerations are why vatwood is generally preferred to directly replicated wood – although vatwood planks are seen as input to larger autofacs.)

Nonetheless, they’re more than good enough for post-material scarcity purposes.


Since we’ve just passed the Matter Replicator trope, and since it may be relevant to an upcoming FAQ question, I thought I’d throw out some definitions relating to such things that may make things clear. Well, clearer.

A nanofac is the basis of nanofacturing technology. Think of it as essentially a 3D printer which can handle arbitrary molecular components with single-atom resolution. (It doesn’t have to: a lot of the time it can simply place pre-assembled multi-atom components picked out of its feed, but the point is that it can.) While it can use free-floating assembler nanites as part of its operation, the vast majority of the work is done in a supercooled vacuum chamber by an array of distant descendants of the atomic force microscope. The materials supply it needs is fed to it as a suspension of molecular components called nanoslurry available in a variety of forms, supplied as a utility from a central nanosource that makes the stuff from raw materials and recycles the return feed of all the stuff that the nanofacs don’t use.

Most important to note is that a nanofac is not a discrete thing you can buy itself – it’s just the term for the central construction array as a module.

What you can buy, on the other hand, is a cornucopia, which is a general-purpose construction device that comes in sizes ranging from desktop-printer-sized (the ubiquitous nanoforge) to dishwasher/fridge size. These are common household, etc., appliances, packaged as vending machines by companies like Valuematic Vending, and are basically a user interface/power supply/etc. wrapped around an appropriate nanofac. They can make pretty much anything you can describe in a recipe, or conceptual seed, to give it its formal name, although if it’s something too big to fit into its vacuum chamber what you’ll get is a heap of parts over several runs which you have to assemble manually following the v-tags after you get them out. (They may or may not bond permanently once you do this.)

A specialized cornucopia, on the other hand, is a fabber. These exist because in nanofacturing, there’s essentially a scale with versatility at one end and efficiency at the other. A cornucopia is a magical device that can make everything, but isn’t the fastest or most efficient way to make anything in particular.

So there are fabbers, which trade off that ability for greater speed and efficiency and customized user-friendliness in doing one particular thing. So while you want a cornucopia available to you, certainly, what you want in your wardrobe is a clothing fabber, in your kitchen is a food fabber, in your sickbay is a pharmafabber, in your wet bar is a cocktail fabber, etc., etc.

And finally, it’s worth noting that assembling things atom by atom, or molecule by molecule, is not actually a terribly efficient way to do things in the first place. It works fine for small objects, sure, where the convenience outweighs the inefficiency, and especially for those made out of lots of tiny components with fine detail to assemble. But large things, especially large things with large areas of relatively homogeneous structure, you really don’t want to make that way.

Which is where autofacs come in. An autofac is a automated assembly system that contains an array of nanofacs for making individual detailed components, but which also contains lathes and drills and presses and kilns and extruders and all manner of other macroscale manufacturing-process equipment, along with plenty of motile robots whose job is to do the assembly of all the different outputs of these processes into the end product. (So they take in nanoslurry for the ‘facs, but also metal ingots, ceramic powder, plastic granules, etc., etc., as their raw materials.)

These vary in size from the relatively modest autofacs you’ll find in most neighborhoods, belonging to companies like Ubiquifac, whose job is to construct large goods people have ordered on-line at a point relatively local to them for immediate delivery, up through larger factories – such as the ones that take nanoslurry and sheet metal in at one end and have finished vehicles drive out the other – all the way to truly giant many-square-miles really-can-build-anything complexes like the Hive.


Yes, folks, it’s that time again for the first time when I answer y’all’s background questions!

We have one question this month. James Sterrett asks:

What precursor elements do autofacs require for fabrication? The same elements in the same proportions as the finished product (plus waste etc), or can they synthesize required elements?

Well, now, that’s an interesting question with quite a complicated answer, inasmuch as autofacs are rather complicated things in themselves..

Let me first suggest that this might be a good time to re-read Things That Make Things, since it covers a lot of the terminology I’m about to be throwing about.

So let’s start at the small end, with one of the most common working parts of an autofac, and which is also the core component of a cornucopia, including the ubiquitous desktop nanoforge, the portable nanolathe, and the specialized fabbers.

These, themselves, can’t synthesize elements, or indeed produce any other part of their feedstock – which is to say, you can’t just throw trash into them and have them rearrange it into what you want (you need specialized disassemblers for that, that are hardened to the job. Throw trash into a cornucopia, you have a good chance of wrecking the delicate internal components). They’re just glorified 3D printers. They’re absolutely dependent on a supply of feedstock, which is called nanoslurry.

(One exception to this is that you can also get what is called a nanobrick, which is basically dehydrated nanoslurry and formed together with a mass of simple assemblers. You use it together with a programming nanolathe for field construction, after mixing it with a suitable solvent, usually water, to form a nanopaste. But that’s not what we’re talking about here.)

Nanoslurry itself is a complex suspension of materials useful in nanoconstruction, designed to make it as easy and efficient as possible for nanofacs to pick out the bits they need. It comes in a variety of different kinds and grades, most of which are intended for one specialized industrial application or another. Standard-grade, which is what is shipped out as a public utility down municipal nanopipe systems, comes in two forms, informally referred to as “gray” and “green”.

The nanopipe you have plugged into your domestic cornucopia, for that matter, is actually a four-pipe system. The first supplies gray nanoslurry – which is water, long-chain alcohols, sulphur and nitrogen compounds, a suspension of iron and copper oxides, heavy metals, silicates, acetats, nanograins of industrial plastics, ceramics, and alloys, and prefabricated molecular components, or to put it another way, everything you might need to perform “common mechanosynthetic applications”. The second supplies green nanoslurry, which is specialized towards organic synthesis applications – what this means, of course, varies from world to world. And the third is the special-order pipe, which gets aliquots of specialized feedstock shot down it upon request, because while you may occasionally need, say, 2.1 g of technetium, it’s something specialized enough that there’s no point in including it in the regular feedstock.

(The fourth is the return pipe, that pumps what’s left after the nanofac has picked out what it needs back to the nanosource for recycling.)

And what the nanofacs need is, well, exactly what elements are in the finished product. (Plus a certain degree of in-process waste that ends up squirted down the outgoing pipe back to the nanosource.)

So, so far, we’ve just pushed the problem back to the nanosource; after all, nanoslurry doesn’t exist in nature, so it has to be manufactured. Which is what nanosources do: out of a variety of sources. Air mining, for worlds with atmospheres that have useful components. The bactries of chemical companies, refining volatile asteroid-liquor into useful chemicals with bacterial aid. Giant metal ingots shipped from smelters, which are reduced to slurry components. Reclaimed and purified chemicals from recycling plants and biocleaning cascades. In short, from the ends of all the conventional supply chains. Larger autofacs, like the Hive, will usually have their own nanosource(s) to produce all the specialized feedstocks that they want, especially since autofacs use a bunch of those raw materials elsewhere in their non-nanotech manufacturing processes.

So now we’ve just pushed the question back another level, haven’t we, to “can the people the nanosources use as suppliers synthesize elements?”

To which the answer is, finally: yes, but they usually don’t.

Nucleosynthesis is possible. There’s an entire engineering discipline, alchemics, that specializes in this sort of thing. But it’s neither cheap nor convenient, inasmuch as it still involves banging nucleons together and trying to get the wee buggers to stick, a process that tends to involve particle accelerators and nucleonic furnaces and isotopic separators and mucking about at absurdly high energy densities and low efficiencies. That said, it is now regular non-experimental engineering, and a large enough autofac might well include the equipment.

…but economically, it is almost always cheaper to dig the stuff up and have it shipped to you for nanosource processing than try to manufacture it on site from other elements. Nature’s production process may be slow and uncomfortably explosive for anyone within a couple of hundred light-years, but, damn, does it have economies of scale.

This effect is only amplified, of course, by the fact that alchemics equipment is also what you use to produce gluonic string, muon metals, and various other kinds of exotic matter that genuinely don’t occur in nature anywhere. Now that’s what you call comparative advantage!

Self-replicating Machine

A self-replicating machine or Von Neumann device is an independent robot that can create a duplicate of itself from materials scavenged locally. The little monsters can multiply exponentially (i.e., like cancer) so it is best you have some kind of control or kill switch on them.

They are used when you have a really big job, so you want the robot work force to scale itself up to a size suitable to the task. For example: covering the entire equator of the planet Mercury with solar power cells in only a few years. Or sending robot space probes to every planet in the entire galaxy.

Of course it can be weaponized as either an anti-ship or planetary attack weapon. They may even evolve into a sort of robotic life form.


A self-replicating machine is a type of autonomous robot that is capable of reproducing itself autonomously using raw materials found in the environment, thus exhibiting self-replication in a way analogous to that found in nature. The concept of self-replicating machines has been advanced and examined by Homer Jacobsen, Edward F. Moore, Freeman Dyson, John von Neumann and in more recent times by K. Eric Drexler in his book on nanotechnology, Engines of Creation and by Robert Freitas and Ralph Merkle in their review Kinematic Self-Replicating Machines which provided the first comprehensive analysis of the entire replicator design space. The future development of such technology is an integral part of several plans involving the mining of moons and asteroid belts for ore and other materials, the creation of lunar factories, and even the construction of solar power satellites in space. The possibly misnamed von Neumann probe is one theoretical example of such a machine. Von Neumann also worked on what he called the universal constructor, a self-replicating machine that would operate in a cellular automata environment.

A self-replicating machine is an artificial self-replicating system that relies on conventional large-scale technology and automation. Certain idiosyncratic terms are occasionally found in the literature. For example, the term "clanking replicator" was once used by Drexler to distinguish macroscale replicating systems from the microscopic nanorobots or "assemblers" that nanotechnology may make possible, but the term is informal and is rarely used by others in popular or technical discussions. Replicators have also been called "von Neumann machines" after John von Neumann, who first rigorously studied the idea. However, the term "von Neumann machine" is less specific and also refers to a completely unrelated computer architecture that von Neumann proposed and so its use is discouraged where accuracy is important. Von Neumann himself used the term universal constructor to describe such self-replicating machines.

From the Wikipedia entry for SELF-REPLICATING MACHINE

(ed note: "rollers" are Lunar cargo hauling vehicles, with built-in pressurized habitat modules)

     “That reminds me of something I’ve been meaning to ask," Ben said, a little embarrassed and eager to change the subject. “Why are rollers so cheap? You’ve kept telling me that machines are expensive on the Moon. We bought a used one, but even the new rollers I priced were cheap. I'm not arguing with the price—but there’s a contradiction there.”
     Garrison thought for a moment. “Okay, lemme see if I can explain it. Suppose you wanted to sell eggs on the Moon. What would you import, eggs or chickens?”
     “Chickens, of course,” Ben said. “Or probably fertilized eggs, let ’em hatch here.”
     “Minor detail. A fertilized egg is just a chicken in compact packaging. But the point is the same. Since eggs are something everyone can use, it’s worth shipping in a whole egg factory—that is, the chicken—rather than importing the finished product, the egg. You sell a million eggs cheap and make your money on volume. Now, suppose you wanted to important caviar. What do you ship in—the fish eggs or the sturgeon?”
     “The caviar, obviously.”
     “Right. Not only is the fish harder to take care of than a chicken, but a lot fewer people are going to want the product. So if you’re going to make a living selling caviar on the Moon, you’re not going to have much volume. You send the price through the roof to compensate.”

     Garrison took a sip of his beer and went on. “What’s expensive on the Moon are the caviar machines, things you only need a few of if you need them at all. Specialized high-tech stuff. Like the automated ID booth you were expecting. Or maintenance robots. Or luxury items, like those automatic hairdressers that were such a big fad when we were in New York. They’d cost too much, so the Conners get along without them and tell themselves they're better olf without them. If we ever get back to Central, ask Mrs. Lombroska her opinion of automated hotel systems. That’s the caviar stulf.
     “What we've been riding in is a chicken’s egg. There are thousands of rollers around, because some UNLAC lab on Earth designed a whole roller factory. They built it, shipped it here, switched it on, and stepped back. It s been cranking out rollers ever since. They’ve tweaked up the design a few times, and they put out a few different models for dilferent jobs. Cargo versions, oversize jobs like that big guy out front. Besides dialing in what model to make, they just leave the factory alone. Which makes rollers cheap. Basic supply and demand.

     “The kicker is that the factory is totally automated, right down to raw materials. I got a tour of it once. They have robot bulldozers that feed dirt into the hopper of a soil-cracker, and dedicated long-distance robot haulers that supply ore for lunar-rare elements like nitrogen from nearby mines. They fabricate the parts right from raw materials. Robot labor assembles it. They call it a half-von Neumann machine.”
     “Meaning what?"
     “A von Neumann is any machine capable of replicating itself. Then the relicas replicate themselves, and soon you’re up to your keister in von Neumanns. There have been some gimmicky lab gadgets that could copy themselves but no one has ever built a worthwhile true von Neumann. A half-von Neumann can endlessly duplicate a machine simpler than itself. ”
     “They can’t do that with an entire roller, Ben protested. “It’s too complex.”
     “You open up that control panel when we re back aboard and see how many parts there are in it. They ve got the things boiled down to absolute simplicity."

     Ben frowned for a second. “Wait a minute. Solar powered factory, right?”
     “So they've got free power. And they’re getting the raw materials essentially for free, digging the dirt out of the ground and refining it; Robots mine the raw materials and build the rollers, so you don’t have any labor costs. Power, material and labor free. Aside from the cost of building the factory, it doesn’t cost them anything to make the rollers! So what do they base the cost on?”
     “ ‘The value of a thing is that which it will bring,’ ” Garrison quoted. “They sell for whatever the market will bear. At the moment, the market is flooded, so rollers are cheap. Another good run of immigration, and thesupply will dry up. Or else they can just turn off the factory for a while until it’s needed again, and drive the price up a bit. But you've spotted a real problem there. How do you run an economy when things of value cost nothing to produce? The roller plant is an example, not an exception. Lots of common items on the Moon are produced that way.”
     “So how do they run the lunar economy?”
     Garrison shrugged. “UNLAC? I think they just ignore it and hope it will go away. I mean, Christ, you’ve got a whole planet here that doesn’t even have its own currency. Instead we use everyone else’s money—and half of that is in confetti-denominations. From an economic standpoint, the whole Moon is sheer chaos, but it works somehow. I think the policy people are afraid that if they try to fix it, people will notice it can’t possibly work, even though it's been working for years. Confidence would collapse, and the whole place would go to hell in a hand basket.”

(ed note: Garrison is asking about why deep-drilling gear is being hauled to the Lunar farside)

     “Why do they need deep-drill gear for way the hell out there?’ Garrison asked.
     “Laser array. ”
     “The comm laser array they’re building out there.”
     “Right, we’d heard about that," Garrison replied. “But what do comm lasers have to do with deep-drill mining?”
     “They're shipping a whole laser factory out there, and they need a bunch of materials that aren’t available on the surface. It’s the old factory that built the Nearside array."
     “A whole factory just to build a few comm lasers?” Ben objected.
     Garrison shrugged. “You have to put the factory somewhere. Cheaper to ship it than haul the final products halfway around the Moon, even if you only need a few lasers.

     “They need more than a few,” Mohammed said. “With the space traflic density they’re projecting, they'll need to track maybe two, three dozen comm-targets at once—most of the interplanetary shipping, plus the Settlements on Mars, the Belt, and the inner and outer planets. Plus you need back-ups for all of those, and you have to have lasers at several frequencies for various dull reasons. Right off the bat, they’ll need about a hundred frequency-tunable comm laser units. More with eventual expansion, and for replacement of broken units. Makes sense to build them on the spot. Besides which, the laser units aren't small. Each comm laser unit will be self-contained. Each one with its own solar cells and storage coils and pointing mechanism and so on.

     “If they had built ’em at Central Colony, and shipped ’em from there, maybe I’d have been able to fix two of them on my roller at a time. Shipping two per trip, and one trip a month, it would take over four years to get ’em all out there. That would cost UNLAC plenty, more so if they hired several oversize rollers to get it done faster. There aren’t that many outsizers around, and they cost plenty to hire. It’s cheaper, faster, and easier to ship a robot factory out there. In fact, most of the factory is already out at Farside Station. The deep-drill mining gear is the last component to go in.”

From FARSIDE CANNON by Roger MacBride Allen (1988)

(ed note: A self-replicating lunar factory (SRLF) would dramatically accelerate the rate of space industrialization. The problem is establishing the first one. A "seed" hardware has to be developed, then delivered to the lunar surface.

A 1982 study figured you'd have to deliver about 100 metric tons of seed hardware, basically the mass of an entire SRLF. This is far too much mass to deliver, short of an Orion Drive spacecraft. This can be reduces a bit by using 3D printing, but not enough.

A more drastic measure to reduce the seed mass is to avoid full "closure." A SRLF with full closure can totally replicate itself using local materials. A SRLF lacking full closure would require some of the components for the new machine to be shipped from Terra. The main missing item is the local manufacture of computer chips and electronics. A chip fabrication unit requires many tons of seed mass. The advantage of shipping computer chips from Terra is drastically lowered seed mass and faster SRLF replication time (since it doesn't have to make a new chip fab). The disadvanage is lack of full closure does not scale. As more SRLFs are produced, Terra will have to boost more and more computer chip mass.

The paper proposes a new approach: "bootstrapping."

The seed mass for the initial SRLF is low (about 12 metric tons) because the seed will only be able to create equipment that is much more primitive and crude than the seed, say 1700s-era technology. The 1700s tech will construct new equipment that was 1800s-era tech. The 1800s tech will construct 1900s-era tech and so on. It will bootstrap itself until it reaches the point of being an actual full closure SRLF. It will then start cranking out daughter factories and space industrialization goes into high gear. According to their mathematical model, the fully developed 7th generation SRLF will have a total mass of about 100 metric tons, which is in close agreement with the 1982 study. But this is from only 12 metric tons shipped from Terra, instead of 100 MT. Abet with several years of tech advancement.)

Once successfully bootstrapped, a robotic network can access, process, transport, and utilize the solar system’s resources for mankind’s benefit. Appropriately designed robots will not have the problems traveling the vast distances of the solar system that humans have, and they can set up the infrastructure that will enable us to follow. Within the first several decades a vital industry could be established on the Moon and in the asteroid belt using technologies that are for the most part only modestly advanced beyond today’s state-of-the-art. After that, human outposts, laboratories, and observatories can spring up everywhere between the Kuiper belt and Mercury. It can grow exponentially and provide mankind the ability to do things that today are only dreams...

...There are several additional strategies to reduce the launch mass of a seed replicator. The first is to identify and use only the simplest system capable of replication. The second is to avoid full “closure”. Closure is the ability to replicate all aspects of the system in space so that nothing further is required from Earth to build replicas. Nearly full closure is vastly easier to achieve than full closure (Freitas and Gilbreath 1980), because the manufacture of electronics and computer chips requires heavy, high-tech equipment that would be expensive to launch from Earth and would command much of the industry’s resources during replication. However, incomplete closure results in very high launch masses later as the industry grows exponentially, as we show below. A third strategy, which to our knowledge has not been discussed in the literature, is to begin with a simpler, sub-replicating system and evolve it toward the self-replication capability. In this strategy, the evolving system might never become a “self-replicator” even after it reaches full closure, because each generation can continue creating something significantly more advanced than itself. This is the strategy adopted here.

The first hardware sent to the Moon will be high-tech equipment built on Earth. However, the high launch costs demand that it be mass-limited so it will have insufficient manufacturing capability to replicate itself. It will construct a set of crude hardware made out of poor materials, so the second generation is actually more primitive and inefficient than the first. The goal from that point is to initiate a spiral of technological advancement until the Moon achieves its own mature capabilities like Earth’s. This evolving approach will provide several benefits. First, industry on the Moon can develop differently than on Earth. The environment, the manufacturing materials, the operators (robots versus humans), and the products and target markets are all different. Allowing it some reasonable time to develop will allow it to evolve an appropriate set of technologies and methods that naturally fit these differences. Second, the evolving approach supports the development of automation so that industry can then spread far beyond the Moon. The technological spiral will develop the robotic “workers” in parallel with the factories. It will also improve automated manufacturing techniques such as 3D printing. The third and probably most important benefit is the economic one. As we show here, a space economy can grow very rapidly, and it will quickly require massive amounts of electronics and robotics unless there is full closure. The tiny computer chips alone become too expensive to launch within a few decades as the industry grows exponentially, and therefore we will quickly need lithography machines on the Moon to make the computer chips. The evolving approach sends only a small and primitive set of machines as “colonists”, and the nascent lunar industry develops over time – but still rapidly – toward the full sophistication that Earth cannot afford to launch...

