This page is for starships that travel at conventional speeds. The fact that interstellar distances are so astronomically huge means the main problem is the voyages will take many thousands of years. And that's for the nearby stars, others will take millions of years.

The main problems are that human astronauts will die of old age long before the voyage ends, and most spacecraft are not built to last that long.

The starships in this page come under the headings of "Go Slow", "NAFAL" (not as fast as light) and "Apocee" (far from c). For arbitrary reasons I am defining an Apocee starship as one which cruises at a speed below 14% of the speed of light (0.14c). This is because that is the speed where the relativistic gamma factor reaches 101% (γ = 1.01). I warned you it was arbitrary.

Go Slow

The first of Gordon Woodcock's methods of interstellar travel is "go slow".

Distance between stars is huge, traveling said distance slower-than-light will take a huge amount of time, human beings have a very limited lifespan. And it is much easier to travel at 10% the speed of light than it is to travel at 99.99999% the speed of light

"Go Slow" means to focus on the limited human lifespan problem, and be content to travel slowly at 10% c or so.


Just to be annoying, I'm going to revisit that ever-giving fount of joy, slower than light (STL) interstellar travel. You may think that, because it's not physically impossible, that it's inevitable that humans will travel this way one day. Sadly, it looks like blasting your way between the stars the hard way requires magical technology too, just as FTL does.

We've talked about this before on the blog, but unfortunately, the really good conversation was about 800 comments in and about 8 (?) years ago, so you can't just google it. Here, I'm going to cover two points: why canned monkeys don't ship well, and what the precursors to STL would look like, so that we'll know if our society ever starts preparing itself to expand into space at less than the speed of light.

"Canned monkeys don't ship well" refers to the problem of keeping people alive in interplanetary or interstellar space (this for the two people who didn't know it already). There are a lot of problems, what with providing air, water, food, radiation protection, decent meteor defenses, a working clothes cleaner, producing food reliably, recycling trash, keeping people healthy and able to step onto a planet again, and last but not least, completing a human life cycle from conception through birth to maturity and senescence. Many of these are provided by Earth, and the rest require more space than anyone currently has on the International Space Station (ISS). That's why, for instance, they don't have a clothes washer on the ISS. They wear their clothes for a week or so depending on what it is, and throw them out. More insidious problems have to do with what the lack of gravity does to the health of humans, plants, and animals. Correct me if I'm wrong, but I don't think any plant or animal has successfully completed a life cycle (seed to seed or animal to animal) entirely in freefall. And, if you read Chris Hadfield's An Astronauts Guide to Life on Earth, he's quite candid about how extendedly unpleasant it was to reacclimatize to Earth after spending a year in space. It wasn't just reflexes--his feet couldn't tolerate the weight on them, he had rashes and all sorts of weird symptoms that took days to go away, and weakened bones that took at least a year to go away. It's uncomfortable to get into freefall, it's painful to get out of freefall after an extended time in it, it's not just humans that have problems, it seems to be most eukaryotes do, and we still need to figure out how to work around this. Magic, obviously. Just wave that wand, and the problems go away. But what exactly is the wand you're waving? CRISPR? Vibrating pants? Some wonderful pharmaceutical suitable for plants, humans, and fish (gravipramine? (he said that medication name was a weak joke, apparently a drug that prevents zero gee nausea))?

Fine, you say, my interstellar ark will spin to make up for this problem. And the ship will be huge, so you can have not just your damned washing machine but vast pools of water as radiation shields (as in Anathem). This is great. Heck, we'll even assume that you have steering and propulsion systems that can handle pushing a great sloshing gyroscope in a precise direction for centuries. Yeah, that. It'll be fun to steer your spindizzy when there's a lot of weight moving around inside it. The wobbling thrust to compensate will be fun too, and keeping this coupled set of chaotic oscillators from going out of control will be easy, of course. All we need is a magic navigation system, magic because it doesn't just steer a wobbling gyroscope impeccably, it does it for centuries without error, and with rapid collision avoidance too. Isn't magitech wonderful?

This is where we get into the engineering challenges. I'm certain, for instance, that we can build computers that last 50 or 100 years. After all, the Voyager space probes are still kind of working, 40 years later. Actually, there's a fun little problem here: a few bespoke resilient computers for expensive space probes won't disrupt a consumer electronics market built on planned obsolescence, but what if you're building an effectively immortal (to a first approximation) system? Won't that decimate the local computer industry, when everybody wants a computer that they can pass on to their kids, rather than discarding, just so that a team of engineers can stay employed making replacements? After all, STL voyages last decades to centuries, and the electronics all have to work forever, with only onboard repairs. This is actually be one of the precursors to deep space colonization, that computers stop being made to fall apart, but instead are built simultaneously rugged, long-lived, and easy to repair. Or, of course, we could put an entire computer fabrication facility on every spaceship. I'm sure that won't take much weight. And swapping out the navigation system every few years should be really easy, too.

That's just one subsystem. If we're talking about a century or millennium long voyage (and note that these are optimistic given our current state of propulsive affairs), then to a first approximation, every bit of hardware either has to last the entire trip, or has to be totally repairable using the (recycled?) supplies brought along. Yes, yes, I know, 3-D printing. That will certainly be part of it, but don't you think that there's going to be critical infrastructure that just can't be reprinted ad nauseum, like critical structural elements and parts of the hull? You'll need really good (dare I say magical?) printing capabilities to reprint a big chunk of the ship from inside the ship. You'll also need a really efficient materials recycling facility to sort all the waste materials and efficiently remanufacture all the printer feedstocks. But heck, sorting stuff into pure materials streams and rebuilding it only takes lots of energy, time, know-how, and specialized technology, which is why we don't yet do this with municipal trash. Actually, trash management is another one of those little precursors: if idiot-proof urban recycling becomes a thing, we'll be one small step closer to space.

Then we've got the big noise, the interstellar medium and the fun of ramming into it at high speeds. Raising your kids in the middle of a firing range or next to the containment shell of a nuclear reactor is positively tame in comparison. Interplanetary and interstellar space are astonishingly good vacuums (better than we can readily make on Earth), but they're not empty. Worse, the stuff in space tends to move really, really fast, which means it has a lot of energy. Bullets travel at around 1 km/sec, but meteors travel at 10 km/sec and above, and an STL spaceship needs to get moving much faster than this to make decent time between the stars. Even the best steering system can't get a ship (especially a huge, spinning, sloshing ship) rapidly out of the way of some bit of interstellar debris. No, we need shields, and those shields need to be fixable or replaceable from inside the ship, because humans aren't going to survive very well either out in that shooting gallery. Yes, anti-micrometeorite armor (like Whipple shields) works on a different principle than terrestrial armor and wouldn't stop a bullet, but even it needs to be replaced, and a starship will occasionally run into bullet-sized space junk at ultraballistic speeds. So we need magical armor. And magical radiation shielding too, preferably in the shape of a mobile cowling, so that robots and humans can get outside and work on the starship hull, under cover, and not die rapidly. More magic! Or heck, I'd settle for a force field at this point.

And yes, there's a rocket firing for years to centuries to push the starship up to speed. How long do real world rockets fire for, again? The starship engine is another one of those magical technologies. While yes, ion engines have fired continuously for years (on the Dawn space probe, for example), their thrusts are tiny, equivalent to the weight of a piece or two of paper in your hand. Since space is effectively frictionless, those tiny thrusts add up, but only on relatively light-weight spacecraft, over interplanetary distances, and over a few years. We need extremely high thrust and for centuries, and it's not clear how to get this. The closest we might want to get is an Orion drive powered by hydrogen bombs, but then we've got to store those beasts indefinitely. And I'm sure everyone wants to grow up immediately adjacent to a nuclear test range, protected by some really, really good shielding that will have to be repaired in house, even though it's a wee bit radioactive.

Then, once we get to the new planet, we've got to land on it, repeatedly. So we need landers that can boost themselves back up to orbit, ideally in a single stage. That's easy, we're developing SSTO (single stage to orbit) technology now. Right? Well, the little interesting challenge is that your lander has to be full of fuel to take off again. Indeed, without magic fusion rockets or some such, almost all of the lander's weight when it lands has to be fuel. And it's going to be really hot on landing, as it decelerates from orbital speed (Mach 10+) down to zero. So you're flying the equivalent of an ostrich egg full of rocket fuel, and decelerating it from Mach 10 to zero, landing on a totally unimproved landing spot (so the lander either has to be able to hover or land in the water, take your pick), and then take off from that spot (or the water) again. And if you think the water launch of a supersonic plane is easy, you really should google "XF-2Y Sea Dart." Anyway, making conventional rocket landers more magitech to work. We could use Orion technology to land and take off, but then the lander is going to have to land, erm, quite a long distance from wherever the colony is. That's going to be a bit tedious, especially the part where they have to repair the road after every launch or landing.

Finally, we've got the problem of using the toilet. Yes, I know space toilets have come a long way. Here I'm talking about recycling nutrients, all 17 of them. People have tried living in closed ecosystems since the 1970s, and it's a chore. I saw a description of one DIY experiment that said that the man involved had to produce feces of the correct weight and composition every day, just to feed the recycling system that fed the plants that fed him. If you've got a small, closed ecosystem, shit can't just happen, it has to be excreted in precise amounts and on schedule. Earthly ecosystems are resilient to when poop happens because there are huge surpluses of some nutrients (like nitrogen in the air). This gives us a fair amount of slack in how nutrients get processed. Dead wood can lie around for centuries in the desert without causing all the plants around it to die from lack of carbon. Unfortunately, when you get into a smaller ecosystem, the surplus nutrient pools are smaller. So, if there's too much dead wood around (or unprocessed feces) you really could starve, and if you don't have enough oxygen for the microbes to break down the dead wood, you could suffocate as the microbes got to work recycling your waste. Biosphere 2 ran into problems associated with this. Ideally, you want the starship's biosphere to be as big as possible for stability. Simultaneously you need to minimize its weight and size to make it easier to send to another star system. Magic ecosystem handling? That's the easy solution. The hard solution is making sure that everyone on the ship is more capable of running an ecosystem than are almost all PhD ecologists currently working (that would include me, incidentally).

Speaking of which, the crew: all astronauts, the best of the best, right? Good breeding stock and all? And their grandkids are going to settle the new planet? Well, erm, yeah. There are problems here too. One is that humans don't breed true, so amazingly talented people tend to have less talented kids; it's called regression to the mean. In a multigenerational setting, you have to allow for incredibly talented initial crew getting old, becoming incompetent, and passing off their responsibilities to their less-talented offspring. That's tricky. You also have to allow for people being incapacitated, whether they are young, old, pregnant, sick, or drunk. Yes, drunk. One of my proposals for dealing with the shortcomings of a closed ecosystem was to designate 10% of the grain crop to making beer, so that people could get drunk occasionally. The point wasn't that alcoholism was good, it's that if your nutrient cycling is so tight that you can't afford the surplus crop needed for an occasional party, then you're absolutely incapable of dealing with problems that incapacitate part of the crew, let alone storing surplus food for when it's needed. Having a system that's resilient to people getting drunk occasionally is one way to make sure that your system can also deal with more serious problems. And, if there's a crop failure, the grain that would have gone to alcohol can be used for food.

I could go on, but there are three points that really need to be made instead. One is that STL can involve as much magic technology as FTL. It doesn't involve breaking Einstein, but Einstein's not the only scientific hurdle out there. All sorts of things are permitted by general relativity but physically or logistically impossible.

The second is that our species isn't ready for the stars. We're not magical enough. If we were getting close, the precursors for interstellar technology would already be around, changing our lives. For instance, if we could almost build a starship, it would be possible for an (evil) magnate to build a secret lair that was impenetrable to anything including a nuclear blast (starship shielding). His minions could take shelter in that lair, seal the entrances, and live in there under his dynasty for centuries, with no problem at all (closed ecosystem with indefinite recycling, plus social engineering). Climate change would be a non-issue for the super-rich, because their castles would be proof against anything the climate or outsiders could throw at them. And we'd have the equivalent of the GNP of Russia to literally throw away in making a starship that would send a few hundred people on a one way trip to a nearby star, since that's about the level of resources you'd need for a starship. So yeah, we're not there yet. This isn't to say that you can't write a story using STL, but it would be good if you spread the magic technology more widely than just in your ship. Why should people have starships in space, but only the Whole Earth Catalog planetside? Starship tech makes for great secret bases and mechanized armor, if nothing else. And every character won't be able to just fix a toilet, they'll have the whole system piped through their closet composting and growth chamber to feed them a treat a few weeks later. In an STL enabled world, proving you can take care of your own crap should be a rite of passage akin to getting a driver's license today.

The final point is one that I'm sure is well-known to SF cognoscenti: there's a reason so many SF writers have used FTL, gravity control, reactionless drives, and force fields. They make things easy. Instead of getting into the aeroponic weeds about how everybody must cycle their nutrients through the system for centuries, you just wave at least two of those magic wands and all of the difficult STL technical challenges go away. You can speed from star to star before your life support runs out, land on planets and take off as many times as you want, and interplanetary and interstellar meteors won't kill you, because you're not about to run into them at high speed without proper shielding. They're not stupid tropes, just overexposed because they're so gosh darned useful. I may be wrong, but I believe that the SF writers who originally proposed this tetrad knew enough about science and engineering to have a good idea of the problems they were avoiding by using them. Sadly, we've since discovered that the problems were even worse than they originally thought. Perhaps later generations of SF aficionados have forgotten and need to be reminded?

What did I miss? Heat, did you say? Power plants? Shipping corpsicles and thawing on arrival?

From CANNED MONKEYS DON'T SHIP WELL by Frank Landis (2018)


There are several ways of dealing with the lifespan issue. See sub-sections below

The paper World ships: Feasibility and Rationale suggested this terminology. I am unsure though how common this useage is .

Cruise Velocity
Population Size
< 1,000< 100,000> 100,000
>10%SprinterColony Ship
< 10%Slow BoatColony ShipWorld Ship
< 1%Colony ShipWorld Ship
World ship designs from the literature with key values
DesignPopulation SizeDry Mass
Propellant Mass
Cruise Velocity
Enzman world ship [20]20,000 - 200,000300,0003×1060.9%
Torus world ship [38]100,0001075×1071%
Dry world ship - Mark 2A [15]250,0002.0×10118.2×10110.5%
Dry world ship - Mark 2B [15]250,0005.7×10112.3×10120.5%
Wet world ship [15]250,0002.2×10129.0×10120.5%
[15] A Bond and AR Martin. World Ships-An Assessment of the Engineering Feasibility. Journal of the British Interplanetary Society, 37:254, 1984.

[20] A. Crowl, K. F. Long, and R. Obousy. The Enzmann Starship: History & Engineering Appraisal. Journal of the British Interplanetary Society, 65(6):185, 2012.

[38] Andreas M Hein, Mikhail Pak, Daniel Pütz, Christian Bühler, and Philipp Reiss. World Ships Architectures & Feasibility Revisited. Journal of the British Interplanetary Society, 65(4):119–133, 2012.

      SLOWBOAT. A STARSHIP that has no FTL, and therefore take a long, long time to get where it's going.

     This poses the problem of what the crew and passengers do in the meantime. If the Slowboat can go nearly the speed of light, and the destination is a nearby star, they may simply endure a very tedious voyage of a decade or so. But if the Slowboat is even slower, or the destination farther away (and usually both are the case), more drastic measures have to be taken.

     One common option is hybernation. Everyone goes into suspended-animation sleep, to be awakened in a few hundred years. The alternative is a Generation Ship. Here the ship is basically a HABITAT with a DRIVE engine attached. Crew and passengers form a self-sustaining community, and the Ship (usually a huge one) is their world till they finally reach the destination.

     It often happens that, after a few centuries en route, the inhabitants of a Generation Ship completely forget where they came from, much less where they're going.

Tough Guide to SF entry for SLOWBOAT

Digital Crew

Since every atom of mass is a penalty, the logical starship would just carry a master computer and no human crew. This avoids the payload mass of the crew, the habitat module, the life support system, food, water, and everything. The starship might be under a meter long, which would make this concept the lowest mass of all the slowboat starships.

However, nobody wants wants to read about the adventures of a computer (yes, I know there have been a couple of SF stories on this theme, but it requires extraordinary skill on the part of the author, and the stories are not wildly popular. With the exception of the Bolo stories by Keith Laumer et al.).

Enter the "digital crew" concept. You postulate technology capable of "uploading" human brain patterns into a computer. In essence, the ship's computer is running incredibly advanced simulations of the crew, creating a virtual reality much like that found in the movie The Matrix. This also allows the author to pontificate upon the nature of reality, ask if we are actually unaware virtual people in a virtual reality, and stuff like that. Authors who have used this concept include Sean Williams, Shane Dix, and Greg Egan.

The point is the author is allowed to write stories about human beings, but the digital humans and their digital environment take up zero mass.

One could add equipmment capable of manufacturing artificial bodies for the crew from local materials upon arrival at the destination. However, the advantage of a digital crew ship over a seed ship is the lower ship mass due to the absence of frozen embryos, artificial wombs, and robot mommies. Adding artifical body manufacturing facilites would reduce or remove the advantage. The only remaining advantage is that the new bodies inhabited by adults instead of babies.

You could regain the advantage if the manufacturing equipment is really tiny. Say a couple of grams worth of nanotechnology self-replicating machines, intended to work on handy asteroids or other free materials lying around the destination solar system. The nanotechnology bootstraps itself by replicating using in-situ resources as feedstocks until it has mass of a few tons, then shifts gears to start manufacturing artificial bodies.


But there was a catch: Living humans could not be sent. Even with the Earth’s vastly expanded resources—cheap fusion power and the new tools of nanotechnology seeming to exponentially expand the horizons every year—there was simply no way to send thousands of people light-years away from Earth in every direction. Quite aside from the colossal cost, there was also the issue of lost time as well as the physical and mental well-being of the individuals undertaking such voyages. Instead, the first wave of survey vessels would represent humanity in the best way possible but would carry no actual live specimens.

At first it was hoped that sophisticated artificial intelligences would fill the pilot seats, but AI research took longer to deliver than its engineering counterpart. While vast orbital shipbuilding facilities evolved new generations of drives, power supplies, and protective magnetic bubbles, programmers explored dead end after dead end, never quite succeeding in creating the right sort of mind to ensure even one mission’s success, let alone thousands. UNESSPRO could not afford to throw away trillions of dollars on ships that might die or go AWOL at any moment. With 5 percent of the Earth’s gross product being channeled into the project, there had to be some sort of guarantee of returns. So they were forced to explore other options.

By 2048, it was clear that only one of these options promised anything like the sort of reliability required, and that was to send out electronic facsimiles of humans to the stars, as opposed to flesh and blood. Consciousness research had not yet managed to re-create an entire person’s mind in an electronic environment, except by inefficient neuron-by-neuron simulation, but they could decipher a great deal that had once been thought a mystery. The processes underlying consciousness could be emulated, as could the way emotions and other impulses ebbed and flowed throughout the body. Memory alone had proven elusive under such reduced conditions, defying all attempts to record it indirectly. The only efficient way it could be captured and simulated was secondhand, by interviewing the original at length about his or her past and using physical records to supply the images. Emotions could be attached later, during the fine-tuning phase, to color the recollection correctly, even though the details might still be slightly askew. Preawakening memory in such a mind was, at best, a patchwork quilt pieced together from a million isolated fragments.

But that was enough. So-called “engrams” behaved more or less the same as their template minds, the flesh-and-blood originals who had devoted six months of their lives to the task of being effectively taken apart and rebuilt inside a computer. When left to run for long periods, the engrams displayed no greater tendency toward unreliability than those same originals, neither failing at familiar tasks nor unable to learn. They were, in fact, ideal candidates for any space-faring crew: They did not eat, breathe, excrete, sleep, or grow sick; they took up very little space—less than a cubic decimeter (as measured in the new Adjusted Planck units created for the international venture)—and weighed less than half a kilogram; they could adjust easily to the long stretches of time during which nothing happened on an interstellar mission; and they could be trained as easily as a real person. In fact, it proved no great difficulty to train sixty real astronauts, then copy them as many times as was required to fill the crew registers of 1,000 survey vessels.

It was the latter detail that aroused the greatest ire among those still concerned about matters of the soul. Each survey vessel had a crew of thirty; there were one thousand ships; that meant a total survey crew of 30,000 individuals had been selected from that initial pool of just sixty. Roles on each mission were allocated randomly—while Caryl Hatzis might be the civilian survey manager on the Frank Tipler, on another ship she might have a junior role—but that didn’t remove the fact that there were in total over 500 Caryl Hatzises in the bubble of surveyed space surrounding the Earth. Were they really all the same person?

From ECHOES OF EARTH by Sean Williams and Shane Dix (2002)

Seed Ship

The next higher mass class of slowboat is the Seed ship aka Embryo Space Colonization via an embryo-carrying interstellar spaceship (EIS). It will tend to have more mass than a Digital Crew ship and less than a Sleeper Ship.

The starship is tiny, containing a payload of millions of frozen fertilized eggs, artificial wombs, robot factory, and a master computer. No mass is needed for life-support, habitat modules, or any human crew.

After traveling for thousands of years, the ship lands in a good spot for a colony. The robot factory starts cranking out robots. Robots build the settlement buildings and start growing food (if the planet is really nasty they might have to spend a few centuries terraforming the planet first). Then the master computer thaws out enough eggs for the available artificial wombs, brings the babies to term, then tries to convince the babies that the robots are mommy and daddy.

I don't know about you but I suspect that the first generation is going to grow up a little bit emotionally stunted.

The most straightforward method is to cryogenically preserve human embryos. The more difficult but more flexible method is to carry frozen sperm and egg cells, and do in vitro fertilization at the destination. The most unobtainium method is to carry genetic information in computer files, then synthesize the required genetic sequences at the destination.

As with all interstellar colonization proposals, there are quite a few technological challenges to solve:

Artificial Intelligence
The ship's computer has to be smart enough to not only pilot the ship, plan the settlement, and coordinate the building; but also be smart enough to perform parenting duties for all the children. This includes teaching the children survival skills, cultural heritage, and healthy psychological functioning. Its hands are going to be real full when the children become teenagers.

The ship robots will have to be advanced enough to raise and nurture the children, as well as building the settlement and growing crops. They could be teleoperated drones controlled by the ship's computer.

The side problem is they will have to be manufactured at the destination using in situ resources. The whole idea behind the Seed Ship is to minimize the payload mass, carrying an army of robots negates this.

Artificial Wombs
An artificial uterus is way beyond our current technology, but it is being worked on. Brave New World is just around the corner. The techno-wombs will be working overtime until the first generation is old enough to make babies the old fashioned way.
Long-duration Hardware
As with other starship proposals, but with the ship's Artificial Intelligence in particular, the equipment will have to reliably operate for how ever many thousands of years the journey will take.
The frozen-embryos/frozen-gametes/genetic-computer-files will need to be protected from cosmic rays and other damaging influences. In addition, the human gut microbiota is a critical part of the body. On Terra it is obtained from the mother and/or the general environment. At the Seed Ship destination, neither will be available. The microbiota will have to be recreated along with the babies.
Unsurprisingly this entire process opens a can of worms with several sticky moral questions. For one, you are deliberately creating children who will grow up without (human) parents. Should the children be taught/programmed behaviour biased to colony success, or biased towards freedom? Should their records of Terra's history be censored? If so, who decides what gets cut? Some of these issues are mentioned in Clarke's The Songs of Distant Earth.

Examples of Seed Ships in science fiction include The Songs of Distant Earth by Sir. Arthur C. Clarke, 2001 Nights chapter Night 4 by Yukinobu Hoshino, Long Shot by Vernor Vinge, and the movie Interstellar.

If men, and not merely their machines, are ever to reach the planets of other suns, problems of much greater difficulty will have to be solved. Stated in its simplest form, the question is this: How can men survive a journey which may last for several thousand years? It is rather surprising to find that there are at least five different answers which must be regarded as theoretical possibilities—however far they may be beyond the scope of today’s science.

One cannot help feeling that the interstellar ark on its thousand-year voyages would be a cumbersome way of solving the problem, even if all the social and psychological difficulties could be overcome. There are, however, more sophisticated ways of getting men to the stars than the crude, brute-force methods outlined above. (Fourth Solution) After the hardheaded engineering of the last few paragraphs, what follows may appear to verge upon fantasy. It involves, in the most fundamental sense of the word, the storage of human beings. And. by that I do not mean anything as naive as suspended animation.

A few months ago, in an Australian laboratory, I was watching what appeared to be perfectly normal spermatozoa wriggling across the microscope field. They were perfectly normal, but their history was not. For three years, they had been utterly immobile in a deep freeze, and there seemed little doubt that they could be kept fertile for centuries by the same technique (this was amazing back in 1955, nowadays we take semen cryopreservation for granted). What was still more surprising, there had been enough successes with the far larger and more delicate ova to indicate that they too might survive the same treatment (oocyte cryopreservation was not perfected until 1986). It this proves to be the case, reproduction will eventually become independent of time.