...So the objective is for the first robotic “colonists” on the Moon to fabricate a set of, say, 1700’s-era machines and then to advance them steadily through the equivalent of the 1800’s, 1900’s, and finally back into the 2000’s. We argue that this can be accomplished in just a few decades. There are reasons why this technological spiral will be both easier and faster than when we accomplished it on Earth. First, the majority of the technology does not need to be re-invented. The knowledge will be provided by technologists on Earth. Second, the Earth will provide material support in the early stages. We will send teleoperated robots and complex electronic assemblies prior to achieving closure. On the other hand, there will be new challenges. For example, we must gain experience in the lunar environment to learn how to adapt terrestrial technologies to it...

...For the concept of lunar industry presented here we do not think the term “self-replicator” is appropriate and so we will avoid the term. A self-replicator is by definition self-contained with all of its parts co-located in a complete set. That entire set fabricates a new complete set that is situated in a new location before the next replication cycle begins. This is unlike industry or biology on Earth: neither businesses nor industries are self-replicators. Although biological species are self-replicators, they require a vast number of other species in a highly networked ecology to survive, and the ecology does not operate on a synchronized replication cycle. We think the networked complexity of these examples is the more successful topology for space industry because it is the one that naturally occurs and hence is probably the more efficient and adapted for survival, as well as the more easily bootstrapped through an evolutionary process. We therefore avoid any requirement that the various hardware assets remain together in a closed set, and we allow instead for transportation to develop naturally between multiple, specialized production sites. Thus, lithography machines to make computer chips can be located in just one laboratory on the Moon, and their products can be transported to other sites for incorporation into robots and machines. The original facility to house that equipment can be built larger than necessary to allow for expansion and to gain economies of scale...

The set of assets within each generation is described below. To be conservative, we usually assume that each asset is retired at the end of its generation so that only the more modern assets of the new generation are involved in producing the generation after that (except as noted below for solar cells and robonauts). This is overly conservative, but it allows that hardware failures could disable some new assets that are unable to be repaired while assets from the prior generation continue to operate to take their place.

In Generation (“Gen”) 2.5, the use of the decimal place (rather than incrementing to 3.0) indicates that the assets of Gen 2.0 and Gen 2.5 are added cumulatively rather than retiring the Gen 2.0 hardware. We do this because it is necessary to vastly diversify materials manufacturing as quickly as possible, and this is accomplished by creating Gen 2.5 hardware that is no more sophisticated than Gen 2.0 although capable of making different materials.

The technologies needed for mining, chemical processing, and metallurgy are for the most part already existent in Earth’s industry. The feasibility of adapting them to the lunar environment has been and is currently being demonstrated by the wide variety of successful space utilization projects described elsewhere in this issue of the journal.

Excavators. The excavators will travel between the digging site and the resource processing site, delivering sufficient lunar regolith each hour to maintain production rates of the other assets. The details of the excavators are unimportant. In our modeling we assumed for specificity that they are small and operate in a swarm. They may also be fitted with paving attachments (Hintze and Quintana 2012).

Chemical plants for volatiles. Dry regolith or an ice/regolith mixture will be deposited into hoppers and then fed into chemical plants. Electrical power for the processes is augmented with thermal power from solar concentrators. One type of chemical plant will be concerned with producing gases and liquids. These fluids will include oxygen, hydrogen, water, hydrocarbons such as methane, and (in more advanced generations) solvents for industrial processes. So far, NASA has developed and field tested only basic oxygen production systems, including hydrogen reduction and carbothermal systems. More complex chemical processes have been developed for terrestrial applications, and by adapting the lessons-learned from the lunar projects it should not be difficult to adapt the other processes to the lunar case, too. For specificity, we have described the chemical plants using particular masses, power levels, and production rates after examining several sources of data. These include analyses of lunar chemical plants (Mendell 1985, Taylor and Carrier 1993) and the actual construction and performance of lunar chemical plants that our team and collaborators have recently field tested on Mauna Kea in 2008 and 2010 (Boucher et al. 2011; Captain et al. 2010; Gustafson et al. 2010a; Gustafson et al. 2010b; Muscatello et al. 2009). The specifics are not too important as we will vary these numbers over wide ranges to demonstrate general feasibility of the bootstrapping process. Gen. 3.0 and subsequent will have larger throughputs than the earlier generations, and they will gain from economies of scale by building much larger chemical plants rather than reproducing a large number of smaller plants (Lieberman 1987; Gallagher et al. 2005). However, to be conservative we ignore the economies of scale and instead describe the chemical plants as though they were units identical to the originals. These represent “units” of chemical processing capability in larger plants rather than standalone assets.

Chemical plants for solids. Chemical plants are also needed to produce plastics and rubbers from the lunar polar ice. This is possible because we now know that the ice contains large quantities of carbon molecules (CO, CO2,…) as well as nitrogen-bearing and hydrogen-bearing molecules (Colaprete et al. 2010; Gladstone et al. 2010). These materials may be needed for gaskets, seals, and insulators, for example. Later diversity will introduce sheet materials, fabrics, and layered and complex materials. Again, economies of scale are ignored in the model to be conservative.

Metal and Ceramics Refinery. It will be crucial to manufacture metals and metal-oxide ceramics and to improve the properties of the various alloys with subsequent generations. Processes to do this from lunar soil have been described (Rao et al. 1979; Jarrett et al. 1980; Sargent and Derby 1982; Lewis et al. 1988; Stefanescu et al. 1988; Landis 2007; Lu and Reddy 2008), and some development work is on-going by our colleagues and collaborators. Notionally, the early generations in our model will produce crude “mongrel alloys” by electrowinning or other methods. Hardware constructed from those alloys will need to be massive to add strength to make up for their poor mechanical properties. (This will be partially offset by the reduced forces in low lunar gravity.) Subsequent generations of metal refineries will add processes and material streams to improve the mechanics of the materials. Oxygen and other gases produced by metal refining will be sent to the chemistry plants. Electrical power is augmented with thermal power collected by solar concentrators.

Manufacturing. Additive manufacturing will have two forms: 3D printers that make parts that are small enough to fit inside the printer, and larger units that move about robotically and add material onto large structures external to themselves. The printers may eventually have multiple material streams including metals and plastics to make complex assemblies in a single pass. However, the earlier generations will require import from Earth of the most complex assemblies, such as electronics packages and the assembly robots. Furthermore, “appropriate technology” will mandate the design of simpler assets that can function without too many complex or miniaturized components, simplifying their manufacture and reducing imports. To achieve the final generations, the additive manufacturing technologies require advancement beyond the current state of the art. However, gains are being made rapidly and it is very likely the advancements will support the bootstrapping strategy presented here.

Solar Cell Manufacturer. Power will be provided mainly by solar cells. Ignatiev et al. (2001) and Freundlich et al. (2005) have shown how these may be manufactured on the Moon even in the earlier, more primitive generations of lunar industry. For specificity, we have described the mass, power, and throughput of the solar cell manufacturers as per those earlier studies. We show that these devices in the first and subsequent generations produce far more available power than needed by the following generation. This excess power capacity grows exponentially. We assume that solar cells are added cumulatively from one generation to the next. Failure of solar cells by radiation damage and micrometeoroids has not been modeled explicitly, but can be deducted from the exponentially growing excess.

Power Station. In the first generation, a power station is included in the mass of hardware shipped to the Moon. This station includes power conditioning, docking stations, and cabling to manage and distribute the solar power. It might also include a small nuclear reactor to support human presence and as a backup system to support re-bootstrapping in case of system failure.

Robonauts. Robotic astronauts, or “robonauts”, will perform the assembly and maintenance tasks. The name is borrowed from a particular robot developed by General Motors and the NASA Johnson Space Center, with the assumption that robonauts in future lunar industry will be the direct descendants of the current ones. The number of robonauts must grow rapidly as the industry itself grows. At first the robonauts are imported from Earth. To keep the strategy slightly more economical, they are not retired with each subsequent generation. Beginning with the third generation their structural components are made on the Moon, while Earth continues to send their cameras, computers, motors, and sensors. Eventually they are made completely on the Moon.

At first the robonauts will be teleoperated from Earth. The approximately 2.5 second round-trip communication time delay can be managed even for fine motor tasks, such as screwing parts together by hand, by having a teleoperator on Earth interact with a virtual world that models the robonaut and its environment rather than interact with the reality itself. The robonauts on the Moon will mimic the behaviors they observe in the virtual reality as closely as possible using existing levels of robotic autonomy. Resynchronization will occur in the virtual world using a rubric designed to prevent operator confusion. Similar schemes are being developed for telesurgery with large communication latency (Haidegger and Benyó 2003). This will make teleoperation manageable for lunar operations, but it will require a growing and expensive workforce of teleoperators on Earth plus sufficient communications bandwidth, and it will not be extensible to the asteroid main belt or beyond. Therefore, with each generation, progress will be made toward full autonomy.

Table 1 describes the autonomy in terms developed by Hans Moravec (Moravec 1999; Moravec 2003). Moravec’s “insect” level is when robots perform simple pre-programmed responses to sensor inputs. Many machines operate at the insect level today. The “lizard” level is when robots identify objects functionally to guide their motor tasks. Lizard-level robotics is already appearing in laboratories on Earth and is making steady progress toward greater capability. “Mouse” level is when the robots learn and improve the performance of their tasks through simple positive and negative feedback. This is important because human industry is only adapted to terrestrial conditions, but learning robots can adapt it to the multitude of environments they will experience in the solar system. “Monkey” level is when the robots maintain a mental model of the world including other agents. This provides them with insight into the intent of other agents as well as foresight. “Human” level is when the robots have the mental ability to reason abstractly, generalizing from specific learning situations to a broader class of applications, and thus to make decisions in the face of the unexpected. These higher levels of robotics will be necessary in the distant future when, for example, a robotic construction crew is building a science lab on Pluto, many hours of time delay away from human help. Extrapolating the computing speed of small, inexpensive microprocessors that are commercially available, we expect by the year 2023 they will reach the speed Moravec predicted as necessary to support human-level robotics. Even if Moore’s Law ended today, that computer power is easily achieved by paralleling inexpensive microprocessors and by other advances planned by computer chip manufacturers (Gargini 2005). On-going advances in robotic software and artificial intelligence present a very optimistic picture that these levels of robotics will be achieved as Moravec predicted, with lizard-level occurring by 2020, mouse-level by 2030, monkey-level by 2040, and human-level by 2050. Only mouse-level is needed by the end of bootstrapping on the Moon, but depending on how fast the strategy is carried out the robotics sent to the asteroid belt may be at the monkey-level or higher.

Electronics Manufacturing. In the baseline model, when Gen. 2.5 is fabricated its assets include some electronics manufacturing machines. Those machines themselves are built with electronics imported from Earth, and they are capable of making only the crudest and simplest of electronics components such as resistors and capacitors, which will not be miniaturized or efficient. Gen. 3.0 and subsequent possess a greater diversity of electronics manufacturing machines with increasing sophistication. By Gen. 5.0 we aim to have basic lithography machines on the Moon, built using computer chips sent from Earth, so that by Gen. 5.0 all computer chips can be made in space. The early computer chips will lack the transistor density of chips made on Earth, but they will be adequate for “appropriate technology” in space. Later generations (not modeled here) continue spiraling the sophistication of space industry so that eventually the lithography machines and computer chips match the best of Earth’s.

(ed note: the reseachers then created a mathematical model to determine how rapidy the lunar industry would grow. Maximum production rate "Max." is when the plants run full bore trying to make the next generation. Reduced production rate "Red." is when the plants pause manufacturing to allow other tasks, such as technological advancement, manufacturing experiments, or to allow slow robotnauts to catch up assembling parts. "Red." reduces the manufacturing rate by one-half. As it turns out, the reduced rate is the minimum necessary to meet survival and growth goals. Once the SRLF reaches the final generation, it can switch over to Max rate thereafter.)



In theory, a self-replicating spacecraft could be sent to a neighbouring planetary system, where it would seek out raw materials (extracted from asteroids, moons, gas giants, etc.) to create replicas of itself. These replicas would then be sent out to other planetary systems. The original "parent" probe could then pursue its primary purpose within the star system. This mission varies widely depending on the variant of self-replicating starship proposed.

Given this pattern, and its similarity to the reproduction patterns of bacteria, it has been pointed out that von Neumann machines might be considered a form of life. In his short story, "Lungfish", David Brin touches on this idea, pointing out that self-replicating machines launched by different species might actually compete with one another (in a Darwinistic fashion) for raw material, or even have conflicting missions. Given enough variety of "species" they might even form a type of ecology, or – should they also have a form of artificial intelligence – a society. They may even mutate with untold thousands of "generations".

The first quantitative engineering analysis of such a spacecraft was published in 1980 by Robert Freitas, in which the non-replicating Project Daedalus design was modified to include all subsystems necessary for self-replication. The design's strategy was to use the probe to deliver a "seed" factory with a mass of about 443 tons to a distant site, have the seed factory replicate many copies of itself there to increase its total manufacturing capacity, over a 500-year period, and then use the resulting automated industrial complex to construct more probes with a single seed factory on board each.

It has been theorized that a self-replicating starship utilizing relatively conventional theoretical methods of interstellar travel (i.e., no exotic faster-than-light propulsion, and speeds limited to an "average cruising speed" of 0.1c.) could spread throughout a galaxy the size of the Milky Way in as little as half a million years.

Implications for Fermi's paradox

In 1981, Frank Tipler put forth an argument that extraterrestrial intelligences do not exist, based on the absence of von Neumann probes. Given even a moderate rate of replication and the history of the galaxy, such probes should already be common throughout space and thus, we should have already encountered them. Because we have not, this shows that extraterrestrial intelligences do not exist. This is thus a resolution to the Fermi paradox – that is, the question of why we have not already encountered extraterrestrial intelligence if it is common throughout the universe.

A response came from Carl Sagan and William Newman. Now known as Sagan's Response, it pointed out that in fact Tipler had underestimated the rate of replication, and that von Neumann probes should have already started to consume most of the mass in the galaxy. Any intelligent race would therefore, Sagan and Newman reasoned, not design von Neumann probes in the first place, and would try to destroy any von Neumann probes found as soon as they were detected. As Robert Freitas has pointed out, the assumed capacity of von Neumann probes described by both sides of the debate are unlikely in reality, and more modestly reproducing systems are unlikely to be observable in their effects on our Solar System or the Galaxy as a whole.

Another objection to the prevalence of von Neumann probes is that civilizations of the type that could potentially create such devices may have inherently short lifetimes, and self-destruct before so advanced a stage is reached, through such events as biological or nuclear warfare, nanoterrorism, resource exhaustion, ecological catastrophe, or pandemics.

Simple workarounds exist to avoid the over-replication scenario. Radio transmitters, or other means of wireless communication, could be used by probes programmed not to replicate beyond a certain density (such as five probes per cubic parsec) or arbitrary limit (such as ten million within one century), analogous to the Hayflick limit in cell reproduction. One problem with this defence against uncontrolled replication is that it would only require a single probe to malfunction and begin unrestricted reproduction for the entire approach to fail – essentially a technological cancer – unless each probe also has the ability to detect such malfunction in its neighbours and implements a seek and destroy protocol (which in turn could lead to probe-on-probe space wars if faulty probes first managed to multiply to high numbers before they were found by sound ones, which could then well have programming to replicate to matching numbers so as to manage the infestation). Another workaround is based on the need for spacecraft heating during long interstellar travel. The use of plutonium as a thermal source would limit the ability to self-replicate. The spacecraft would have no programming to make more plutonium even if it found the required raw materials. Another is to program the spacecraft with a clear understanding of the dangers of uncontrolled replication.

Applications for self-replicating spacecraft

The details of the mission of self-replicating starships can vary widely from proposal to proposal, and the only common trait is the self-replicating nature.

Von Neumann probes

A von Neumann probe is a spacecraft capable of replicating itself. The concept is named after Hungarian American mathematician and physicist John von Neumann, who rigorously studied the concept of self-replicating machines that he called "Universal Assemblers" and which are often referred to as "von Neumann machines". While von Neumann never applied his work to the idea of spacecraft, theoreticians since then have done so.

If a self-replicating probe finds evidence of primitive life (or a primitive, low level culture) it might be programmed to lie dormant, silently observe, attempt to make contact (this variant is known as a Bracewell probe), or even interfere with or guide the evolution of life in some way.

Physicist Paul Davies of Arizona State University has even raised the possibility of a probe resting on our own Moon, having arrived at some point in Earth's ancient prehistory and remained to monitor Earth (see Bracewell probe), which is very reminiscent of Arthur C. Clarke's The Sentinel.

A variant idea on the interstellar von Neumann probe idea is that of the "Astrochicken", proposed by Freeman Dyson. While it has the common traits of self-replication, exploration, and communication with its "home base", Dyson conceived the Astrochicken to explore and operate within our own planetary system, and not explore interstellar space.

Oxford-based philosopher Nick Bostrom discusses the idea that future powerful superintelligences will create efficient cost-effective space travel and interstellar Von Neumann probes.


A variant of the self-replicating starship is the Berserker. Unlike the benign probe concept, Berserkers are programmed to seek out and exterminate lifeforms and life-bearing exoplanets whenever they are encountered.

The name is derived from the Berserker series of novels by Fred Saberhagen which describe a war between humanity and such machines. Saberhagen points out (through one of his characters) that the Berserker warships in his novels are not von Neumann machines themselves, but the larger complex of Berserker machines – including automated shipyards – do constitute a von Neumann machine. This again brings up the concept of an ecology of von Neumann machines, or even a von Neumann hive entity.

It is speculated in fiction that Berserkers could be created and launched by a xenophobic civilization (see Anvil of Stars, by Greg Bear or could theoretically "mutate" from a more benign probe. For instance, a von Neumann ship designed for terraforming processes – mining a planet's surface and adjusting its atmosphere to more human-friendly conditions – might malfunction and attack inhabited planets, killing their inhabitants in the process of changing the planetary environment, and then self-replicate and dispatch more ships to attack other planets.

Replicating seeder ships

Yet another variant on the idea of the self-replicating starship is that of the seeder ship. Such starships might store the genetic patterns of lifeforms from their home world, perhaps even of the species which created it. Upon finding a habitable exoplanet, or even one that might be terraformed, it would try to replicate such lifeforms – either from stored embryos or from stored information using molecular nanotechnology to build zygotes with varying genetic information from local raw materials.

Such ships might be terraforming vessels, preparing colony worlds for later colonization by other vessels, or – should they be programmed to recreate, raise, and educate individuals of the species that created it – self-replicating colonizers themselves. Seeder ships would be a suitable alternative to Generation ships as a way to colonize worlds too distant to travel to in one lifetime.

From the Wikipedia entry for SELF-REPLICATING SPACECRAFT

“Figured out what?” Sondra asked. “A theory about what?”

“About what the Charonians are,” she said.

“They’re von Neumanns. That’s it. That’s got to be it.”

“That’s what?”

“The answer, the explanation. The key to it all.

Not all by itself, but it’s a start.” Marcia stood up, still holding the pages of the message, and stared off into space, carefully thinking it all out. “It makes sense,” she said. “They’ve got to be von Neumanns.”

“Will you please quit saying ‘von Neumanns’ and explain what they are?” Sondra demanded.

“It’s very simple,” Marcia said. “How did we miss it? A von Neumann machine is any device that can exactly duplicate itself out of locally available raw materials. A toaster that could not only toast bread but build more toasters out of things found in the kitchen would be a von Neumann toaster. It’s a very old concept, named for the scientist who dreamed it up.

“But von Neumann’s real idea was to build a von Neumann starship,” Marcia said. “A robot explorer that could fly from one star system to another, explore the system—and then duplicate itself a few dozen times, maybe mining asteroids for materials. It would send out new von Neumanns, duplicates of itself, from there. Then each new exploration robot would travel on to a nearby star, duplicate itself, and start the cycle again. Each machine would report back to the home planet on what it found. Even given a fairly slow transit speed between stars, you could explore a huge volume of space in just a few hundred years. Traveling, exploring, reproducing, over and over again.”

Von Neumann Machine. Any machine that can precisely duplicate itself. A Swiss army knife that included a Swiss-army-knife-making attachment would be a von Neumann machine.

From THE RING OF CHARON by Roger MacBride Allen (1990)

He was saying to Frad: "The arrangements with the machinery are cumbersome, but not difficult in principle. We can lend you our Brood assembly until she replicates herself; then you reset the daughter machine, feed her scrap, and out come City Fathers (computers) to the number that you'll need—probably about a third as many as we carry, and it'll take maybe ten years. You can use the time feeding them data, because in the beginning they'll be idiots except for the computation function.

From A LIFE FOR THE STARS by James Blish (1962)

Space Plastic

Plastics are organic polymers, which means they are composed of huge chains of carbon and hydrogen molecules. The raw materials can be found in carbonaceous asteroids and in the hydrocarbon lakes of the Saturnian moon Titan.

Inside the closed ecology of a spacecraft's or base's CELSS some of the carbon and hydrogen can be diverted to brewing up some plastics. The source can be from carbon dioxide in the air or from agricultural waste.


I've mentioned several times that plastic can be produced from agricultural waste and/or CO2.

 The plastic of interest is polyethylene, or rather ultra high molecular weight polyethylene (UHMWPE, UHDPE, polyethene or trade names Spectra or Dyneema). It is formed of very long single chains of carbon, so the unit formula is CH2. This material is a thermoplastic, melting around 130 °C (or less if it contains solvents and/or crystalline defects). In normal use it should be kept between -150 °C and about 80 °C, so in-space applications will typically require a protective coating such as thin-film aluminum.

 So, we will assume we have available a quantity of ethanol (C2OH6) from other processes. Fermentation of sugar and/or cellulose is one possible source, as is syngas fermentation. It does not have to be perfectly pure; ethanol-water eutectic produced by distillation is acceptable. See the link for chemical structure and other data. This is passed through a fluidized bed reactor with alumina or zeolite dehydration catalyst that removes one molecule of water from each molecule of ethanol, yielding ethylene (C2H4) at high purity. The catalyst has to be regenerated periodically to recover the water and remove carbon and trace contaminants. The ethylene can be stored in high-pressure tanks.

 An alternative is to use the reverse water gas shift reaction to convert H2 and CO2 to CO; with additional H2 added this is syngas. The H2 would come from electrolysis of water. If methane is available it can be used with some O2 to produce syngas directly. Apply the methanol process, then apply something like the Mobil methanol to gasoline process to yield ethylene. Proper choice of zeolite will yield pure gaseous ethylene.

 The ethylene can be polymerized using titanium tetrachloride as a catalyst. This process has been used commercially for 60 years and currently yields high-quality resin with masses of 5.5 to 6 x10^6 grams per mol and contaminants of titanium (<40 ppm), aluminum (<20 ppm) and chlorine (<30 ppm), a total of 110 grams of catalyst lost per ton of plastic {UHMWPE biomaterials handbook, Steven M. Kurtz; chapter 2, tables 2.1 and 2.2}. Of these, chlorine is the hardest to replace unless there are convenient chloride salt deposits available; even so, 10kg of chlorine would be sufficient for 333 tons of plastic. The catalyst also requires magnesium chloride and a structural scaffold (usually microporous silica beads, sometimes zeolite or activated carbon), both of which can be completely recovered. The catalyst must be activated with triethylaluminum (TEA); this material is very dangerous and also useful as a rocket igniter. It can be produced using metallic aluminum, hydrogen gas and ethylene gas; if no TEA is available to jump-start the reaction then a small amount must be made using another process involving lithium hydride or ethyl chloride.
 A related catalyst, metallocene titanium, zirconium or hafnium chloride is used in solution with methylaluminoxane (MAO). Recent work has developed related catalyst systems using MAO; it is also very dangerous and is related to TEA. Synthesizing metallocene looks straightforward, but the input materials are fairly complex.
 These reactions can be terminated by hydrogen, so care must be taken to avoid any excess hydrogen gas in the ethylene or catalyst bed (particularly avoid water gas shift). The reactions normally occur in a solvent which does not react with the catalysts; this allows the catalyst active sites to remain exposed as the polymer molecules are carried away. Details about separation of metallocene from solvent appear to be kept secret, but supported catalyst in solvent should be straightforward as a continuous process.

 Solvents are a tricky question. The ideal solvent will be an organic oil which can be extracted from biomass, can dissolve PE, boils above 150 °C, does not thermally polymerize below that temperature and is nontoxic. Orange oil (mixed terpenes) fit that description, but the yield of terpene per m² of growing area is so low that it is disqualified for use in space. Proven solvents are xylene and toluene. This is an input that requires additional research; plant oils, silicone oils and possibly alcohols should be examined. Prototype production of PE could use Earth-sourced solvents while demonstrating the process is feasible, as could projects with a specific mass of plastic as a goal (early tethers for example).

 The product of this polymerization process is purified to contain only long-chain polymer and solvent. If necessary this can be done by centrifuge but the proportion of shorter chains and branches should be very low with these catalysts. Solvent is removed until the concentration of polymer is around 20% by mass. This results in a gel which is loaded into a ram extruder or similar heat and pressure device with spinneret holes. As the gel is passed through these holes the molecules are drawn into alignment; the resulting gel fibers are cooled and passed through water or ethanol to gradually remove the rest of the solvent as the fiber is drawn further. This gel spinning causes a very high degree of alignment and crystallization and can even reduce lattice defects; this is the secret of the incredibly high strength of Spectra fibers. The fiber is eventually wound onto a bobbin and then heat treated to drive off any remaining solvent and to further improve strength. Individual fibers are woven into yarn which is then used as needed. Again, fibers that will be exposed to space or to monatomic oxygen (low-Earth orbit) should be coated with a protective layer such as vapor deposited aluminum or a UV and oxygen resistant polymer film.

 For bulk use the freshly made polymer will have all solvent removed, yielding a powder that can be pressed into pellets for later use or pressed directly into shape. The powder can be melted under inert gas or redissolved in solvent and cast or extruded into the necessary shapes; pellets can sometimes be difficult to redissolve without shredding or patience.

 The bulk plastic is suitable for cutting boards, artificial joints, low-friction or low-wear contact surfaces, storage containers for water or moderate acids / bases, lightweight mechanical parts like rollers, cams, gears, etc., etc. Extruded rods can be used in 3d prototyping machines for additive manufacturing.

 The fiber is suitable for composite overwrap pressure vessels, tensile reinforcing members (including in regolith blocks), habitat hulls, meteoroid hull patches, cut-resistant cloth, ballistic armor, sutures and space tether strands.

Actually doing any of this would require the services of a good process chemist / chemical engineer at the least (plus possibly other engineers) to set up the various reactions, required equipment and input streams. Many of these processes if operated in batch mode will have unreacted or partially reacted inputs, and all of the processes will have some byproducts; these materials will need to be destroyed, most probably in a wet gasifier (supercritical water + O2 reactor) to recover the C, H and O. Any trapped catalyst material will find itself in the ash/salts, so any metals in the waste stream will end up as oxides or chlorides. Because the inputs are generated slowly, the overall process does not need to be particularly time-efficient as long as it is reliable, low-maintenance and low-mass-loss. For edge cases where a large supply of water and methane or CO2 are available a different solution might be used that takes less time but more manual effort.

Space Concrete


Lunarcrete, also known as "mooncrete", an idea first proposed by Larry A. Beyer of the University of Pittsburgh in 1985, is a hypothetical aggregate building material, similar to concrete, formed from lunar regolith, that would reduce the construction costs of building on the Moon.


Only comparatively small amounts of moon rock have been transported to Earth, so in 1988 researchers at the University of North Dakota proposed simulating the construction of such a material by using lignite coal ash. Other researchers have used the subsequently developed lunar regolith simulant materials, such as JSC-1 (developed in 1994 and as used by Toutanji et al.). Some small-scale testing, with actual regolith, has been performed in laboratories, however.

The basic ingredients for lunarcrete would be the same as those for terrestrial concrete: aggregate, water, and cement. In the case of lunarcrete, the aggregate would be lunar regolith. The cement would be manufactured by beneficiating lunar rock that had a high calcium content. Water would either be supplied from off the moon, or by combining oxygen with hydrogen produced from lunar soil.

Lin et al. used 40g of the lunar regolith samples obtained by Apollo 16 to produce lunarcrete in 1986. The lunarcrete was cured by using steam on a dry aggregate/cement mixture. Lin proposed that the water for such steam could be produced by mixing hydrogen with lunar ilmenite at 800 °C, to produce titanium oxide, iron, and water. It was capable of withstanding compressive pressures of 75 MPa, and lost only 20% of that strength after repeated exposure to vacuum.

In 2008, Houssam Toutanji, of the University of Alabama in Huntsville, and Richard Grugel, of the Marshall Space Flight Center, used a lunar soil simulant to determine whether lunarcrete could be made without water, using sulfur (obtainable from lunar dust) as the binding agent. The process to create this sulfur concrete required heating the sulfur to 130–140 °C. After exposure to 50 cycles of temperature changes, from -27 °C to room temperature, the simulant lunarcrete was found to be capable of withstanding compressive pressures of 17MPa, which Toutanji and Grugel believed could be raised to 20MPa if the material were reinforced with silica (also obtainable from lunar dust).

Casting and production

There would need to be significant infrastructure in place before industrial scale production of lunarcrete could be possible.

The casting of lunarcrete would require a pressurized environment, because attempting to cast in a vacuum would simply result in the water sublimating, and the lunarcrete failing to harden. Two solutions to this problem have been proposed: premixing the aggregate and the cement and then using a steam injection process to add the water, or the use of a pressurized concrete fabrication plant that produces pre-cast concrete blocks.

Lunarcrete shares the same lack of tensile strength as terrestrial concrete. One suggested lunar equivalent tensioning material for creating pre-stressed concrete is lunar glass, also formed from regolith, much as fibreglass is already sometimes used as a terrestrial concrete reinforcement material. Another tensioning material, suggested by David Bennett, is Kevlar, imported from Earth (which would be cheaper, in terms of mass, to import from Earth than conventional steel).

Sulfur based "Waterless Concrete"

This proposal is based on the observation that water is likely to be a precious commodity on the Moon. Also sulfur gains strength in a very short time and doesn't need any period of cooling, unlike hydraulic cement. This would reduce the time that human astronauts would need to be exposed to the surface lunar environment.

Sulfur is present on the moon in the form of the mineral troilite, (FeS) and could be reduced to obtain sulfur. It also doesn't require the ultra high temperatures needed for extraction of cementitious components (e.g. anorthosites).

"Sulfur "concrete" is an established construction material. Strictly speaking it isn't a concrete as there is little by way of chemical reaction. Instead the sulfur acts as a thermoplastic material binding with a non reactive substrate. Cement and water are not required. The concrete doesn't have to be cured, instead it is simply heated to above the melting point of sulfur, 140 °C, and after cooling it reaches high strength immediately.

The best mixture for tensile and compressive strength is 65% JSC-1 lunar regolith simulant and 35% sulfur, with an average compressive strength of 33.8 MPa and tensile strength of 3.7 MPa. Addition of 2% metal fiber increase the compressive strength to 43.0 MPa Addition of silica also increases the strength of the concrete.

This sulfur concrete could be of especial value for dust minimization, for instance to create a launching pad for rockets leaving the Moonp>

Issues for "Sulfur Concrete"

It provides less protection from cosmic radiation, so walls would need to be thicker than concrete walls (the water in concrete is an especially good absorber of cosmic radiation).

Sulfur melts at 115.2 °C, and lunar temperatures in high latitudes can reach 123 Celsius at midday. In addition, the temperature changes could change the volume of the sulfur concrete due to polymorphic transitions in the sulfur. (see Allotropes of sulfur).

So unprotected sulfur concrete on the Moon, if directly exposed to the surface temperatures, would need to be limited to higher latitudes or shaded locations with maximum temperatures less than 96 °C and monthly variations not exceeding 114 °C.

The material would degrade through repeated temperature cycles, but the effects are likely to be less extreme on the Moon due to the slowness of the monthly temperature cycle. The outer few millimeters may be damaged through sputtering from impact of high energy particles from the solar wind and solar flares. This may however be easy to repair, by reheating or recoating the surface layers in order to sinter away cracks and heal the damage.


David Bennett, of the British Cement Association, argues that lunarcrete has the following advantages as a construction material for lunar bases:

  • Lunarcrete production would require less energy than lunar production of steel, aluminium, or brick.
  • It is unaffected by temperature variations of +120 °C to −150 °C.
  • It will absorb gamma rays.
  • Material integrity is not affected by prolonged exposure to vacuum. Although free water will evaporate from the material, the water that is chemically bound as a result of the curing process will not.

He observes, however, that lunarcrete is not an airtight material, and to make it airtight would require the application of an epoxy coating to the interior of any lunarcrete structure.

Bennett suggests that hypothetical lunar buildings made of lunarcrete would most likely use a low-grade concrete block for interior compartments and rooms, and a high-grade dense silica particle cement-based concrete for exterior skins.

From the Wikipedia entry for LUNARCRETE


What sort of space clothing will space colonists wear? Something lightweight, to save on mass. No spandex, please.

Clothing is difficult to manufacture in microgravity, from growing the plant fibers to spinning, weaving, dying, and tailoring. All of those processes are much more difficult when things are floating around. This will limit the supply of available clothing, and make them expensive.

On the ISS, the crew wears garments made of cotton. These have a tendency to shed lint which can clog up ISS machinery and air filters. They are experimenting with Merino wool shirts and polyester shorts, which are lighter and do not shed lint.

The clothing might be treated with anti-microbial agents to make them odour resistant, since a microgravity clothes washer is so problematic that the ISS does not have one. On the ISS, clothing is worn and re-worn without washing until they get too stinky. Then they are put on the next cargo supply ship to burn up in re-entry. Actually, in microgravity, clothing does not actually touch the wearer's body as much as it does under Terra's gravity. For a crew of six, the ISS requires about 400 kilograms of clothing per year.

In classic Star Trek, the laundry renders the clothing back down to its chemical components, filters out the dirt, then refabricates the clothing. Nowadays we would think in terms of a 3D printer. Later versions of Star Trek would use unobtainium "replicators", but they have unintended consequences.

There are two basic ways to enable textiles to kill microbes. The first is to coat the fabric in a liquid solution that contains metals like silver ions; metal oxides like copper oxide; or compounds of ammonium. The other way is to impregnate the threads themselves with these kind of antimicrobial agents. Some testers said that the clothing would not stink, but it did tend to get noticeably heavier the more times it was worn. Presumably from the accumulation of perspiration and cast-off skin cells.

The ISS solution of "rely upon resupply from Terra" for the clothing problem won't work for a Mars Mission. Terra will be too far away, so you'll have to carry all the required clothing. In and effort to reduce the clothing payload NASA commissioned the UMPQUA Research Company in 2011 to produce the Advanced Microgravity Compatible Integrated Laundry System. The prototype worked on a vomit comet test flight, but UMPQUA is trying reduce the unit's water and power supply requirements.

Skirts or kilts are discouraged because [a] it is difficult to impossible to keep them in a modest position in free fall, and [b] if the decks are open gratings instead of solid floors, people on the next deck down will be treated to an up-skirt view. No panchira allowed.

NASA ISS astronauts wear clothes with lots of pockets and strips of velcro, as a handy place to carry gear.

In Larry Niven's Protector, the Belters of the asteroid belt spend most of their lives inside their space suit. They have a tendency to paint their suits in extravagant colors. One of the characters had Salvador Dali's Madonna of Port Lligat on the front of their suit. In an interesting psychological quirk, Belters also tend to be nudists when in a pressurized environment. This could also be a response to the difficulty of making clothing, or a reaction to the how expensive clothing is.


I'd waived clearance while still under ascent thrust on our original trajectory to a 200-kilometer parking orbit. Our delta-vee margin was excellent even though the Tomahok was running with a full cargo bay of—would you believe it?—cotton underwear.

Clothing wears out, and we hadn't established any clothing industry in space yet. Spinning, weaving, dyeing, and tailoring are ancient technologies, but they were among the last to be adapted to the weightlessness of space. As for cotton, one of the Commonwealth's primary products, nobody has yet developed an artificial fiber quite as comfortable.

We had a lot of leeway in changing our flight path because the Tomahok had "bulked-out" before she "grossed-out"—the hold was filled long before maximum weight was attained.

From MANNA by Lee Correy (G. Harry Stine) 1983

APPAREL. The clothing worn in most of the KNOWN GALAXY, at least that worn by EARTH HUMANS, is for the most part extremely dull, or in very bad taste. Usually both. At least, this is the case in HOLLYWOOD SCIFI, and so far as I can tell from book covers, in most written SF as well. The general norm, especially for advanced societies, is essentially long underwear; the most common alternative is a jumpsuit, equally unflattering and harder to get into or (especially) out of.  These may be tarted up - as in the era c. 2000 CE - with racing stripes, corporate-logo-style swooshes, and so forth. But no matter what you do to them, basically they are all butt-ugly. To be sure, a babelicious actress will still look good, especially since her outfit is invariably skin-tight and low-cut. But she'd look good in anything, and even better in a more flattering costume.

Things are slightly better when WARFARE is involved (which may be why it is so common). Uniforms tend to follow 20th century CE practice, more or less, and so are at least crisp if still basically long underwear. Original Trek did go boldly where no one has gone since, putting the female crew members in those miniskirts. Alas, the feminists did away with that. Their logic might be impeccable, as Spock would say, but 'tis still a pity.

The one social system in the Known Galaxy that allows people to actually look good is NEOFEUDALISM. In Neofeudalist cultures (at least in the ruling class), the men get to wear that most dramatic of male costume accessories, a sword, while the women are all a major eyeful in long, tight, low-cut dresses. There is simply no way to look better without looking tacky. This of course is achieved in the final, decadent stages of the FIRST EMPIRE, when the women - at least those of the Imperial Court - run around looking like the Victoria's Secret catalog. But when you see that much flesh on display, you know that the FALL OF EMPIRE is at hand.

On the whole, ALIENS WITH FOREHEAD RIDGES get to dress better than Earth Humans. The same odd rule applies as with Earth Humans, though; on the whole, the more civilized they are the worse they dress. In general, Space SF Apparel is in a bad way. Give us some help here, people! With at least billions, maybe trillions of intelligent, civilized beings throughout the Known Galaxy, someone ought to be able to come up with a few decent outfits.


Space Clothes: Averted; even in space, people just wear regular clothes. (Sure, they have lots of pockets, but that’s not specific to spacer culture.) The only difference is that the pure-skirt option is eliminated for both sexes (because microgravity), and the cloaks have to come with MEMS and occasional microfan thrusters to let them manage themselves as people move.

And spandex is not used for regular, day-to-day clothing anywhere. Even not in space.


 One of the basic necessities not yet mentioned is clothing.

 Most clothing is made, not surprisingly, of cloth. That in turn is made of woven threads, which are made either of plastic, animal hair or plant fiber.

 I'm going to eliminate animal fibers simply on an efficiency basis. The varieties of animals that are raised for fur (sheep, rabbits, goats, yaks) can also produce meat and milk, but are much less efficient than purpose-bred varieties. Sheep are probably the best all-round performers (~5.4kg wool per animal per year eating about 2kg/day of feed), but we can still do much better than that. Of course any incidental fur, skin, etc. will be used for filling, lining, leather and such, but we cannot assume there will be enough of that material to clothe everyone.

Read on for the rest. This turned out to require a lot more growing space than I would have thought, 10.26m² per person.