The social implications of this make anything in Brave New World seem like child’s play, but I am not concerned here with the interesting results which might have been obtained by, for example, uniting the genes of Cleopatra and Newton, had this technique been available earlier in history. (When such experiments are started, however, it would be as well to remember Shaw’s famous rejection of a similar proposal: “But suppose, my dear, it turns out to have my beauty and your brains?”) *

* We have Shaw's word for it that the would-be geneticist was a complete stranger and not, as frequently stated, Isadora Duncan.

The cumbersome interstellar ark, with its generations of travelers doomed to spend their entire lives in empty space, was merely a device to carry germ cells, knowledge, and culture from one sun to another. How much more efficient to send only the cells, to fertilize them automatically some twenty years before the voyage was due to end, to carry the embryos through to birth by techniques already foreshadowed in today’s biology labs, and to bring up the babies under the tutelage of cybernetic nurses who would teach them their inheritance and their destiny when they were capable of understanding it.

These children, knowing no parents, or indeed anyone of a different age from themselves, would grow up in the strange artificial world of their speeding ship, reaching maturity in time to explore the planets ahead of them—perhaps to be the ambassadors of humanity among alien races, or perhaps to find, too late, that there were no home for them there. If their mission succeeded, it would be their duty (or that of their descendants, if the first generation could not complete the task) to see that the knowledge they had gained was someday carried back to Earth.

(ed note: in latter science fiction utilizing this technique, they either use scouting robot probes to make blasted sure there exist a habitable planet at the target site before sending a seedship, or the seedship scans the planet for habitability before creating any embryos. Having the children starve to death because the planet is uninhabitable is just a little too cruel)

Would any society be morally justified, we may well ask, in planning so onerous and uncertain a future for its unborn—indeed unconceived—children? That is a question which different ages may answer in different ways. What to one era would seem a cold-blooded sacrifice might to another appear a great and glorious adventure. There are complex problems here which cannot be settled by instinctive, emotional answers.

From THE PLANETS ARE NOT ENOUGH by Arthur C. Clarke (1955)

“I want to talk about matters that are of global significance and which affect every individual alive on this planet, and indeed the generations yet to be born—assuming there will be future generations.” He paused. “I want to talk about survival—the survival of the human species.”

Congreve went on. “We have already come once to the brink of a third world war and hung precariously over the edge. Today, in 2015, twenty-three years have passed since U.S. and Soviet forces clashed in Baluchistan with tactical nuclear weapons, and although the rapid spread of a fusion-based economy at last promises to solve the energy problems that brought about that confrontation, the jealousies, mistrusts, and suspicions which brought us to the point of war then and which have persistently plagued our race throughout its history are as much in evidence as ever.

“Today the sustenance that our industries crave is not oil, but minerals. Fifty years from now our understanding of controlled-fusion processes will probably have eliminated that source of shortages too, but in the meantime shorter-sighted political considerations are recreating the climate of tension and rivalry that hinged around the oil issue at the close of the last century. Obviously, South Africa’s importance in this context is shaping the current pattern of power maneuvering, and the probable flashpoint for another East-West collision will again be the Iran-Pakistan border region, which our strategists expect the Soviets to contest to gain access to the Indian Ocean in preparation for the support of at war of so-called black African liberation against the South.”

Congreve paused, swept his eyes from one side of the room to the other, and raised his hands in resignation. “It seems that as individuals we can only stand by as helpless observers and watch the events that are sweeping us onward collectively. The situation is complicated further by the emergence and rapid economic and military growth of the Chinese-Japanese Co-Prosperity Sphere, which threatens to confront Moscow with an unassailable power bloc should it come to align with ourselves and the Europeans. More than a few Kremlin analysts must see their least risky gamble as a final resolution with the West now, before such an alliance has time to consolidate. In other words, it would not be untrue to say that the future of the human race has never been at greater risk than it is at this moment.”

Congreve pushed himself back from the podium with his arms and straightened. When he resumed speaking, his tone had lightened slightly. “In the area that concerns all of us here in our day-to-day lives, the accelerating pace of the space program has brought a lot of excitement in the last two decades. Some inspiring achievements have helped offset the less encouraging news from other quarters: We have established permanent bases on the Moon and Mars; colonies are being built in space; a manned mission has reached the moons of Jupiter; and robots are out exploring the farthest reaches of the Solar System and beyond. But”—he extended his arms in an animated sigh—“these operations have been national, not international. Despite the hopes and the words of years gone by, militarization has followed everywhere close on the heels of exploration, and we are led to the inescapable conclusion that a war, if it comes, would soon spread beyond the confines of the surface and jeopardize our species everywhere. We must face up to the fact that the danger now threatening us in the years ahead is nothing less than that.”

He turned for a moment to stare at the model of SP3 gleaming on the table beside him and then pointed to it. “Five years from now, that automated probe will leave the Sun and tour the nearby stars to search for habitable worlds … away from Earth, and away from all of Earth’s troubles, problems, and perils. Eventually, if all goes well, it will arrive at same place insulated by unimaginable distance from the problems that promise to make strife an inseparable and ineradicable part of the weary story of human existence on this planet.” Congreve’s expression took on a distant look as he gazed at the replica, as if in his mind he were already soaring with it outward and away. “It will be a new place,” he said in a faraway voice. “A new, fresh, vibrant world, unscarred by Man’s struggle to elevate himself from the beasts, a place that presents what might be the only opportunity for our race to preserve an extension of itself where it would survive, and if necessary begin again, but this time with the lessons of the past to guide it.”

An undercurrent of murmuring rippled quickly around the hall. Congreve nodded, indicating his anticipation of the objections he knew would come. He raised a hand for attention and gradually the noise abated.

“No, I am not saying that SP3 could be modified from a robot craft to carry a human crew. The design could not feasibly be modified at this late stage. Too many things would have to be thought out again from the beginning, and such a task would require decades. And yet, nothing comparable to SP3 is anywhere near as advanced a stage of design at the present time; let alone near being constructed. The opportunity is unique and cannot, surely, be allowed to pass by. But at the same time we cannot afford the delay that would be needed to take advantage of that opportunity. Is there a solution to this dilemma?” He looked around as if inviting responses. None came.

“We have been studying this problem for some time now, and we believe there is a solution. It would not be feasible to send a contingent of adult humans, either as a functioning community or in some suspended state, with the ship; it is in too advanced a stage of construction to change its primary design parameters. But then, why send adult humans at all?” He spread his arms appealingly. “After all, the objective is simply to establish an extension of our race where it would be safe from any calamity that might befall us here, and such a location would be found only at the end of the voyage. The people would not be required either during the voyage or in the survey phase, since machines are perfectly capable of handling everything connected with those operations. People become relevant only when those phases have been successfully completed. Therefore. we can avoid all the difficulties inherent in the idea of sending people along by dispensing with the conventional notions of interstellar travel and adopting a totally new approach: by having the ship create the people after it gets there!”

Congreve paused again, but this time not so much as a whisper disturbed the silence.

Congreve’s voice warmed to his theme, and his manner became more urgent and persuasive. “Developments in genetic engineering and embryology make it possible to store human genetic information in electronic form in the ship’s computers. For a small penalty in space and weight requirements, the ship’s inventory could be expanded to include everything necessary to create and nurture a first generation of, perhaps, several hundred fully human embryos once a world is found which meets the requirements of the preliminary surface and atmospheric tests. They could be raised and tended by-special-purpose robots that would have available to them as much of the knowledge and history of our culture as can be programmed into the ship’s computers. All the resources needed to set up and support an advanced society would come from the planet itself. Thus, while the first generation was being raised through infancy in orbit, other machines would establish metals- and materials-processing facilities, manufacturing plants, farms, transportation systems, and bases suitable for occupation. Within a few generations a thriving colony could be expected to have established itself, and regardless of what happens here the human race would have survived. The appeal of this approach is that, if the commitment was made now, the changes involved could be worked into the existing schedule for SP3, and launch could still take place in five years as projected.”

By this time life was flowing slowly back into his listeners. Although many of them were still too astonished by his proposal to react visibly, heads were nodding, and the murmurs running around the room seemed positive. Congreve nodded and smiled faintly as if savoring the thought of having kept the best part until last.

“The second thing I have to announce tonight is that such a commitment has now been made. As I mentioned a moment ago, this subject has been under study for a considerable period of time. I can now inform you that, three days ago, the President of the United States and the Chairman of the Eastern Co-Prosperity Sphere signed an agreement for the project which I have briefly outlined to be pursued on a joint basis, effective immediately. The activities of the various national and private research institutions and other organizations that will be involved in the venture will be coordinated with those of the North American Space Development Organization and with those of our Chinese and Japanese partners under a project designation of Starhaven.”

Congreve’s face split into a broad smile. “My third announcement is that tonight does not mark my retirement from professional life after all. I have accepted an invitation from the President to take charge of the Starhaven project on behalf of the United States as the senior member nation, and I am relinquishing my position with NASDO purely in order to give undivided attention to my new responsibilities. For those who might believe that I’ve given them some hard times in the past, I have to say with insincere apologies that I’m going to be around for some time longer yet, and that before this project is through the times are going to get a lot harder.”

Several people at the back stood up and started clapping. The applause spread and turned into a standing ova- tion. Congreve grinned unabashedly to acknowledge the enthusiasm, stood for a while as the applause continued, and then grasped the sides of the podium again.

“We had our first formal meeting with the Chinese yesterday, and we’ve already made our first official decision.” He glanced at the replica of the star-robot probe again. “SP3 now has a name. It has been named after a goddess of Chinese mythology whom we have adopted as a fitting patroness: Kuan-yin—the goddess who brings children. Let us hop/e that she watches over her children well in the years to come.”

From VOYAGE FROM YESTERYEAR by James Hogan (1982)

‘At first, it was believed that Rama was dead - frozen for so many hundreds of thousands of years that there was no possibility of revival. This may still be true, in a strictly biological sense. There seems general agreement, among those who have studied the matter, that no living organism of any complexity can survive more than a very few centuries of suspended animation. Even at absolute zero, residual quantum effects eventually erase too much cellular information to make revival possible. It therefore appeared that, although Rama was of enormous archaeological importance, it did not present any major astropolitical problems.

‘It is now obvious that this was a very naïve attitude, though even from the first there were some who pointed out that Rama was too precisely aimed at the Sun for pure chance to be involved.

‘Even so, it might have been argued—indeed, it was argued—that here was an experiment that had failed. Rama had reached the intended target, but the controlling intelligence had not survived. This view also seems very simple-minded; it surely underestimates the entities we are dealing with.

‘What we failed to take into account was the possibility of nonbiological survival. If we accept Dr Perera’s very plausible theory, which certainly fits all the facts, the creatures who have been observed inside Rama did not exist until a short time ago. Their patterns, or templates, were stored in some central information bank, and when the time was ripe they were manufactured from available raw materials—presumably the metallo-organic soup of the Cylindrical Sea. Such a feat is still somewhat beyond our own ability, but does not present any theoretical problems. We know that solid state circuits, unlike living matter, can store information without loss, for indefinite periods of time.

‘So Rama is now in full operating condition, serving the purpose of its builders—whoever they may be. From our point of view, it does not matter if the Ramans themselves have all been dead for a million years, or whether they too will be re-created, to join their servants, at any moment. With or without them, their will is being done and will continue to be done.

From RENDEZVOUS WITH RAMA by Arthur C. Clarke (1973)

Sleeper Ship

Sleeper ship tend to have more mass than a Seed Ship and less than a Generation Ship.

The crew is frozen into suspended animation, so they do not age nor require food and oxygen during the thousand year journey. Or spacious living accomodations. The Sleeper Ship does require the mass of the crew, enough mass for a spartan habitat module, and only enough consumables for the time the crew will be awake.

Poul Anderson warned that frozen crew have a limited shelf life. Naturally-occurring radioactive atoms in the human body will cause damage. Normally the body will repair such damage, but one in suspended animation cannot. After a few hundred years, enough damage will accumulate so that a corpse instead of a living person is thawed out at journey's end. This may force one to thaw each crew member every fifty years or so to allow them to heal the damage, then freezing them again.

If men, and not merely their machines, are ever to reach the planets of other suns, problems of much greater difficulty will have to be solved. Stated in its simplest form, the question is this: How can men survive a journey which may last for several thousand years? It is rather surprising to find that there are at least five different answers which must be regarded as theoretical possibilities—however far they may be beyond the scope of today’s science.

Medicine may provide two rather obvious solutions…

(ed note: the first medical solution is immortality)

Perhaps a better answer is that suggested by the story of Rip Van Winkle. Suspended animation (or, more accurately, a drastic slowing down of the body’s metabolism) for periods of a few hours is now, of course, a medical commonplace. It requires no great stretch of the imagination to suppose that, with the aid of low temperatures and drugs, men may be able to hibernate for virtually unlimited periods. We can picture an automatic ship with its oblivious crew making the long journey across the interstellar night until, when a new sun was looming up, the signal was sent out to trigger the mechanisms which would revive the sleepers. When their survey was completed, they would head back to Earth and slumber again until the time came to awake once more, and to greet a world which would regard them as survivors from the distant past.

From THE PLANETS ARE NOT ENOUGH by Arthur C. Clarke (1955)

      It had been a long time since Terrans had first reached toward other worlds. Three hundred years since the first recorded pioneer flight into the Galaxy. And even before that there were legends of other ships fleeing the nuclear wars and the ages of political and social confusion which followed. They must have been either very desperate or very brave, those first explorers—sending their ships out into the unknown while they were wrapped in cold sleep with one chance in perhaps a thousand of waking as their craft approached another planet. With the use of Galactic overdrive such drastic chances were no longer necessary. But had his kind paid too high a price for their swifter passage from star to star?

From STAR GUARD by Andre Norton (1955).
Collected in Star Soldiers (2001), currently a free eBook in the Baen free library.

      “We are leaving this star now. We have already pumped the solution back into the upper reservoir. I fear we will be hard-put this time, for the enormous radiation of this sun declines so slowly, our stored power may not last till we are safely out in the cold regions where we may enter the Sleep. We have discovered a new radioactive material whose half-life is twenty times that of element ninety-two, We are going again into the Sleep. The atmosphere above has not yet frozen, but our energy is exhausted. We have been able to freeze sufficient air in the cold rooms of the sleeping quarters to freeze our bodies. We will probably be safe enough.

     “Investigation of those who are sleeping indicates that many of them have died. Tharsarn suggests a twofold reason for this. Many of course did not survive the original action of the drugs. This was indetectable at the time. Many more have possibly been killed by the atom-smashing rays from space. Even under our Great Seal (a half-mile thick saturated solution of water and lead nitrate), and half a mile more of solid rock, in the enormous times that have passed, these rays might well have been deadly. They do not influence the machines, since machines are not as delicate as body chemistry.

     “That I have survived, Tharsarn believes to be due to a peculiar susceptibility on my part to the action of the drugs, and to the fact that during the periods of awakening I have renewed the entire chemical structure of my body, replacing the destroyed atoms with fresh material from the foods I have eaten. He says that we who awaken have a better chance of ultimate survival.

From THE INCREDIBLE PLANET by John W. Campbell, Jr. (1949)

Generation Ship

The highest mass type of slowboat tends to be the Generation ship. This is because it has to carry the mass of an entire community as crew, a habitat module at the minimum the size of a small town, and enough life support for the people for however many hundreds of years the journey takes. As the ship crawls to its destination, generations of people are born, have children, and die of old age.

A flying asteroid can be defined as a spaceship, using the principle that anything can be a spaceship if you throw it hard enough.

If the generation ship is escaping from some Terra-destroying catastrophe; carrying Terra's scientific and cultural heritage, a representative sample of animal species, colony equipment and supplies, and a fertile representative sample of humanity, the craft is termed an Interstellar Ark.

Problems include the later generations refusing to cooperate with their forefather's vision, civil wars that wreck the ship, failure of the closed ecological life support system, ship arriving at the destination but the current generation does not want to colonize an icky dirty planet because they like living in their ship and the later generations forgetting where they came from, forgetting where they are going, and indeed forgetting the fact that they are in a starship. The classic "forgetting you are on a ship" stories are Robert Heinlein's Orphans of the Sky (1941) and Brian Aldiss' Non-Stop (1958).

In Larry Niven and Jerry Pournelle's FOOTFALL, the aliens deal with the "forgetful generation" problem by including a group of original crew frozen in suspended animation. Members of the original crew are periodically woken so they can ensure that the generational crew keeps the faith. The concept is sort of a combination of sleeper starship and generation starship.

The concept was sort of touched on in Don Wilcox's The Voyage that Lasted 600 Years (1940), though in that story only the captain was frozen. Since he was only a single person he had a limited influence on the generational tribes.

There are also disturbing ethical questions about the morality of condemning several generations of people to living inside a space-going rock. This can lead to political problems starting a Generation ship development project in the first place.

Noted SF author Kim Stanley Robinson apparently got very annoyed at people who dismissed the generation ship ethical questions while simultaneously being infected with a blind faith techno-utopianism over the feasibility of building a generation ship. Currently our technology has utterly failed to build a closed ecological life support system that can operate longer than a year or so, much less a couple of centuries. The same goes for spacecraft components. The second law of thermodynamics is a harsh mistress. The techno-utopians have to understand that there is no plan B.

As a sort of wakeup call, Mr. Robinson wrote the deliberately pessimistic novel Aurora about a disastrous generation ship heading for Tau Ceti and all the things that could and did go wrong. And the novel featured some techno-utopian characters in a most unfavorable light. The more optimistic members of the science fiction community have been raging at Mr. Robinson ever since.

If men, and not merely their machines, are ever to reach the planets of other suns, problems of much greater difficulty will have to be solved. Stated in its simplest form, the question is this: How can men survive a journey which may last for several thousand years? It is rather surprising to find that there are at least five different answers which must be regarded as theoretical possibilities—however far they may be beyond the scope of today’s science.

The third solution was, to the best of my knowledge, suggested over thirty years ago by Professor J. D. Bernal in a long out-of-print essay, The World, the Flesh, and the Devil, which must rank as one of the most outstanding feats of scientific imagination in literature. Even today, many of the ideas propounded in this little book have never been fully developed, either in or out of science fiction. (Any requests from fellow authors to borrow my copy will be flatly ignored.)

(ed note: I read that above paragraph in the early 1960s when I was about ten. I struggled hard to find a copy. One of my fondest memories is when a librarian friend of my mother managed to find a copy and gift it to me for my birthday. Clarke was not exaggerating, the book contains all sorts of fascinating ideas)

Bernal imagined entire societies launched across space, in gigantic arks which would be closed, ecologically balanced systems. They would, in fact, be miniature planets, upon which generations of men would live and die so that one day their remote descendants would retum to Earth with the record of their celestial Odyssey.

The engineering, biological and sociological problems involved in such an enterprise would be of fascinating complexity. The artificial planets (at least several miles in diameter) would have to be completely self-contained and self-supporting, and no material of any kind could be wasted. Commenting on the implications of such closed systems, Time magazine’s able, erudite science editor Jonathan Leonard once hinted that cannibalism would be compulsory among interstellar travelers. This would be a matter of definition; we crew members of the two-billion-man spaceship Earth do not consider ourselves cannibals despite the fact that every one of us must have absorbed atoms which once formed pan of Caesar and Socrates, Shakespeare and Solomon.

One cannot help feeling that the interstellar ark on its thousand-year voyages would be a cumbersome way of solving the problem, even if all the social and psychological difliculties could be overcome. (Would the fiftieth generation still share the aspirations of their Pilgrim Fathers who set out from Earth so long ago?)

From THE PLANETS ARE NOT ENOUGH by Arthur C. Clarke (1955)

Kim Stanley Robinson’s recent piece in Scientific American marks the second time he’s written in as many months about the viability of generation ships as mankind prepares to explore the stars. That’s not surprising, considering that Robinson’s new book Aurora (which was published in July 2015) tracks a massive generation ship and its seven or so generations of humans as they make their way to the Tau Ceti system (“only” 12 light-years away) to start a new human colony. What’s interesting about his two pieces are that they’re both pragmatic verging on pessimistic: He lists so many biological, psychological, and sociological barriers and complications that readers—of these articles, at least—will be convinced to stay firmly put.

In both pieces (the first published in Boing Boing late last year), Robinson comes to the same conclusion: “There is no Planet B.” For all that generation ship stories have been a long-enduring subgenre of science fiction, the deck is stacked against us in a myriad of ways: Getting to a habitable planet will take generations. The humans who keep a generation ship running are most likely not the same ones who will see their new home. Keeping an ark—because it’s so much more than a mere ship—running is filled with so many variables involving radiation exposure, social systems, and the fragility of the human mind and spirit. With each point, Robinson returns to the notion that Earth is our only home.

And yet, we can’t stop looking upward and projecting ourselves—in thought, if nothing else—outward to other systems. So, let’s look at each of his obstacles, because you can bet there’s been a generation ship story that addresses (if not also tries to solve) it.

The ark itself must be:

  • Big enough to support ecology… Most important, Robinson says, is a fully recycling ecosystem. Not surprisingly, he addresses this in Aurora: The generation ship is made up of twenty-four biomes recreating different areas of Earth, and carries about two thousand passengers.
  • …but small enough to travel at quick speeds. This limits the humans’ exposure to cosmic radiation ( put together this neat infographic explaining just how huge a problem radiation is to space flight.) and minimizes breakdowns in the ark itself. But when Aurora opens, the ship’s chief engineer and de facto leader, Devi, is finding more problems than she has time to fix. Most of them couldn’t have been anticipated by those who created the ships on Earth, understandably, but it’s the latter generations who must bear that responsibility. Which brings us into the most vital part of the ark…

Culture of the ship:

  • More than one generation is needed to keep the ship going. Rather than busy themselves with the effort it takes to raise unique people, generation ship crews should just take a page from George Zebrowski’s Macrolife and clone everyone! Or you can go the route of Beth Revis’ unsettling but oh-so-compelling Across the Universe, in which 100 VIPs from Earth are cryogenically frozen on the generation ship Godspeed. Multiple generations spool out during Godspeed‘s voyage, but their real purpose is to ensure that these cryo-pods stay perfectly preserved. Once unfrozen, these Earthlings will be the first to step onto their new planet.
    • Enforced reproduction to maintain population control. You can make this very clear, like on the Syfy miniseries Ascension, which made reproduction a privilege handed out through computer algorithms and annual fertility festivals. Or you can go the route of Across the Universe‘s Elders, who pump pheromones into the air and water, and establish mating seasons.
    • Mandatory jobs. In addition to strictly controlling breeding, Rob Grant’s farcical book Colony sees crew members inheriting their parents’ jobs on the ship… which goes about as well as you would expect, with later generations developing personal beliefs that distance them from their duties to an alarming degree.
    • The establishment of a totalitarian state. Most of the stories try this, and it never works out well—especially when there’s a murder, as in David Ramirez’s The Forever Watch, and the totalitarian state is trying to cover it up. James P. Hogan’s Voyage from Yesteryear, in particular, shows what happens when a generation ship full of an authoritarian regime tries to rein in the Chironian branch of humans who have created their own society on a distant planet.
    • Psychology of enclosed spaces. A Million Suns, the sequel to Revis’ Across the Universe, addresses the chaos and depression of realizing that neither you nor your children will ever see anything but the inside of a ship. Long before that, Robert A. Heinlein took this notion to the ultimate extreme with Orphans of the Sky, in which the remaining survivors on generation ship Vanguard believe that the ship is the entire universe.
    • Untrustworthy AI. This isn’t in Robinson’s argument, but it’s a useful point. If we trust an artificial intelligence with anything concerning our fate, and it evolves as we evolve over the generations, the power dynamic will undoubtedly shift. Just ask the crew members in Pamela Sargent’s Earthseed.

Getting to a new planet:

  • The rights of preexisting life. If the planet is “alive,” Robinson says, humans will have to learn how to exist with any preexisting lifeforms, in ways that will likely range from innocuous to fatal. We’re talking anything from the prions (essentially “bad” proteins that cause neural degeneration) in Aurora to pterodactyl-like creatures in the conclusion to Revis’ trilogy, Shades of Earth.
  • The struggle to terraform. This will take centuries, and will require that the ark, after getting its crew to the planet, continue to function as a shelter and ecosystem. And if your planet has no sun, like the unfortunately-named Eden in Dark Eden, your generation ship will become a strange place—part prison, part home base as you wait for a rescue from Earth that may never come.

So, yeah, there are a lot of barriers to generation ships even getting in the sky, let alone to colonizing a new planet. But we’ll keep writing and reading these stories, because they hold up a mirror to what we need to fix about our own society before we can contemplate starting over on a new world. Personally, I hope we’re still able to make generation ships a reality, even if I’m long-dead when it happens. While Robinson’s first piece on Boing Boing makes it sound like there is absolutely no alternate planet for us, his conclusion in Scientific American is more hopeful, or at least conditional:

The preparation itself is a multi-century project, and one that relies crucially on its first step succeeding, which is the creation of a sustainable long-term civilization on Earth. This achievement is the necessary, although not sufficient, precondition for any success in interstellar voyaging. If we don’t create sustainability on our own world, there is no Planet B.