 Plastic has many benefits. Some routes to plastic production don't involve living organisms as a bottleneck. Others use byproducts of food production. Depending on the choice of plastic the resulting fabric can be quite strong, durable and chemical-resistant. Dyes can be incorporated directly into the fibers for simpler, more durable coloring. Plastics can also have drawbacks like static buildup or melting, and tend to be degraded by UV exposure.
 On Earth, the main clothing fibers are nylonpolyurethane (as Spandex), acrylicrayon and polyester. Nylon and polyurethane are fully synthetic, typically at the end of a long chain of processes that start with benzene. Acrylic is also fully synthetic, most commonly encountered as cheap knitting yarn. Rayon is a cellulose polymer that typically starts life as wood or bamboo fiber; it is considered semisynthetic even though it is just as heavily processed as nylon. Polyester refers to a family of plastics, some of which are found in nature. Notable polyesters include PET (PET as Dacron, BoPET as Mylar), Vectran and PLA.
 Notice that none of those are my favorite plastic, polyethylene. I suspect PE fabric for clothing would be uncomfortable, with no stretch and a slightly oily feel. Excellent as an outer layer in a protective garment but terrible for underwear.

 Plant fibers also have many benefits. Softness and durability are at the forefront. Natural fibers tend to withstand flexing better than synthetic fibers. These tend to char rather than melt. They also have a softer feel. On the other hand, natural fibers are more difficult to dye and can be more labor-intensive to produce.
 On Earth the main clothing fibers are cotton, flax (linen) and hemp. All three are grown for their fibers and seeds; the seeds can be eaten, milled into flour or pressed for oil. All three are suitable for hydroponic cultivation, mechanical harvesting and mechanical processing.

 Regardless of type, the various fibers are spun into yarns. Often the yarn is composed of several different fibers in order to combine desirable properties. For example, it is very common to include a few percent Spandex with cotton or nylon to produce a yarn with some elastic response.

 From yarn, some types of clothing are knitted directly. Machine-knit sweaters and socks are common. Other types require that the yarn is woven into fabric, which is then cut and sewn. One key variable is the weight of the fabric, typically specified in ounces per yard or grams per square meter. I'll be using GSM / grams per square meter. For example, a thin t-shirt might use 100 gsm material while a resort hotel might offer super-thick 800 gsm bath towels. I've used values below that are in the middle of the range for each fabric type.

 Fabric amounts are handled oddly in the US. People specify some number of yards of length, but often fail to mention that it's not square yards. Fabric is typically sold in widths of 45" or 60" (sometimes 32"). If a pattern calls for 2 yards of 45" fabric, that's actually 2.5 square yards. I've listed values below in square meters. I used a variety of sources; there was one excellent sewing blog with tables for the first three items, then I found a pattern for coveralls. For the bath and bedroom items I used actual dimensions, added an allowance for hems and converted to metric.

Typical fabric requirements:
pants, 1.5-4.2m² (2.5m² avg.) x 250gsm = 625g
shirt, 0.7-3.4m² (2m² avg.) x 150gsm = 300g
dress, 1.3-7.7m² (3m² avg) x 200gsm = 600g
coveralls, 4.3-6m² (5.3m² avg.) x 350gsm = 1855g
sheet (queen): 6m² x 150gsm = 900g
sheet (full): 5m² x 150gsm = 750g
blanket (single-layer fleece): 7m² (90x110"/ 230x280cm) x 350gsm = 2450g
quilt (2-layer without filling): 14m² (90x110"/ 230x280cm) x 200gsm = 2800g
pillowcase: 1.4m² x 150gsm = 210g
napkin: 0.4m² x 200gsm = 80g
towel (face / washcloth): 0.1m² x 350gsm = 35g
towel (hand): 0.4m² x 350gsm = 140g
towel (bath): 0.9m² x 500gsm  = 450g (also worth a read)

 Let's assume each person has a full set of bedding and bath linens, a blanket, a napkin and one week of clothing. The linens come to 20.2m² of various weights, 4.865kg. Clothing depends a lot on the person's size, preference and occupation; I will add the first four values and divide by three to get an 'average' mass for a day's clothing. This is likely to be high as I doubt that a third of the crew will need heavy denim coveralls for daily work. Still, that comes to 29.9m² or 7.887kg for seven outfits. That's 12.75kg in total; add a bit of leeway for knits and undergarments (~1.5kg), thread, sizing, etc. and call it 15kg per person. Specifics may differ; I'm assuming a full-size bed and all single people. A couple could use the same sheet and bathroom set. A more regimented facility might use Navy-style hot bunks and hot air drying after bathing to cut most of the linens. I suppose wealthy tourists might want more amenities.

 These things wear out. Socks and undergarments typically last perhaps half a year. Outerwear lasts 1-3 years depending on circumstances, so let's use one year. Bedding and bath items can last many years, but let's call it three. Heavy-duty work outfits like denim coveralls might last a decade or might wear out in three months; call it one year. On an annual basis using these numbers each person needs about 12.5kg of replacement fabric. These estimates are a bit conservative; many people replace their clothing far less often than this and only when they are actually worn through. I assume that the 'replaced' garments may not necessarily be discarded and people may simply build up a wardrobe over time. Sufficient space could be available to store 2-3 weeks worth of clothing, while worn or damaged pieces would be used as rags or as fiber sources for filter paper.

 Let's look at yields. I simply don't have enough information available to predict the material requirements for plastics, so this will focus only on natural fibers.

 Cotton can be grown at 1000-1500 lb per acre in open fields. In a hydroponic environment with no pests and tight nutrient control it should be possible to significantly exceed that mark, but let's use 1500lb/ac for this estimate. 4047m² per acre gives us about 0.37lb per m² or about 168g/m². Estimates range from 150-180 days for a growth cycle, so let's use 165 days. That works out to almost exactly 1 gram per square meter per day. Each person needs 34.25 grams per day, which would require 34.25m² per person. If we use a record yield of 6.31 bales per acre, that would be 3,536kg/ha or 354 grams per m² or 2.15g/m² per day > ~16m² per person. Seed production is about 1.62 x cotton production, or 3.48g/m² per day.
  Flax can be grown to yield over 1800kg/ha or 186g/m². Growth cycle is 90-125 days (in Canada), so we will use 108 days. That gives 1.72g/m² per day or 19.89m² per person.
 Hemp has yielded 6 tonnes of fiber per hectare or 600g/m² (again, Canada). Growth cycle is 70-90 days for fiber only (80) or 110-150 days (130) for dual crop. Using the dual crop number we get 4.62g/m² per day or 7.4m² per person. Seed production is about 0.7g/m² per day.

 Based on those numbers it sounds like a mix of 2:1 hemp and cotton would be ideal. That would be 4.93m² per person of hemp and 5.33m² per person of cotton to produce a total of 12.5kg of fiber. The process would co-produce about 1.25kg of hempseed and 6.77kg of cottonseed plus another few kg of plant waste.

It's worth mentioning that the record land yields tend to be twice the high averages, if not more, for most crops; even so, hydroponic methods can often surpass the record yields simply by preventing stress. Crops that have not been optimized for performance (like flax and hemp) have the potential to double or triple their yields given a dedicated breeding project. Have a look at dwarf rice and wheat yields vs. the varieties that existed before the 60's for an example of this in action. In other words, within a decade any number I post here will be obsolete as long as someone is actively developing the potential of these species. I think a combination of longer 'useful life' assumptions for clothing and improved hydroponic yields could cut the required area down below 3m² per person. That research and breeding could be applied to commercial crops on the ground, providing improved incomes for farmers around the world.

Bamboo has bast fibers, similar to flax and hemp. Most things referred to as 'bamboo fiber' are actually rayon made with bamboo as a feedstock, but true bamboo fiber is possible. The raw material is high in lignin, so processing is often a combination of mechanical chopping and chemical or enzyme boosted bacterial retting. Lots more details here:

The tenacity of those fibers is less than half that of cotton. The resulting cloth won't be as durable or tough, but still falling between wool and rayon. That is to say, still useful and with a reportedly pleasant feel.

The yield of bamboo basts can be as high as 53 tonnes per hectare (5.365kg/m² or 14.7g/m² per day). As long as the net fiber yield is at least 15% of the gross bast mass then it is competitive with cotton. If it is over 31% then it is competitive with hemp.

yield source:

If we look back a ways (1909), at least one author noted yields as high as 44 tons per acre (an eye-popping 98.6 tonnes per hectare) and fiber yields as high as 44%.

source: Congressional Serial Set: pulp and paper investigation hearings. 1 January 1909. (available free)

Let's go with the modern optimistic yield (while noting that high-intensity hydroponics can almost certainly double that value) and a fiber yield of, say, 38%. That would give a fiber yield of 5.6g/m² per day or about 6.1m² per person. The plants require a three-year lead time and a third of the stand is harvested each year. Waste from this process would be suitable for paper or fuel alcohol.

I think the main drawback would be that bamboo grows very tall. A dwarf species could be found that grows to 4m after three years, but it's not certain the yield numbers would still apply. Even so, that's about 25m³ per person. Cotton by contrast grows to perhaps 130cm. Allowing 20cm for lighting and nutrient systems, that same 4m space could house two stacked crops. If a mild dwarf variety of cotton was developed that matured to 110cm or less, three stacked crops could occupy the same space and would become competitive again on a floor-space and volume basis.

Another drawback is that bamboo requires several years to develop for this purpose. Peak fiber yields occur at three years. Peak structural strength is typically seen around 5 years. For paper pulp or wattle it can be taken at 1-2 years.

Those drawbacks are certainly opposed by several advantages unique to bamboo. It's a structural material, can be used similar to wood (buttons, flooring, furniture) and resists bacterial and fungal attack. Bamboo resists high-pH environments and can be embedded in concrete; it could serve as the tension member in a reinforced regolith-block construction if metal is scarce.

John Powell: Beta cloth is also a good candidate for durable outer wear. It's non-flammable, completely recyclable and made from silica fibers, a major component of lunar regolith. The Apollo/Skylab spacesuits used it.

Chris Wolfe: Excellent idea. I'd probably not want to wear that against the skin and there's a potential inhalation hazard from shed fiber fragments, but as an outer layer with heat and chemical resistance it would be very useful for engineering coveralls. Equipment to make silica fibers would also be useful for making rockwool insulation and rooting media.


The Advanced Microgravity Compatible, Integrated Laundry (AMCIL) is a microgravity compatible liquid / liquid vapor, two-phase laundry system with water jet agitation and vacuum assisted drying.

Umpqua Research Company previously developed a complete microgravity compatible Single Phase Laundry System (SPLS). Single-phase operation during the wash cycle facilitated microgravity compatible fluidics and eliminated problems associated with foams. Pulsed water jets were utilized to agitate the clothing. Drying was achieved with microwave assisted vacuum drying followed by a tumble cycle that greatly enhanced softness in the previously vacuum pressed clothing. Tumbling was achieved by an array of three air jets, two to generate a cyclonic effect and a third to induce tumbling by blowing perpendicular to the plane of rotation. This concept was successfully demonstrated during a KC-135 microgravity simulation flight.

The proposed AMCIL concept will build on the SPLS technology and incorporate key design improvements to reduced water requirements and lower power consumption. Specific advancements include a redesigned wash cycle that consumes less water and reduces power demand.

The Phase I effort will demonstrate the feasibility of the microgravity compatible liquid / liquid vapor, two-phase washing concept in a laboratory scale system. A complete, automated prototype unit that incorporates the system parameters established during the Phase I tests will be designed, fabricated, and tested during the Phase II program.


The ship's loudspeaker blatted out, "All hands! Free flight in ten minutes. Stand by to lose weight." The Master-at-Arms supervised the rigging of grab-lines. All loose gear was made fast, and little cellulose bags were issued to each man. Hardly was this done when Libby felt himself get light on his feet — a sensation exactly like that experienced when an express elevator makes a quick stop on an upward trip, except that the sensation continued and became more intense. At first it was a pleasant novelty, then it rapidly became distressing. The blood pounded in his ears, and his feet were clammy and cold. His saliva secreted at an abnormal rate. He tried to swallow, choked, and coughed. Then his stomach shuddered and contracted with a violent, painful, convulsive reflex and he was suddenly, disastrously nauseated. After the first excruciating spasm, he heard McCoy's voice shouting.
     "Hey! Use your sick-kits like I told you. Don't let that stuff get in the blowers." Dimly Libby realized that the admonishment included him. He fumbled for his cellulose bag just as a second temblor shook him, but he managed to fit the bag over his mouth before the eruption occurred. When it subsided, he became aware that he was floating near the overhead and facing the door. The chief Master-at-Arms slithered in the door and spoke to McCoy.
     "How are you making out?"
     "Well enough. Some of the boys missed their kits."
     "Okay. Mop it up. You can use the starboard lock." He swam out.

     McCoy touched Libby's arm. "Here, Pinkie, start catching them butterflies." He handed him a handful of cotton waste, then took another handful himself and neatly dabbed up a globule of the slimy filth that floated about the compartment. "Be sure your sick-kit is on tight. When you get sick, just stop and wait until it's over." Libby imitated him as best as he could. In a few minutes the room was free of the worst of the sickening debris. McCoy looked it over, and spoke:

     "Now peel off them dirty duds, and change your kits. Three or four of you bring everything along to the starboard lock."
     At the starboard spacelock, the kits were put in first, the inner door closed, and the outer opened. When the inner door was opened again the kits were gone — blown out into space by the escaping air. Pinkie addressed McCoy.

     "Do we have to throw away our dirty clothes too?"
     "Huh uh, we'll just give them a dose of vacuum. Take 'em into the lock and stop 'em to those hooks on the bulkheads. Tie 'em tight." This time the lock was left closed for about five minutes. When the lock was opened the garments were bone dry — all the moisture boiled out by the vacuum of space. All that remained of the unpleasant rejecta was a sterile powdery residue. McCoy viewed them with approval. "They'll do. Take them back to the compartment. Then brush them — hard — in front of the exhaust blowers."

From MISFIT by Robert Heinlein (1939)

(ed note: Our protagonists are the crew of the passenger transport spacecraft Eurydice. Solid-core nuclear thermal rocket engine. The passengers do not know it yet, but the engine has suffered a severe malfunction and may be starting to melt down. In the control room, away from the passengers, the crew are trying to figure out exactly how much of a disaster they are looking at. Minus the captain, he is in sickbay suffering from explosive decompression and contamination with radioactive fragments of reactor core elements)

      (First Officer) Prescott took a deep breath and turned to (Ship's Medic) Mercer. "You may have thought that I was about to compliment you back there. Don't set too much store by that—I just can't abide outsiders criticizing one of the family, even a new, untrained, foundling member like you.
     "But I've a job for you," he went on. "Go back and recheck the tank temperature. You'll find insulated bottles in the bulkhead locker beside the outer seal. Take one. You will see that it has a snap fastening at the neck, that it is double-walled, and that there is a thermometer and a yellow disc, which changes color in certain circumstances, between the walls.
     "Go into the lock chamber," he continued. "No need to go into the tank itself at this stage until we have some idea of how much radioactive contamination you left behind after your first bath. Open one of the inner valves, which are plainly labeled with operating instructions, and press the neck of your bottle against the outlet and keep it there until it is nearly full. In free fall the water will not pour out, so you may have to wait a few minutes for it to fill…" (the Eurydice uses water for reaction mass, during the trip bored passengers can "swim" in the zero-gravity blob of water)
     "I should do this," said Neilson suddenly. "After all, I'm still dressed for the job."
     "Don't think I haven't noticed," said Prescott sourly. "Pull up your shorts, dammit. I have enough problems on this ship without having my sensibilities blasted by the sight of your hairy navel. And I don't want you or your eyes to leave that board. MacArdle will monitor the Captain's breathing (in sickbay) and watch your board, Mercer, so move."

     As he would not have to go into the tank itself, Mercer did not bother to change, but he put on a purposeful expression and pretended not to notice the passengers who spoke to him on the way. The weightless dancers were not noticing anyone but each other. He found an insulated bottle and entered the chamber quickly, pressed the mouth against the outlet and began turning the valve.

     The metal felt very warm.

     Suddenly the bottle thumped against the palm of his hand. He stared at it stupidly, realizing that it was already full and that it should not have filled so quickly.
     As he withdrew and sealed the bottle, steam and scalding gobbets of water spurted from the outlet, filling the chamber with a hot, blinding fog. Mercer let go of the bottle, wrapped his hand in his cap and twisted shut the outlet valve, while with his other hand he groped for the evacuation button. He heard the combination suction pump and air blower—the only means of rapidly emptying a compartment full of weightless water—making rude, gurgling sounds. But the chamber did not clear completely—steam and a fine spray of scalding droplets were spurting from the edges of the inner seal. Mercer retrieved his cap and test bottle, whose thermometer showed a temperature close to boiling point and a disc, which had turned from yellow to muddy brown. He felt like a half-boiled lobster with an icy cold lump of fear in its belly. Even though he did not know what exactly was happening, he did know that it was deadly serious and that he had to get back to Prescott fast.

     The passengers outside had other ideas, however.
     As soon as he came out they surrounded him, laughing and trying to grab him.
     "There's a black crow among the lovebirds," said one of the men. "A wet, black crow."
     "That isn't fair," said one of the girls. "You promised us a swim, and now you've had two and—"
     "With your clothes on!" added the other girl, who had succeeded in grabbing his ankle.
     He wanted to yell at her to let go or he would kick her pretty, laughing face that he had no time for horseplay at a time like this. But instead he said, "No ma'am, space-washing. I dump my wet uniform in a lock, open it to space and the moisture boils off. It takes out the wrinkles, too. Excuse me, I mustn't catch cold…"

     When he entered the control room a few minutes later, Prescott, with one hand gripping the engineer's headrest, was hovering over Neilson's board. He said, "Mercer, you do not launder your uniform in that incredible fashion, unless you don't mind ice crystals in your underpants—and your ability to lie convincingly under pressure worries me…"
     He broke off as he saw Mercer's face, then put out his free hand for the bottle.
     "It's hot," said Mercer.
     Prescott's features went stiff. "In both senses of the word."

From DARK INFERNO by James White (1972)

Hyperpower Stations

There are just some industrial applications that demand power approaching Kardashev Type I levels. Hyperpower stations will supply you with massive amounts of power (along with a massive electricity bill).

At the orbital radius of the planet Mercury the solar flux is about 9,121 watts per square meter, a whopping 6.7 times the 1,366 W/m2 available at Terra's orbital radius. So a ten kilometer square solar photovoltaic array that was 100% efficient would crank out about one terawatt of power.

Titanic solar power stations covering huge areas on the surface of Mercury or Luna are called "Asimov Arrays", name bestowed by James Powell and Charles Pellegrino after Isaac Asimov pointed out several serious errors in their design. Such as "You do know that Mercury is not tidally braked with respect to the Sun, do you not?"

Do keep in mind that it is not mandatory for the solar cells to be mounted on Mercury, they can be orbital. You could even place them closer to the sun if you really need the power. The only problem is that light pressure will tend to push them away, Mercury's gravity can anchor them.
Vulcanoids are a hypothetical population of asteroids that orbit the Sun in a dynamically stable zone inside the orbit of the planet Mercury. No astronomer has ever discovered any, but admittedly they would be rather hard to observe. They wouild not have the total surface area of a Mercury Asimov array, but [a] they would be closer to Sol and [b] if enemy nations(s) held claim(s) to the entire surface of Mercury, vulcanoids would be a worth-while alternate site.
An Asimov Array of solar power stations around the lunar equator could supply the all the energy needs of inhabited Terra. The power would be beamed to Terra using microwaves.
There is 2.0 × 1013 watts (20 terawatts) potential between Jupiter and Io. This can be harvested with electrodynamic tethers.
Alternatively, you mount lasers on copper rods and launch them from Io at Jupiter. As the rods cut the magnetic lines of force they generate electricity. This is converted into laser light and beamed back to Io. Rod is destroyed when it hits Jupiter, but so what, they are cheap.
Saturn, Uranus, and Neptune have atmospheres rich in Helium-3, useful for 3He+D fueled fusion reactors (although there is some evidence that Saturn only has 1/5th the 3He of the others). This can be harvested by atmospheric scooping. Jupiter has 3He as well, but the heavy gravity makes it uneconomical to harvest. You need a freaking solid core NTR to boost the harvest into Jovian orbit.
Naturally huge arrays of fusion power plants are going to require a huge supply of fusion fuel nearby.

“Getting the power may not cost us that much after aIl,” said Tuna ("Richard Tuna" is actually Charles Pelligrino). “Not when you consider where computer science and robotics are headed during the next fifty to seventy years. A few days ago, at Brookhaven, we came to realize that it may cost us only the expense of developing about thirty small, self-replicating factories—which build factories, which build factories—which, when they reach a certain population density, switch over from building factories to building solar panels. We simply send them like a viral infection tolthe planet Mercury, and carpet one hemisphere with panels. You’d get more than fifty thousand times the U.S. energy budget there.”

“Mercury rotates!” Colby called out.


“Sorry, Richie,” Colby said, “but you can’t build your panels on one hemisphere only, because Mercury rotates. Where have you been? People have known that for ten years. I’m sure it rotates. Have I introduced you to the natural resource? Ask him.”

“He’s right,” said Dr. Isaac Asimov. “Mercury rotates with aperiod of about three months, and the energy received per square foot, when Mercury is most distant from the Sun, is seven times what Earth receives. It goes up to ten times during closest approach. The interesting thing is that the one-eighty-degree and zero-degree longitudes do become gravitationally locked, alternately, during apihelion, and are exposed to the Sun almost three times longer than the longitudes ninety degrees away from them.”

“Oh, no,” Tuna groaned, flushing with embarrassment. “I don’t believe it. We have to redesign the whole thing.”

“Please,” Asimov said, raising his hands above his head, as if to show that he bore no weapons. “Don’t blame me. I didn’t make Mercury rotate. Better to find out now than when you get there and look up in the sky and—‘Hey! Why is the Sun moving?’ ”

Tuna groaned. “I don’t believe I missed that. We can redesign it, though. We’ll just have to cover a bit more surface area, and perhaps send three times as many machines to get the project started … make sure at least one cluster of them is in sunshine at any given moment…”

“And you’ll certainly want to test and perfect the machines on the Moon first,” added Asimov. “You know, it’s just one more argument for building a permanent Moon base. By the time you perfect solar panel builders for Mercury, you’ll already be beaming large quantities of clean energy down to Earth from the Moon…” He paused and smiled. “Incidentally, I wrote a science fiction story, way back in 1940, about an interrupted array of panels around the Moon’s equator (I have yet to figure out which story this is. The closest I can find is Reason). So if you ever do build the thing, Richard, you can call it the Asimov Array.”

“We can?”

“I insist on it.”

(ed note: The following is from the technical appendix to the novel.

Keep in mind that this was written in 1993 so adjust US dollars and national power consumption for inflation)

The technology for producing antimatter using particle accelerators is presently under development at American and European laboratories. At CERN’s seven-kilometer-circumference synchrotron near Geneva, Switzerland, antiprotons are routinely produced by firing a high-energy beam of protons into a block of tungsten. A trillion (1012) antiprotons can be created in this way. A trillion antiprotons may sound like a lot, but they contain the potential annihilation energy of only three hundred joules (roughly equivalent to the “bang” from a cap gun), and the CERN facility gets slapped with a $40,000 electric bill every time the accelerator is turned on (more like $68,000 US in year 2017).

Clearly, simpler and more efficient accelerators are needed. Machines with the proper requirements are presently under development in the United States (and soon to be under intensive development in Japan) for use in fusion reactor research.

“As an example,” explains physicist George Mueller, “in one design being studied, the particle accelerator will produce short bursts of protons with a power beam of 1014 watts, about one hundred times the present power output of the entire world! (in 2013 the world's average energy consuption was about 12.3×1012 watts, 1014 is still one hundred times as much) Of course, since the machine will be operated in 10-8-second bursts, the average power is very much lower.”

The natural location for antimatter factories, in view of their large power requirements, is in space, where continuous and, from an industrial perspective, limitless solar power is available. “Using the solar flux at the Earth’s distance from the Sun,” adds Mueller, “a light collector about three hundred kilometers on a side could provide the power for a 1014-watt factory (9×1010 m2. Solar flux at Terra orbit is 1,366 W/m2. At 100% efficiency that is 1.2×1014 watts).

If the efficiency of antiproton production from each high-energy proton in the initial beam could be made as high as 0.1 percent (efficiency of 0.001.) [presently Hiroshi Takahashi is predicting higher efficiencies] (Dr. Robert Forward thinks realistically it will be more like 0.0001)

…then this machine would produce 1020 antiprotons per second (each antiproton requires 1.5×10-10 Joules, 1.2×1014 watts at efficiency of 0.001 can produce 7.98×1020 antiprotons per second),…

…or about one kilogram of antimatter per month.” (antiprotons have a mass of 1.7×10-27 kilograms. At a rate of 1020 antiprotons/sec 1 kilogram would take 69 days or 2.3 months)

From our earliest brainstorming sessions emerged proposals for a solar panel array, in orbit around Earth, covering an area in excess of ten thousand square kilometers. Even if it should one day become economically feasible to mine, refine, and transport materials from the Moon to Earth orbit, such an array would literally become a gigantic solar sail, requiring a considerable expenditure in thrust (presumably from rockets that would have to be refueled from somewhere) just to keep it from blowing away on the solar wind.

Our attention tumed elsewhere, to a power source more firmly anchored, yet overlooked, perhaps because it is so large that no one noticed it before. If we are correct, the planet Mercury is destined to become the most valuable piece of real estate in the solar system.

Presently, we are eyeing self-replicating, solar-panel-building machines. If humanity plays its cards right, prototypes could be tested on the Moon near 2020, and the rewards that the descendants of these first machines can bring—cheap, clean, and unlimited power for all mankind—are yet another argument for a permanent Moon base. Using the materials available at the lunar surface, they will build solar panel farms, and new solar-panel-building machines. In time, the farms will girdle the Moon’s equator to form the Asimov Array (named, exactly as described in Chapter 4, after an early contributor to the concept). The Asimov Array will provide power for Earth.

However, we dare not use that power for producing and storing large quantities of antimatter on the Moon, or anywhere near Earth, because even a single kilogram of the stuff—a mere handful—carries the explosive potential of forty hydrogen bombs, along with the moral responsibilities that go hand in hand with the possession of such power. (1 kilogram of antimatter + 1 kilogram of matter will make about 1.8×1017 joules, about 43 megatons. B83 nuclear bomb has highest yield of any in US nuclear arsenal, it has a maximum yield of 1.2 megatons. So 43 MT is about thirty-six hydrogen bombs)

Once perfected, the descendants of the original Asimov Array robots can be sent like a viral infection to the planet Mercury. Assembling replicas of themselves from the substance of their host, their first half decade of habitation will be a latent, incubation phase, during which most of the solar panels manufactured by the machines will be used to power an ever-accelerating chain reaction of machines building more self-replicating machines. As their population approaches a predetermined critical density, more and more of them cease reproduction and join to form solar panel factories, with the result that almost three decades after the arrival of the original twenty or thirty machines, uncountable square kilometers of generator, with an area the size of Rhode lsland being added daily.

When Mercury is farthest from the Sun, each panel will receive 6.7 times as much solar energy as it would receive on the surface of the Moon. This figure rises to fully ten times the lunar surface value as the planet’s eccentric orbit dips twenty-four million kilometers nearer the Sun. In time, self-replicating machines will carpet the Mercuran landscape from pole to pole with solar panels, giving mankind more than 50,000 times the present U.S. electrical energy budget, a power capable of launching at least two interstellar missions per year.(1990 U.S. annual electrical output 2.7×1016 Joules. ×50,000 = 1.3×1021 Joules. Divided by 2 per year = 6.7×1020 Joules = 7.5 kilogram antimatter per interstellar mission.)

Using self-replicators, the world’s future energy problems and even the excess energy required for relativistic flight can be solved for a very small initial investment: the cost of developing as few as a dozen ancestral machines on the Moon. Of course, we should not trouble ourselves to begin immediate development. They would be too expensive to build today, and too inefficient if built from the equipment now at hand—just as a trans-Atlantic airline service and videocassette recorders, though technologically feasible, would have been prohibitively expensive to build in 1925. We must wait, not only for technology to catch up with the idea, but for the idea to become economically viable.

From FLYING TO VALHALLA by Charles Pellegrino (1993)

The main dome of Ganymede City covered three square kilometers of the moon's surface. Here on the top level, the observation screens covered the interior of the dome; they could be turned on individually, or en masse to show a panorama of sky and surrounding terrain. A direct view would have left the observation level with too little shielding against the leakage of solar radiation trapped by Jupiter's magnetic field; even though Ganymede City sat in the moon's radiation shadow, the aboveground portion was protected, as an added precaution, by meters of water in the outer shell, piped in from the nearby ice field. Natives called Ganymede City "the big igloo,” because of the liquid that was kept frozen in its insulating space. In addition to the physical shielding against stray radiation and occasional meteors, the Laser-Fed Fusion Reactor powered the super conducting units which cast a magnetic shield over the domes.

Ganymede plowed through a sea of death; but despite this, ships had visited all the Galilean satellites by 2015. Built at the Martian space docks on Phobos and Deimos, the water- and magnetically-shielded tin cans, as the ships came to be called, had penetrated into Jupiter's radiation belts, setting up research bases on Callisto and Ganymede, as well as temporary facilities on a few of the close-in rocks whipping around the edges of the gas giant’s atmosphere.

To make power for building the first underground living quarters, the tin cans had deployed a giant sun mirror. The collector was no more than a few molecules thick, but its huge size and focusing capacity made up for the fact that the sun's intensity was only about four percent of what it was in the vicinity of earth (51 W/m2 instead of 1,366 W/m2).

While Ganymede City's first levels were being built, a mass driver track had been constructed beyond what was now the tug port. Using the three-kilometers-per-second escape velocity from the Jovian moon, the track began to toss copper ingots toward Jupiter, whose escape velocity was twenty times greater; this large energy difference was expressed in the form of eddy currents of electricity forming in the copper as it rushed through Jupiter's powerful magnetic field; these were lased back to Ganymede by a small disposable unit, continuing right up to the moment when the ingot hit the atmosphere for a final, dramatic surge of electricity. Current flow was evened out by storage facilities at the receiving station on Ganymede.

As a result of this and other systems, Ganymede became one of the energy-self-sufficient places for science, attracting research and development from earth. The solar mirror was still working; the lofter still threw ingots into Jove's face; and a second fusion LFR had recently been completed.

Ironically, Sam thought, success on Ganymede had slowed the building of facilities on Saturn's moons, as well as delaying development of the larger asteroids such as Ceres. A whole system of worlds waited to come alive out here, offering conditions for industry and research, room for a civilization to grow.

From MACROLIFE by George Zebrowski (1979)

Hyperpower Uses

Here are a few industrial applications that require hyperpower stations.

Interstellar Mission Laser Sail
     Such as in Rocheworld, which needed an outrageous 1,300 terawatts.
     Solar system wide network of laser power transmission and laser thermal rocket energizers.
Project Daedalus
     This slower-than-light unmanned starship would get up to 0.12 c and cruise for 46 years before flashing through the Barnard's Star system frantically snapping pictures. It would require 30,000 metric tons of Helium-3, harvested from Jupiter's atmosphere over a 20 year period. It also needs 20,000 metric tons of deuterium, but you can get that out of seawater.
Antimatter Creation
     Antimatter factories producing commercial quantities of antimatter, are hideously inefficient power hogs. But antimatter has a thousand and one uses, it will be a valuable commodity. In Michael McCollum's Thunderstrike antimatter is used as super-duper rocket fuel and to move valuable asteroids to more convenient locations. Pelligrino and Powell's Valkyrie and Frisbee's starship use antimatter as starship fuel.
     Antimatter distribution is administered by the Antimatter Guard because is it so much easier to misuse than mere plutonium..
Valkyrie Antimatter Starship
     These ultralight starships can deltaV up to 0.92c and back down to zero. But my slide rule says they'll need about 4,200 metric tons of antimatter.
Frisbee Antimatter Starship
     This brute can delta V up to 0.125c and back down to zero, with an acceleration of 0.01 g. And the freaking thing is 700 kilometers long (not meters, kilometers). It will require 159,450 metric tons of antimatter liquid hydrogen, because it ain't no ultralight starship. 500 kilometers of the ship is just heat radiators.

Fission Fuel Plant

Atomic Rockets need Atomic Fuel. Raw uranium or thorium ore is worthless as fuel for your nuclear thermal rocket or nuclear power reactor (the same goes for nuclear weapons). The stuff the asteroid miners haul in will have to be enriched before it can be used as fuel.

Common power reactors require enrichment from 1% to 20%, fast-neutron power reactors and nuclear thermal rockets need 20% to 85%, above that is the weapons-grade fissionables needed for nuclear weapons, SNRE-class propulsion, Orion pulse units, and nuclear salt water rockets burning 90% UTB.

Enrichment requires a sizable high-tech factory, they will have to be strategically placed around the inhabited solar system. Along with security forces provided by the astromilitary of a select group of nations, to ensure that none of the weapons-grade plutonium gets stolen (the Nuke Guard).

It is a lamentable fact that fission engine fuel elements clog up with nuclear poisons and stop working while there is still lots of fuel in them. After about 15% of the fuel is burnt (85% unburnt) the rod stops fissioning. Since is it a criminal waste of scarce fuel to throw the rod away when 85% is still yet to be used, you have to take it to a nuclear reprocessing plant. The plant will filter out all those nuclear poisons and use the unburnt fuel to make new fuel rods. A distressing by-product is lots of weapons-grade plutonium, which the Nuke Guard will also have to deal with.

Obviously reprocessing will only be needed for solid core and closed-cycle gas core NTRs. Other nuclear rockets blow the nuclear fuel out their tail pipes, burnt and unburnt.

There will have to be reprocessing plants strategically placed around the inhabited solar system, perhaps inside enrichment plants. Perhaps with a network of fuel transport ships shuttling fuel rods (fresh and spent) between plants and spaceports for convenience. Said ships will undoubtedly be a part of the Nuke Guard.

Radioactive Waste Disposal

Nuclear power unfortunately produces radioactive waste. The low-level cesium-137 and strontium-90 waste has a half-life of 30 years or so (decaying to 1% of it original deadly strength in about 180 years). But the plutonium is freaking transuranic waste with a half-life of around 24,000 years (decaying to 1% of it original strength in about 144,000 years, about the time separating us from Neanderthal Man).

Where are you going to dispose of this death-metal? Pretty much every intelligent being will scream in your face "NOT IN MY BLASTED BACK YARD, YOU AIN'T!!!"

A common simplistic solution is to lob the stuff into outer space (since there currently are no back yards in space). You may have seen the concept in the scifi show Space 1999. Understand that the bit where the Space 1999 disposal site blows up and kicks the moon out of orbit is utter bovine excreta.

Granted that space is so freaking huge that it is pretty much impossible to contaminate it with glowing pollution. But the transport cost makes this solution impractical. It would be several orders of magnitude cheaper to drown the stuff in vats of computer printer ink mixed with Dom Pérignon champagne and wrap them with diamond-encrusted iPhones tied with ropes of saffron.

For this to work the cost to boost payload into orbit will have to come way down, or the cost of terrestrial disposal will have to go way up. Or both.

If the boost cost becomes reasonable, an old NASA report recommends disposal in Lunar or Solar graveyard orbits from an overall mission safely standpoint. Some sort of remote-controlled space rescue capability will be needed, in case a rocket malfunction sticks the radioactive waste rocket into the wrong orbit.

No, don't even think about trying to hurl the radwaste rockets into the Sun. In delta V terms it will be far cheaper to accelerate the waste to Solar escape velocity and into intergalactic space.


      The objective of this option is to remove the radioactive waste from the Earth, for all time, by ejecting it into outer space. The waste would be packaged so that it would be likely to remain intact under most conceivable accident scenarios. A rocket or space shuttle would be used to launch the packaged waste into space. There are several ultimate destinations for the waste which have been considered, including directing it into the Sun

     The high cost means that such a method of waste disposal could only be appropriate for separated HLW (high level waste)i.e., long-lived highly radioactive material that is relatively small in volume – rather than spent fuel. The question was investigated in the USA by NASA in the late 1970s and early 1980s. Because of the high cost of this option and the safety aspects associated with the risk of launch failure, it was abandoned.

From STORAGE AND DISPOSAL OF RADIOACTIVE WASTE by the World Nuclear Association (2018)


Disposal of refined waste was described in section 3.9 of the technical report. It was shown, concurring with earlier NASA studies, that refined waste disposal is practical using the space shuttle and a modified full-capability tug for transportation.

Nuclear waste is presently processed to a solidified form consisting of about 25 percent fission product oxides, less than 1 percent actinides, the remainder being inert (nonradio-active) material. The waste is typically canned in "pots" 0.3m in diameter by 2.4m in length (1 x 8 ft). It would be desirable, if economically practical, to dispose of total waste in this form, eliminating completely the need for long-term Earth storage. Accordingly, a brief study of total waste disposal was performed.


This concept assumes disposal of total so!idified waste, based on current waste solidification technology. The total waste is roughly 1110th as radioactive per unit rass as the partially refined waste discussed above. The total waste package is illustrated in figure 3-1. It appears practical to provide a portable shield for safe handling and for flight crew protection. It is unlikely, however, that such a massive shield could be designed lo survive abort entry and impact. Thc launch system and operational procedures must provide protection from public exposure. The shield is assumed returned to Earth for reuse.

Requirements are stated in table 3-1. Data shown are typical. Waste can be repackaged to some degree in order to tailor the mass per package to capabilities of the transportation system.


3.2.1 Transportation Mode Candidates

The total waste requirement is very demanding, both in terms of total mass and in terms of economics, i.e., transportation cost. Consequently, only very low cost Earth launch options were considered. Orbit transfer options included 1-½ stage and common stage (slingshot mode) LO2/LH2 OTV's and an electric propulsion option powered by decay heat of the waste itself. The low cost Earth launch options included a low cost heavy lift vehicle (LCHLV) and a second generation single stage-to-orbit (SSTO) shuttle. Where the LCHLV is used as the only Earth launch option, gliders similar to the shuttle orbiter, but without main propulsion systems, delivered to orbit by the LCHLV, are used as waste carriers to provide the needed intact-abort capability. The LCHLV is described in Appendix 2. SSTO concepts have been published in the literature, notably by Salkeld, and have been studied by Boeing on IR&D. The Boeing concept is illustrated in figure 3-2. No effort was spent on SSTO concepts by this study.

3.2.2 Transportation Sequences

Figures 3-3 and 3-4 show the transportation sequences investigation for the SSE destination. The first mode employs a LCHLV and a common-stage LO2/LH2 OTV. Intact abort capability during Earth launch is provided by the gliders shown. One shielded waste package is camed in each glider. In orbit, the waste packages are extracted from their shields and installed on the OTV system. The shields are returned to Earth by the gliders. The OTV's operate in slingshot mode with the boost stage recovered and the second stage expended along with the payloads to solar system escape.

The second mode employs a SSTO to launch the waste packages and small OTV/drop tank systems to orbit. The waste package goes up last; the shield is recovered by the SSTO. The OTV operates in a perigee kick mode; the drop tanks contain enough LO2/LH2 to establish a one day elliptic orbit. At first perigee the injection stage fires to SSE with the payload. All OTV elements are expended.

Table 3-2 (missing) provides a summary mission history for the 1-½ stage OTV system.

Thp LCHLV was assumed to have a low orbit payload capability of 200 000 kg as for the power satellite program. The SSTO was assumed to have 30 000 kg low orbit capability, with return payload capability of 24 000 kg. The gliders used with the LCHLV were also assumed to have 24 000 kg return payload capability.

3.2.3 Earth Launch Summary

A summary of Earth launch and OTV requirements for the various options and modes is shown in table 3-3. The ROM busbar surcharge values shown are in cents/kWh, 1975 dollars, and are transportation cost only. They do not include waste processing or packaging costs. Numbers of flights per waste package are indicated with flights per year in parentheses based on 50 and 1,100 waste packages per year, respectively.