NOTE FOR 2018 READERS: This is the fourth in a series of open letters to the next century. The series marks a little-known chronological milestone. According to UN data, life expectancy at birth in 18 countries now exceeds 82 years — meaning babies born in 2018 are more likely than not to see the year 2100.

What will the world be like at the other end of those kids' lives? Today's scientific discoveries, Silicon Valley visions, and science fiction can give us glimpses — and in this series of digital time capsules, we also recognize that our hopes and fears can shape what the future will become.

Dear 22nd century,

I have no doubt that many of you have lived in, worked in, or at least visited space. To us, it seems likely you'll have at least one moon base, that you're mining ridiculously mineral-rich asteroids, and that the long hard work of terraforming Mars has begun. You've probably set foot on the more interesting moons of Jupiter and Saturn. Space may even seem a bit mundane to you — the solar system variety, at least.

The real space question is this: Have you given up on the whole idea of traveling farther, of visiting or settling planets other than the ones in our solar system? I fear you have.

I'm not talking about the faster-than-light dream. The speed of light is likely just as impassable in your century as it is in ours. Science fiction that says otherwise is mostly magical thinking. It's likely we can't even get close, given the titanic amounts of energy it takes to accelerate to even a fraction of the speed of light.

No way around it, each light year — or 6 trillion miles — will take decades to cross, and there are barely any stars in a 10-light year radius. We're in a celestial suburb. The only way humans have ever seriously considered getting to the realm of other star systems is in vast starships designed to run for a century or two. Because they would probably take multiple generations to reach their destination, space nerds call them generation ships.

But the last few years have not been kind to the generation ships dream. The science seems to be telling us that generation ships are a dream too far — and science fiction is starting to follow suit.

This is a wrenching thing to have to write. Tales of generation ships have fired my imagination for as long as I can remember. I love all the tropes — the ones where the Earth-like ships are so vast and old that the inhabitants, descendants of the original crew, have forgotten they're on their way to another star (such as Robert Heinlein's classic Orphans of the Sky, in which the vessel is launched in the 22nd century). Or the ones where the colonists are all in cryogenic storage and one is accidentally woken decades early (like Chris Pratt in the much-maligned 2016 movie Passengers).

My favorite of this last kind of story is also my favorite explanation for why we write. In Allen Steele's Coyote (2002) — also set in the 22nd century — a guy in a generation ship is accidentally awakened a century too early and can't resume his cryogenic storage. He goes mad from loneliness, tries to drink himself to death, almost throws himself out of an airlock.

So far, so much like Passengers. But instead of doing what Pratt does — creepily stalking and waking a writer played by Jennifer Lawrence, dooming her to the same fate — this guy decides to become a writer himself. He fills the interior walls of the ship with a fantasy epic, which after his death becomes a myth beloved by the children of the colonists on their brand-new planet.

Hell of a legacy, right? I think it reflects the feelings of many of us space-loving nerds in the 21st century: we may not get to see the promised land of interstellar settlement, but we're happy to consume the stories and dream of the days to come when our descendants fly in generation ships through the heavens.

Because it's almost an article of faith, in these faithless times, that this is our future. For example, in every version of the bestselling computer game Civilization, in which you lead your people over thousands of years from the dawn of history, you win the game by building a generation ship and launching it at Alpha Centauri. It's literally the most advanced thing the game makers could think of doing.

In 2011, NASA and DARPA (the Pentagon agency that gave birth to the internet) lent their names to a project called The Hundred-Year Starship. Its aim was to coalesce the space community behind a goal of launching an interstellar mission by your time: 2112. For generation ship dreamers, it was like Christmas had come.

And then along came Kim Stanley Robinson to stomp over all our dreams and tell us that Santa Claus didn't exist.

Robinson is best known in our time as the author of the Mars trilogy (Red Mars, Green Mars, Blue Mars, all written in the 1990s, the latter two set in your century). He's what they call a hard sci-fi guy — he talks to the experts in the field, reads the latest research, does his level best to get the science exactly right. For example, all of his novels set in the 22nd century feature a flooded, post-climate change Earth. (Again, we're really sorry about that.)

Watching a Hundred-Year Starship conference incensed Robinson. "It was a combination of a scam and a religious meeting," he says, "presented with such an authoritative sheen, such pseudo-science." So he went to NASA Ames, discovered a lot of internal consternation about the agency's involvement with the project, talked actual planetary science with actual planetary scientists, and published Aurora in 2015.

Aurora was a mic drop of a book — not merely a rebuttal to decades of generation starship stories, but the ultimate case for why we couldn't and shouldn't try to settle a planet in another star system. It caused a feud between Robinson and elder science fiction authors that hasn't died down since.

"It was like the Starship was made out of plastic, and I had a baseball bat, and I smashed it to smithereens," Robinson told me. "If people had not reacted in anger, I would have done something wrong. People think humanity going to the stars is a sign that our species has succeeded. But if it's impossible, if it will consistently fail, then there will be people wandering around feeling like humanity is a failure. It needed to be said."

Because of all this, it's worth taking a little time to recap the first half of Aurora. (I'll try not to spoil too many of the details, but come on — depending on when you're reading this, you've had between 3 and 185 years to pick it up already.)

It starts with a generation ship getting close to Tau Ceti, which at 12 light years away is probably the nearest inhabitable star system. (In real life, we know now that Alpha Centauri is "a weird star with weird planets," Robinson says, and probably unsuitable for humans.) Robinson has given his ship every advantage: two rings full of vast biomes, a Noah's Ark full of every kind of Earth life, and a chief engineer who's a wiz at solving problems.

Still, problems persist. Stuff breaks down. It's a closed biological system, and the essential element phosphorus is leaking out of the soil somewhere — perhaps, the engineer muses, in the few grams of ash a family is allowed to keep when a loved one dies. Decelerating the ship as it arrives at Tau Ceti is as big a problem as accelerating it, and is putting all sorts of weird strains on the ship that its designers didn't anticipate. And the next generation of kids is noticeably sicker and less smart — probably a result of the island effect.

The colonists arrive at the fourth moon of the fifth planet of Tau Ceti, which they name Aurora. It's Earth-like and oxygen-rich, but everything's a little bit off; they have to contend with near-constant 60 mph winds, a planet in the sky that makes everything way too bright at night, and week-long eclipses. The first arrivals get to work building shelters and grumble about wanting to take their helmets off — until someone rips their spacesuit, gets sick and promptly dies.

Pretty soon everyone's sick. Something's alive on the planet, something at the bacterial level, and nobody can identify it. Whatever's in the air or water of this alien planet, it remains beyond the reach of human science — and that's scarier than a thousand bug-eyed monsters.

Back on the ship, a murderous argument breaks out between the people who want to leave Tau Ceti altogether and head back to Earth, and those who want to remain. (Think Brexit, but with more kidnappings.) A compromise is brokered, and here's where it gets heartbreaking. As the ecology on board continues to break down, those who head back to Earth are faced with the nightmare of famine — starvation, rations, eating their pets, suicides, the whole bit.

Just in time, news of new technology arrives via radio waves from Earth — not cryogenic storage, exactly, but slowing down the body's system enough that they can hibernate like bears. But that doesn't translate into perfect Passengers-style sleep. "Bears can and do die in hibernation," Robinson points out, ever the buzzkill. The difficult maneuvers the ship has to undertake in order to decelerate back home without shooting through the solar system and out the other side manages to kill a lot of hibernating humans, too.

Back on Earth, folks are blind to the difficulties the ship went through, and at a gathering not unlike the 100-year Starship conference (rather pointedly filled by men with beards), one guy insists on sending more ships out into the cosmos. One of Robinson's characters punches him in the face.

You can see why pro-colonization authors such as Gregory Benford vehemently objected to Aurora. But they haven't been able to tear down its central argument: stuff breaks down. The second law of thermodynamics is unbreakable. You can't just put a closed system in space for a century and hope for the best.

And either the planet at the other end is dead, in which case we'll have to spend centuries terraforming it, or it's alive, and its unseen assassins could kill us in a hundred ways — just as the invaders in War of the Worlds were killed by Earth's bacteria.

I asked Robinson to respond to Benford's accusation, that he put his thumb on the scale to make interstellar colonization look as unworkable as possible. "Well, God put his thumb on the scale," he said.

In recent years, science has come down hard on Robinson's side of the debate. And that's all thanks to some grisly experiments on mice.

First researchers blasted their little mouse brains with particles similar to cosmic rays, the kind that will bathe any astronaut on a deep space mission. Result: the mice became noticeably slower, more forgetful, more confused. It's hard to escape the conclusion that space basically gives you dementia. Send a ship to Tau Ceti and you may not even have an engineer smart enough to fix all the problems by the end of the voyage.

Maybe we can clad our ships with enough protective material to keep our brains safe, but it's hard to keep cosmic rays at bay without Earth's protective magnetosphere. "There really is no escaping them," said the oncology researcher in charge of the study.

Besides, there's also a problem with our guts. Earlier this month, NASA published more research where mice were blasted with cosmic radiation, this time in their small intestines. Result: massive GI damage and tumors.

So much for ships that stay out of planetary magnetosphere range for a century or more. At this point, we're not even sure we can make it to Mars without giving astronauts cancer. No hibernation device is going to save you from a pre-existing disease.

So is that it for the generation ships dream? Will you remember it as a quaint but unworkable sci-fi idea, like Jules Verne sending his fictional explorers to the moon via a giant cannon? Do we have to be content with remaining in our solar system unless a convenient wormhole to another one pops up, the way it did in Interstellar?

Kim Stanley Robinson would like to say yes. His position is that the whole concept is a distraction from the essential task of fixing Earth, our current and only starship. Aurora contains several translations of a single poem about how you have to learn to be less restless and more satisfied with living in one place, but they all boil down to one succinct phrase: "there is no Planet B."

But then again, as Robinson's environmental chemist wife Lisa Howland Newell gently chides him when he says this, "never say never. Never say impossible. You don't know."

Perhaps we can colonize other planets in the equivalent of a slow boat to China — a hollowed-out asteroid, with miles of rock providing the best possible cosmic ray protection. Robinson invented this form of transport, which he called Terraria, in another book, 2312. If we could learn to be self-sufficient within Terraria, we could travel to the stars in them, albeit in thousands of years rather than hundreds.

There's the option of sending ships with DNA printers that could reconstruct human beings at the other end, although that too would require technology we don't currently have. Or perhaps we'll locate the perfect planet before we go by sending hundreds of so-called StarChips — tiny, lightweight sensors that could reach Tau Ceti in a matter of decades. Because you can go a lot faster when you don't have to take humans with you.

Regardless, there's one side of space Kim Stanley Robinson can't kill and doesn't even want to: the imaginary version. "The galaxy is a great story space in the same way that Middle Earth is a great story space," he says. "You don't have to give up on the galaxy. You can tell these stories, and you can do a little handwaving, and you can let your imagination roam — and you can say, look, this story takes place 20,000 years from now. Who the hell knows what we will have done in that time, if we're still around."

May you continue to create and consume tales of the galaxy for as long as you draw breath.

Yours in interstellar imagination,



Human starflight yawns as a vast prospect, one many think impossible. To arrive in a single lifetime demands high speeds approaching lightspeed, especially for target stars such as Tau Ceti, about twelve light years away.

Generation ships form the only technically plausible alternative method, implying large biospheres stable over centuries. Or else a species with lifetimes of centuries, which for fundamental biological reasons seems doubtful. (Antagonistic plieotropy occurs in evolution, ie, gene selection resulting in competing effects, some beneficial in the short run for reproduction, but others detrimental in the long.) So for at least for a century or two ahead of us, generation ships (“space arks”) may be essential.

Aurora depicts a starship on a long voyage to Tau Ceti four centuries from now. It is shaped like a car axle, with two large wheels turning for centrifugal gravity. The biomes along their rims support many Earthly lifezones which need constant tending to be stable. They’re voyaging to Tau Ceti, so the ship’s name is a reference to Isaac Asimov’s The Robots of Dawn, which takes place on a world orbiting Tau Ceti named Aurora. Arrival at the Earthlike moon of a super-Earth primary brings celebration, exploration, and we see just how complex an interstellar expedition four centuries from now can be, in both technology and society.

In 2012, Robinson declared in a Scientific American interview that “It’s a joke and a waste of time to think about starships or inhabiting the galaxy. It’s a systemic lie that science fiction tells the world that the galaxy is within our reach.” Aurora spells this out through unlikely plot devices. Robinson loads the dice quite obviously against interstellar exploration. A brooding pessimism dominates the novel.

There are scientific issues that look quite unlikely, but not central to the novel’s theme. A “magnetic scissors” method of launching a starship seems plagued with problems, for example. But the intent is clear through its staging and plot.

I’ll discuss the quality of the argument Aurora attempts, with spoilers.

Plot Fixes

The earlier nonfiction misgivings of physicist Paul Davies (in Starship Century) and biologist E.O. Wilson (in The Meaning of Human Existence) about living on exoplanets echo profoundly here. As a narrator remarks, “Suspended in their voyage as they had been, there had never been anything to choose, except methods of homeostasis.” Though the voyagers in Aurora include sophisticated biologists, adjusting Earth life to even apparently simple worlds proves hard, maybe impossible.

The moon Aurora is seemingly lifeless. Yet it has Earth-levels of atmospheric oxygen, which somehow the advanced science of four centuries hence thinks could have survived from its birth, a very unlikely idea (no rust?—this is, after all, what happened to Mars). Plot fix #1.

This elementary error, made by Earthside biologists, brings about the demise of their colony plans, in a gripping plot turn that leads to gathering desperation.

The lovingly described moon holds some nanometers-sized mystery organism that is “Maybe some interim step toward life, with some of the functions of life, but not all…in a good matrix they appear to reproduce. Which I guess means they’re a life-form. And we appear to be a good matrix.” So a pathogen evolved on a world without biology? Plot fix #2.

Plans go awry. Backup plans do, too. “Vector, disease, pathogen, invasive species, bug; these were all Earthly terms…various kinds of category error.”

What to do? Factions form amid the formerly placid starship community of about 2000. Until then, the crew had felt themselves to be the managers of biomes, farming and fixing their ship, with a bit of assistance from a web of AIs, humming in the background.

Robinson has always favored collective governance, no markets, not even currencies, none of that ugly capitalism—yet somehow resources get distributed, conflicts get worked out. No more. Not here, under pressure. The storyline primarily shows why ships have captains: stress eventually proves highly lethal. Over half the crew gets murdered by one faction or another. There is no discipline and no authority to stop this.

Most of the novel skimps on characters to focus on illuminating and agonizing detail of ecosphere breakdown, and the human struggle against the iron laws of island biogeography. “The bacteria are evolving faster than the big animals and plants, and it’s making the whole ship sick!” These apply to humans, too. “Shorter lifetimes, smaller bodies, longer disease durations. Even lower IQs, for God’s sake!”

Robinson has always confronted the nasty habit of factions among varying somewhat-utopian societies. His Mars trilogy dealt with an expansive colony, while cramped Aurora slides toward tragedy: “Existential nausea comes from feeling trapped… that the future has only bad options.”

Mob Rules

Should the ship return to Earth?

Many riots and murders finally settle on a bargain: some stay to terraform another, Marslike world, the rest set sail for Earth. The ship has no commander or functional officers, so this bloody result seems inevitable in the collective. Thucydides saw this outcome over 2000 years ago. He warned of the wild and often dangerous swings in public opinion innate to democratic culture. The historian described in detail explosions of Athenian popular passions. The Athenian democracy that gave us Sophocles and Pericles also, in a fit of unhinged outrage, executed Socrates by a majority vote of one of its popular courts. (Lest we think ourselves better, American democracy has become increasingly Athenian, as it periodically whips itself up into outbursts of frantic indignation.)

When discord goes deadly in Aurora, the AIs running the biospheres have had enough. At a crisis, a new character announces itself: “We are the ship’s artificial intelligences, bundled now into a sort of pseudo-consciousness, or something resembling a decision-making function.” This forced evolution of the ship’s computers leads in turn to odd insights into its passengers: “The animal mind never forgets a hurt; and humans were animals.” Plot Fix #3: sudden evolution of high AI function that understands humans and acts like a wise Moses.

This echoes the turn to a Napoleonic figure that chaos often brings. As in Iain Banks’ vague economics of a future Culture, mere humans are incapable of running their economy and then, inevitably, their lives. The narrative line then turns to the ship AI, seeing humans somewhat comically, “…they hugged, at least to the extent this is possible in their spacesuits. It looked as if two gingerbread cookies were trying to merge.”

Governance of future societies is a continuing anxiety in science fiction, especially if demand has to be regulated without markets, as a starship must. (Indeed, as sustainable, static economies must.) As far back as in Asimov’s Foundation, Psychohistory guides, because this theory of future society is superior to mere present human will. (I dealt with this, refining the theory, in Foundation’s Fear. Asimov’s Psychohistory resembled the perfect gas law, which makes no sense, since it’s based on dynamics with no memory; I simply updated it to a modern theory of information.) The fantasy writer China Mieville has similar problems, with his distrust of mere people governing themselves, and their appetites, through markets; he seems to favor some form of Politburo. (So did Lenin, famously saying “A clerk can run the State.”)

Aurora begins with a society without class divisions and exploitation in the Marxist sense, and though some people seem destined to be respected and followed, nothing works well in a crisis but the AIs—i.e., Napoleon. The irony of this doesn’t seem apparent to the author. Similar paths in Asimov, Banks and Mieville make one wonder if similar anxieties lurk. Indeed, Marxism and collectivist ideas resemble the similar mechanistic theory of Freudian psychology (both invented by 19th C. Germans steeped in the Hegelian tradition)—insightful definitions, but no mechanisms that actually work. Hence the angst when things go wrong with a supposedly fundamental theory.

The AIs, as revealed through an evolving and even amusing narrative voice, follow human society with gimlet eyes and melancholy insights. The plot armature turns on a slow revelation of devolution in the ship biosphere, counterpointed with the AI’s upward evolution—ironic rise and fall. “It was an interrelated process of disaggregation…named codevolution.” The AIs get more human, the humans more sick.

Even coming home to an Earth still devastated by climate change inflicts “earthshock” and agoraphobia. Robinson’s steady fiction-as-footnote thoroughness brings us to an ending that questions generational, interstellar human exploration, on biological and humanitarian grounds. “Their kids didn’t volunteer!” Of course, immigrants to far lands seldom solicit the views of their descendants. Should interstellar colonies be different?

Do descendants as yet unborn have rights? Ben Finney made this point long ago in Interstellar Migration, without reaching a clear conclusion. Throughout human history we’ve made choices that commit our unborn children to fates unknown. Many European expeditions set sail for lands unseen, unknown, and quite hostile. Many colonies failed. Interstellar travel seems no different in principle. Indeed, Robinson makes life on the starship seem quite agreeable, though maybe tedious, until their colony goal fails.

The unremitting hardship of the aborted colony and a long voyage home give the novel a dark, grinding tone. We suffer along with the passengers, who manage to survive only because Earthside then develops a cryopreservation method midway through the return voyage. So the deck is stacked against them—a bad colony target, accidents, accelerating gear failures, dismay… until the cryopreservation that would lessen the burden arrives, very late, so our point of view characters do get back to Earth and the novel retains some narrative coherence, with character continuity. Plot Fix #4.

This turn is an authorial choice, not an inevitability. Earthsiders welcome the new cryopreservation technologies as the open door to the stars; expeditions launch as objections to generation ships go away. But the returning crew opposes Earth’s fast-growing expeditions to the stars, because they are just too hard on the generations condemned to live in tight environments—though the biospheres of the Aurora spacecraft seem idyllic, in Robinson’s lengthy descriptions. Plainly, in an idyllic day at the beach, Robinson sides with staying on Earth, despite the freshly opened prospects of humanity.

So in the end, we learn little about how our interstellar future will play out.

The entire drift of the story rejects Konstantin Tsiolkovsky’s “The Earth is the cradle of mankind, but humanity cannot live in the cradle forever.” – though we do have an interplanetary civilization. It implicitly undermines the “don’t-put-all-your-eggs-in-one-basket” philosophy for spreading humanity beyond our solar system. Robinson says in interviews this idea leads belief that if we destroy Earth’s environment, we can just move. (I don’t know anyone who believes this, much less those interested in interstellar exploration.) I think both ideas are too narrow; expansion into new realms is built into our evolution. We’re the apes who left Africa.

Robinson takes on the detail and science of long-lived, closed habitats as the principal concern of the novel. Many starship novels dealt with propulsion; Robinson’s methods—a “magnetic scissors” launch and a mistaken Oberth method of deceleration—are technically wrong, but beside the point. His agenda is biological and social, so his target moon is conveniently hostile. Then the poor crew must decide whether to seek another world nearby (as some do) or undertake the nearly impossible feat of returning to Earth. This deliberately overstresses the ship and people. Such decisions give the novel the feel of a fixed game. Having survived all this torment, the returning crew can’t escape the bias of their agonized experience.

Paul Davies pointed out in Starship Century that integrating humans into an existing alien biosphere (not a semi-magical disaster like his desolate moon with convenient oxygen) is a very hard task indeed, because of the probable many incompatibilities. That’s a good subject for another novel, one I think no one in science fiction has taken up. This novel avoids that challenge with implausible Plot fix #2.

Realistically considered, the huge problems of extending a species to other worlds can teach us about aliens. If interstellar expansion is just too hard biologically (as Paul Davies describes) then the Fermi paradox vanishes (except for von Neumann machines, as Frank Tipler saw in the 1970s). If aliens like us can’t travel, maybe they will expend more in SETI signaling? Or prefer to send machines alone? An even-handed treatment of human interstellar travel could shore up such ideas.

Still, a compelling subject, well done in Robinson’s deft style. My unease with the novel comes from the stacked deck its author deals.


Well, indeed. We sit warm, at ease, breathing sweet air, smoking, drinking, snacking as we feel like it. The artificial gravity is a solid one g underfoot, its vector so aligned that we cannot detect the slight pressure of our acceleration. Nor do we sense the monstrous outpouring of engine energy by which this mass is driven starward. Modern technology is subtle as well as powerful.

We are not yet at Bussard velocity, where we can begin scooping up interstellar hydrogen to burn in the fusion reactors. But we have enough fuel of our own to reach that condition, and afterward to brake at interplanetary speeds as we back down on Alpha Centauri. We have a closed biocycle—every'thing essential to life can be reclaimed and reused indefinitely, for millions of years if need be—which at the same time is expansible. Boats, machines, robots, computers, instruments, and in the microfiles virtually all the knowledge of all the human civilizations that ever were, lie waiting for us like Aladdin’s genie.

“Very well.” Amspaugh turns to me. “I suppose you know that our single agendum today is educational policy.”

“I'd heard mention of that, " I reply. “ But, uh, what's the rush? The first babies are scarcely born.”

“They'll keep that up, though,” McVeagh reminds me.

Missy Blades murmurs: "‘And thick and fast they came at last, and more, and more, and more.’ Right up to the legal limit of population, whatever that may be at any given time. It's still the favorite human amusement. ”

Amspaugh takes pipe and tobacco pouch from various pockets and fumbles with them. “The children will grow," he points out earnestly. "They will require schools, teachers, and texts. The non-controversial basics pose no problem, I imagine—literacy, science, math, et cetera. But even while the pupils are small, they'll also be studying history and civics. Presently they'll be adolescent, and start inquiring into the value of what they've been taught. A few years after that, they'll be franchised adults. And a few years after that, they'll be running the society.

“This isn't a planet, or even an asteroid, where people simply live. The voyage is the ship's entire raison d ’étre. Let the ideal be lost, and the future will be one of utter isolation, stagnation, retrogression, probably eventual extinction. To avoid that, we're uniquely dependent on education.

“We'll only have a thread of maser contact with Sol, years passing between question and answer. We'll only have each other for interaction and inspiration; no fruitful contacts with different countries, different ways of living and thinking. Don't you see how vital it is, Mr. Sanders, that our children be raised right? They must have a proper understanding, not simply of the technology they need, but of the long-range purpose and significance."

Having stuffed his pipe, he pauses to light it. Orloff talks into the silence: “Basically, what we must decide is what the history courses should include. Once we know that, we have writers who can put it into textbooks, actors who can put it on tape, and so forth for every level from kindergarten through college. In the absence of outside influences, those teachings are likely to be accepted, unchallenged, for generations if not forever. So what ought they to be?”

“The truth," I blurt.

“What is truth?”

“Why the facts what really happened—”

"Impossible," Amspaugh says gently. “First, there are too many facts for any human skull to hold, every recorded detail of everybody's day-by-day life since ancient Egypt. You have to choose what's worth knowing, and set up a hierarchy of importance among those data. Already, then, you see your ‘truth’ becoming a human construct. Second, you have to interpret. For instance, who really mattered more in the long-term course of events, the Greeks or the Persians? Third, man being what he is, moral judgments are inevitable. Was it right, was it desirable that Christianity take over Europe, or that it be later faced with such enemies as Mohammedanism and Communism?

"An adult, intellectually trained and emotionally mature, can debate these questions with pleasure and profit. A child cannot. Yet unless you raise the child with a sense of direction, of meaning, you'll never get the adult. You'll get an ignoramus, or else a spiritual starveling frantic for some True Belief—a potential revolutionary. Astra can't afford either kind."

“ The problem was foreseen,” Orloff puts in, “ but we purposely delayed considering it till we should have been en route for a while and gotten some feeling of how this unique community is shaping up."

"I see," I answer. “At least, I think I see.”

“ Details later,” Amspaugh says. “What we must arrive at is a basic educational philosophy. " He gives me a long look. “The original circle of us know each other quite well. I think, by and large, we can predict what stand everybody will take. That isn't good. We need a wider range of thoughts. lt’s a major reason why we're inviting new members in, you the latest.

"So would you like to open the discussion?"

From TALES OF THE FLYING MOUNTAINS (prologue) by Poul Anderson (1970)

The problem with generation ships is that younger generations don’t necessarily respect the concerns of older generations.

Those who initially board the ship may enthusiastically embrace the idea of emigrating to a new planet, even if they won’t live to see the planet themselves. They believe their descendants will thank them for a fresh start away from whatever troubles plagued the old home world.

But children can be ungrateful. Also oblivious. And careless. Numerous generation ships explode or become uninhabitable because the great-great grandchildren of the original crew can’t be bothered to do preventive maintenance, or forget what certain switches and dials are for. Many more such ships reach their destinations but never send out a landing party—the task of building farms and cities sounds like dirty complicated hardship, not to mention that children born in a cozy enclosed vessel may be terrified by the wide open spaces of an entire world. Either the ships remain in orbit indefinitely, or they slingshot once around the planet and head straight back for home.

To avoid such difficulties, the generation ships of Tau Ceti have developed a technique for keeping younger generations mindful of the first generation’s intentions: they paint a line down the middle of the ship, thus dividing the ship’s living areas into “Port-half’ and “Starboard-half.” They then organize contests in which Port children compete with Starboard children for rote memorization of important knowledge (such as how to run the ship, how to survive on an alien planet, and how to construct farms, roads, etc.).

Children may not care about pleasing their parents, but they’ll do anything to defeat a rival. Therefore, they throw themselves into the job of learning whatever is required. They organize themselves into study groups, and use peer pressure on their fellows to make sure everyone is working hard. Each formal competition brings together both sides in keenly fought challenges to remember exactly what they’re supposed to.

Within three generations, violence usually breaks out. Three generations more, and the two halves have calcified into religious orthodoxies that furiously oppose each other on tiny points of received doctrine. By the time the ship actually reaches its destination, the Port and Starboard communities are eager to land and establish themselves so they can wage holy war.

Admittedly, this isn’t a perfect solution to preserving commitment and knowledge down through the generations. However, it has ample historic precedent.



To evaluate the feasibility of long duration, manned spaceflights, it is of critical importance to consider the selection and survival of multi-generational crews in a confined space. Negative effects, such as infertility, overpopulation and inbreeding, can easily cause the crew to either be wiped out or genetically unhealthy, if the population is not under a strict birth control.

In this paper, we present a Monte Carlo code named HERITAGE that simulates the evolution of a kin-based crew. This computer model, the first of its kind, accounts for a large number of free parameters such as life expectancy, age range allowed for procreation, percentage of infertility, unpredictable accidents, etc... to be investigated proactively in order to ensure a viable mission.

In this first paper, we show the reliability of HERITAGE by examining three types of population based on previously published computations. The first is a generic model where no birth/population control has been set up, quickly leading to fatal overcrowding. The second is the model presented by Moore (2003), that succeeds to bring settlers to another Earth under a 200 year-long flight, but the final crew is largely diminished (about a third of the initial crew) and about 20% of them show inbreeding of various levels. The third scenario is the model by Smith (2014) that is more successful in maintaining genetic diversity for the same journey duration.

We find that both the Moore and Smith scenario would greatly benefit from coupling a kin-based crew together with a cryogenic bank of sperm/eggs/embryos to ensure a genetically healthy first generation of settlers. Finally, we also demonstrate that if initial social engineering constraints are indeed needed to maintain an healthy crew alive for centuries-long journeys, it is necessary to reevaluate those principles after each generation to compensate for unbalanced births and deaths, weighted by the inbreeding coefficient and a need for maximizing genetic diversity.


In this section, we present the HERITAGE results for three different cases. The first one concerns an interstellar mission without any control on the population level. The second simulation tests the numbers suggested by Moore [10] and the third simulation investigates the predictions from Smith [12]. For all simulations, the duration of the flight was set to 200 years and includes an equal number of men and women. The age of the initial crew members follows a normal distribution centered around 20 years old. The simulations were looped over 100 voyages to obtain sufficient statistics.

3.1 Results for a Uncontrolled Population

It is an ethical and moral question as to whether a permanent control over the breeding selection and the number of child can be accepted by a multi-generational crew. In the case of an absence of control, the number of children per women might be more important and we fixed the default parameter to a mean of 3 children (with a standard deviation value of unity). By using a Gaussian distribution of children per woman, we ensure that there is a reasonable number of offspring per woman, avoiding illogical situations where the crew would constantly seek for reproduction. However, we let the size of the population and its genetic diversity uncontrolled, so that there is no enforced regulation of breeding and no self-imposed lower limits by the crew either. We allowed the crew members to breed as soon as they turn 18 years old until the natural limit of menopause. We stress that this is something of a worst-case scenario: the crew aren’t concerned with managing resources and have no taboos against incest, which is not realistic. This is a purely theoretical scenario to see how fast the mission could fail without strict controls.

(ed note: start with 75 men and 75 women. Average number of births per woman: 3. Age of start of procreation: 18. Age of end of procreation 45. Ship capacity 500 people)

Figure 2 shows the evolution of the population within the interstellar ship over 200 years. It appears that the number of crew members steadily increases over time until a sudden disaster reduces the populace by a third. This is due to the sudden disaster that happened at the 75th year of travel. The population soon recovers as nothing prevents breeding so the number of offspring continues to rise. The growth of population is almost exponential at the end as nothing, except inbreeding (not restricted in this section), can stop the multiplication of humans.

The limited number of crew members at the beginning of the mission drives the emergence of consanguinity within the offspring. The first generation of crew members born inside the vessel starts to reproduce regardless of the family affiliation and the inbreeding coefficient reaches a high value (21.00%) as soon as 30 years after launch.

In conclusions, an uncontrolled crew is likely to develop at a dangerous rate, inducing potentially high consanguinity. Because of the lack of self-imposed limits by the crew, the colony ship can be overcrowded within 25 years, driving the mission to a fatal end due to the probable development of diseases, a scarcity of food and internal conflicts due to overpopulation and loss of personal space. We agree with the original findings from Moore: the mission necessitates a control of birth rates in order not to saturate the space ship. Postponing parenthood until late in the women’s reproductive life is one viable option to delay overcrowding.

3.2 Results for a Moore-Like Population

Based on ETHNOPOP modeling, Moore speculated that a population of 150 – 180 people could survive a 200 years long travel if their reproductive cycle was restricted to a finite number of child per woman. In this case, the couples were only allowed to reproduce at an advanced age, clustering the population into discrete age groups, limiting the number of non-productive people within the vessel. Additionally, age clustering helps to maintain genetic variation by lengthening the generations, resulting in smaller sibships.

(ed note: start with 75 men and 75 women. Average number of births per woman: 2. Age of start of procreation: 35. Age of end of procreation 40. Ship capacity 500 people)

We find that the social engineering principles of Moore work well: births are clearly identified as peaks in the demographic spectra, compensated by a certain number of deaths shortly after. However the birth/death equilibrium is not maintained using those strict numbers. The number of crew members decreases slowly with time, only slightly impacted by the catastrophic event predicted by Smith (see Fig. 6). Contrary to what Smith postulated [12], a Moore-like population can absorb the loss of 30% of its human population on a short period of time. However, if the social engineering principles are not adapted, the crew is doomed to extinction for longer trips. Nevertheless, at the end of the journey, there is still a crew of about 56 people, with almost the same number of males and females (29 women and 27 men on average).

The biggest problem concerns the appearance of inbreeding at high levels during the interstellar travel. As seen in Fig. 9, age clustering helps reducing the average level of consanguinity: since procreation can only happen when a settler is at least 35 years old, and lasts for 5 years, it becomes impossible for a daughter to mate with her father, as the father will be well beyond the allowed procreation window. However, inbreeding due to cousin/cousin or brother/sister breeding is still possible, together with other combinations. Hence, the maximum inbreeding coefficient reaches 16.51% at the emergence of the second generation of space-born children. The coefficient remains high during the last years of the journey, with an average consanguinity factor (measured from only those who show a non-zero coefficient) ≥ 6%, above the security threshold (5%). More alarming is the fraction of the crew showing inbreeding, about 19.74% at the end of the simulation: this means that about one-fifth of the crew has a deleterious inbreeding coefficient.

In short, the numbers suggested by Moore allow a multigenerational crew to cover a 200 years long journey even if a sudden disaster impacts the space ship. The demographic sampling of the population thanks to well-defined social engineering concepts works as expected but cannot maintain a genetically healthy crew until the end of the mission. In order to help the offspring to develop without genetic disorders, the social principles must be re-assessed at each generation, according to the needs of the crew. As an example, a selective breeding program could potentially fully restore the genetic health of the population.

3.3 Results for a Smith-Like Population

The last model to be investigated is the one presented by Smith [12]. In his paper, statistics applied to large numbers and a unique MATLAB simulation are used to check whether a crew can be healthy after a 150 years long trip. Smith found that it is possible only if the founding population number lies between 14000 and 44000 crew members. In comparison to Moore studies, Smith accounted for effects of mutation, migration, selection and drift, and found that a larger initial population is necessary to compensate genetic catastrophes. To test his predictions, we thus put an initial crew of 7000 women and 7000 men in the code and increased the colony ship capacity to 50000, in order to have a similar crew/capacity ratio with the two previous studies of this paper. All other parameters are the same

(ed note: start with 7,000 men and 7,000 women. Average number of births per woman: 2. Age of start of procreation: 35. Age of end of procreation 40. Ship capacity 50,000 people)

We plot in Fig. 10 the evolution of this crew over 6 new generations. Except for the higher numbers, the evolution of the Moore-like population and the prediction by Smith is very similar, albeit the sharp variation due to the catastrophic event (that we selected to occur at the same time as in the two previous cases, but the reader must bear in mind that it can happen anytime over the whole 200 years). At best the ship is only at about 54.25% of its maximum capacity and, similarly to the Moore-like scenario, the population shows a steady decline to end the journey with about 6171 people, which represents ~44.08% of the initial crew. Compared to the fraction of humans reaching destination in the section above (~37.33%), the Smithlike population appears to be better resilient to a long voyage. However, the lack of revision of the initial social engineering principles throughout the journey causes the number of deaths to be unbalanced by the number of births. It is a consequence highlighted by the Monte Carlo approach, as not every woman will give birth to two offspring while every male and female will eventually die.

Results are quite different regarding inbreeding. While the consanguinity coefficient rises sharply after the second generation (12.38% at maximum), the averaged inbreeding coefficient drops to ~6% at the third generation and stabilizes until the end of the journey. More importantly, the fraction of crew members showing inbreeding is very small in comparison to a Moore-like population (< 0.22%), which is negligible. The gene diversity from a much larger initial crew prevents the appearance of a large inbred subset of the crew.

The Smith-like population appears to be more efficient in mixing the genetic pool in order to ensure a safe sixthgeneration to arrive on an exoplanet without severe inbreeding. The large number of initial crew members is more adapted to resist a sudden disaster and even with a severe catastrophe the viability of the mission is not compromised. However, there is a steadily decline of the population that indicates a potential risk for the long-term health of the colony. This is due to the non-adaptivity of the initial social engineering principles that cannot counterbalance the number of deaths and births.


As a primary test, we explored three different scenarios: 1) a population that is not under a strict birth/population control, 2) a Moore-like (small crew) population, and 3) a Smith-like (large crew) population. The first scenario fails to reach the destination due to overcrowding. Without birth control or population limits, the ship’s crew capacity is exceeded after two generations. Starvation and internal conflicts will lead to the failure of this scenario. The second scenario successfully reaches destination with a reduced crew (about a third of the initial crew) but inbreeding within the crew members drives about one-fifth of the settlers to be genetically unhealthy. It is also very unlikely that a Moore-like population can survive much longer space voyages6. Finally, the third scenario is the only one to achieve the goal of the mission: bringing a genetically healthy crew to another distant planet, despite the fact that a small percentage (< 0.22%) of the spatial settlers have a non-zero inbreeding coefficient.

As noted in the introduction, viable genetic material can be frozen and preserved for long durations (in principle indefinitely, though there could be a failure rate over time). This means that even a tiny crew could carry enough genetic material to entirely prevent inbreeding even on long timescales, and would avoid the potential difficulties of creating artificial wombs and the problems of a lack of human parents to raise the children. Sociologically there’s never been a population born entirely from in vitro fertilization (IVF), which also has a much greater probability of multiple births (though, by staggering when each crew member is fertilized, the population numbers can still be controlled). Although IVF is a complex medical procedure which may not be suitable aboard a very small ship, fertilization from stored sperm is very much simpler. However, it may not be desirable for the mission to have to rely on this method - either sociologically or technologically.

Contrary to what was predicted, all three scenarios are resistant to a sudden disaster that destroys a third of its population. The establishment of social engineering principles can easily compensate for the loss of population. Yet, it is unsure how a real crew would agree with drastic measures. In addition, the Moore-like and Smith-like scenarios have a clear decreasing trend over the trip duration, meaning that the initial social engineering principles should not be carved in stone but must evolve according to the population level at a given time. A potential consequence of this conclusion is that either a hierarchical organization or the entire ship community should work on regular revisions of the initial principles. The need for enforced social rules might drive debates and delays in defining the new social principles if a democratic vote is applied, as there would probably be a panel of non-compatible suggestions. The question of a hierarchical structure has strong implications, as it can result in a military structure with representatives of the non-democratic selection of the breeding principles. However, the social order of the spacecraft is beyond the scope of the paper and we therefore limit ourselves to mentioning this problem.

In conclusion, this paper has shown that a Moore-like or a Smith-like population are both viable prospects, especially if the starship includes a cryogenic bank of frozen sperm, eggs or embryos to ensure the creation of a colony with a new generation of genetically healthy settlers. However, strict social engineering principles are driving the population of the vessel towards extinction. There is a necessity for adaptive (i.e. to be re-assessed every generation) social engineering principles.

10. J.H. Moore, “Kin-based crews for interstellar multi-generational space travel”, in Interstellar Travel and Multi-Generation Space Ships, K. Yoji, F. Bruhweiler, J. Moore and C. Sheffield (Eds.), Interstellar Travel and Multi-Generation Space Ships, Collectors Guide Publishing, Burlington, Ontario, Canada, pp.80-88, 2003.

12. C.M. Smith, “Estimation of a genetically viable population for multigenerational interstellar voyaging: Review and data for project Hyperion”, Acta Astronautica, 97, pp.16–29, 2014. doi:10.1016/j. actaastro.2013.12.013.

Alter Metabolism

A variation of the "Increase Lifespan" technique was in Charles Sheffield's Between The Strokes Of Night. A technique was discovered that would allow human metabolism to enter the "S-state." In this state, humans age at a rate 1/2000th normal, and perceive things at the same rate. There was also a protocol that would return an S-state person back to normal metabolism.

So with ships traveling at a slow 10% light speed, the trip to Proxima Centauri seems to take only a few weeks to an S-state person. Of course to a human in normal state, the trip will take about forty years.

As far as the S-state person is concerned, the ships are travelling faster than light. As long as they always stay in S-state.

To an S-state person, a normal state person moves so fast that they are invisible. To a normal state person, an S-state person appears to be immobile, though they are actually moving very very slowely. Of course to an S-stater all those normal state persons grow old and die 2000 times faster.


"I'll tell you one thing I still don't understand," Peron said. "When I was in S-space, I felt as though I was in a one-gee environment. Now we're in exactly the same part of the ship, but we're in freefall. I don't see how that can happen."

There was silence for a while, then Kallen made a little coughing noise. "T-squared effect," he said softly.


"He's quite right," Sy said calmly. "Good for you, Kallen. Don't you see what he's saying? Accelerations involve the square of the time—distance per second per second. Change the definition of a second, and of course you change the perceived speed. That's why they can travel light-years in what they regard as a few days. But you change perceived acceleration, too—and you change that even more. By the square of the relative time rates—"

"—which is another reason the Immortals don't go down to the surface of planets," said Lum. "They want to spend their time in S-space to increase their subjective lifespans, but then that forces them to live in a very weak acceleration field. They can't take gravity."

"Not even a weak field," added Rosanne. "They'd fall over before they even knew they were off balance. What did you say the time factor was?—two thousand to one? Then even a millionth of a gravity would be perceived by them as a four-gee field. They have to live in freefall. They have no choice about it. But they perceive a four-millionth of a gee as normal gravity."

Peron looked around him in disgust. "All right. So everybody saw it easily except me. Try another one. Tell me what's going on outside the ship. One reason I thought at first that S-space had to be some kind of hyperspace was the view from the ports. When you look out, you don't see stars at all. All you see is a sort of faint, glowing haze. It's yellow-white, and it's everywhere outside the ship."

This time there was not even a moment's pause.

"Frequency shift," said Sy at once. "Let's see. Two thousand to one. So the wavelengths your eyes could see would be two thousand times as long. Instead of yellow light at half a micrometer, you'd see yellow at a millimeter wavelength. Where would that put us?"

There was a hush.

"The Big Bang," whispered Kallen.

"The three degree cosmic background radiation," said Rosanne. "My Lord. Peron, you were seeing leftover radiation from the beginning of the Universe—actually seeing it directly with your eyes."

And it's uniform and close to isotropic," added Lum. "That's why it looked like a general foggy haze. At that wavelength you don't get a strong signal from stars or nebulae, just a continuous field."

"But it can't be that straightforward." Sy frowned. "The pupils of our eyes provide too small an aperture to deal with millimeter wavelengths. There has to be a lot more to S-space modification than the obvious changes."

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

Increase Lifespan

Finally there is the "Methuselah" concept. Advances in medical technology might increase human lifespan to thousands of years. So prolonged interstellar trips are more a problem of boredom instead of life-span.

If men, and not merely their machines, are ever to reach the planets of other suns, problems of much greater difficulty will have to be solved. Stated in its simplest form, the question is this: How can men survive a journey which may last for several thousand years? It is rather surprising to find that there are at least five different answers which must be regarded as theoretical possibilities—however far they may be beyond the scope of today’s science.

Medicine may provide two rather obvious solutions. There appears to be no fundamental reason why men should die when they do. It is certainly not a matter of the body “wearing out” in the sense that an inanimate piece of machinery does, for in the course of a single year almost the entire fabric of the body is replaced by new material. When we have discovered the details of this process, it may be possible to extend the life span indefinitely if so desired. Whether a crew of immortals, however well balanced and psychologically adjusted, could tolerate each other’s company for several centuries in rather cramped quarters is an interesting subject for speculation.

(ed note: the other medical solution is suspended animation)

From THE PLANETS ARE NOT ENOUGH by Arthur C. Clarke (1955)

      "Now we come to one of the more sobering aspects of our journey," said Jinjur. "Dr. Wang, could you please give us a short medical briefing."

     "Certainly," said Dr. Wang, smiling as he rose and took Jinjur's place at the podium. "This expedition is a long one. Longer than the normal life-span of the human body, even with all the medical advances we have made. Therefore, after the initial launch phases of the mission, we will all be treated with the life-extending drug, No-Die. When it has thoroughly saturated our tissues, it will slow our aging process to one-fourth of normal rate. Thus the forty years that it will take for us to travel to Barnard will only produce ten years of aging in our bodies.

     "Unfortunately, our intelligence will also be lowered by roughly the same factor (this complication was added by the author for dramatic purposes). That is why No-Die is not used more on Earth. Fortunately, you all have been picked as persons with higher than normal intelligence, so that the No-Die will merely reduce your functional level to that of a small child. We will have a semi-intelligent computer on board to keep us out of trouble during the trip out. It will stop administering the No-Die as we approach Barnard so that we will be back to normal intelligence when we arrive.

     "As for sexual matters. The engineers cannot make Prometheus go any faster. So even if they designed the system for a round-trip journey, No-Die couldn't stave off death long enough to bring us back alive. Thus, this trip is a one-way journey for all of us. The planets there are not habitable without using highly technical life-support systems to protect us against the poisonous atmospheres, so this cannot be a colonization mission. There must be no children born during the mission, and since we cannot count on your intelligent cooperation during the No-Die phase, all of you will have to undergo surgical operations to ensure that your reproductive organs are blocked."

     "We're finally on our way," said Jinjur. "I guess it's time."
     Everyone looked uncomfortable.
     "I wonder if we'll notice it?" asked George.
     "According to most clinical studies of No-Die," said Doctor Wang. "The effects come on so gradually that most users have no idea they are mentally impaired, unless they are asked to do some difficult task. But even then, there is a tendency to believe it is only because they are 'tired' or 'sick', not because the No-Die has slowed their mental processes."
     "I'll be just as happy to be fooled," said Jinjur. "I don't think I could stand knowing I was a drooling idiot."
     "It won't be that bad," said Dr. Wang. "We all have high IQ's to begin with. Even when we are reduced to twenty-five percent of normal, we'll still be high-grade imbeciles and can probably even button our own clothing."
     David noticed some disgusted expressions and tried to cheer them up.
     "Besides, even if we forget how to tell our right shoe from our left, we still have 'Mother' James and the Christmas Bush to take care of us. It can button our shirts, tie our shoes, and wipe our noses."
     Jinjur spoke to her imp. "Start putting the No-Die in the water, James."
     "It is done," replied a low whisper.

(ed note: several decades into the mission)

     "Laser beam contact!" the computer announced to General Jones, its normally soothing baritone taking an imperative edge.
     "Wha?" murmured Jinjur, rousing from her stupor in front of a video screen displaying an old John Wayne battle movie. Deep within her mind she sensed a martinet screaming at her, "Wake up, you idiot! You're in charge!" She shook her head... This was no way for a commander to act. She floated clumsily across the control deck to pull herself into the central command seat.
     "Report ship status, James!" she rasped in a weak imitation of her parade voice.
     James spoke through her hair imp. "I detect low energy laser beams from Earth. It is time to stop. I quit putting No-Die in your water a month ago. It is now time for the rest of the crew to be taken off." There was a slight pause as the friendly voice of the computer took on a formal note. "As Commander, you have the authority to countermand this prearranged plan, but you will have to elucidate your objections in detail."
     Jinjur blinked at the last few confusing words as James dropped back into his normal voice, "But you do want me to stop the drug, don't you, Jinjur?"
     Appalled by her mental weakness at this critical juncture, Jinjur grabbed her thick cap of fuzzy hair and shook her head with her muscular black arms, trying to wake the numbed brain inside. "Yes! Yes! Do it! Flush out the tanks, get rid of that stuff! I want to be me again!"
     "Take it easy, Jinjur," said James. "I'll do it right away. It will take a few months, however, before everyone recovers completely. I'll be looking forward to it. It sure has been dull playing nursemaid to a bunch of ageless imbeciles."

From THE FLIGHT OF THE DRAGONFLY aka "Rocheworld" by Dr. Robert E. Forward

Laser Sail

In Dr. Robert L. Forward laid it all out in his classic paper "Roundtrip Interstellar Travel Using Laser-Pushed Lightsails," Journal of Spacecraft and Rockets 21 (1984), pp. 187–195.

The secret is using a Laser Sail, which you will recall is a photon sail beam-powered by a remote laser installation.

The advantage is the starship does not have to carry the mass of the engine and the propellant, you leave it at home. This makes the task of designing the starship merely incredibly difficult, instead of utterly impossible.

The home system can also add to the laser batteries gradually after the starship's journey starts, as needed (the inverse-square law will weaken the beam as the range increases). You cannot do this with a self-contained starship, all of its engines have to be built before the journey starts.

And if a home system's laser battery or two break down, no problem! The resources of the home system are available to fix it. If a self-contained starship engine breaks down on the other hand, they are in trouble. They do not have the resources of the home system to help, all alone in interstellar space. They have to fix it themselves with whatever spare parts they managed to bring along. Or die all alone in the night.