3.3 Special Study: Nuclear Waste Disposal in Space Utilization of Waste Decay Heat

It was suggested that the decay heat of nuclear fission waste products might be used to drive a propulsion system to accomplish disposal of the waste to SSE. A typical conceptual system includes a closed-cycle heat engine operating from the decay heat, generating electricity to drive an electric prppulsion system (figure 3-5). Refined and total waste options are examined by the FSTSA study. Only the total waste option appears to be a candidate for this transportation mode because (a) the refined waste as defined by Lewis Research Center has very little thermal power, and (b) it can be handled economically by Shuttle/FCT.

This is an energy-limited problem. The energy available in the waste is finite and must be sufficient to provide the necessary energy change to accomplish the mission. An estimate of the energy available in solidified total waste is presented in figure 3-6. This decay is nearly a straight line on the log/log-plot and therefore may be approximated by q = atb where q is thermal panel at time t after core shutdown and a and b are curve-fit constants. Decay heat data were obtained from a MIT study and adjusted for representative mass properties of solidified waste. The above expression can be readily integrated to determine !otal thermal energy available over any period t1 to t2. Results are shown in figure 3-7.

The energy required for solar system escape from low Earth orbit at low thrust is roughly equivalent to a delta V of 25 km/sec. This large delta V arises because the low thrust system must first escape Earth at nearly the full 7.73 km/sec required at infinitely low thrust plus a large proportion of the additional 30 km/sec required to escape the solar system at infinitely low thrust. (An impulsive maneuver from low Earth orbit with no gravity losses, can reach solax system escape with a delta V of about 8.8 km/sec).

The energy required to achieve a ΔV of 25 km/sec is a function of jet velocity (Isp) and of the efficiency of converting thermal energy to jet energy. The required energy versus Isp has a minimum.

This function is plotted in figure 3-8 for cycle and thruster system efficiencies of 40% and 70%.

Comparing this result with figure 3-7 and recognizing the uncertainties in such a brief analysis, the following observations are made:

  • There is a question as to whether enough energy for self-propulsion is available in nuclear waste as presently processed. Careful examination of this question and its ramifications should precede any system definition activities.
  • A system designed to utilize waste energy for disposal will be sensitive to the "quality," i.e., thermal power, of the waste. It could not dispose of "old" waste and low grade wastes (contaminated shoes, clothing, tools, etc.) except as a payload on high quality wastes.
  • The system will have to combine long life with low cost. Propulsive periods on the order of 5-10 years are required.
  • A large riumber of vehicles will be under powered flights in various stages of the escape mission at any one time. All would presumably require some degree of monitoring. We have not made an estimate of the number of vehicles (the number clearly depends on the size of each) but a number in the range between 1 00 and 1,000 is likely.

When I fly from Texas to Europe, I pay $3–6 a pound, depending on how well I do buying a ticket. When a satellite or shuttle is launched into space, the customer (or taxpayer) pays over $10,000 a pound. That is the major challenge of space flight: until the cost of going into space drastically decreases, the large-scale exploration and exploitation of space will not occur.

The world currently sends approximately 200 tons of payloads, the equivalent of two 747 freighter flights, into space annually. At $50–500 million a launch, very few cargoes can justify their cost. We have here the classic chicken-and-egg situation. As long as space flight remains very expensive, payloads will be small. As long as payloads remain small, rockets will be expensive.

If annual demand were 5,000 tons instead of 200, the equation would shift. Engineers would have the incentive to design more efficient launch systems. Large, guaranteed payloads could significantly reduce the cost of reaching orbit, ushering in a new, affordable era in space for governments, businesses, universities, and, hopefully, individuals.

Where would this much new cargo come from? Fortunately, there is an answer. Unfortunately, it’s not intuitively attractive, at least at first glance: it’s high-level nuclear waste, the 45,000 tons and 380,000 cubic meters of high-level radioactive spent fuel and process waste and detritus (as opposed to the more abundant but far less dangerous and shorter-lived low-level waste) from six decades of nuclear weapons programs and civilian power plants.

There are three good reasons to send nuclear waste into space. First, it is safe. Second, space disposal is better than the alternative, underground burial. Third, it may finally open the door to widespread utilization of space.

Because of the obvious and real concern about moving such dangerous material anywhere, let alone into space, this proposal justly raises the question of safety. Can nuclear waste be safely launched into earth orbit? The answer is yes. By keeping the launch system on the ground instead of putting it on the vehicle, designing and building unbreakable containers, and arranging multiple layers of safety precautions, we can operate in a judicious and safe manner.

The nuclear waste problem

The problem of nuclear waste disposal is real, especially for future generations. Leaving radioactive wastes on earth creates permanent and tempting targets for terrorism as well as threatening the environment. We have a moral imperative to solve this problem now so we do not burden our children and their children.

For twenty years, the federal government’s preferred solution to the nuclear waste problem is underground disposal, specifically, over 11,000 30–80 ton canisters buried in 160 kilometers of tunnels hundreds of meters underneath Yucca Mountain in northern Nevada. Forty-nine states favor this plan. It’s not hard to guess which state does not.

To be fair to Nevada, any site would draw the same objections from anybody who lost this lottery, yet policymakers remain stuck on the idea of burial. Nevada’s fears are justified: researchers cannot guarantee complete environmental isolation for the thousands of years needed for these wastes to decay harmlessly. A recent report by the Government Accountability Office raised nearly 200 technical and managerial concerns about the site. Even the promise of construction and maintenance jobs has failed to sway a skeptical public.

Historically, garbage has been something to bury or recycle. Consequently, nuclear waste disposal has remained the province of the geologists, who are professionally inclined to look down, not up. That’s shortsighted. The permanent elimination of high-level radioactive waste demands a reconceptualization of the problem. We need to look up, not down. Let’s put high-level radioactive waste where it belongs, far out in space where it will not endanger anyone on earth.

The laser launch solution

Neither the space shuttle nor conventional rockets are up to this task. Not only are they expensive, but they lack the desired reliability and safety as insurance rates demonstrate. Instead, we need to develop a new generation of launch systems where the launcher remains on the ground so the spacecraft is almost all payload, not propellant. As well as being more efficient, ground-launched systems are inherently safer than rockets because the capsules will not carry liquid fuels, eliminating the in-flight danger of an explosion. Nor will the capsules have the pumps and other mechanical equipment of rockets, further reducing the chances of something going wrong.

How would disposal of nuclear wastes in space actually work? In the simplest approach, a ground-based laser system will launch capsules directly out of the solar system. In a more complicated scheme, the laser system will place the capsules into a nuclear-safe orbit, at least 1,100 kilometers above the earth, so that they could not reenter for several hundred years at a minimum. Next, a space tug will attach the capsules to a solar sail for movement to their final destination orbiting around the sun, far, far from earth.

The underlying concept is simple: the launcher accelerates the capsule to escape velocity. Like a gun, only the bullet heads toward the target, not the entire gun. Unlike a shuttle or rocket, ground systems are designed for quick reuse. To continue the analogy, the gun is reloaded and fired again. These systems would send tens or hundreds of kilograms instead of tons into orbit per launch.

Of the three possible technologies—laser, microwave, and electromagnetic railguns—laser propulsion is the most promising for the next decade. In laser propulsion, a laser beam from the ground hits the bottom of the capsule. The resultant heat compresses and explodes the air or solid fuel there, providing lift and guidance. Although sounding like science fiction, the concept is more than just an elegant idea. In October 2000, a 10-kilowatt laser at White Sands Missile Range in New Mexico boosted a two-ounce (50 gram) lightcraft over 60 meters vertically. These numbers seem small, but prove the underlying feasibility of the concept.

American research, currently at Rensselaer Polytechnic Institute in New York with previous work at the Department of Energy’s Lawrence Livermore National Laboratory in California, has been funded at low levels by the United States Air Force, NASA, and FINDS, a space development group. The United States does not have a monopoly in the field. The four International Symposiums on Beamed Energy Propulsion have attracted researchers from Germany, France, Japan, Russia, South Korea, and other countries.

The long-term benefit of a ground-based system will be much greater if it can ultimately handle people as well as plutonium. Dartmouth physics professor Arthur R. Kantrowitz, who first proposed laser propulsion in 1972, considers the concept even more promising today due to more efficient lasers and adaptive optics, the technology used by astronomers to improve their viewing and the Air Force for its airborne anti-ballistic missile laser.

Where should the nuclear waste ultimately go? Sending the capsules out of the solar system is the simplest option because the laser can directly launch the capsule on its way. Both Ivan Bekey, the former director of NASA’s of Advanced Programs in the Office of Spaceflight, and Dr. Jordin T. Kare, the former technical director of the Strategic Defense Initiative Organization’s Laser Propulsion Program, which ran from 1987-90, emphasized solar escape is the most reliable choice because less could go wrong.

A second option, a solar orbit inside Venus, would retain the option of retrieving the capsules. Future generations might actually find our radioactive wastes valuable, just as old mine tailings are a useful source of precious metals today. After all, the spent fuel still contains over three-quarters of the original fuel and could be reprocessed. Terrorists or rogue states might be able to reach these capsules, but if they have that technical capability, stealing nuclear wastes will be among the least of our concerns. This approach is more complex, demanding a temporary earth orbit and a solar sail to move it into a solar orbit, thus increasing the possibility of something going wrong.

Addressing safety

The issue of safety has two components. One is the actual engineering of safe operations. This is demonstrable and testable. The other, equally important, part is the public perception of safety. As University of Missouri nuclear engineering professor William H. Miller, a specialist on nuclear fuel cycle and fuel management, noted, “The obvious problem is public perception. No matter how far you go to show that it is safe, there will always be someone to say ‘what if’.” John W. Poston, a Texas A&M nuclear engineering professor with a forty-six year career in nuclear health physics, agrees, considering convincing people of the safety of space-based disposal as challenging, if not more so, than the actual technical questions.

Safety should appropriately dominate public discussion of this proposal. To succeed, space disposal must demonstrate lower risk and uncertainty than underground disposal. This project must be completely safe technically, but nonetheless will not succeed unless potential supporters and opponents are thoroughly convinced about its safety and efficiency.

Assuring safety is possible. The two major concerns are launching the capsule and ensuring the integrity of the capsule. Laser launching is safer and more reliable than rockets. The absence of rocket propellants and its accompanying propulsion systems eliminates the possibility of an explosion. The major problem would be if the laser failed before the capsule reached escape velocity. Because the capsule will be bullet-shaped, its ballistic characteristics are well known. Thus, if a launch failure occurred, the capsule would land only in known recovery zones. Launch trajectories would be designed to avoid populated areas.

One advantage of a laser launch system is that the safe return from these aborted missions can be demonstrated by testing with inert capsules. Scores of launches could test every conceivable scenario, the equivalent of firing a new rifle to understand all its characteristics. This could not be done with a rocket. If another layer of safety is desired, placing the launch system on an island in the Pacific Ocean will further decrease the chance of an aborted flight landing in a populated area. Such isolation would also improve security.

The capsule itself must protect its radioactive cargo not only from the demands of a normal launch with its severe atmospheric heating and aerodynamic loading, but also from potential accidents ranging from reentry into the atmosphere to a seriously flawed launch that would send the capsule into the high pressures of the ocean’s depths or into land. Summing up the engineering challenges, Bob Carpenter, the program manager for Orbital Sciences’ space nuclear power program, cautioned, “I’m not saying they are insurmountable, but they are major technical issues to be solved.”

Jordin Kare, now an independent aerospace consultant, was more optimistic. The laser can accelerate the capsule slowly in the lower atmosphere, reducing heating. Furthermore, noted NASA nuclear engineer Dr. Robert C. Singleterry, the same aerobraking analyses and technologies that use a planet’s atmosphere to slow down a visiting spacecraft as the Mars Global Surveyor demonstrated in 1997 can ensure the control of a capsule leaving the earth’s atmosphere.

The integrity of a capsule can be demonstrated too. The aerospace industry has accumulated decades of research and experience on how to contain radioactive material in containers that can maintain their integrity despite atmospheric re-entry, accidents, explosions, and other potential catastrophes. They are called nuclear warheads. Designing containers for space disposal is well within the state of the art. Dr. Rowland E. Burns, the engineer who led a NASA study in the mid-1970s on this issue, stated it is feasible to design and construct containers that can safely withstand the demands of even a catastrophic explosion, claiming, “I won’t say you would have to nuke the container to break it, but it would take something like that.”

Materials technology has improved since the 1970s, making even tougher capsules possible. Because launch costs will be relatively inexpensive, engineers can overdesign for safety instead of trying to create the lightest possible container. Fail-proof capsules can be built, though the ratio of waste to shielding will be low.

Ensuring safety must have an inclusionary component. A broadly based panel of stakeholders, including skeptics and opponents, should determine the criteria for tests and scenarios that proponents must pass. Computer simulations and controlled tests, however, will not be enough. Convincing demonstrations such as aborting launches with a mock payload and sending test capsules to reenter the atmosphere will be necessary to calm fears and prove the veracity of safety calculations. Minimum danger must be demonstrated, not assumed. Those opponents who unilaterally reject space-based disposal should be asked to propose an alternative. Nuclear waste will not go away on its own volition.

Expensive and inexpensive

What about the economics? Let’s be honest and upfront in our accounting: Space disposal will ultimately cost tens of billions of dollars, but the federal government has already spent $8 billion researching underground disposal and expects the total cost will be $60 billion. The difference is that future generations will not have to worry about the waste and they will have an infrastructure for reaching space. While technologically impressive, developments in tunnel boring have far less potential. Disposal in any form will be expensive. Space disposal at least offers a major spinoff, inexpensive access to space. Putting a small surcharge—a fraction of a cent per kilowatt-hour of electricity—on power generated by nuclear reactors would handle the operational costs.

How can a system be both expensive and inexpensive? Judging by the costs of other high technology projects such as the Airbus 380 and Boston’s Big Dig, developing a laser launch system will require at least $5–10 billion. This is a lot of money, but historically space technologies are expensive: The Apollo program cost over $150 billion in contemporary dollars. Constructing the actual launch system will require a few billion dollars and operations will consume billions more. And even if the price of a pound to escape velocity is only $100, 5000 tons is $1 billion.

We owe the future as well as ourselves the opportunity to determine whether space-based disposal is the best way to handle nuclear waste. Accordingly, over the next few years, NASA and the Department of Energy should establish three research programs. The first will determine the criteria and acceptance for a demonstration program. The second program will design safe capsules and the third program will test the ground-launched system. For the price of a new hotel in Las Vegas or a day or two of the defense budget, we will have enough information to decide whether to commit large resources to space-based disposal.

Space disposal may not appear the obvious solution to the high-level nuclear waste problem. Nor is disposing of nuclear waste the obvious answer to the question of how to reduce the cost of reaching space. But the immense magnitude of nuclear wastes provides the incentive to develop launch systems that will drastically cut the cost of space exploitation. The result will be lower operating costs, more infrastructure, and more skilled personnel able to develop other areas of space.

The development of the computer may offer a good analogy. Government funding, mostly from the military, intelligence community, and NASA, greatly accelerated research, development, and diffusion of computers since the 1940s. The federal government did this to conduct projects of national significance such as the census, Social Security, weapons research (especially nuclear explosions), cryptoanalysis, and space exploration. Not until the 1970s did the civilian market grow large enough to seize the technological initiative.

Space disposal may prove a similar opportunity. Once a ground launcher is developed and built, constructing additional launchers will be far less costly and risky. The dream of affordable access to space may then come true, opening up the final frontier in ways that we have not dreamed of since the 1960s. As important, we will be acting ethically, providing our children a safer earth and inexpensive access to space for people as well as plutonium.

From NUCLEAR WASTE IN SPACE? by Jonathan Coopersmith (2005)

      Tim started the scanner going as soon as I handed over the controls. He thought I'd picked up our discarded fuel container again — which annoyed me since it showed little faith in my common sense. But he soon saw that it was in a completely different part of the sky and his scepticism vanished.
     'It must be a spaceship,' he said, 'though it doesn't seem a large enough echo for that. We'll soon find out — if it's a ship, it'll be carrying a radio beacon.'
     He tuned our receiver to the beacon frequency, but without result. There were a few ships at great distances in other parts of the sky, but nothing as close as this.
     Norman had now joined us and was looking over Tim's shoulder.
     'If it's a meteor,' he said, let's hope it's a nice lump of platinum or something equally valuable. Then we can retire for life.'
     'Hey!' I exclaimed, 'I found it!'
     'I don't think that counts. You're not on the crew and shouldn't be here anyway.'
     'Don't worry,' said Tim, 'no one's ever found anything except iron in meteors — in any quantity, that is. The most you can expect to run across out here is a chunk of nickel-steel, probably so tough that you won't even be able to saw off a piece as a souvenir.'

     By now we had worked out the course of the object, and discovered that it would pass within twenty miles of us. If we wished to make contact, we'd have to change our velocity by about two hundred miles an hour — not much, but it would waste some of our hard-won fuel and the Commander certainly wouldn't allow it, if it was merely a question of satisfying our curiosity.
     'How big would it have to be,' I asked, 'to produce an echo this bright?'
     'You can't tell,' said Tim. 'It depends what it's made of — and the way it's pointing. A spaceship could produce a signal as small as that, if we were only seeing it end-on.'

     'I think I've found it,' said Norman suddenly. 'And it isn't a meteor. You have a look.'
     He had been searching with the ship's telescope, and I took his place at the eyepiece, getting there just ahead of Tim. Against a background of faint stars a roughly cylindrical object, brilliantly lit by the sun­light, was very slowly revolving in space. Even at first glance I could see it was artificial. When I had watched it turn through a complete revolution, I could see that it was streamlined and had a pointed nose. It looked much more like an old-time artillery shell than a modern rocket. The fact that it was streamlined meant that it couldn't be an empty fuel container from the launcher in Hipparchus: the tanks it shot up were plain, stubby cylinders, since streamlining was no use on the airless Moon.
     Commander Doyle stared through the telescope for a long time when we called him over. Finally, to my joy, he remarked: 'Whatever it is, we'd better have a look at it and make a report. We can spare the fuel and it will only take a few minutes.'

     Our ship spun round in space as we began to make the course-correction. The rockets fired for a few seconds, our new path was rechecked, and the rockets operated again. After several shorter bursts, we had come to within a mile of the mysterious object and began to edge towards it under the gentle impulse of the steering jets alone. Through all these manoeuvres it was impossible to use the telescope, so when I next saw my discovery it was only a hundred yards beyond our port, very gently approaching us.
     It was artificial all right, and a rocket of some kind. What it was doing out here near the Moon we could only guess, and several theories were put forward. Since it was only about ten feet long, it might be one of the automatic reconnaissance missiles sent out in the early days of space-flight. Commander Doyle didn't think this likely: as far as he knew, they'd all been accounted for. Besides, it seemed to have none of the radio and tv equipment such missiles would carry.

     It was painted a very bright red — an odd colour, I thought, for anything in space. There was some lettering on the side — apparently in English, though I couldn't make out the words at this distance. As the projectile slowly revolved, a black pattern on a white background came into view, but went out of sight before I could interpret it. I waited until it came into view again: by this time the little rocket had drifted considerably closer, and was now only fifty feet away.
     'I don't like the look of the thing,' Tim Benton said, half to himself. 'That colour, for instance — red's the sign of danger.'
     'Don't be an old woman,' scoffed Norman. 'If it was a bomb or something like that, it certainly wouldn't advertise the fact.'
     Then the pattern I'd glimpsed before swam back into view. Even on the first sight, there had been something uncomfortably familiar about it. Now there was no longer any doubt.
     Clearly painted on the side of the slowly approaching missile was the symbol of Death — the skull and crossbones.
     Commander Doyle must have seen that ominous warning as quickly as we did, for an instant later our rockets thundered briefly. The crimson missile veered slowly aside and started to recede once more into space. At the moment of closest approach, I was able to read the words painted below the skull and crossbones — and I understood. The notice read:




     'I wish we'd got a Geiger counter on board,' said the Commander thoughtfully. 'Still, by this time it can't be very dangerous and I don't expect we've had much of a dose. But we'll all have to have a blood-count when we get back to base.'