The disadvantage is the starship is at the mercy of whoever is in charge of the laser station back in the Solar System. If there is a revolution back home and the Luddites seize power, the starship and crew are up doo-doo pulsar with no gravity generator. Dr. Forward came up with two clever ways of using the home system's lasers to decelerate the starship into the target solar system. In Larry Niven and Jerry Pournelle's classic The Mote In God's Eye the Motie aliens laser sail starship rather pointedly do NOT use Dr. Forwards deceleration methods, because they absolutely do not trust the laser station controllers back at their homeworld. In Dr. Forward's The Flight of the Dragonfly aka "Rocheworld" political foot-dragging and short-sighted policies almost lead to disaster for the laser station and starship. In Buzz Aldrin and John Barnes's Encounter With Tiber politics does kill the laser station and starship.


“Light sail!” Rod shouted in sudden realization. “Good thinking.” The whole bridge crew turned to look at the Captain. “Renner! Did you say the intruder is moving faster than it ought to be?”

“Yes, sir,” Renner answered from his station across the bridge. “If it was launched from a habitable world circling the Mote.”

“Could it have used a battery of laser cannon?”

“Sure, why not?” Renner wheeled over. “In fact, you could launch with a small battery, then add more cannon as the vehicle got farther and farther away. You get a terrific advantage that way. If one of the cannon breaks down you’ve got it right there in your system to repair it.”

Like leaving your motor home,” Potter cried, “and you still able to use it.

“Well, there are efficiency problems. Depending on how tight the beam can be held,” Renner answered. “Pity you couldn’t use it for braking, too. Have you any reason to believe—”

“Captain, look,” he said, and threw a plot of the local stellar region on the screen. “The intruder came from here. Whoever launched it fired a laser cannon, or a set of laser cannon — probably a whole mess of them on asteroids, with mirrors to focus them — for about forty-five years, so the intruder would have a beam to travel on. The beam and the intruder both came straight in from the Mote.”

(ed note: the lightsail was accelerated by lasers from its homeworld. But it braked by diving into New Cal's sun.)

“But that’s the point: it’s not right, Captain,” Renner protested. “You see, it is possible to turn in interstellar space. What they should have done—

The new path left the Mote at a slight angle to the first. “Again they coast most of the way. At this point” — where the intruder would have been well past New Cal — “we charge the ship up to ten million volts. The background magnetic field of the Galaxy gives the ship a half turn, and it’s coming toward the New Caledonia system from behind. Meanwhile, whoever is operating the beam has turned it off for a hundred and fifty years. Now he turns it on again. The probe uses the beam for braking.

“You sure that magnetic effect would work?”

“It’s high school physics! And the interstellar magnetic fields, have been well mapped, Captain.”

“Well, then, why didn’t they use it?”

“I don’t know,” Renner cried in frustration. “Maybe they just didn’t think of it. Maybe they were afraid the lasers wouldn’t last. Maybe they didn’t trust whoever they left behind to run them. Captain, we just don’t know enough about them.”

(ed note: spoiler alert: the answer is they didn't trust who they left behind to run the laser cannons. So when it came to braking, they did it the hard way.)

(ed note: In Mallove and Matloff's The Starflight Handbook, they note that if the interstellar magnetic fields have not been well mapped, this scheme could potentially doom the starship to a lonely death.)

From THE MOTE IN GOD'S EYE by Larry Niven and Jerry Pournelle (1975)

(ed note: the laser transmitter lens was big enough to launch the starship. But the size of the lens has to be increased for the deceleration phase. Evil political hack Senator Winthrop manages to steal GNASA's budget for expanding the mirror so he can funnel it into tobacco farmer subsidies in his home state.)

Senator Beauregard Darlington Winthrop III was in his third term of office, and as Chairman of the Senate Appropriations Committee he wielded an influence only slightly less potent than the Senate Majority Leader. GNASA officials winced when they heard that budget-hearing time was coming around again.

"Now. Ah'm sure you honorable gentlemen realize that this nation, as rich and as glorious as it is, cannot afford every space boondoggle there is. Ah trust that you've come up with a budget that realizes that there are people here on the ground that desperately need money to keep their family businesses alive..."

"He probably means subsidies for the tobacco farmers," thought the Honorable Leroy Fresh, as he prepared to defend GNASA's budget before the committee.

"There is one item that the Chairman noticed in the preliminary reports that he would like to question the Honorable Dr. Fresh about, if he may." Without waiting for a reply, Winthrop continued. "I notice this line-item number one hundred eight, for four hundred million dollars to expand the transmitter lens for the Barnard laser propulsion system. I didn't notice that in the previous year's budget, and since the mission is not slated to reach Barnard for another twenty years or so, surely this item could be deferred a year or two to release a few funds to succor the poor people of this nation?"

Leroy was ready for this one. "May I remind the Chairman, the reason the item was not in last year's budget was that it was removed by the Senate Appropriations Committee, as it has each year for nearly the past decade. The transmitter lens doesn't have to be full size at the start of the mission, and can be built slowly as time passes and the Barnard expedition moves further away, but the lens must be made ready for the deceleration phase, which requires it to be at maximum diameter. The amount of money in the budget is that needed to bring us back on schedule."

"But the lasers are turned off, and the Barnard lightsail is merely coasting on its way to its destination. Surely we can defer work on the lens expansion since it's not being used. Especially since I notice in line-item one hundred ten the fifty million dollars for the construction of the Tau Ceti lens. The increase in diameter planned for each lens is fifty kilometers. Surely that indicates that they should have equal budgets. Perhaps we should just make those two lens-construction items both equal in size at fifty million?" Senator Winthrop looked around at his committee and smiled.

"Is that agreeable, gentlemen? ...Oh, yes. Excuse me, Madam Ledbetter. Is that agreeable, gentlemen and lady?" He raised a blue pencil and scratched away at his copy of the budget.

"But Senator Winthrop, Sir," Leroy protested. "The Ceti lens is going from a diameter of twenty kilometers to seventy kilometers, while the Barnard lens is going from three hundred and twenty to three hundred and seventy kilometers. Even though both have the same increase in diameter, the increase in area of the Barnard lens is eight times larger than that of the Ceti lens. The cost goes as the square of the diameter."

"Well, Ah must admit Ah'm a little 'square' when it comes to that scientific math, Dr. Fresh, but Ah'm pretty good at figures when they have a dollar sign in front of them." There was a polite laugh at the Chairman's joke from the committee and staff. Fresh was silent, knowing that he had lost another skirmish. "After all," said Senator Winthrop with a smile that seemed entirely sincere over the TV cameras. "That's what we have you scientific types at GNASA for, to take care of all that 'square root' and 'cube root' type math stuff. And Ah must say," he said, with only a trace of sarcasm, "You've been doing an excellent job on an austere budget—like the true Greater American patriots that you are. Now, let's go on to line-item one hundred thirty-three, the million-channel receiver to search for signals from aliens. Surely a single channel is all that you need. It's obvious. One receiving antenna, one receiving channel..."

(ed note: the stupid, it burns!)

(ed note: things start to unravel, and Evil Senator Winthrop frantically looks for a way to avoid his fate )

"As Chairman of the Senate Appropriations Committee, what are your plans for completing the transmitter lens for the Barnard star expedition so the crew can be brought safely to a halt?"

Winthrop didn't know the details, but he wasn't stupid. There was no way that the transmitter lens could be completed in time. Twenty years ago the construction of the lens had been stopped just short of one-third-diameter. The lens had to be nearly full-sized if the deceleration technique were to work. Since the diameter had to be tripled, the area had to increase nine times. Although the ship would not need to start decelerating at Barnard for nearly eight years, the light beam to carry out that deceleration had to be on its way across the six lightyears between here and there only twenty-four months from now. There wasn't enough time. That g*******d Gudunov was doomed.

(ed note: Evil Senator Winthrop gets his just desserts, and the new pro-space congress tries to repair the damage )

The first action of the new chairman was to call for testimony from the newly appointed head of the Space Agency, the Honorable Perry Hopkins.

"I'm pleased to have you with us today, Dr. Hopkins," said Senator Rockwell. "I know we're all concerned about our brave crew that are approaching Barnard, ready to stop. Now, in the past, this committee, under the leadership of our distinguished Minority Leader..." here Senator Rockwell turned to nod to Senator Winthrop down near the end of the table, "...found it expedient for the sake of the small farmers of this nation to defer certain items of expenditure for the space program. We realize that this may have caused you some problems in the past and we want you to know that the time has come for the space program to receive the resources that it needs to carry out its mission. Tell us. What do you need to bring this great nation's crew of astronauts to a successful conclusion of their epic voyage?"

"I wish I could tell you, Senator Rockwell," he started. "But I'm afraid I can't. And I can't because there is no answer. The previous GNASA administrators have reported to this committee an infinite number of times that work needed to be done on the Barnard transmitter lens. But that... (careful now, Perry, calm down)... the previous chairman always felt it could be postponed until some future date. Well, gentlemen, that date was two years ago."

"Do you mean to tell me that there is no way to allow our brave crew to come to a safe landing at their destination?" said Senator Rockwell.

"I don't mean to be melodramatic, Mr. Chairman. And I have exercised my staff for alternatives, but unless someone comes up with a miracle, that crew is as good as dead."

"But surely with a crash effort..."

"There are only so many robots in space, and due to the low demand for space robots, there is only one space robot factory," said Perry. "Even if we could speed up the production line by five times, and even if we had some magical way to transport those robots instantly over the ten astronomical units to the transmitter lens and put them all to work, there isn't enough web and plastic in the solar system to make up for twenty years of neglect. At best we could get the lens up to sixty percent of the necessary diameter. Even if the lasers were up to power, that would only suffice to strand the crew some two lightyears beyond Barnard, with no hope of getting back. I'm sorry to bring you such bad news, gentlemen, but it's the best I have!"

(ed note: spoiler alert: they managed to save the crew. Somebody invented a nonlinear material that would frequency triple, turning three infrared photons {at fifteen hundred nanometer frequency} into one green photon {at five hundred nanometer frequency}. If a laser beam has its wavelength cut by one-third then a lens of a given size can sent it three times as far. So while the existing transmitter lens is only ⅓rd the size it need be for an infrared laser beam, it is just the right size for a green laser beam.)

From THE FLIGHT OF THE DRAGONFLY aka "Rocheworld" by Dr. Robert E. Forward


Starwisp is an ultra-low mass interstellar probe, a tiny sail driven by a beam of microwaves. The concept was invented by Dr. Robert L. Forward, and expanded upon by Dr. Geoffrey A. Landis.

Dr. Forward assumed that the microwave beam would be efficiently reflected by starwisp, so he calculated it would be a superconducting metal mesh with a sail mass of 16 grams and a payload mass of 4 grams; total mass of probe is 20 grams. Dr. Landis found this turned out not to be the case, it would absorb quite a bit of microwaves and heat up (i.e., the design is thermally limited). In Dr. Landis' design the starwisp is woven out of carbon wires with a sail mass of 1,000 grams, a payload mass of 80 grams, and a diameter of 100 meters.

Acceleration is 24 m/s2, microwave lens 560 km in diameter transmitting 56 GW of power, accelerating the probe to 10% of the speed of light.

Yes, it probably could be weaponized. See Accelerando by Charles Stross.

Another form of beamed power propulsion uses beams of microwaves to drive the starship. Microwave energy has the great advantage that it can be made and transmitted at extremely high efficiencies, although it is difficult to make narrow beams that extend over long distances. Because of the short transmission range, the starship being pushed by the microwave beam must accelerate at a high rate to reach the high velocities needed for interstellar travel before the starship gets too far from the transmitting system (which means it can be weaponized). The accelerations required are larger than a human being can stand, so microwave pushed starships seem to be limited to use by robotic probes. There is one design that looks quite promising. I call it Starwisp, because of its extremely small mass.

Starwisp is a light-weight, high-speed interstellar flyby probe pushed by beamed microwaves. The basic structure of the Starwisp robotic starship is a wire mesh sail with microcircuits at the intersection of the wires. The microwave energy to power the starship is generated by a solar powered station orbiting Earth. The microwaves are formed into a beam by a large fresnel-zone-plate lens made of sparse metal mesh rings and empty rings. Such a lens has very low total mass and is easy to construct.

The microwaves in the beam have a wavelength that is much larger than the openings in the wire mesh of the Starwisp starship, so the very lightweight perforated wire mesh looks like a solid sheet of metal to the microwave beam. When the microwave beam strikes the wire mesh, the beam is reflected back in the opposite direction. During the reflection process, the microwave energy gives a push to the wire mesh sail. The amount of push is not large, but if the sail is light and the power in the microwave beam is high, the resultant acceleration of the starship can reach hundreds of times Earth gravity. The high acceleration of the starship by the microwave beam allows Starwisp to reach a coast velocity near that of light while the starship still close to the transmitting lens in the solar system.

Prior to the arrival of Starwisp at the target star, the microwave transmitter back in the solar system is turned on again and floods the star system with microwave energy. Using the wires in the mesh as microwave antennas, the microcircuits on Starwisp collect enough energy to power their optical detectors and logic circuits to form images of the planets in the system. The phase of the incoming microwaves is sensed at each point of the mesh and the phase information is used by the microcircuits to form the mesh into a retrodirective phased array microwave antenna that beams a signal back to Earth.

A minimal Starwisp would be a one kilometer mesh sail weighing only sixteen grams and carrying four grams of microcircuits. (The whole spacecraft weighs less than an ounce—you could fold it up and send it through the mail for the cost of first class postage.) This twenty gram starship would be accelerated at 115 times Earth gravity by a ten gigawatt (10,000,000,000 watt) microwave beam, reaching twenty percent of the speed of light in a few days. Upon arrival at Alpha Centauri some twenty years later, Starwisp would collect enough microwave power from the microwave flood beam from the solar system to return a series of high resolution color television pictures during its fly-through of the Alpha Centauri system.

Because of its small mass, the ten gigawatt beamed power level needed to drive a minimal Starwisp is about that planned for the microwave power output of a solar power satellite. Thus, if power satellites are constructed in the next few decades, they could be used to launch a squadron of Starwisp probes to the nearer stars during their "checkout" phase.

From Indistinguishable from Magic by Robert Forward (1995)

Laser-Pushed Lightsail

So your gigantic laser battery at home pushes the laser sail starship to its destination, accelerating it to about half the speed of light. Presumably you want to stop at your destination instead of streaking through it at 0.5c. But how?

If you were going about an order of magnitude slower, you might be able to use the sunlight from the destination star to put on the brakes. However that ain't gonna be enough at 0.5c. You'll just pancake into the star at a substantial fraction of the speed of light and be vaporized.

Dr. Philip Norem had a clever idea. Interstellar space has large magnetic fields. So one can use large electrical charges on the starship to make huge light-year wide sweeping turns by the Lorentz force.

Say you were going to Alpha Centauri. You aim the starship not at the destination, but instead off to one side. How far off depends upon the starship's turning radius. The laser battery back at the solar system pushed the starship up to relativistic velocities over the next 27 years or so. Then the lasers turn off.

The starship deploys one hundred metal cables, each about 50,000 kilometers long. It then charges them up to 800,000 volts and 3.7×104 coulombs. This is timed to interact with the interstellar magnetic field (as mapped) so that the starship makes a huge gradual turn, until it is approaching Alpha Centauri from the back door. That is, so that a line drawn from the starship to Alpha Centauri will pass directly through the solar system and the laser battery.

Meanwhile, the solar system laser battery starts up its barrage long enough in advance so that the leading edge of the laser wavefront will reach the starship just as it is aligned properly. It then continues the barrage for the years required to bring the starship to a halt exactly at Alpha Centauri.

In Mallove and Matloff's The Starflight Handbook, they note that if the interstellar magnetic fields have not been well mapped, this scheme could potentially doom the starship to a lonely death. If the starship misses the beam, it just goes sailing off into the Big Dark. The Starflight Handbook has the equations for a starship using the Lorentz force, if you are interested.


“Why will we use green lasers for starwisp and light sail propulsion?

“Because stars aren’t green. Can’t be green, in fact, because a black-body spectrum that peaks in the green is broad enough that there’s plenty of other-colored light to make it not sum to green. That makes green the least stealthy color in space.

“So when you’re going to be shining a few hundred terawatts into someone else’s star system, a monochromatic 530 nm green is as good as it gets in letting them know up front that you aren’t trying to sneak something in on them.”

– Argil Medanis-ith-Medanis, Laserider Network

From SUPERGREEN by Alistair Young (2020)

Laser sail propulsion is the one method for achieving star travel with human crews that is closest to reality. It will be some time before our engineering capabilities in space will be up to building the laser system needed, but there is no new physics involved, just a large scale engineering extrapolation of known technologies. In laser sail propulsion, light from a powerful laser is bounced off a large reflective sail surrounding the payload. The light pressure from the laser light pushes the sail and payload, providing the needed thrust. The laser sail starship is about as far from a rocket as is possible. The starship consists of nothing but the payload and the lightweight sail structure. The rocket engine of our starship is the laser, powered by an energy source such as the Sun. The reaction mass is the laser light itself.

For interplanetary operation and interstellar flight, the lasers would be in near-Earth space and powered by sunlight collected by large reflectors, sending their beams out to push the sails of the interplanetary fleet with the light pressure from their powerful beams. For pushing an interstellar starship, the lasers might work better if they were in orbit around Mercury. There is more sunlight there and the gravity attraction of Mercury would keep them from being "blown" away by the back reaction from their light beams. The lasers would use the abundant sunlight at Mercury's orbit to produce coherent laser light, which would then be combined into a single coherent beam and sent out to a transmitter lens floating between Saturn and Uranus.

The transmitter lens would be a fresnel-zone-plate lens with dimensions tuned to the laser frequency and consisting of wide rings of one-micrometer-thick plastic film alternating with empty rings. The transmitter lens would not be in orbit, but would either be freely falling (very slowly at that distance from the Sun), or "levitated" in place by rockets or by the momentum push from a portion of the laser light passing through it. The lens would be 1000 kilometers in diameter (as big as Texas) and mass about 560,000 tons. A lens this size can send a beam of laser light over forty lightyears before the beam starts to spread.

The first interstellar mission that could be performed with this laser and lens system would be a one-way flyby robotic probe mission to the nearest star system. The robotic probe would have a total mass of one metric ton, about one-third each of payload, support structure, and thin aluminum film reflecting panels. The sail portion of the probe would have a diameter of four kilometers.

The probe would be pushed at an acceleration of three percent of Earth gravity by an array of solar-pumped lasers with a total power of 65,000 megawatts or 65 gigawatts. While this is a great deal of laser power, it is well within our future capabilities. Power levels of this magnitude are generated by the Space Shuttle rocket engines during liftoff, and one of the ways to make a high power laser is to put mirrors across the exhaust of a high power rocket. If the acceleration is maintained for three years, the interstellar probe will reach the velocity of eleven percent of the speed of light at a distance of only one-sixth of a lightyear. At this distance it is still within range of the transmitter lens and all of the laser power is still focused on the sail. The laser is then turned off (or used to launch another robotic probe) and the robotic starship coasts to its target, flying through the Alpha Centauri system forty years after launch.

When I first invented the concept of laser-pushed lightsails back in 1962, I thought it was obvious that since all the laser can do is push the lightsail, it would not be possible to use a solar system laser to stop the lightsail at the target system. The idea seemed to be limited to fly-by precursor robotic probe missions. It wasn't until twenty years later, while trying to find a new way of traveling to the stars for a novel I was writing, I realized that if the lightsail were separated into two parts, then one part could be used as a mirror to reflect the laser light back toward the solar system. That retrodirected light could then be used to decelerate the other portion of the lightsail. When I worked out the equations and put numbers into it, I found that not only was it a good science fiction idea, but it would really work. The concept has since been published as a scientific paper in the Journal of Spacecraft and Rockets, and one of the references to prior work in the scientific paper is my novel, The Flight of the Dragonfly, later reissued by Baen Books in a much expanded version as Rocheworld.

If the reports from the unmanned probes are favorable, then the next phase would be to send a human crew on an interstellar exploration journey. More than just the nearest star system will ultimately need to be explored, so I designed the laser lightsail starship to allow a roundtrip exploration capability out to twelve lightyears, so Tau Ceti or Epsilon Eridani can be visited within a human lifetime. I assumed the diameter of the lightsail at launch to be 1000 kilometers in diameter, the same size as the transmitting lens. The total weight would be 80,000 tons, including 3,000 tons for the crew, their habitat, their supplies, and their exploration vehicles. The lightsail would be built with three stages. There would be a disc-shaped inner "return stage" portion, 100 kilometers in diameter, that would carry the payload and crew, and return them to Earth. This would be surrounded by a ring-shaped "accelerator stage" portion, 320 kilometers in diameter with a 100 kilometer diameter hole. Together, these two sails constitute the "rendezvous stage" that would stop at the target star. This in turn would be surrounded by the "decelerator stage", 1000 kilometers in diameter with a 320 kilometer diameter hole. [See Figure 7.]

All three portions of the lightsail would be accelerated together at thirty percent of Earth gravity by 43,000 terawatts of laser power. At this acceleration, the lightsail would reach a velocity of half the speed of light in 1.6 years. The expedition would reach Epsilon Eridani in twenty years Earth time and seventeen years crew time, and it would be time to stop.

At a half-lightyear from the target star, the 320 kilometer rendezvous stage would be detached from the center of the lightsail and turned to face the large ring-shaped decelerator stage that remains. The laser light coming from the solar system would reflect from the decelerator stage acting as a retro-directive mirror. The reflected light would decelerate the smaller rendezvous sail and bring it to a halt at Epsilon Eridani.

After the crew explored the system for a few years (using their rendezvous stage lightsail as a solar sail), it would be time to bring them back. To do this, the 100 kilometer diameter return stage would be separated out from the center of the 320 kilometer ring-shaped accelerator stage. The laser light from the solar system would hit the accelerator stage and be reflected back on the return stage. The laser light would then accelerate the return stage and its payload back toward the solar system. As the return stage approached the solar system twenty Earth-years later, it would be brought to a halt by a final burst of laser power. The members of the crew would have been away 51 years (including five years of exploring), have aged 46 years, and would be ready to retire and write their memoirs.

From Indistinguishable from Magic by Robert Forward (1995)

Smart Pellets

Smart Pellets

When Clifford Singer proposed in his 1980 paper that a stream of pellets could be used to drive an interstellar vehicle, the idea emerged at a time when Robert Forward had already drawn attention to a different kind of beamed propulsion. Forward’s sail missions used a beamed laser from an array near the Sun, and he explored the possibility of building a Fresnel lens in the outer Solar System to keep the beam tightly collimated; i.e., we want the narrowest possible beam to put maximum energy on the sail.

It was an era when huge structures in space defined interstellar thinking. Forward’s lasers were vast and he envisioned a 560,000-ton Fresnel lens in deep space, a structure fully one-third the diameter of the Moon. Such a lens made collimating the laser beam a workable proposition, to say the least — at 4.3 light years, the distance of Alpha Centauri A and B, such a beam is still converging, and would not reach the size of its 1000 kilometer transmitting aperture until an amazing 44 light years out.

Singer’s ideas were just as big, of course, and we saw yesterday that they demanded not only a series of stations to keep the pellet beam collimated but also an accelerator in the outer Solar System that would be 105 kilometers long. If we’re building enormous structures to begin with, wouldn’t it be easier to just send laser photons than a stream of particles or pellets? The answer, and it’s surely one that occurred to Singer as he examined Forward’s ideas, is that there is an inherent downside to photon propulsion. Let Gerald Nordley explain it:

The pellet, or particle, beam propulsion system is conceptually similar to photon beam propulsion systems discussed by Forward and others. While the concept is feasible, the reflected photons must still move at the speed of light and so carry away much of the energy used to generate them. The velocity of a beam of particles, however, can be varied so that the reflected particles are left dead in space and thus waste much less energy.

Geoffrey Landis described the same problem in his 2004 paper “Interstellar Flight by Particle Beam.” For all their size, Forward’s laser-propelled lightsails have extremely low energy efficiency, which is why the laser installations have to be so large in the first place. Some of Forward’s proposals reach lasers with power in the range of 7.2 terawatts. So we have an inefficient mechanism forcing not just huge lasers but spectacular lenses in the outer system. I don’t rule out huge structures in space — nanotech assemblers may some day make this possible — but finding ways to eliminate the need for them may bring the day of actual missions closer.

The Nordley quote above is drawn from his website, where slides from a presentation he made at a workshop in 1993 are made available. Nordley had already addressed the matter of particle beam propulsion in a 1993 paper in the Journal of the British Interplanetary Society, in which he discussed a magnetic sail, or ‘magsail,’ as the reflector for the incoming particles. The magsail reflects the particles and, as Nordley notes, thereby gains some fraction of twice their momentum, although he adds that reflector concepts are not limited to magnetic sails.

A retired Air Force officer, Nordley is an astronautical engineer who also writes science fiction (under the name G. David Nordley), author of the highly regarded novella “Into the Miranda Rift” along with numerous other stories mostly in Analog. It was in that magazine in 1999 that he pursued the work on magnetic sails that Dana Andrews and Robert Zubrin had developed, combining their insights with Clifford Singer’s pellet concepts. The result: Mass beam drivers driven by solar power that shoot pellets to a spacecraft whose laser system ionizes them, reflecting the resultant plasma by a magnetic mirror to produce thrust. Or perhaps a self-destruct mechanism within each pellet that would be triggered by proximity to the starship.

Nordley’s pellet stream added a significant new wrinkle to Singer’s in that it would be made up of pellets that could steer themselves to the beam-riding spacecraft. Remember the scope of the problem: Singer needed those stations in deep space to make course adjustments for the pellet stream, which had to hit the spacecraft at distances of several hundred AUs. Nordley talks about nanotech-enabled pellets in the shape of snowflakes capable of carrying their own sensors and thrusters, tiny craft that can home in on the starship’s beacon. Problems with beam collimation thus vanish and there is no need for spacecraft maneuvering to stay under power.

In “Beamriders,” a non-fiction article in the July/August, 1999 Analog, he sees these pellets as weighing no more than a few micrograms, although here again the question of interstellar dust comes into play. Singer had found in his second JBIS paper (see citation at the end of yesterday’s entry) that pellets over a gram in size should be impervious to large-scale dispersion. It would obviously have to be demonstrated that much lighter ‘smart pellets’ like these would not suffer from dust strikes. But the beauty of lighter pellets is that they would rely on shorter accelerators than the 100,000 kilometer behemoth Singer described.

Efficient delivery of the pellet stream can also make for smaller magsails because the incoming stream is tightly concentrated. The pellet concept Singer introduced is thus significantly enhanced by Nordley’s application of nanotechnology, and forces us to ask the question that has infused this entire series of posts: Given the rapid pace of miniaturization and computing, can we imagine a paradigm shift that takes us from smart pellets all the way to self-contained probes the size of bacteria? Developing the technologies by which such minuscule craft would travel in swarms, combining resources for scientific study and communications, will surely energize one stream of interstellar studies in coming decades.

The Geoffrey Landis paper cited above is “Interstellar Flight by Particle Beam,” in Acta Astronautica Vol. 55, pp. 931-934 (2004). The earlier Nordley paper on particle beam propulsion is “Relativistic Particle Beams for Interstellar Propulsion,” JBIS, 46-4, April 1993. See also his “Interstellar Probes Propelled by Self-steering Momentum Transfer Particles” (IAA-01-IAA.4.1.05, 52nd International Astronautical Congress, Toulouse, France, 1-5 Oct 2001).


This is from Spacecraft With Interstellar Medium Momentum Exchange Reactions: The potential and limitations of propellantless interstellar travel (2019).

Bottom line: a starship using this propulsion could manage velocities of about 0.5 c, and expends energy at about the same rate as a laser lightsail. It is suitable for carrying massive payloads on multi-century long missions. It could manage to propel a generation ship or bulk materials for an interstellar colony.

Of course a generation ship would have to hope that the people back at Sol will be willing to constantly supply them with 10 terawatts of laser beam for 370 years.

Spacecraft with interstellar medium momentum exchange reactions (SWIMMERs) is another propulsion system attempting to avoid the need to carry reaction mass. Much like the laser light sail. Because when you are trying to delta-V your spacecraft up to fractional c velocity the Tyranny of the Rocket Equation will kneecap you like a loan shark's henchman, and for similar reasons. The "interest rate" on the propellant is just too freaking high.

SWIMMERs are actually more energy efficient than light sails. It is very similar to Robert Zubrin's dipole drive, and the two might actually be different designs for the same concept. Zubrin's design has a constant electric field while the SWIMMERs use a pulsed electric field.

SWIMMERs take advantage of the fact that while the interstellar medium (ISM) could pass for a good vacuum in a laboratory, that is NOT the same as having no atoms in it at all. It still has about a million atoms per cubic centimeter.

Electric sails are propelled by solar wind, Magsails are propelled by the solar magnetic field. Both are very low mass structures composed of charged or current carrying wires. They both can use the heliosphere or ISM as drag, to slow the sail down.

SWIMMERs attempt to use low mass structures composed of charged wires to somehow interact with the ISM and create thrust instead of drag. The exact opposite of electric sails.

The idea behind SWIMMERs is to set up "inhomogeneities" in the surrounding ISM that the SWIMMERs electric field can push upon to create thrust.

The pusher plate is composed of tether segments, much like a standard electric sail. However, unlike the e-sail, the tethers are two identical layers of wire separated by strong insulator. The insulator keeps the wire layers physically apart and electrically isolated. The tethers are constructed with fine superconducting wire. The pusher plate is spun to create tension and keep the tether grid extended without requiring a heavy support structure.

In the Primer phase of the operational cycle, the front layer of the tethers are charged up to positive potential φ, where φ is a bit above the stopping potential of the ions in the local interstellar medium. The back layer of tethers are charged to negative potential .

Due to something called the "edge effects of the finite plates and the self-shielding behavior of plasmas", ISM ions streaming toward the front positively charged layer slow down. This creates an over-dense clump in front of the pusher plate. Simultaneously an underdensity forms between the clump and the front of the tethers.

Next comes the Pull phase of the operational cycle. The charges on the front and back layers are reversed and significantly increased. The ion clump in front of the pusher plate is attracted to the (now) negative front layer, pulling the spacecraft forwards (creating thrust).

However, if the ion clump actually strikes the pusher plate the plate charge will create drag and negate the thrust. This has been taken into account. Remember the underdensity that was formed right over the pusher plate? There ain't hardly no ions in that zone, so they will cause only minimal drag. This gives time for the approaching ion clump to create plate thrust. When the ion clump is close to actually striking the pusher plate, the plate charge is turned off. This allows the ion clump to pass through the plate, again with only minimal drag.

Finally comes the Push phase of the operational cycle. The plate charge is turned back on with the same polarity. Since the ion clumps are now behind the plate, they are repelled and create more thrust by the law of action and reaction.

Due to symmetry, while the positively charged ions are moving front to back, the negatively charged electrons are moving back to front. Ordinarily this would cause reverse thrust that would cancel out the forward thrust, with a net thrust of zero. Luckily, ions weigh 1,836 times more than electrons, so they create 1,836 times more thrust. The 1/1,836 reverse thrust of the electrons is negligible.

It may be possible to use multiple layers of pusher plates to accelerate ions to a higher thrust. The report says that may be worth looking into.

Laser sail can only be pushed by the laser beam from Sol. This means if the laser sail has to brake to a halt at the destination, or return back home, it will have to do all sorts of weird things.

Since SWIMMER can direct its thrust independently of the energizing laser beam, it has no problem at all either braking to a halt at destination nor returning home. Or even braking to a halt back home at journey's end. Normal mode is used for traveling to destination, Destination Braking mode is used to halt, Tractor Beam mode is used to return to Terra, and Home Braking mode is used to stop at Terra.

Vmaxmaximum velocity
tcruisetime to traverse 1 parsec (3.26 ly) in the ISM, ignoring time spent in Sol's heliosphere and time spent decelerating near α Cen A.
tcruise/tsailcompares cruise time with time required by ideal massless light sail with equal payload pushed by equal amount of power
Pdeliveredpower delivered to SWIMMER by laser beam based back at Sol
Mpaymass of payload
Mpowermass of power conversion system (converts laser light into electricity)
Summed tether lengthtotal of all the individual pusher plate tethers added together
Mpushermass of pusher plate tethers (decreases during journey as excess plate mass is jettisoned)
Mission parameters
Space probeArk ship
Vmax0.020 c0.014 c
tcruise260 years370 years
Pdelivered10 MW10,000 GW
Mpay1000 kg8×109 kg
Mpower2500 kg1×109 kg
Initial pusher plate parameters
Summed tether length4.1×109 m2.0×1016 m
Mpusher7400 kg3.7×1010 kg
Final pusher plate parameters
Summed tether length2.9×108 m1.0×1015 m
Mpusher520 kg1.9×109 kg

For the sample missions, the paper assumes that a standard interstellar sail laser array will work equally well with SWIMMER. Since there have been quite a few papers about such laser arrays the paper doesn't bother doing any analysis.

The paper didn't go in deep analysis of the power conversion system that transforms the laser light into electricity. It assumes the system will have a specific power of 4 kW/kg. If the system has a worse specific power, the trip times will be increased.

For the Sol to α Cen A trip, a simplified model of the ISM was used. It was assumed to be uniform with a density of 0.07/cm3 and a temperature of 7000 K. The heliospheres of both Sol and α Cen A were assumed to have a density of 7.3/cm3 and a temperature of 140,000 K. Furthermore the solar wind in both heliospheres are assumed to be uniformly streaming outward at 5×105 m/s to a distance of 100 AU, at which point the medium abruptly transitions to a stationary ISM.

Space Probe

This is to deliver a 1,000 kg payload to α Cen A, decelerating it into gravitational capture to become a permanent space telescope observing the α Cen system. It will only require 10 megawatts of laser beam. Constantly supplied for 260 years of course. If back at Terra there is a war obliterating the laser array, a drastic cutback of the space exploration budget, or other interruption of the beam; the space probe will just go sailing into the big dark. Which will be a waste of space probe dollars but that's life in the big city.

Space Ark

SWIMMER would work well for a generation ship due to its extremely favorable performance at lower powers and velocities. For purposes of example the paper sets the payload mass at 8×109 kg, the mass of Freeman Dyson's Super Orion ship. The paper assumes that for such an ambitious mission the power conversion system would have an improved specific power of 10 kW/kg.

It will require a 10,000 gigawatt laser beam. Constantly operated. For 370 years. If anything interrupts the beam the space ark will also go sailing into the big dark. Only this time there will be generations of people living inside. For a while at least...

Mechanical Reliability

A related issue is mechanical reliability. Currently the best space probe NASA can build cannot be guaranteed to properly function past about forty years. The starship will need an extensive self-repair capability or have some way of having humans periodically available to fix things.

Jumping The Gun

A common science fiction gag is the "jumping the gun" plot. A slower than light ship departs on a 500 year journey to Alpha Centauri. About 100 years after launch, some joker on Terra invents a faster-than-light starship. Fleets of FTL ships fly to Alpha Centauri and colonize the place. The slower than light ship arrives to find not the virgin planets they were expecting, but instead 400 year old colonies. Har, har, silly slowboat.

The earliest example of this trope that I could find was A. E. van Vogt's "Far Centaurus" (1944)

In 2006, scientist Andrew Kennedy actually studied the problem. He published his analysis in a paper called Interstellar Travel: The Wait Calculation and the Incentive Trap of Progress. In it, he introduced his solution: the Wait Calculation.

The Wait Calculation allows future space explorers to avoid the "jumping the gun" problem (and also avoid being paralyzed with indecision by terror of jumping the gun). The equation shows that, assuming technology develops in such a way that there is exponential growth in the velocity of travel, there is an optimal departure time for arriving earliest. Kennedy states that the equation applies even if somebody invents faster-than-light travel.

You see, there comes a time when although technological advances continues to produce higher speeds, the waiting time for that advance is too long to make up the velocity difference. If you wait too long for a higher speed, a slower ship launched sooner will have enough of a head start to beat you.

For details about the equation, see the Wikipedia article. It is also discussed in a blog post (and in the comment section) at Centauri Dreams.


When we get into space, we can note Voyager 1’s 17 kilometers per second as it leaves the Solar System. The Helios solar probes launched in 1974 and 1976 set the current record at 70.22 km/s. And looking forward, the Solar Probe Plus mission is to perform a close flyby of the Sun, reaching a top heliocentric speed of 195 kilometers per second, which works out to 6.5 × 10 −4 c. If Breakthrough Starshot realizes its goal, an interstellar lightsail may one day head for Proxima Centauri at fully 20 percent of the speed of light.

Part of what occupies René Heller in his new paper is the exponential growth law we can construct between the 1804 Penydarren locomotive and the 17 kilometers per second of Voyager 1 in 2015. From wind- to steam-driven ships and into the realm of automobiles, then aircraft and, finally, rockets, we can extrapolate speeds that may take us into interstellar probe territory some time in this century or the next. Given that an interstellar mission may take longer than the average human lifetime, we thus need to ask a key question. When do we launch?

For the problem, a classic in science fiction, is to work out the most efficient timing. If we launch a starship at a particular level in our technology, will it not be caught by a faster ship launched at a much later date? Given sufficient technological improvements, a later launch (incorporating the necessary ‘wait time’) could result in an earlier arrival.

Those who have read A. E. van Vogt’s story “Far Centaurus” will recall precisely that scenario, when an Alpha Centauri mission reaches destination only to find it populated by humans who arrived by faster means. It’s a theme that shows up in Heinlein’s Time for the Stars and many other places.

Heller calls this problem ‘the incentive trap.’ And he refers back to Andrew Kennedy’s 2006 paper, which looked at the problem with the assumption of an exponential growth of the interstellar travel speed. Kennedy was assuming a 1.4 % average growth rate, under which a minimum time to reach Barnard’s Star could be calculated: some 712 years from 2006.

What that means is this: There is a total time that includes the waiting time (waiting for improved technology) and the actual travel time, and we can calculate a minimum value for this total time by using our assumption about the exponential growth of the interstellar travel speed. Calculating the minimum value shows us when we can launch without fear of being overtaken by a faster future probe, in hopes of avoiding that “Far Centaurus” outcome.

But was Kennedy right? Heller’s own take on the incentive trap takes into account the possibility that Breakthrough Starshot may achieve a velocity of 20 percent of lightspeed within several decades, an outcome that would, in Heller’s words, “…fundamentally change both the assumptions and the implications of the incentive trap because the speed doubling and the compounded annual speed growth laws would collapse as v approaches c.” And whatever happens with Breakthrough Starshot, the speed growth of human-made vehicles turns out to be much faster than previously believed.

Intriguing results flow out of Heller’s re-examination of what Kennedy had called the ‘wait equation,’ and tomorrow I want to go deeper into the paper to explain how the scientist uses exponential growth law models to show us a velocity which, once we have attained it, will no longer be subject to the incentive trap of faster, later technologies. The results are surprising, particularly if Breakthrough Starshot achieves its goal in the planned 30 years. The implications for our reaching well beyond Alpha Centauri, as we’ll see, are striking.

The Heller paper is “Relativistic Generalization of the Incentive Trap of Interstellar Travel with Application to Breakthrough Starshot” (preprint). The Kennedy paper is “Interstellar Travel: The Wait Calculation and the Incentive Trap of Progress,” Journal of the British Interplanetary Society Vol. 59, No. 7 (July, 2006), pp. 239-247.


When to launch a starship, given that improvements in technology could lead to a much faster ship passing yours enroute? As we saw yesterday, the problem has been attacked anew by René Heller (Max Planck Institute for Solar System Research), who re-examined a 2006 paper from Andrew Kennedy on the matter. Heller defines what he calls ‘the incentive trap’ this way:

The time to reach interstellar targets is potentially larger than a human lifetime, and so the question arises of whether it is currently reasonable to develop the required technology and to launch the probe. Alternatively, one could effectively save time and wait for technological improvements that enable gains in the interstellar travel speed, which could ultimately result in a later launch with an earlier arrival.

All this reminds me of a conversation I had with Greg Matloff, author of the indispensable The Starflight Handbook (Wiley, 1989) about this matter. We were at Marshall Space Flight Center in 2003 and I was compiling notes for my Centauri Dreams book. I had mentioned A. E. van Vogt’s story “Far Centaurus,” originally published in 1944, in which a crew arrives at Alpha Centauri only to find its system inhabited by humans who launched from Earth centuries later. I alluded to this story yesterday.

Calling it a ‘terrific story,’ Matloff discussed it in terms of Robert Forward’s thinking:

“Bob had a couple of concepts of technological advancement. He had a famous plot of the velocity of human beings versus time. And he said if this is true, and you launch a thousand-year ship today, in a century somebody could fly the same mission in a hundred years. Theyre going to be passed and will probably have to go through customs when they get to Alpha Centauri A-2.”

Customs! Clearly, we’d rather not be on the slow starship that is superseded by new technologies. What Heller and Kennedy before him want to do is to figure out a rational way to decide when to launch. If we make assumptions about the exponential growth in speed over time, we can address the question by adding the time we spend waiting for better technology to the time of the actual journey. We can then calculate a minimum value for this figure based on the growth rates we find in our historical data.

This is how Kennedy came up with a minimum figure of 712 years (from 2006) to reach Barnard’s Star, which is about 6 light years away. The figure would include a long period of waiting for technological improvement as well as the time of the journey itself. Kennedy used a 1.4 percent annual growth in speed in arriving at this figure but, examining 211 years of data on historical speed records, Heller finds a higher annual growth, some 4.72 percent.

From the Penydarren steam locomotive of 1804 to Voyager 1, we see a speed growth of about four orders of magnitude. Growth like this maintained for another 112 years leads to 1 percent of lightspeed.

But how consistent should we expect the growth in speed over time to be? Heller points out that the introduction of new technologies invariably leads to jumps in speed. We are now in the early stages of conceptualizing the Breakthrough Starshot project, which could create exactly this kind of disruption in the trend. Starshot aims at reaching 20 percent of lightspeed.

Working with the exponential speed doubling law we began with, we would expect that a speed of 20 percent of c would not be achieved until the year 2191. But if Starshot achieves its goal in the anticipated time frame of several decades, its success would see us reaching interstellar speeds much faster than the trends indicate. Starshot, or a project like it, would if successful exert a transformative effect as a driver for interstellar exploration.

We know that speed doubling laws cannot go on forever as we push toward relativistic speeds (we can’t double values higher than 0.5 c). But as we move toward substantial percentages of the speed of light, we see powerful gains in speed as we increase the kinetic energy beamed to a small lightsail like Starshot’s. Thus Heller also presents a model based on the growth of kinetic energy, noting that today the Three Gorges Dam in China can reach power outputs of 22.5 GW. 100 seconds exposure to a beam this powerful would take a small sail probe to speeds of 7.1 percent of c. Further kinetic energy increases could allow relativistic speeds for at least gram-to-kilogram sized probes within a matter of decades.

Usefully, Heller’s calculations also show when we can stop worrying about wait times altogether. The minimum value for the wait plus travel time disappears for targets that we can reach earlier than a critical travel time which he calls the ‘incentive travel time.’ Considered in both relativistic and non-relativistic models, this figure (assuming a doubling of speed every 15 years) works out to be 21.6 years. In Heller’s words, “…targets that we can reach within about 22 yr of travel are not worth waiting for further speed improvements if speed doubles every 15 yr.”

Thus already short travel times mean there is little point in waiting for future speed improvements. And in terms of current thinking about Alpha Centauri missions, Heller notes that there is a critical interstellar speed above which gains in kinetic energy beamed to the probe would not result in smaller wait plus travel times. His equations result in a value of 19.6 percent of c, an interesting number given that Breakthrough Starshot’s baseline is a probe moving at 20 percent of c, for a 20-year travel time. Thus:

In terms of the optimal interstellar velocity for launch, the most nearby interstellar target α Cen will be worthy of sending a space probe as soon as about 20 % c can be achieved because future technological developments will not reduce the travel time by as much as the waiting time increases. This value is in agreement with the 20 % c proposed by Starshot for a journey to α Cen.

We can push this result into an analysis of stars beyond Alpha Centauri. Heller looks at speeds beyond which further speed improvements would not result in reduced wait times for ten of the nearest bright stars. The assumption here would be that Starshot or alternative technologies would be continuously upgraded according to historical trends. Plugging in that assumption, we wind up with speeds as high as 57 percent of lightspeed for 70 Ophiuchi at 16.6 light years.

Thus the conclusion: If something like Breakthrough Starshot’s beaming capabilities become available within 45 years — and assuming that the kinetic energy transferred to the probes it pushes could be increased at the historical rates traced here — then we can reach all ten of the nearest star systems with an interstellar probe within 100 years from today.

Just for fun let me conclude with a snippet from “Far Centaurus.” Here a ship is approaching the ‘slowboat’ that has just discovered that Alpha Centauri has been reached by humans long before. The crew has just puzzled out what happened:

I was sitting in the control chair an hour later when I saw the glint in the darkness. There was a flash of bright silver, that exploded into size. The next instant, an enormous spaceship had matched our velocity less than a mile away.

Blake and I looked at each other. “Did they say,” I said shakily, “that that ship left its hangar ten minutes ago?”

Blake nodded. ‘They can make the trip from Earth to Centauri in three hours,” he said.

I hadn’t heard that before. Something happened inside my brain. “What!” I shouted. “Why, it’s taken us five hund… ” I stopped. I sat there.

“Three hours!” I whispered. “How could we have forgotten human progress?”

The René Heller paper discussed in the last two posts is “Relativistic Generalization of the Incentive Trap of Interstellar Travel with Application to Breakthrough Starshot” (preprint).

(ed note: Bill, Blake, and Jim depart in the world's first slower-than-light starship on a trip to Alpha Centauri. They use hibernation to sleep for the 500 year trip.)

Blake rose to his feet. "Bill, after I'd read your reports about, and seen the photographs of, that burning ship, I got an idea. The Alpha suns were pretty close two weeks ago, only about six months away at our average speed of five hundred miles a second. I thought to myself: 'I'll see if I can tune in some of their radio stations.'"

"Well," he smiled wryly, "I got hundreds in a few minutes. They came in all over the seven wave dials, with bell-like clarity." He paused; he stared down at me, and his smile was a sickly thing. "Bill," he groaned, "we're the prize fools in creation. When I told Renfrew the truth, he folded up like ice melting into water."

Once more, he paused; the silence was too much for my straining nerves.

"For Heaven's sake, man" I began. And stopped. And lay there, very still. Just like that the lightning of understanding flashed on me. My blood seemed to thunder through my veins. At last, weakly, I said: "You mean ."

Blake nodded. "Yeah," he said. "That's the way it is. And they've already spotted us with their spy rays and energy screens. A ship's coming out to meet us.

"I only hope," he finished gloomily, "they can do something for Jim."

I was sitting in the control chair an hour later when I saw the glint in the darkness. There was a flash of bright silver, that exploded into size. The next instant, an enormous spaceship had matched our velocity less than a mile away.

Blake and I looked at each other. "Did they say," I said shakily, "that that ship left its hangar ten minutes ago?"

Blake nodded. 'They can make the trip from Earth to Centauri in three hours," he said.

I hadn't heard that before. Something happened inside my brain. "What!" I shouted. "Why, it's taken us five hund... " I stopped. I sat there. "Three hours!" I whispered. "How could we have forgotten human progress?"

From FAR CENTAURUS by A. E. van Vogt (1944)

"Exactly, Mr. Grimm. But I spent most of those years asleep, in suspended animation...aboard the first American rocket bound for the stars. I left the earth in 1988, travelling at a velocity of a million miles per hour. Even at that speed, the journey to Earth's nearest stellar neighbor required a millenium."

"But a mere 200 years after I left, a man named Harkov came along with a new theory of physics that made faster-than-light space travel a reality. When I came out of my ship on Centauri-IV, a colony of Earthmen was there to greet me!"

"I felt as you must have, Cap, when you were released from that iceburg back in 1964, after slumbering for two decades. Only I wasn't as lucky. As with you, my world, all the people I'd known and loved, were dead and gone. But you hadn't aged during your sleep — I had!"

"I am a prisoner of the copper foil suit that sustains my life. Once punctured, any exposure of my sky to fresh air ... and I crumble into dust. Martinex, our group's scientist, designed this additional outer shel I wear to prevent just that."

Ten years we had been on our way when they found a hyper-drive
And man spread to a thousand stars while we were half alive
"Space is Dark", Bill Roper

The brave explorers or colonists set out in their spaceship to spread humankind to the stars. You can't travel faster than light, so they're going to spend most of the trip on a Sleeper Starship as Human Popsicles, or it's a Generation Ship and it'll be their descendants who step out at the other end of the trip. Either way, they're saying goodbye forever to everyone and everything they know. Decades and centuries pass, and eventually they arrive at their destination—

—and there's people there waiting for them. Turns out, Faster-Than-Light Travel is possible, and it got sorted out while they were in transit. Now the same trip that took them centuries can be done and be back in time for Christmas. And that planet you were all set to colonise? Done already, and actually we're not sure there's any room for you...

Expect the brave pioneers to be upset about this.

An in-universe Sub-Trope of Science Marches On. Can also be related to Humans Advance Swiftly.

(ed note: see TV Trope page for list of examples)

Sublight Starships


The Prometheus is a laser sail starship from The Flight of the Dragonfly aka Rocheworld, written by Dr. Robert L. Forward who wrote the classic real-world scientific reseach paper on the subject.


Interstellar Laser Propulsion System 

The payload sent to the Barnard system consisted of the crew of twenty persons and their consumables, totalling about 300 metric tons; four landing rockets for the various planets and moons at 500 tons each; four nuclear powered VTOL exploration airplanes at 80 tons each; and the interstellar habitat for the crew that made up the remainder of the 3500 tons that needed to be transported to the star system.