     'How long do you think it's been up here, Sir?' asked Norman.
     'Let's think — I believe they started getting rid of dangerous waste this way back in the 1970s. They didn't do it for long — the Space Corporations soon put a stop to it! Nowadays, of course, we know how to deal with all the by-products of the atomic piles, but back in the early days there were a lot of radioisotopes they couldn't handle. Rather a drastic way of getting rid of them — and a short-sighted solution, too!'
     'I've heard about these waste-containers' said Tim, 'but I thought they'd all been collected and the stuff in them buried somewhere on the Moon.'
     'Not this one, apparently. But it soon will be when we report it. Good work, Malcolm! You've done your bit to make space safer!'
     I was pleased at the compliment, though still a little worried lest we'd received a dangerous dose of radiation from the decaying isotopes in the celestial coffin. Luckily my fears turned out to be groundless — we had left the neighbourhood too quickly to come to any harm.

     We also discovered, a good while later, the history of this stray missile. The Atomic Energy Commission is still a bit ashamed of this episode in its history, and it was some time before it gave the whole story. Finally it admitted the dispatch of a waste-container in 1981 that had been intended to crash on the Moon but had never done so. The astronomers had a lot of fun working out how the thing had got into the orbit where we found it — it was a complicated story involving the gravities of the Earth, Sun and Moon.

From ISLANDS IN THE SKY by Arthur C. Clarke (1952)

Aldrin Cyclers

A trip to Mars is very expensive in terms of propellant. Rockets are very sensitive to mass. Remember that Every Gram Counts.

The Mars mission requires hydroponics for food and air for the astronauts, nine months worth. The habitat module. And worst of all, the massive anti-radiation storm cellar. All of this takes mass. Then you have to add the mass for the lander and the other equipment you'll need on Mars. Just think about the propellant bill.

Then if you have a second expedition, you have to pay for it all again. And for each subsequent expedition.

About this time, astronautics experts had the thought "what if we could re-use some of the required equipment?" More specifically, re-use the delta-V.

Take the habitat module, the hydroponics, and the storm cellar and make it into a space station. Spend enough propellant to delta-V it up into an orbit that passes by Mars and eventually returns to Earth. It will regularly pass by Earth and Mars for the rest of eternity, with a little mid-course correction now and then. So you now have a habitat module delta-Ved for a Mars mission that can be re-used. It is a Cycler.

For your next Mars mission, you have a transfer vehicle that will carry the crew and mission specific payload. It rendezvous with the cycler, more or less paying the same delta-V cost as the start of a Mars mission. Except it only pays the propellant cost for the crew and the mission payload, it does not have to pay for the habitat module. You will be re-using the delta-V for the hab module by using the cycler. When the cycler passes by Mars, the transfer vehicle leaves the cycler and burns enough propellant (or aerobrakes in the Martian atmosphere) to delta-V into Mars orbit. The cycler goes on its merry way, still full of delta-V, still available for re-use by a future expedition.

Keep in mind that you still need the propellant for the people and mission payload. But saving the propellant needed for the habitat module is a huge help.

Understand that since a cycler is a clever way to reuse the delta V of the habitat module, the hydroponics, and the storm cellar, the implication is that a cycler is worthless for sending inert payloads to Mars. It will take the exact same amount of delta-V to send the inert payload to Mars regardless of whether you use the cycler or not, so what's the point? This is because inert payloads do not need habitat modules, hydroponics, nor storm cellars.

Hop David has computed the orbits for Earth-Asteroid cyclers, discovering the existence of virtual "railroad towns".

UCLA Cycler

UCLA Cycler
Storm Cellar15,000
x2 Crew
Refrig Cycle31
Reactor and
1,390 kg/m3)
x60 Ion
x16 Crew,

This is from Ion engine propelled Earth-Mars cycler with nuclear thermal propelled transfer vehicle. It is a preliminary study by California University School of Engineering and Applied Science.

The report makes a few assumptions:

  • There is a space station in LEO to be a base for construction of the cycler, and a rendezvous spot for the "taxi" (spacecraft that ferries astronauts to and from the cycler)
  • There is some kind of transportation system between Terra and the space station (a Space Shuttle or Soyuz spacecraft)
  • Previous missions has already established habitats on the Martian surface, as well as landing/launch pads for the taxi
  • Previous missions has already established an in-situ resource utilization plant to produce liquid hydrogen propellant for the NTR taxi. The cycler cannot a transport all the propellant the taxi needs, it has to refuel on Mars.
  • Previous missions have already established a fuel ship capable of transporting liquid hydrogen from the Martian ISRU plant to the taxi in low Mars orbit

The heart of a cycler system is the Cycle; that is, the orbit it follows.

The study looked at various orbits, trying to optimize for:

  • More frequent encounters between Mars and Terra
  • Smaller detal V angles and Terra and Mars approaches
  • Shorter stay time on the cycler
  • Easy predictability of the position of the cycler

Having just one cycler and rotating its orbit to meet the two planets seems attractive, but there are major drawbacks. The fuel required to rotate the orbit are expensive, about 85% the mass of the cycler. This requires constant refueling. Also since the cycler is not in a predictable orbit the motion will have to be constantly monitored and mid-course corrections applied. With no corrections the orbit error will propagate to future trips.

To deal with the predictability problem a proposed solution was to rotate the orbit every 2.143 years (the delay between times the relative positions of Mars and Terra repeat) by 51.429 degrees (360° / 7, giving 7 discrete orbits). This would cover the entire range with only seven passes. The orbits would be rotated by a burn performed at the closest approach to Terra in order to get a gravity assist. The drawback is that the fuel requirements would be about the same, and there would be periods of more than ten years before the cycler returned to Mars and allowed the Mars explorers to return to Terra.

So this option was rejected.

The Up/Down Escalator orbit was ruled out because: the Taxi would need excessive amounts of delta V to catch a ride and the orbit would have to go way further past Mars in order to encounter Mars on the inward swing (which drastically increases the cycler orbital period).

This option was rejected as well.

The report concluded that the optimum cycle was using three cyclers with VISIT-like orbits. One at zero degrees, one at +130° and one at -130° (230°). This allows squeezing the most mission into each 20 year period while optimizing the other factors.

In Table 2, row Cycler DELTA V (row 19) shows the taxi delta V needed to leave the cycler and enter close Mars orbit. This varies from 5.27 to 6.32 kilometers per second. Row Hyperbolic delta v (27) shows the taxi delta V needed to leave the cyclear and enter close Terra orbit. This varies from 9.49 to 10.46 km/s. They figure that to perform these maneuver the taxi will need a thrust of 5.639e+5 Newtons, which is good because the planned taxi engine will have a thrust of 6.98e+5N.

Taxi Requirements
ORBIT 1 Mars Approach
ΔV6.3152 km/s
Propellant16,108 kg
Burn Time378 s (6.3 min)
ORBIT 1 Terra Approach
ΔV9.4889 km/s
Propellant20,564.34 kg
Burn Time432.7 s (8.05 min)
ORBIT 2 & 3 Mars Approach
ΔV5.2707 km/s
Propellant14,220 kg
Burn Time333.78 s (5.56 min)
ORBIT 2 & 3 Terra Approach
ΔV10.4554 km/s
Propellant21,613 kg
Burn Time507.3 s (8.46 min)

Table 3 shows three Terra-Mars mission opportunities over an 18 year period. Trip 1 starts at year Zero, uses the zero degree cycler, and has a mission duration of 5.41 years. Trip 2 starts at year 4.75 and has a mission duration of 6.73 years. Trip 3 starts at year 14.89 and has a mission duration of 2.856 years.

The Reactor produces 10 MWe (electrical) power. The Power Conversion is a Stirling cycle with an efficiency of 0.254 so the reactor has to produce 40 MWth (heat). To reject waste heat 1,000 m2 of Heat Radiators operating at 1,000K are used.

The Storage / Experiment / Greenhouse module is above the hub. It contains space for microgravity experiments, food storage, and the life support reclamation systems. The hygiene/gray water reclamation system uses various filters, as well as Waldman's dark green lettuce in the greenhouse.

The cycler provides artificial gravity by spinning as a dependent centrifuge, where the spin axis is parallel to the thrust axis. The habitat modules are set at a radius of 50 meters from the spin axis, the spin rate is 2.32 rpm (at the nausea limit for the untrained), the resulting gravity is 0.3 g. The report assumes this will be enough gravity to prevent muscle atrophy, since it will be very hard to explore Mars if the astronauts are too weak to walk. For what it is worth the surface gravity of Mars is 0.376 g.

The bulk of the cycler's mass is on the spin axis (with the exception of the habitat modules) to make the smallest moment of inertia. This reduces the amount of reaction control jet fuel needed to spin up or spin down the cycler. The jets are located at the ends of each habitat module. The cycler must be despun for docking and releasing the taxi.

Each of the two Habitat Modules has two levels: command level above and residential level below.

The command level has the control/communication room, the kitchen, the communal room (including exercise equipment), and the infirmary. On such a multi-year mission a dedicated sickbay is needed. And two of the sixteen crew are doctors.

The residential level has the crew quarters, toilets and showers. Each crew member has their very own 2 x 3 meter private room, with bed and desk.

The Storm Cellar is located below the hub. It can hold all sixteen astronauts and is designed to ensure that a solar proton storm does not inflict a dose higher than 0.5 Sieverts. The astronauts are seated in semi-reclined chairs so they can sleep or do work, since the ceiling is too low to stand up (2.5 meters floor to ceiling).

Aluminum shielding instead of water was chosen due to ease of construction and maintenance. The shielding is 20 grams per square centimeter of aluminum (thickness of 7.4 centimeters). The largest proton storm ever recorded was the August 1972 solar event and the most harmful spectrum was the February 1956 solar event. If the cycler suffers a solar event with the duration and intensity of the 1972 event coupled with the deadly spectrum of the 1956 event the storm cellar will ensure the astronauts only suffer a dose of 0.43 SV.

The storm cellar has an expensive mass of 15,000 kilograms, but radiation shielding always has a painful amount of penalty mass.

The Communication Array is de-spun. It is mounted on a coupling with rings and brushes (perhaps this could be replaced by a Canfield Joint).

The Ion Thrusters use argon propellant. Each engine has a mass of 165 kg, a diameter of 0.85 m, a specific impulse of 10,000 sec, a propellant mass flow of 1.579E-3 kg, and produce 4.4 Newtons of thrust. The reactor produces 10 MW of electricity but for safety and to leave power for the rest of the ship only 9.5 MW are used by the engines. For the 130,000 kg cycler, 35 ion engines were deemed adequate. Since ion engines tend to fail, 60 engines are carried.

Ion Requirements
ΔV10.5 km/s
Propellant13,195 kg
Burn Time8,356,554.78 s (96 days 17 hours 15.9 minutes)
ORBIT 2 and 3
ΔV11.46 km/s
Propellant14,333 kg
Burn Time9,077,264.09 s (105 days 1 hour 27.7 minutes)

Stage 1
EngineSolid core
Isp836 s
Ve8,200 m/s
Thrust349,000 N
Mass Flow42.6 kg/s
Accel11.65 m/s2
Dry Mass11.65 m/s2
TOTAL21,614 kg
Stage 1
TOTAL8,386 kg
Wet Mass30,000
Taxi Requirements
ORBIT 1 Mars Approach
ΔV6.3152 km/s
Propellant16,108 kg
Burn Time378 s (6.3 min)
ORBIT 1 Terra Approach
ΔV9.4889 km/s
Propellant20,564.34 kg
Burn Time432.7 s (8.05 min)
ORBIT 2 & 3 Mars Approach
ΔV5.2707 km/s
Propellant14,220 kg
Burn Time333.78 s (5.56 min)
ORBIT 2 & 3 Terra Approach
ΔV10.4554 km/s
Propellant21,613 kg
Burn Time507.3 s (8.46 min)

The taxi has two stages: a nuclear powered first stage with a solid core NTR and a chemical powered second stage (based on a McDonnell Douglas DC-X).

Contrary to what you'd expect, the nuclear stage never lands on Mars. Its purpose is to ferry the chemical stage from the cycler (as is whizzes past Mars) to low Mars orbit. The chemical stage separates and lands on Mars while the nuclear stage stays in Mars parking orbit. It seems that the designers were hesitant to bath a part of the Martian surface with deadly radiation from the nuclear engine. It was also a challenge to protect the astronauts from getting a bad dose of radiation as they crawled down the ladder along the taxi's side to step on the Martian surface.

While the astronauts explore the Martian surface, a robot propellant transport containing a full load of ISRU liquid hydrogen blast off and refuels the nuclearr stage in parking orbit. The chemical stage on the surface also has its liquid hydrogen tanks topped off.

When the return cycler approaches, the astronauts blast off from Mars in the chemical stage, rendezvous with the nuclear stage, and use the nuclear engine to rendezvous with the cycler.

The report is very vague on the chemical stage, other than stating it has a maximum mass of 8,386 kg. Doing some back of the envelope calculations I figure it will need about 3,550 m/s of delta V to land or blast off from Mars. Using LH2/LOX chemical engines with 4,905 m/s of exhaust velocity, the chemical stage will need a mass ratio of 2.06 in order to produce enough delta V. If the wet mass is 8,386 kg, then the propellant mass is 4,315 kg and the dry mass is 4,071 kg.

I find the figures for the Taxi Requirements puzzling. It says the Orbit 2 Terra Approach burn requires 21,613 kg of propellant. However, if the total wet mass is 30,000 kg, minus the 8,386 kg for the second stage gives us a wet mass for the first stage of … 21,613 kg. Subtact the required propellant from that and you discover the dry mass of the first stage is zero. Which is impossible. I am re-reading the report to try and figure this out. It could be that they are assuming that at the Terra Approach Burn the chemical stage will have empty fuel tanks.

Hildas As Cyclers


      The above image was made from screen captures of Scott Manley's beautiful animation Asteroids In Resonance With Jupiter.

     Jupiter is the dot off to the left, the sun is the yellow dot in the middle. Within the Main Belt can be seen Mercury, Venus, Earth and Mars. I colored the different asteroid populations so we can tell them apart.

     The Sun Jupiter Trojans have a 1 to 1 resonance with Jupiter. They co-rotate with Jupiter. The leading Trojans remain in a neighborhood 60 degrees ahead of Jupiter and the trailing Trojans stay in a neighborhood 60 degrees behind.

     The Hildas have have 3 to 2 resonance with Jupiter meaning they circle the sun three times for every two Jupiter orbits. Jupiter's orbital period is about 12 years and the Hildas have 8 year periods.

     The Hilda orbits only look triangular in Manley's animation because they're being viewed in a rotating frame. You can see Jupiter remains on the left side of the image. In an inertial frame, a Hilda orbit is an ordinary elliptical orbit with aphelion passing through the Trojans and perihelion passing through the main belt.

     I envision the Hilda biomes playing a similar role as Marco Polo's caravans shuttling people and goods between east and west. But the Hildas travel between the Trojans and the Main Belt.

     The would be a series of regular fly bys for a Hilda Cycler:

  1. Main Belt to trailing Trojans — 4 years
  2. Trailing Trojans to Main Belt — 4 years
  3. Main Belt to leading Trojans — 4 years
  4. Leading Trojans to Main Belt — 4 years
  5. Main Belt to Sun Jupiter L3 — 4 years. But there is no asteroid population at SJL3
  6. From SJL3 to Main Belt 4 years

     Then back to step 1). The cycle repeats itself.

     So not only can a Hilda be a go between between the Main Belt and Trojans, but it can also move stuff between the trailing and leading Trojan populations. Trailing to leading takes 8 years and leading to trailing takes 16 years.

     As can be seen from Manley's animation, there is a steady stream of Hildas traveling the circuit.

Delta V

     The Hildas have a variety of eccentricities. I will look at a Hilda orbit having an eccentricity of .31. That would put the aphelion at 5.2 A.U. and the perihelion at 2.74 A.U. (The perihelion is in Ceres' neighborhood, Ceres' semi-major axis is 2.77 A.U.).

     Assuming a circular, coplanar orbit at 2.74 A.U., it would take 2.6 km/s to leave a Main Belt Asteroid and board a Hilda.

     Assuming a circular, coplanar orbit at 5.2 A.U., it would take 2.2 km/s to depart the Hilda and rendezvous with a Trojan.

     However, coplanar orbits is a very optimistic assumption. Asteroids have a large variety of inclinations. Making a 10 degree plane change from a Hilda's orbit can cost 2 to 3 km/s.

Ways to mitigate delta V expense

     Many asteroids spin about pretty fast. This plus their shallow gravity wells make them amenable to bean stalks, also known as space elevators.

     "Why would an asteroid need a space elevator?" I'm sometimes asked. The questioner will assert "It's very easy to get off an asteroid's surface, and getting off the body's surface is the only reason for an elevator." Which is wrong, of course.

     Speed of a body on an elevator is ωr where ω is angular velocity in radians per time unit and r is distance from center of rotation. If r is large, the elevator can fling a payload at high velocity with regard to the asteroid. It is quite plausible for an asteroid's bean stalk to provide .5 to 1 km/s delta V.

     Also an asteroid bean stalk allows rendezvous with an ion propelled space craft. Ion ships have great ISP but minute thrust. Soft landings with an ion craft are not possible on larger asteroids like Ceres, or Vesta.

     And ion propelled ships are more viable in the outer system. When a ship's acceleration is a large fraction of the local gravity acceleration, an ion burn is more like a chemical impulsive burn. See General Guidelines for Modeling a Low Thrust Ion Spiral. In the outer Main Belt, the sun's gravity is about 1 millimeter/sec2. Sun's gravity at the Trojans is about .2 millimeters/sec2.

Jupiter's Trojans

     Not much is known about Jupiter's Trojans. Japan hopes to launch a mission in the early 2020's. Another proposed mission is Lucy which would also launch in the early 2020's.

     These bodies are on average 5.2 A.U. from the sun and so receive only 1/27 the sunlight earth enjoys. For this reason I am hopeful they are rich in volatile ices. I'd give better than even odds they have lots of water and carbon dioxide ice. Nitrogen compounds like ammonia and cyano compounds are a possibility. Aside from earth, Nitrogen is in short supply throughout the inner solar system and these would be a great export to the Main Belt biomes.

     Their numbers are speculation. According to Wikipedia:

Estimates of the total number of Jupiter trojans are based on deep surveys of limited areas of the sky. The L4 swarm is believed to hold between 160–240,000 asteroids with diameters larger than 2 km and about 600,000 with diameters larger than 1 km. If the L5 swarm contains a comparable number of objects, there are more than 1 million Jupiter trojans 1 km in size or larger. For the objects brighter than absolute magnitude 9.0 the population is probably complete. These numbers are similar to that of comparable asteroids in the asteroid belt. The total mass of the Jupiter trojans is estimated at 0.0001 of the mass of Earth or one-fifth of the mass of the asteroid belt.

Two more recent studies indicate, however, that the above numbers may overestimate the number of Jupiter trojans by several-fold. This overestimate is caused by (1) the assumption that all Jupiter trojans have a low albedo of about 0.04, whereas small bodies may actually have an average albedo as high as 0.12; (2) an incorrect assumption about the distribution of Jupiter trojans in the sky. According to the new estimates, the total number of Jupiter trojans with a diameter larger than 2 km is 6.3 ± 1.0×104 and 3.4 ± 0.5×104 in the L4 and L5 swarms, respectively. These numbers would be reduced by a factor of 2 if small Jupiter trojans are more reflective than large ones.

The number of Jupiter trojans observed in the L4 swarm is slightly larger than that observed in L5. However, because the brightest Jupiter trojans show little variation in numbers between the two populations, this disparity is probably due to observational bias. However, some models indicate that the L4 swarm may be slightly more stable than the L5 swarm.

The largest Jupiter trojan is 624 Hektor, which has an average diameter of 203 ± 3.6 km. There are few large Jupiter trojans in comparison to the overall population. With decreasing size, the number of Jupiter trojans grows very quickly down to 84 km, much more so than in the asteroid belt. A diameter of 84 km corresponds to an absolute magnitude of 9.5, assuming an albedo of 0.04. Within the 4.4–40 km range the Jupiter trojans' size distribution resembles that of the main-belt asteroids. An absence of data means that nothing is known about the masses of the smaller Jupiter trojans. The size distribution suggests that the smaller Trojans are the products of collisions by larger Jupiter trojans.