This payload was carried by a large light sail 300 kilometers in diameter. The sail was of very light construction, a thin film of finely perforated metal, stretched over a lightweight frame. Although the sail averaged only one-tenth of a gram per square meter of area, the total mass of the sail was over 7000 tons. The payload sail was not only used to decelerate the payload at the Barnard system, but also for propulsion within the Barnard system.

The 300 kilometer payload sail was surrounded by a larger ring sail, 1000 kilometers in diameter, with a hole in the center where the payload sail was attached during launch from the solar system. The ring sail had a total mass of 71,500 tons, giving a total launch weight of the sails and the payload of over 82,000 tons.

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. 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. The configuration of the lasers, lens, and sail during the launch and deceleration phases can be seen in Figure 6.

The accelerating lasers were left on for eighteen years while the spacecraft continued to gain speed. The lasers were turned off, back in the solar system, in 2044. The last of the light from the lasers traveled for two more years before it finally reached the interstellar spacecraft. Thrust at the spacecraft stopped in 2046, just short of twenty years after launch. The spacecraft was now at two lightyears distance from the Sun and four lightyears from Barnard, and was traveling at twenty percent of the speed of light. The mission now entered the coast phase.

For the next 20 years the spacecraft and its drugged crew coasted through interstellar space, covering a lightyear every five years. Back in the solar system, the laser array was used to launch another manned interstellar expedition. During this period, the Barnard lens was increased in diameter to 300 kilometers. Then, in 2060, the laser array was turned on again at a power level of 1500 terawatts and a tripled frequency. The combined beams from the lasers filled the 300 kilometer diameter Fresnel lens and beamed out toward the distant star. After two years, the lasers were turned off, and used elsewhere. The two-light-year long pulse of high energy laser light traveled across the six lightyears to the Barnard system, where it caught up with the spacecraft as it was 0.2 lightyears away from its destination.

Before the pulse of laser light had reached the interstellar vehicle, the vehicle had separated into two pieces. The inner 300 kilometer payload sail detached itself and turned around to face the ring-shaped sail. The ring sail had computer-controlled actuators to give it the proper optical curvature. When the laser beam from the distant solar system arrived at the spacecraft, the beam struck the large 1000 kilometer ring sail, bounced off the mirrored surface, and was focused onto the smaller 300 kilometer payload sail as shown in the lower portion of Figure 6. The laser light accelerated the massive 71,500 ton ring sail at 1.2 percent of Earth gravity and during the two year period the ring sail increased its velocity slightly. The same laser power reflecting back on the much lighter payload sail, however, decelerated the smaller sail and the exploration crew at nearly ten percent of Earth gravity. In the two years that the laser beam was on, the payload sail slowed from its interstellar velocity of twenty percent of the speed of light to come to rest in the Barnard system. Meanwhile, the ring sail sped on into deep space, its job done.



The interstellar spacecraft that took the exploration crew to the Barnard system was called Prometheus, the bringer of light. Its configuration is shown in Figure 7. Although quite large, from a distance it would be difficult to see Prometheus in the vast expanse of shining sail that carried it to the stars.

A major fraction of the spacecraft volume was taken up by four units. They consisted of a planetary lander called the Surface Landing and Ascent Module (SLAM), holding within itself a winged Surface Excursion Module (SEM). Each SLAM rocket is forty-six meters long and six meters in diameter, and masses 600 tons including the SEM.

Running all the way through the center of Prometheus is a four-meter-diameter, sixty-meter-long shaft with an elevator platform. Capping the top of Prometheus on the side toward the direction of travel is a huge double-decked compartmented area that holds the various consumables that will be used in the 50-year mission as well as the workshop for the spaceship's computer motile. At the very center of starside is a small port with a thick glass dome that is used by the star-science instruments to investigate the star system they are moving toward. There is enough room for one or two people under the dome, but the radiation level is high enough that the port is mostly used by machines, not people.

At the base of Prometheus were five decks. These were the home for the crew. Each deck is a flat cylinder twenty meters in diameter and three meters thick. The bottom control deck contains the consoles that run the lightcraft, with the earthside science dome at the center. The living area deck is next. This contains the communal dining room, lounge, and recreational facilities. The next two decks are the crew quarters decks that are fitted out with individual living quarters for each of the twenty crew members. Above that is the hydroponics deck with four air locks that allow access to the four SLAM spacecraft. The water in the hydroponics tanks added to the radiation shielding for the crew quarters below.

From THE FLIGHT OF THE DRAGONFLY aka "Rocheworld" by Dr. Robert E. Forward

Bond and Martin World Ships

DesignPopulation SizeDry Mass
Propellant Mass
Cruise Velocity
Dry world ship - Mark 2A [15]250,0002.0×10118.2×10110.5%
Dry world ship - Mark 2B [15]250,0005.7×10112.3×10120.5%
Wet world ship [15]250,0002.2×10129.0×10120.5%

The Bond and Martin [15] world ships are the largest proposed world ships in terms size and mass. Figs. 3 and 4 show a artist conceptions of the Bond and Martin world ships [15]. In particular Fig. 4 provides a size comparison of the ”Dry World” (habitable area is mostly land) and ”Wet World” (habitable area is mostly water) world ships.

It can be seen that the habitat (large cylindrical section) and the propulsion module (second cylinder in the back with nozzle) are dominating the designs. In addition, a flat, circular dust shield is attached to the front of the world ship.

[15] A Bond and AR Martin. World Ships-An Assessment of the Engineering Feasibility. Journal of the British Interplanetary Society, 37:254, 1984.

Colonized Interstellar Vessel

This is from Colonized Interstellar Vessel: Conceptual Master Planning by Steve Summerford (2012)

The focus of the report is on the habitable space of the starship. It is assumed that any suitable interstellar propulsion system can be attached, so that is outside the scope of the report. Some sort of fusion power generation is assumed, since the inverse-square law will solar photovoltaic panels worthless long before the edge of the solar system is reached.

Mr. Summerford notes that all those images of generational starships built inside hollow asteroids are very pretty, but it will be centuries before our technology is up to actually building one. Let alone paying for it. Yes, it would be real nice to have a starship environment that looks like Terra. But in the spirit of learning how to walk before you run, perhaps it would be best to design a more modest starship. True, the enviroment will probably look more urban and less like rural. However it will be pleasant to have a design we can actually build. And pay for.

As a bonus, a generational starship has far fewer ethical and technological issues than, say: seed ships, sleeper ships, and digital crews.

The drawback is that while the other methods only have to worry about breakdowns of the ship's technology, generational ships have to also worry about social breakdowns of generations of inhabitants. So attention must be paid to preservation of the physical, spatial, and psychological needs of the colony. It wouldn't do for the starship to fail because the fifth generation snapped psychologically due to living for 125 years inside miserable little steel boxes. These design problems are just as important as matters of radiation shielding, fuel suppply, and food production.

Therefore the report does devote some space to issues such as architecture, healthy community living, privacy needs, and other "soft" problems. Many of the principles come from basic urban planning.

For a starting point Mr. Summerford uses the Stanford torus space colony design. Summerford suggest that the reader peruse NASA SP-413 for details about the Standford torus (86.3 MB PDF, Online).

To expand a Standford Torus, the logical method would be to stack several toruses. But this would be an engineering nightmare, and a wasteful increase of mass. Instead, a banded cylinder design is used. Flat-bottomed faceted cylinders called "bays" are used. This allows sufficient colonial living space while maintaining a compact vehicle design.

Each bay contains dwellings, agicultural areas, civic structures, open spaces, an places of work and research.

To avoid spin nausea, the spin rate should be no more than about 2 rpm. The design goes with 1.5 rpm, spin radius of 400, meters, and an artificial gravity of 1.0 g. A rate of 2.0 rpm was looked at to reduce the spin radius, but the increase in the coriolis effect was considered unacceptable.

For urban planning, they use a metric called "floor-to-area ratio" or FAR. In a site, you take all the buildings therein, and add up the surface area of all the floors in all the buildings. This is the "floor". Divide by the surface area of the site and you have the FAR. Example: if a site with a surface area of 10,000 m2 had a single bulding, and it was a four story building with each floor containing 2,500 m2, then the FAR would be 1.0. The FAR of the generation ship was set to from 1.25 to 1.75 in an attempt to enforce some park like open areas with no buildings.

Enzmann Starship

The Enzmann starship is a concept for a manned interstellar spacecraft proposed in 1964 (date is disputed) by Dr. Robert Enzmann. Over the years the basic design has evolved, and there were several types in the initial design. It was quite popular in the science fiction community. An analysis of the Enzmann starship can be found here.

In 1972 space artists Don Davis and Rick Sternbach worked with Dr. Enzmann to develop the idea. This refined the "lollypop" look of the ship. For some odd reason most paintings of the Enzmann starship show two of them in formation. The original design had a naked sphere of frozen deuterium as fuel. Calculations with Sternbach and Davis revealed that the deuterium could not be kept frozen and was too structurally weak to be accelerated. So the redesign encased the deuterium in a huge tank.

The Enzmann exploded into the science fiction community with the October 1973 issue of Analog magazine. G. Harry Stine wrote an extensive article about the concept, accompanied by a stunning piece of cover art by Rick Sternbach. Stine said the ships were 12 million tonnes, could reach 0.30 c (highly unlikely), had 8 engines, and used spinning habitats for artificial gravity.

  • Command center 30 meters in diameter
  • Central core load bearing struture 15 meters diameter
  • Frozen deuterium 300 meters diameter
  • Living modules 90 meters diameter × 90 meters long
  • Engineering compartments 70 meters diameter

In Science Digest, Rick Sternbach's 1972 piece depicts a pair of Enzmanns departing from an asteroid factory. The number of engines was increased from 8 in the original design to 24. Modular sections were created that can separate from the starship. Height of starship was 690 meters. 3 million tonnes of deuterium, with metal shell (doubling as a radiation shield). Magnetic confined fusion propulsion. 20 decks per habitat, 100 rooms per deck. Cruising speed 0.09c.

Thomas Schroeder wrote an article entitled "Slow Boat to Centauri" in Astronomy Magazine. Claimed a cruising speed of 0.1c, and an advanced design might reach 0.3c. 12 million tonnes deuterium. The outer layers of the habitats were composed of bulk material as radiation shielding for the inner layers. Bulk means nuclear reactor, store rooms, heat exchangers, airlocks, landing craft, observation areas, communication equipment. Eight Project Orion nuclear pulse units.

  • Frozen deuterium 305 meters diameter
  • Height 609.6 meters
  • Living modules 91.5 meters diameter × 91.5 meters long
  • From bottom of fuel sphere to top of Orion engines 305 meters

In the 1980's Dr. Enzmann started designing variants.

In 2011, K. F. Long, A. Crowl, and R. Obousy did a study on the Enzmann starship, and tried to rationalize it with recent developments in astronautics. First they took the historical concepts:

Historical Concepts
Length610 msamesame
Sphere Diameter305 msamesame
Total Habitat Length273 msamesame
Individual Habitat length91 msamesame
Habitat Diameter91 msamesame
Core Diameter15 msamesame
Num Habitats113
Num Engines8824
Exhaust VelocityUnspecifiedUnspecifiedUnspecified
Specific ImpulseUnspecifiedUnspecifiedUnspecified
Structural MassUnspecifiedUnspecifiedUnspecified
Propellant Mass3×106 metric tons12×106 metric tons3×106 metric tons
Cruise Speed27,000 km/s
90,000 km/s
27,000 km/s
Starting Colony200samesame
Final Colony2000samesame

Long et al created three variants. The primary difference is the size of the population carried. The rest of the design was re-sized to handled the population. As near as I can calculate, the cruise and mission times for all three are for a mission to Alpha Centauri.

Specific Power11.5 MW/kg
Thrust Power344 terawatts
Length620 m979 m1752 m
Dry Mass30,000 MT300,000 MT3,000,000 MT
Propellant Mass3 × 106 MT3 × 106 MT3 × 106 MT
Mass Ratio
Mass Ratio10.053.321.41
Exhaust Velocity11,700 km/s11,260 km/s12,119 km/s
ΔV54,000 km/s
27,000 km/s
8,400 km/s
Cruise Velocity27,000 km/s
13,500 km/s
4,200 km/s
Total Acceleration
18.95 yrs98.67 yrs84.9 yrs
Total Cruise
41.05 yrs51.33 yrs265.1 yrs
Total Mission
60 yrs150 yrs350 yrs
Mass Flow Rate5.02 kg/s0.96 kg/s1.12 kg/s
0.019 m/s2
0.003 m/s2
0.004 m/s2
Thrust58,730 kN10,810 kN13,573 kN

Project Daedalus

Project Daedalus
TargetBarnard's Star
(5.9 ly)
Mission Duration48 years
Cruise Phase44 years
Boost Phase2.05y (stage 1)
+ 1.78y (stage 2)
(35,975,000 m/s)
Height190 m
Width50 m
Wet Mass5.4×107 kg
Propellant Mass5.0×107 kg
Dry Mass4×106 kg
Payload Mass450,000 kg
Min Kinetic Energy to cruise speed3.5×1022 J
Min Power to cruise speed291TW
Pellet pulse freq250Hz
Stage 1
Stage Dry Mass1.69×106 kg
Num Propellant Tanksx6
Propellant Tank Mass7.666×106 kg
Total Propellant Tank4.5996×107 kg
Stage Wet Mass4.7686×107 kg
Thrust7.54×106 N
Accel (1st stage ignition)0.14 m/s2
0.014 g
Burn Time2.05y
mdot0.71 kg/s
ICF capsule Q63
Exhaust Velocity1.06×107 m/s
Specific Impulse1.08×106 sec
Stage 2
Stage Dry Mass9.8×105 kg
Num Propellant Tanksx4
Propellant Tank Mass1.0×106 kg
Total Propellant Tank4.0×106 kg
Stage Wet Mass4.98×106 kg
Thrust6.63×105 N
Accel (2nd stage ignition)0.13 m/s2
0.013 g
Accel (2nd stage burnout)0.676 m/s2
0.0689 g
Burn Time1.78y
mdot0.0072 kg/s
ICF capsule Q35
Exhaust Velocity9.21×106 m/s
Specific Impulse9.39×105 sec

This was a study conducted between 1973 and 1978 by the British Interplanetary Society to design a plausible uncrewed interstellar spacecraft. Yes, it is naught but a mere space probe, but it is a starship.

Given the mission parameters and state-of-the-art of various propulsion designs, the study chose an inertial-confinement fusion engine. In an effort to cut down on radiation shield mass, they selected the close-to-aneutronic deuterium—helium-3 fuel. A neutron radiation shield protecting a starship 190 freaking meters long will savagely cut into the payload mass. Since there isn't enough helium-3 in the entire world, the study proposed a titanic mining operation to gather the stuff from Jupiter's atmosphere. The study details about 3He skimming is why the Traveller role-playing-game allows "Wilderness Refueling". But I digress.

Even using a two stage fusion propulsion system, Daedalus could only get up to 12% the speed of light, and could not slow down. The good news is it only takes 48 years to reach Barnard's Star (so some of the scientists who sent it might actually live long enough to see the data). The bad news is since it cannot slow down, it will streak through the Barnard's system at 12% c and have only a couple of days to frantically take pictures and do all the science it possibly can. Basically if Daedalus was designed so it could come to a halt at the Barnard system, the trip time would double to about 96 years. This would guarantee that all the scientists who sent the blasted ship would be long dead of old age, which makes it hard to work up any enthusiasm to work on the project. Or to convince the public to fund it.


Project Daedalus was a study conducted between 1973 and 1978 by the British Interplanetary Society to design a plausible unmanned interstellar spacecraft. Intended mainly as a scientific probe, the design criteria specified that the spacecraft had to use existing or near-future technology and had to be able to reach its destination within a human lifetime. Alan Bond led a team of scientists and engineers who proposed using a fusion rocket to reach Barnard's Star 5.9 light years away. The trip was estimated to take 50 years, but the design was required to be flexible enough that it could be sent to any other target star.


Daedalus would be constructed in Earth orbit and have an initial mass of 54,000 tonnes including 50,000 tonnes of fuel and 500 tonnes of scientific payload. Daedalus was to be a two-stage spacecraft. The first stage would operate for two years, taking the spacecraft to 7.1% of light speed (0.071 c), and then after it was jettisoned, the second stage would fire for 1.8 years, taking the spacecraft up to about 12% of light speed (0.12 c), before being shut down for a 46-year cruise period. Due to the extreme temperature range of operation required, from near absolute zero to 1600 K, the engine bells and support structure would be made of molybdenum alloyed with titanium, zirconium, and carbon, which retains strength even at cryogenic temperatures. A major stimulus for the project was Friedwardt Winterberg's inertial confinement fusion drive concept, for which he received the Hermann Oberth gold medal award.

This velocity is well beyond the capabilities of chemical rockets or even the type of nuclear pulse propulsion studied during Project Orion. According to Dr. Tony Martin, controlled-fusion engine and the nuclear–electric systems have very low thrust, equipment to convert nuclear energy into electrical has a large mass, which results in small acceleration, which would take a century to achieve the desired speed; thermodynamic nuclear engines of the NERVA type require a great quantity of fuel, photon rockets have to generate power at a rate of 3×109 W per kg of vehicle mass and require mirrors with absorptivity of less than 1 part in 106, interstellar ramjet's problems are tenuous interstellar medium with a density of about 1 atom/cm3, a large diameter funnel, and high power required for its electric field. Thus the only suitable propulsion method for the project was the nuclear pulse rocket.

Daedalus would be propelled by a fusion rocket using pellets of a deuterium/helium-3 mix that would be ignited in the reaction chamber by inertial confinement using electron beams. The electron beam system would be powered by a set of induction coils trapping energy from the plasma exhaust stream. 250 pellets would be detonated per second, and the resulting plasma would be directed by a magnetic nozzle. The computed burn-up fraction for the fusion fuels was 0.175 and 0.133 producing exhaust velocities of 10,600 km/s and 9,210 km/s respectively. Due to scarcity of helium-3 on Earth, it was to be mined from the atmosphere of Jupiter by large hot-air balloon supported robotic factories over a 20-year period, or from a less distant source, such as the Moon.

The second stage would have two 5-metre optical telescopes and two 20-metre radio telescopes. About 25 years after launch these telescopes would begin examining the area around Barnard's Star to learn more about any accompanying planets. This information would be sent back to Earth, using the 40-metre diameter second stage engine bell as a communications dish, and targets of interest would be selected. Since the spacecraft would not decelerate, upon reaching Barnard's Star, Daedalus would carry 18 autonomous sub-probes that would be launched between 7.2 and 1.8 years before the main craft entered the target system. These sub-probes would be propelled by nuclear-powered ion drives and would carry cameras, spectrometers, and other sensory equipment. The sub-probes would fly past their targets, still traveling at 12% of the speed of light, and transmit their findings back to the Daedalus' second stage, mothership, for relay back to Earth.

The ship's payload bay containing its sub-probes, telescopes, and other equipment would be protected from the interstellar medium during transit by a beryllium disk, up to 7 mm thick, weighing up to 50 tonnes. This erosion shield would be made from beryllium due to its lightness and high latent heat of vaporisation. Larger obstacles that might be encountered while passing through the target system would be dispersed by an artificially generated cloud of particles, ejected by support vehicles called dust bugs about 200 km ahead of the vehicle. The spacecraft would carry a number of robot wardens capable of autonomously repairing damage or malfunctions.


A quantitative engineering analysis of a self-replicating variation on Project Daedalus was published in 1980 by Robert Freitas.[8] The non-replicating design was modified to include all subsystems necessary for self-replication. Use the probe to deliver a seed factory, with a mass of about 443 metric tons, to a distant site. Have the seed factory replicate many copies of itself on-site, to increase its total manufacturing capacity, then use the resulting automated industrial complex to construct probes, with a seed factory on board, over a 1,000-year period. Each REPRO would weigh over 10 million tons due to the extra fuel needed to decelerate from 12% of lightspeed.

Another possibility is to equip the Daedalus with a Magnetic sail similar to the magnetic scoop on a Bussard ramjet to use the destination star heliosphere as a brake, making carrying deceleration fuel unnecessary, allowing a much more in-depth study of the star system chosen.

From the Wikipedia entry for PROJECT DAEDALUS

      THE REFERENCE MISSION for Project Daedalus is an un­manned, undecelerated fly-by of Barnard's Star at a distance of six light years. A time limit of about 40 years was initially set. this being the active career of a person connected with the Project at the launch of the vehicle, this giving contin­uity during the mission phase of the project. This implies a mission velocity during coast of 15% the speed of light (0.15с).
     The attainment of such high velocities by targe payloads is beyond the capability of present day rocket technology, and for this reason the propulsion system involves most of the extrapolations of technology involved in the Project. The various advanced propulsion systems which have been suggested for interstellar flight are briefly reviewed in the next section.


     A survey of most of the propulsion systems which may be capable of interstellar flight application has been given by Bond [9]. Here we briefly note some of the reasons for the choice of system for Project Daedalus.
     The nuclear-electric system where electrical energy is produced by a nuclear-powered generator and used to accelerate a working fluid, has a very high mass associated with the hardware required to implement the system, and hence the achievable acceleration is very low. This results in a long acceleration time before the desired final velocity is reached, and long journey times.
     The same problem arises with systems propelled by beamed energy from arrays of space-based lasers, and solar sail systems where particles in the solar wind impart momentum to the vehicle. Both these systems have low masses, compared with the nuclear electric system, but the rate of energy transfer to the vehicle is so low that, again, long acceleration times are required.
     High thrust nuclear rockets, such as the thermodynamic nuclear engines of the NERVA type, could achieve this desired acceleration. However, the exhaust velocities of such systems are typically 104 m/s maximum, and this would mean that very large mass ratios would be required to reach the final velocity. The amount of propellent required in such a system would render it impractical.

     All the above systems are within the capabilities of present day technology, but are not able to perform the reference mission. The remaining systems are beyond modern capabilities and Involve extrapolation to a greater or lesser degree.

     The photon rocket appears to be the ultimate rocket propulsion system possible, as there is no hope of a vehicle containing its own energy source and propellant being any better than this system (within our present-day knowledge of physics). Here, a direct beam of electromagnetic radiation is used to produce thrust, and an exhaust velocity equal to the speed of light results. However, the system is completely beyond modern capabilities. The rocket would require powers of 300 MW per Newton of thrust, and for acceleration at one Earth gravity it would have to produce powers of the order of 3 × 109 W per kilogramme of vehicle mass. Having generated this power it would be necessary to reflect it away from the vehicle with efficiencies such that only 1 part in 106 was absorbed, otherwise the vehicle would be destroyed. The only method of achieving this so far suggested is by using an electron gas as a mirror, and even this raises many doubts.
     In order to achieve suitable mass ratios in a photon jacket it is necessary to convert mass very efficiently into energy. One way of doing this, which is also relevant to a propulsion system in its own right, is matter-antimatter annihilation. At present, only minute quantities of antimatter have been produced in laboratory conditions, and this material has had a short lifetime due to the problems of isolating it from ordinary matter and avoiding annihilation. Here again, large extrapolations of modem day capabilities are required.
     The interstellar ramjet, in contrast with most other systems, does not carry its propellant along with the vehicle, but scoops up the gas in interstellar space. This gas is then fed into fusion reactors to produce power from the energy released when the nuclei of hydrogen are fused. Thus, the vehicle is envisaged as a relatively small craft carrying only a small amount of propellant and power-producing materials. This vehicle is the centre of a magnetic scoop spreading out thousands of kilometres from the vehicle. The gas in space is ionised (for example, by shining lasers at it), and the ions are then swept up by the magnetic field. The potential of such a system is enormous, and it is the only method proposed which really carries hope of travel among the stars.
     However, at present there are many limitations which must be taken into account when considering application to interstellar missions in the near future. These limitations have been discussed previously, and will only be briefly mentioned here.
     The relative kinetic energy of the interstellar gas must not be dissipated by mass or energy losses from the intake, as any such loss will set a limit to the acceleration which can be achieved. Thus, the scoop must be very efficient at energy containment.
     If a magnetic field is used to create the scoop, then the section of the vehicle which contains the sources of the magnetic field must be strong enough to withstand the forces exerted upon these sources by the fields which they create. That is, the magnetic field energy must be balanced by mechanical forces. The magnetic flux density of the scoop must increase as the velocity increases, in order to contain the Ions, and the strength of known materials will set a limit to the acceleration period, and the attainable velocities, before the structure is destroyed by the magnetic field forces. Evan if a field of sufficiently high flux density can be achieved, at low velocities the effective scoop radius must be very large in order to collect sufficient ions to fuel the reactors from the very diffuse gas of interstellar space.
     In a variant of the pure ramjet, the Ram Augmented Interstellar Rocket (RAIR), the vehicle carries its own nuclear fuel supply and exhausts the reaction products to produce thrust. However, it enhances it performance by scooping up the interstellar medium and using its momentum and kinetic energy to augment that of the rocket. It still suffers from the intake problems associated with the pure ramjet, but avoids the reactor problems inherent in the proton cycle.
     The augmentation of relatively modest magnetic fields by electrostatic fields has been considered in some detail, but formidable problems still remain and it is not dear that such augmentation would allow a reasonably sized vehicle to perform the reference mission in any case. Thus, to build an interstellar ramjet, even to operate at 0.15c , would require a level of technology capable of building and working with very high strength materials, producing and controlling to a very fine degree intense magnetic and electric fields which project over very large distances, controlling the stability of plasmas in such fields and keeping the energy losses in these plasmas at a low level, and constructing efficient thermonuclear fusion reactors whfch run on the proton (and possibly the deuterium) cycle. These achievements are not within a reasonable extrapotation of modem technology.