     I'd love to see science fiction stores set on 624 Hektor.

     This article written in memory of Hilda Alvarez May 5, 1929 - July 20, 2016

From HILDAS AS CYCLERS by Hollister David (2016)

Interplanetary Internet

This section is for a telecommunications network around a planet or within a given solar system.

For an interstellar faster-than-light telecommuications network see here.

The Interplanetary Internet (based on IPN, also called InterPlaNet) is a conceived computer network in space, consisting of a set of network nodes which can communicate with each other. Communication would be greatly delayed by the great interplanetary distances, so the IPN needs a new set of protocols and technology that are tolerant to large delays and errors. While the Internet as it is known today tends to be a busy network of networks with high traffic, negligible delay and errors, and a wired backbone, the Interplanetary Internet is a store-and-forward network of internets that is often disconnected, has a wireless backbone fraught with error-prone links and delays ranging from tens of minutes to even hours, even when there is a connection.


NASA is about to make it a little easier to check your Instagram in zero gravity. Two teams, Science Mission Directorate and Human Exploration and Operations, are working together to finally make interplanetary internet a thing. Previous efforts to bring WiFi throughout the solar system haven’t always been successful, but this time, it could become reality.

It will work using something called Delay/Disruption Tolerant Networking, which is pretty similar to the internet you’re familiar with. But conventional internet doesn’t do well in space, plagued with long delays, noisy channels, and high error rates.

With DTN, even if your connection gets disrupted, it will guarantee data packet delivery once the next communication path opens. Normally, if you lose connection, the data gets dumped. But by removing the need to retransmit during a lag, it saves time and frees up the limited memory used by spacecraft.

Cosmic WiFI

Getting WiFi in space is complex, especially given typical extreme distances and fragile connection links. Even if your internet is traveling at lightspeed, it can take considerable time to send a message from Earth to Mars, for example. NASA previously proposed bringing the internet to the Red Planet in 2009, but due to budget constraints, the Mars Telecommunications Orbiter was scrapped. It would have used high-speed radio signals and laser light beams to send the equivalent of three compact disks of data each day.

DTN will now be deployed with the launch of PACE, or the Plankton, Aerosol, Cloud, ocean Ecosystem Mission, an Earth-monitoring satellite operation that will advance our understanding of climate change. The satellite, slated for launch in 2022, will surveil everything from massive storms to algal blooms to carbon cycles, teaching us more about the health of the planet’s oceans.

But testing DTN goes back to 1998, when the U.S. Defense Advanced Research Projects Agency (DARPA) began its Next Generation Internet initiative, which financed a small team at NASA’s Jet Propulsion Laboratory in Pasadena, California. Their goal was to build a “specialized deep space backbone network of long-haul wireless links” under the guidance of internet pioneer Vinton Cerf. It built upon the Space Communications Protocol Specifications developed by the late Adrian Hooke.

Since then, DTN has been used on Deep Impact, a space probe that in 2005 launched an impactor at a comet called Hartley 2, which caused an explosion equivalent to 4.8 tons of TNT. More recently, NASA tested DTN to remotely drive a Lego car in Germany from the International Space Station and to send a photo at the National Science Foundation’s McMurdo Station in Antarctica, where internet connection is spotty.

“DTN represents a shift in how data will get delivered in the future,” NASA engineer David Israel said in a statement.

DTN is just a part of NASA’s Decade of Light initiative, a growing endeavor to build an internet in the Solar System, including NASA’s Near Earth Network, Space Network and Deep Space Network. As Space Race 2.0 heats up, we’re going to need faster, more reliable ways to connect online — and technologies like DTN will be the path forward to exploring the stars.


/* Increase the timeout each time we retransmit. Note that
* we do not increase the rtt estimate. rto is initialized
* from rtt, but increases here. Jacobson (SIGCOMM 88) suggests
* that doubling rto each time is the least we can get away with.
* In KA9Q, Karn uses this for the first few times, and then
* goes to quadratic. netBSD doubles, but only goes up to *64,
* and clamps at 1 to 64 sec afterwards. Note that 120 sec is
* defined in the protocol as the maximum possible RTT. I guess
* we'll have to use something other than TCP to talk to the
* University of Mars.

* PAWS allows us longer timeouts and large windows, so once
* implemented ftp to mars will work nicely. We will have to fix
* the 120 second clamps though!



Actually, given the great time lags in communications (and probable bandwidth issues), I would not expect interplanetary Tweets, phone calls or even emails.

Point to point communications would most likely resemble texting (including the incomprehensible abbreviations), but you would have anywhere from 1.4 seconds delay to the moon to many hours to Uranus.

This means another possible driver for high performance spacecraft would be mail delivery. (Consider a small car delivering 100 DVDs for Netflixx probably is carrying more information than you can access through your home internet connection in a day.) Contracts, magazines, movies, personal messages and anything else which needs more detail than a text message will all be loaded aboard mail servers on fast packets, and blasted to their destinations via the fastest means possible.

This of course leads to interesting scenarios where protecting and intercepting mail becomes important for intelligence agencies, business and criminals. Mail delivery will involve high levels of security and screening of the mail delivery personnel. I doubt anyone will hijack a mail packet in flight, but having a covert operative on board to hack the mail server and download the interesting information is a very real possibility.

Jim Baerg:

This sounds very implausible to me.

The light speed delays mean that all interplanetary communication will resemble emails with attachments more than phone calls.

The immense power required for torchship performance would make interplanetary communication lasers much more economical for sending even terabytes of data, much less an email greeting with a 1 MB photo.


Email in space — Should be no problem with this, even if the distances slow it down to telegraph speeds.

Text messaging in space — Will be restrained to within a few light-seconds, probably no more than 10 or so. Any longer than that and you might as well send a longer message.

Instant messaging — Will probably be restrained to 2-3 light-seconds. Like a phone call, any more delay defeats the whole purpose.

Physical packets — Documents that you don't want easily copied, high-density data storage for non-essential information (Movies, music, etc), items that can't be replicated at the destination (Due to lack of facilities), items that shouldn't be replicated at the destination (Gifts), and people (Diplomats, managers, mediators, etc).


To expand on Citizen Joe's point, would you like to receive "Male enhancement" emails with the olympusmon.mars address? How about "Dear Sir, I am the last surviving member of the Europan Resistance front and need your help to transfer $10 billion solars from the Bank of Callisto...."

On a more serious note, the high bandwidth links would probably be reserved for ship traffic, government and military communications and corporate communications (for companies with the financial clout to get in line for email). Certainly the Uranus Space Navy would not want the high bandwidth links clogged during their showdown with the Imperial Jovian Navy, nor would they want to risk malware or botnet attacks coming through those links; which suggests interplanetary comms would be tightly controlled and subscribers carefully vetted.


Since power and surface area aren't problems on the ground (and to a lesser extent, in orbit), communications would likely be predominantly TO ships while ship to shore (home base) comms would be limited to a confirmation of receipt. On Earth, we can afford to put up huge arrays to catch the smallest radio signal. Not so much in space. Likewise we can pump a lot of energy into the antenna to send back a longer message with a lot of bandwidth and strength even at stupendous ranges.

Now there are some tricks, like omnidirectional beacons and antenna that act as targets for the directional antennae. But you can only listen to data in the direction of the directional antenna. That might be limited to a single stream. Comm relays would likely have at least 4: Signal in, Signal out, Previous relay, Next Relay. By using multiple relays (at least 4 would get you around the sun) you wouldn't have black outs (except at ship orbit). And then there is the problem with the fragile gimbals needed for all the antennae.

In the end, yes, you can communicate via radio. No, it isn't broadband. No, it won't service a population comparable to the internet. It will likely be biased communication. It will still be expensive. There will be extreme needs that keep every carrier busy. So, although you COULD send a digital copy of Pluto Nash to Pluto, you would never get enough priority to use the carriers and thus it would be simpler to put it on a torch or fling it out the airlock.

Jean Remy:

What about relays at Lagrangian points?

Everyone knows where those are, and if someone needs to connect they can just link into the network. I don't even think lasers would even be needed. The relays would have high gain antennas to receive the data on broadband signals, and when a ship or colony needs to link in they can query the closest available platform with a much lower-gain antenna.

If we can be in contact with various probes (like Voyager probes) at interplanetary distances, on 1970s technology, then I hardly think you would need a giant technological leap to create a system-wide comm network.

Granted you won't have the bandwidth of fiberoptic cables, so no browsing for Earth-porn from Callisto, but I don't see communication as much of an impediment. The only real reason to use lasers (that I see) would be for private (read: military) communications that you don't want intercepted, but with a target area of 90 km in radius, it's not really very private anymore.


Surfing the interplanetary web would be a unique experience. Given the reply times measured in minutes or hours, getting a response for a search query would be more akin to making a request at the Library of Congress and then waiting for the attending librarian to go and bring you a copy of whatever they think you asked for (which may or may not be what you were actually looking for).

Since we survived the dark ages of dial-up modems, I think we could probably put up with the connection speed of the interplanetary net.


I think that interplanetary communications networks would be more like a cluster of 'webs'; one on Earth, one on Mars, one on Luna, ect. The different webs would send updates to each other on a regular schedule via dense-data/high-priority channels, and all other inquries/messages being sent via lower-priority channels. Most of your routine web activities would be with your local internet, but occationally you'd connect with another world's internet via the systen-wide web.

Jean Remy:

Interestingly enough, this is the way the "extranet" is postulated to work in the Mass Effect games. Essentially all colonies pack with them a web server full of the most needed and demanded data. If the information you are looking for is not on the server the system opens a communication link (FTL in the game) and hits the closest large colony server, and so on, until it finds the info, and it uploads it into the colony's server for easy retrieval later. In the game data storage is no longer an issue, but I assume if storage space is needed and a specific file hasn't been looked at for a while, it would be deleted.

However, since most early colonies will be science bases, I would assume large amounts of data will be passed back and forth as scientists on Callisto and on Earth look over the data, form theories and send them back and forth. It seems like a very fluid form of data exchange is needed even with local "nets". I also doubt early colonies will be very large, or widespread, and it is far more likely the Callisto colony terminals will be linked on a LAN to a single server rather than a network of servers in the first place.

Citizen Joe:

Viral propagation is another option. It doesn't guarantee speed or privacy. The idea would be that a message would be sent to any ship that is heading in the right direction that is within range. This could be a very long route. However, since there would likely be relatively few interplanetary vessels (compared to airplanes), Solar Space Traffic Controllers would have the full list of vessels and thus able to chart a route based on predicted paths. While the InterPlanetary Space ships would carry parcels, they would probably also serve as communication hubs for the viral network.


That sounds pretty close to how the modern internet functions, with data tracing geographically indirect paths as it goes from place to place. I like the efficiency of such a scheme: it would use the infrastructure that's already there to create an ad-hoc network backbone. On the other hand, you'd be trading out data security unless you've got some very good encryption (or just plain don't care who else reads your messages).

Jean Remy, I was thinking along those same lines. A lot of business/corporate/government entities would end up hosting proxies of their sites and databases on the far-flung colony servers, with periodic updates being beamed back and forth (ultra-secure data being handled differently).

With multiple proxies being hosted for a single database, with data exchange rates measured in minutes, how long do you think the system will last until Murphy's Law takes a server down through version conflicts?

Jean Remy:

Viral dissemination works on Earth because of thousands and thousands of privately owned servers.

However, even in the best-case scenario of a very developed interplanetary infrastructure, I don't see a lot of traffic in space. Say two cyclers between every major colonial epicenters (say 2 for Mars, 2 for the Jovian colonies, 2 for Saturn etc...) and a few "moon hopper" shuttles, but those would be so close to their giant primaries getting a Line of Sight on them would be an issue.

However the Comm relay platforms suggested are basically Voyager probes without the scientific instrumentation and a known stable orbit. Rather that throwing your message out omnidirectionally and hope that eventually it will reach your destination (because viral dissemination is kind of like shooting blind) you simply bounce the signal of a set number of known (and if want, secure) predetermined platforms. If your goal is to reach as many people as possible (the entire point of viral dissemination in the first place) then target the Cyclers. Chances are good the passengers en route back and forth have personal computers linked in to the ship's server, which keeps updated by linking in to the platforms.


The general outlines of communications in the plausible mid-future look a bit like the Victorian era. Fast and reliable close to home, a little slower if you want to contact the next 'city' over, slower again but still reliable if you want to contact another 'country', and best of luck to you if you want to contact someone in the backbeyond. In this case city, country, and backbeyond are defined by both linear distance and orbit.

You could even end up with a Pony Express situation. Someone sets up a Planet Express torchship delivery system to carry high information-density packages or mail that requires high-security delivery... And then nine months later someone else sets up a tightbeam relay network that can handle multiple high-bandwidth high-security messages... And suddenly you've got a lot of cheap ponies on the market.

Except in this case the ponies have 45 gW/30 milligee legs.

From On Torchships comments (2010)

This is a topic post referring to Purdue University's project Destiny.
Here is my introductory post for the series.
The subject is section 4, Interplanetary Communications Network.

 Headline results: I believe that the cost of this system can be reduced by nearly 50% without altering the underlying performance assumptions.


 This study's communications design is a major factor in overall cost. Every effort should be made to reduce this cost; the headline price is $228 billion, which works out to $4,560 per colonist per year for connectivity.

 The basic design using radio links for surface to orbit and optical links for orbit to orbit is sound. The performance numbers seem well-researched, as do the fault rates. The need for a relay is well-established. The amount of data considered is also reasonable: a single HD video stream for each 100 colonists plus two minutes of SD video (or its equivalent in other data) per person per day. My disagreement is that the use of small disposable satellites is an oldspace norm which unnecessarily drives cost.

 What's the alternative? A large multiuser platform with human maintenance missions. ITS has more than enough dV to deliver a maintenance crew and 300 Mg of cargo from Earth to ESL-5 or from Mars to areosynchrous orbit. By delivering discrete components like transponders, processors, batteries, apertures, etc., the mass of the thrust tube and propellant masses for orbital insertion and decommissioning are eliminated. Beyond that, a shared facility offers options for cost recovery. Science missions such as large telescopes may draw funding for shared services like electrical power and maintenance. Organizations like the L5 society may choose to dock habitats to the platforms for similar reasons.

Relay Node

 Consider for example the ESL-5 relay. Baseline costs for the node are $47.5 billion. The baseline plan requires 53 separate Falcon Heavy launches, 12 of which are contingency satellites. Each vehicle has dedicated avionics, GNC, optical apertures, structural bus, thrust tube, propulsion, etc., etc., all of which are discarded once any one system fails.

 For redundancy and to minimize interference, two platforms should be deployed. The physical structures are not significantly affected by exposure to deep space, so their effective service lifetime is greater than the length of the study. By allowing one ITS launch per cycle and alternating the visited platform, each platform is visited roughly every five years. Components can be tested, repaired or replaced if necessary and certified for another 5-year period. System expansion is done by adding more components than are removed.

 The baseline Falcon Heavy launch and operation costs for this location are $6.2 billion in current dollars. 47 fully-refueled ITS launches (at $46 million each, using a higher lifetime flight rate for near-Earth operation and the same $25 million operations cost) would run $2.16 billion. Total savings: $4.04 billion.

 The solar panels are a major portion of that cost, $240 million for ~360 kWe on one satellite, or $12.72 billion for the relay satellite system. The project considers SAFE-series reactors in other contexts, for example a SAFE-800 design producing 240 kWe for about $1.4 million. At steady state, six satellite equivalents will be operational at each platform; 9 SAFE-800 reactors plus one spare would therefore cost $28 million initial plus $4.3 million per refuel for the L5 relay system. Reactors are assumed to have a 60-year operational life, so two complete sets of hardware (40 total reactors) must be built and deployed during the project along with eight refueling cycles. Total cost $90.4 million, assuming launch costs are covered in the per-cycle maintenance visit cost. Total savings: $12.63 billion. Even if the development process for these reactors took several billion dollars it would be a net gain.

 The thrusters, propellant tanks, lines, valves, avionics, guidance and other systems are shared across the entire platform. Because no insertion or deorbit burns are necessary, the amount of stationkeeping propellant required is a small fraction of the baseline amount. I'll assume that mass and cost of these systems is instead spent on a structural truss for mounting components, on a stationkeeping system with ion thrusters or PITs to eliminate toxic fuels, and on a platform pointing system with very long lever arms and robust reaction wheels. No net change.

 The electronic communication components would be unchanged, and remain the primary expense at $268.8 million each ($11.02 billion total without flight spares). However, because a failure of other critical systems does not lead to loss of use of the comm systems, we are able to eliminate the 12 extra satellites and comm sets. Human servicing missions will be able to repair, repurpose or replace as needed, while one extra set on each platform will provide short-term protection against an outage. Total savings, $3.22 billion.

 This node in the communication network can be implemented as a shared facility for a savings of $19.89 billion, a total cost of $27.65 billion. The average colonist population over 100 years is half a million, so this node costs $553 per colonist per year.

 Similar logic can be applied to the Earth-orbit and Mars-orbit nodes.

Earth Node

 The Earth-orbit node would consist of three geosynchronous shared platforms. Due to the high population of GEO satellites, these could be useful places for permanent satellite servicing facilities. A rotating workforce of technicians could keep the platforms operational while also servicing other customers' hardware to offset costs. Over the life of the node, 66 satellite equivalents are required; 15 satellites are required at steady state, or 5 per platform. For simplicity these platforms should be identical to the ESL-5 platforms, though they will use one fewer reactor due to reduced power requirements. To maintain a five-year service interval, ITS flights are required every 20 months which equals 60 total flights. An alternative is to place a permanent manned maintenance facility at EML-1 and use an orbital tug to service platforms as needed; this would provide the added benefit of a one-week response time to problems.

 The baseline cost of this node is $61.1 billion with 14 flight spares. Electrical savings are ($19.2b baseline - $75.6m capital - $46.44m refueling) = $19.08 billion. Launch savings are ($9.36 billion baseline - $390 million ITS/$6.5m) = $8.97 billion. Spares savings are 14 x $189.86 million = $2.66 billion. This node's total savings are $30.71 billion, total cost is $30.39 billion.

Mars Node

 The Mars-orbit node would consist of three areosynchronous shared platforms. Due to the study's power limitations, bandwidth through each satellite is quite a bit lower than through the relay satellites and so more craft are specified. Over the life of the node, 135 satellite equivalents are required; 33 satellites are required at steady state, or 11 per platform. These would ideally be two standard platforms linked together, with 19 reactor modules. The hardware would be delivered via cargo ITS, one per cycle. This vehicle would aerocapture and then rendezvous with a central maintenance facility on Phobos (or possibly Deimos) for offloading, followed by a descent to the surface with remaining cargo and a standard return. Service missions would be dispatched from the Martian surface using one of several Mars-dedicated ITS vehicles, dock with facilities on Phobos, transit to the appropriate platform, perform maintenance and installation tasks, then return first to Phobos and then the surface. The Phobos base may eventually be permanently manned and used as a port for handling electric-propulsion cargo vehicles via docking tether.

 The baseline cost of this node is $119 billion with 21 flight spares. I believe there is a potential savings from increasing the transmission power of the Mars node to match the data rates of the other nodes, but let's stick with the baseline for now. Electrical savings are ($37.44b baseline - $159.6m capital - $98.04m refueling) = $37.18 billion. Launch savings are ($18.25 billion baseline - $1,457 million ITS/$31m) = $16.79 billion. Spares savings are 21 x $189.86 million = $3.99 billion. This node's total savings are $57.96 billion, total cost is $61.04 billion.


 These changes bring the total I-Comm network costs to $119.08 billion, or $2,382 per colonist per (Earth) year / $198.47 per (Earth) month. That's a combined savings of $108.92 billion, or 47.8%. I believe there may be another $30-$60 billion of potential savings in the actual comms components due to standardization and large production runs, and at the Mars end by adding more power through fewer apertures.

 The use of shared facilities encourages the further development of space. Permanent crewed maintenance bases near the three nodes would revolutionize the way satellites are built, launched and operated, and would open the road to asteroid mining and the colonization of free space.

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