     Turning, therefore, to nearer the present day in terms of our technical capabilities, there are two systems left to discuss. Both systems derive their power from the energy released by thermonuclear fusion, and exhaust velocities of about 107 m/sec (specific impulses of 106 sec) can probably be achieved with such a process.

     The first system is the continuous fusion rocket (magnetically confined fusion), where hydrogen isotopes are fused in an open (mirror) or closed (toroidal) system, the confinement of the high temperature plasma being achieved, for sufficient time for the reactions to take place, by the use of magnetic fields. The hardware mass associated with the engine would be targe, in common with the case of the nuclear-electric system, and trip times would be long unless a large mass ratio could be tolerated. This does not pose too severe a penalty for the reference mission case. Spencer and Jaffe have shown that velcociucs ~0.15 c can be reached with a mass ratio of 10, this being relatively insensitive to the number of stages used or the stage structural masses achieved. Only when attempting to reach velocities of the order of 0.6 — 0.8c do mass ratios of 103 — 104 become necessary.
     Sustained thermonuclear fusion has not yet been demonstrated on Earth, although many laboratories are persuing this goal as a means of producing power for Earth-based consumption. It is generally hoped that magnetically-confined fusion reactors burning a mixture of deuterium and tritium will be operational, in a prototype form at least, before the end of the century. The deuterium-deuterium reaction, upon which Spencer and Jaffe's results were based, is more difficult to ignite, and would not be achieved for some years after that.
     Thus, it would appear that a continuous fusion rocket could perform the mission, with a reasonable mass ratio and a relatively modest extrapolation of modem technology, it has not been chosen for the Daedalus vehicle because it is thought that the last system to be discussed, the pulsed fusion rocket, can perform the mission more easily with less technological extrapolation. This system is briefly described in the next section, followed by the detailed theoretical considerations and calculations which are believed to justify this choice.


     The nuclear pulse rocket, which uses nuclear explosions for propulsion, was first proposed by Everett and Ulam in 1955. The concept was investigated in detail between 1958—1965 in Project Orion. In the final concept, fission explosions were to be detonated behind a spacecraft to accelerate propellant material that would impart momentum to a massive pusher plate. This momentum in turn was to be transmitted more gradually from the pusher plate to spacecraft through a pneumatic spring system. The study was terminated because of the size of the vehicle, which was inherent in the use of large explosions, and the limitations imposed by the nuclear tett ban treaty and the difficulties of testing the system.
     The possibility of producing small fusion explosions (inertially confined fusion), by laser or electron beam ignition, removes two of the major problems with the Orion concept — large size and radioactive pollution. Consequently, interest in nuclear pulse rocket designs for Solar System applications was shown hy the two main groups in the USA involved in laser fusion work.
     The Los Alamos work still retained the concept of a pusher plate, accepting the limitations that this imposes on specific impulse, due mainly to limits set on the impingment velocity of the propellant cloud by pusher plate ablation. In contrast, the Livermore work used the concept of magnetically redirecting the charged particles produced in the explosion out of the rear of the thrust chamber. Thus, the flux of charged particles never comes into contact with the structure of the vehicle , thereby easing specific impulse limitations considerably. Hence, while the Los Alamos studies were of a system with a specific impulse of ~104 sec, the Livermore work noted that values ~106 sec. appeared attainable.
     At the same time as this work, Winterberg published his concept of a system which used relativistic electron beams to initiate the fusion explosions and a concave mirror reflector open on one side to redirect the particle fluxes. Specific impulses of 106 sec were again considered. As already noted in the introduction, this paper was of major importance in evolving the concept of the Daedalus propulsion system.

     A schematic diagram of the system is shown in Fig. 1 , The propellent is carried is small preformed spheres, at cryogenic temperature, in several disposable tanks, The size of the individual spheres is basically limited by the weight of the ignition system needed, A larger size of sphere requires a heavier system and the present design represents a partial optimisation of explosion repetition rate against system weight.
     The fuel pellets are injected into the reaction chamber at high velocity in order to reach the target point at the correct time. It is proposed to achieve this by providing each pellet with a thin superconducting shell and accelerating them into the chamber with an electromagnetic gun. This would simply involve an array of coils and capacitors producing a travelling magnetic wave on which the pellet would be accelerated.
     As the pellets reach the target point they are hit simultaneously by high power electron beams. The outer layers of the pellets are ablated away due to the high heating rates and energy deposition In these outer regions. This ablation results in very high surface pressures being generated and the fuel is compressed and shock heated, with the central core attaining temperatures at which thermonuclear fusion reactions can occur.
     The resulting plasma ball is highly conductive and sweeps aside the magnetic field in the reaction chamber. This field has a cusp-shaped geometry within the reaction chamber, so that there is sufficient flux within the chamber wall to protect it from the plasma. The field diverges slowly downstream of the chamber to direct the exhaust products axially, and field lines must also go from the electron beam generators to the target point, so that the electrons do not have to cross any field lines.
     Due to the rapidity of the deformation of the magnetic field it would effectively be confined and compressed between the conducting shell of the chamber wall and the plasma ball. The kinetic energy of the plasma would be temporarily stored in the magnetic field, which would then reverse the motion of the plasma and eject it at high velocity along the axis of the engine. The momentum of the exhaust would be transferred to the reaction chamber during this process and subsequently, by means of a thrust structure, to the vehicle.
     The energy required to ignite the following pellets would be extracted from the reacting pellet via on induction loop situated it the reaction chamber exit. This loop would charge a large number of transmission lines to a high potential. These transmission lines are imaged in a circle around the chamber axis and focussed on the target point. This configuration should give good illumination of the target, and hence an efficient implosion.


     There an many energy producing fusion reactions which are theoretically possible and which have been studied increasingly over the past few years in order to determine whether their use in ground-based fusion reactor power stations is possible. Such "exotic" fuels include p-6Li, D-6Li, p-9Be, D-9Be, p-10B, p-11B and p-15N .However, not all of these reactions can be ignited to give a net energy production (power produced greater than power required to sustain the reaction), and others require driving to temperatures much higher than those which can easily be attained.
     These problems are due in part to the use of elements with high atomic numbers (relative to hydrogen and helium), leading to enhanced bremsstrahlung losses in the plasmas which are created. Therefore, it is still the case that only three reactions appear to have practical application, even in the long term. These are the reactions occurring between the two hydrogen isotopes deuterium (2D) and tritium (3T) and the light isotope of helium (3He).
     The reactions of interest can be written (where the figures in brackets following the reaction products are the particle energies in MeV, and α is the mass friction converted to energy in the reaction):
2D + 3T4He(3.5) + 1n(14.1) α=0.0038
2D + 2D
3T(1.01) + 1p(3.02) α= 0.0011
2D + 2D
3He(0.82) + 1n(2.45) α=0.0009
2D + 3He
4He(3.6) + 1p(14.7) α=0.0039
     It can be seen that the D-T reaction and the neutron branch of the D-D reaction release about 80% and 35% of the reaction energy in the form of neutrons. This factor has important implications for ground-based fusion reactor technology, in that first generation reactors are expected to use D-T, which is the easiest reaction to ignite, and the neutrons can be used to breed the tritium in lithium blankets surrounding the reactor core.
     The use of D-T and D-D reactions in space propulsion applications, however, leads to weight penalties in the form of shielding mass, to stop the neutrons damaging structural components of the vehicle and causing radiation-induced faults in sensitive areas, and waste heat radiation equipment to dissipate the heat produced in the shields during the stopping process. As an example, the Livermore designed vehicle. which dissipated ~2×1010 W of heat due to neutrons, required 20 tons of shield mass and 14 tons of radiators to dispose of this heat alone. This can be compared with a payload mass of 100 tons, and a total propulsion system moss of 300 tons. The power produced in Daedalus is much larger than the above figure, and it can immediately be appreciated that severe disadvantages would exist if DT or DD was used.
     In addition, of course, the reactions ire also wasteful, in the sense that a large percentage of the energy produced does not go into the exhaust from the engine, and does not contribute to the thrust.
     We have thus chosen the D-3He reaction for the Daedalus propulsion system (in common with the earlier continuous fusion rocket study of Hilton. Luce and Thompson). The reaction products are all charged particles which can be directed into the exhaust by the use of magnetic fields, and so all the reaction energy may by utilised. The reaction is more difficult to ignite than DT or DD, but the energy production rate becomes comparable to DT at higher burning temperatures.
     There are always unavoidable neutron-producing reactions in the system, due to D-D reactions in the primary fuel and D-T reactions between the primary deuterium and secondary tritium. This production need not be large, however. Petrie noted that neutron reactions can be less than 5% of the D-3He reactions, and the Hilton, Luce and Thompson vehicle produced less than 1.5 % of the total system power in neutrons. Also, in the Daedalus system the propellant will be in a highly compressed state for a large fraction of the thermonuclear burn time and a large number of the neutrons produced will be removed via neutron absorption by the He, i.e.
3He + n1p + 3T
in the cool outer layers of the pellet there will be few D-T reactions as the relevant reaction rate is very small under the conditions found. The reactions can be regarded entirely as a "mopping up" of neutrons.     The major problem with using the D-3He reaction is the fact that 3He is virtually non-existent on Earth. Several possible methods of production have been suggested, and these are discussed elsewhere in the Report, where it is shown that atmospheric processing in Jupiter should be able to provide ample 3He for many vehicles.
     Subsequent discussion will, therefore, be concerned with the use of D-3He, although reference will be made to the results of the large volume of work carried out on D-T, not only as a means of comparison of the two systems, but also because the use of a small "trigger" core of D-T will be shown to be an effective method of igniting a D-3He reaction.

(*.DOC file download) by Anthony R. Martin and Alan Bond (1978)
Inertial Confinement Fusion
1Pellet Injection Gun
2Superconducting field coils (4)
3Electron beam generators
4Plasma exhaust jet
5Magnetic field
6Energy extraction coils
7Frozen nuclear pellet
8Nuclear explosion
9Reaction chamber

Project Icarus

Project Icarus is a design study to create an unmanned probe starship to investigate a nearby star. It was inspired by the old Project Daedalus study. In 2013 they had a "contest" for research groups to submit proposed designs given some pre-defined limitations.

Firefly Starship
Firefly Starship
2013 design
ΔV2.698×107 m/s
Wet Mass17,800 metric tons
Dry Mass2,365 metric tons
Mass Ratio7.526
Payload150 metric tons
PropulsionZ-Pinch DD Fusion
Exhaust Velocity1.289×107 m/s
Thrust1.9×106 N
Acceleration0.11 m/s
(0.01 g)
Accel time4 years
Coast time93 years
Decel time1 years
Firefly Starship
2014 design
ΔV2.998×107 m/s
Wet Mass45,000 metric tons
Dry Mass3,000 metric tons
Mass Ratio15.0
Payload150 metric tons
PropulsionZ-Pinch DD Fusion
Specific Impulseone million seconds
Thrust855,000 N
Acceleration0.019 m/s
(0.002 g)
Accel time25 years
Coast time70 years
Decel time5 years
Length~1,0000 m

Icarus Interstellar has a project to design a fusion-rocket based interstellar spacecraft. They call it "Firefly". The technical lead director is Robert Freeland.

Most of the other Icarus fusion designs use inertial confinement fusion. That's because IC fusion is easier to get halfway worthwhile power levels. Magnetic confinement fusion would be nicer but once you get enough nuclear fusion going to to be worthwhile, the magnetic bubble pops like a cheap balloon.

The drawback to IC fusion is that the confinement time is pathetic. The longer you confine the fusion reaction, the more of the fusion fuel actually burns and generates energy. But in IC fusion the first bit of fusion acts to blast the pellet apart, scattering the un-burnt fuel to the four winds.

Back in the olden days of fusion research, the darling was Z-Pinch fusion. You send a bolt of electricity (about 5 mega-amps) down the center of a long tube full of ionized plasma, creating magnetic field which compresses the plasma enough to ignite nuclear fusion. One of the big advantages with Z-Pinch was that the confinement time (and net energy output from the burn) can be increased by simply making the reaction chamber longer.

Unfortunatley, the disadvantage is that Z-Pinch fusion suffers from several hydrodynamic instabilities which disrupt the plasma. So researchers stopped working on it in.

But in 1998 Dr. Uri Shumlak discovered you could eliminate the instabilities if you made the plasma move at high velocities. Based on this work, Z-Pinch was selected for the Icarus design.

The Firefly's long thin tail is the Z-Pinch tube, frantically fusing and radiating x-rays like a supernova. So the starship was given its name for similar reasons as the one on the TV show: it is a flying thing whose tail lights up.

The spacecraft profile is long and skinny, for two reasons:

  • Its cruise velocity is a substantial fraction of the speed of light (4.5% c for the 2013 version). This make interstellar dust grains impact with about 9.1×10-4 joules worth of damage, the equivalent of 46,000 cosmic ray photons. You want to reduce the ship's cross section as much as possible to minimize the number of grain impact events.
  • The longer the ship is, the farther the payload can be placed from the deadly radioactive Z-Pinch drive, taking advantage of distance shielding.

Many other starship designs use 3He-D fusion, because all the reaction products are charged particles that can be easily shieldied. The drawback is that 3He is rare, you'd have to harvest the atmosphere of Jupiter for twenty years in order to get enough.

Instead, Firefly uses D-D fusion, since deuterium can be easily found in common seawater. Of course then you have to deal with all the nasty neutrons and x-rays produced by that reaction. Firefly's approach is to forgo the use of massive radiation shields, and instead try to let as much of the radiation escape into space. The Z-Pinch core is almost totally open to space with only a triad of support rails connecting the aft electrode and magnetic nozzle to the rest of the vessel.

Even with that, the waste heat is going to be titanic. That's where the heat radiators come in. Notice how they are the bulk of the ship. Makes the thing look like a garantuan lawn-dart. The radiators use beryllium phase-change technology, and are positioned as close as possible to the heat loads on the tail.

A long conical shield forwards of the reactor core deflects x-rays away from the payload using shallow-angle effects. The electrodes, rails, and other structure near the core are constructed of zirconium carbide (which is capable of surviving the intensely radioactive environment.

The 2014 design had a total length of just under one kilometer, half of which is the fuel tanks. The forward part of the ship uses the old fuel tank in lieu of spine trick in an effort to save on ship mass.

A fission reactor provides secondary power.

Ghost Ship
Ghost Ship
EngineIC Fusion
Pellet Mass2 grams @
Ignition Rate150 Hz
Specific Impulse540,240 s
Exhaust Velocity5,299,750 m/s
Thrust1,600,000 N
Payload150,000 kg
Dry Mass7,444,000 kg
Propellant Mass247,100,000 kg
Wet Mass263,991,000 kg
Mass Ratio35.5
ΔV18,912,190 m/s
0.01 m/s
Mission Duration118.5 years
Mass Budget
TOTAL TANK927,000 kg
   Magnetic Sail1,780,000 kg
   Truss200,000 kg
   Dust Shield15,000 kg
   Power20,000 kg
   Communication40,000 kg
   ADCS40,000 kg
   Tritium Prod (decel)95,000 kg
   D/T tanks, pellet manufacture20,000 kg
   Radiators2,500,000 kg
   Laser system1,000,000 kg
   Coils500,000 kg
   UO29,537,000 kg
   Neutron Shield82,000 kg
   Accelerator25,000 kg
TOTAL DRY MASS7,444,000 kg
TOTAL PROPELLANT247,100,000 kg
   accel242,500,000 kg
   decel4,600,000 kg
TOTAL WET MASS263,991,000 kg

This was the winner in Project Icarus' 2013 contest to design an interstellar starship using current technology. The entry was created by the Munich Ghost Team headed up by Andreas Hein. The basic rules were to design a spacecraft which was mainly fusion powered and on a mission to Alpha Centauri carrying a 100 to 150 tonnes payload and reaching the destination in no more than 100 years.

The design uses Deuterium-Deuterium fuel, even though it has only half the exhaust velocity of Deuterium-Tritium and Deuterium-Helium3, and about 38% of the energy expresses itself as nasty neutron radiation. They rejected D-T because blasted Tritium has a freaking half-life of only 12 years so most of it would decay away during the 15.6 year acceleration phase and the 54 year coast phase. You'd have to carry a huge excess penalty mass of extra Tritium to allow for decay. They also rejected D-He3 because there probably isn't enough He3 on all of Luna, and harvesting it from a gas giant's atmosphere would require a huge space infrastructure.

Forced to use D-D, the designers looked for ways to turn that pesky neutron flux from a liability into an asset.

Standard inertial-confinement fusion engines use a circular firing squad of lasers to implode the fuel pellet. The compression ignites the fusion fuel. The designers note this is a bit inefficient. By analogy, it is possible to detonate a stick of TNT by squeezing it but you have to squeeze real hard. It takes a lot less energy to use a match to light the fuse on the TNT.

So the designers used a so-called "fast ignition scheme". The circular firing squad just has enough laser power to confine the fusion fuel, but not the extra energy needed to compress it to ignition. A secondary high-powered laser acts as the fuse, piercing the pellet and igniting it. You get the same energy from the pellet, but you need a whole lot less input laser energy.

Alas, a "whole lot less" is still freaking huge. Lasers are power hogs.

The standard method is to harvest some of the fusion energy and convert it into electricity. This is stored in huge banks of heavy capacitors, to be used for the next laser pulse. The initial capacitor charge comes from a nuclear reactor or something which trickle charges the capacitor banks. The problem is the mass of all those capacitors is a punishing amount of penalty-weight.

That's when the designers turned the D-D waste neutron flux into an asset. Have you ever heard of Nuclear-pumped lasers?

All lasers consist of a lasing medium which emits laser light when it is pumped. Conventional lasers pump the medium with electricity or light. Nuclear lasers on the other hand pump with the awesome might of nuclear fission. Uranium-235 is exposed to neutrons, undergoes fission, the energy pumps the lasing medium, and a rather powerful laser beam emerges.

The main draw-back of nuclear-pumped lasers is the lack of convenient sources of high neutron flux. A nuclear reactor can provide a bit of neutrons, and in theory a fission warhead detonation can provide lottsa neutrons (see Bomb-Pumped Lasers). The light-bulb went off over most of your heads while you were reading the previous paragraph. Yes, the neutron flux from the detonating D-D fuel pellets would work splendidly.

The designers used a solid-core nuclear laser instead of liquid-core, since solid-core is more suitable for generating extremely short high-powered pulses. A ring-shaped chamber circles the thrust chamber, centered on the fusion pellet detonation point. The chamber is filled with a uranium dioxide aerosol and a fluorescent gas acting as the lasing medium. Some of the neutrons from the fusion detonation enter the chamber, causing fission reaction with the uranium 235 atoms, the fissile products then excite the fluorescent gas thus pumping it. The light flash from the fluorescent gas is transmitted through a light pipe into the laser amplifier. This creates the laser beam.

This system is about 8% efficient, which is pathetic for a general device but actually fantastic for a laser. And it is using all those otherwise worthless neutrons. The drawback is the uranium dioxide aerosol and fluorescent gas are expended with each laser bolt, that is, they are consumables. Which adds to the mass load.

However, even with the consumables the total mass of the ignition system is less than 1,000,000 kilograms, which is far less than all those banks of capacitors.

The report states an exhaust velocity of 0.018 c, which is considerably smaller than the theoretical maximum of D-D fusion (0.043 c). Which is probably very realistic.

As with all high-energy propulsion there is huge amounts of waste heat to get rid of, and this system does not lend itself to open-cycle cooling (not enough mass in the exhaust to carry off enough heat). Meaning you actually need plenty of heat radiators. The design use liquid-droplet radiators with a total area of 7.6 square kilometers. It has a very high heat rejection rate of 500 kW/kg by using liquid aluminum as the heat-conducting liquid.

The spine of the ship is a cylindrical truss structure composed of carbon nanotubes. This material has an exceptionally high tensile strength at a very low density, prime spacecraft building material. And you are going to need it. An engine thrust of 1.6 megaNewtons is 160 metric tons of compressive force which the spine has to endure for a bit more than 15 years. Thin spines tend to buckle so the design has a spine with a fat diameter of 100 meters. I did some analysis of the above image, if the spine is 100m in diameter, the pictured ship is about 1.1 kilometers long.

Mission Profile

Starting wet mass is 153,940,000 kg, of which 150,000,000 kg is propellant. Outrageous mass ratio of freaking 45.9!

It accelerates for 25.63 years, reaching a velocity of 0.05c. It then coasts for 56.43 years.

Upon approaching Alpha Centauri, it deploys a magnetic sail. This drags on the interstellar medium, decelerating the spacecraft for 36 years. Once the velocity is down to 0.01c the fusion engine is used to finish the job of bringing the ship to a halt. The ship is now totally out of fuel. It then deploys lots of scientific probes and drones to gather as much scientific information as it possibly can, and transmits it back to Terra.

Distance0.37 ly3 ly0.02 ly0.0024 ly
End Velocity5% c-3,470 km/s
150 km/s
Duration25.63 yrs56.43 yrs36 yrs0.229 yrs
Starship Resolution
Length2,000 m
Width50 m
(1st Stage)
20,700,000 kg
(Deburn Stage)
3,900,000 kg
Engine318,000 kg
Structure1,000,000 kg
Num Propellant Tanks
(1st stage)
Num Propellant Tanks
(Deburn stage)
Bombardment Materialberyllium
Bombardment Shield53,650 kg
Bombardment Shield Thickness9.01 mm
Payload150,000 kg
TypeIC Fusion
Engine Mass318,000 kg
Fusion Pellet0.000288 kg
9.97 mm dia
Pulse Rate150 Hz
Exhaust Velocity9,210,000 m/s
Mass Flow0.0432 kg/sec
Jet Power1.832 TW
Thrust398,000 N
Boost Phase15.184 yrs
0.274 ly from Sol
Dist Boost Burn0.274 ly
Cruise Phase81.475 yrs
3.936 ly from Sol
Deburn Phase2.861 yrs
4.4 ly from Sol
Dist Deburn0.189 ly
Total Duration99.519 yrs
Cruise Velocity14,481,476 m/s
(4.83% c)
Excess Velocity at Dest1,280 m/s

The original Project Daedalus starship was a two-stage rocket with each stage having an inertial confinement fusion rocket. Starship Resolution is a conservative extrapolation. Instead of two engines each in a stage, it has a single engine fed by lots of drop-off tanks.

If for some obsessive-compulsive reason you need to know how many fuel pellets each stage has, simply divide the propellant mass in a stage by 0.000288.

Ultra-Dense Deuterium
Deuterium Starship
EngineIC Fusion
Ignition Laser
3 kJ
Ignition Laser
1 PW
Specific Impulse550,000 s
Exhaust Velocity5,400,000 m/s
Propellant Mass75,000,000 kg
Cruising Velocity15,000,000 m/s

This was the fourth entry in Project Icarus' 2013 contest to design an interstellar starship using current technology (it didn't win, the Ghost ship did). The basic rules were to design a spacecraft which was mainly fusion powered and on a mission to Alpha Centauri carrying a 100 to 150 tonnes payload and reaching the destination in no more than 100 years.

For reasons similar to those raised by the Ghost ship, this design also uses the relatively feeble Deuterium-Deuterium fusion reaction. Both designs use laser ignited inertial confinement fusion engines.

The main drawback to IC fusion engines is since beams of light do not push very hard, you need metric-assloads of laser energy to crush the fuel pellet to fusion ignition. Which requires lots of heavy lasers, savagely cutting into your payload mass budget. Since the laser pulse has to be microscopically short, the lasers have to be powered by huge banks of weighty capacitors, further slashing your payload budget.

The Ghost ship gets around this by replacing the capacitor banks with nuclear-pumped lasers, using the waste neutrons from the prior detonation.

The Ultra-Dense Deuterium starship gets around this with something even more tricky. It uses a weird fuel called, you guessed it, ultra-dense deuterium.

Ultra-Dense Deuterium

Ultra-dense deuterium (UDD) is an exotic form of metallic hydrogen called Rydberg matter. As you can probably figure out from the name the stuff is dense. Real dense. As in 1028 to 1029 grams per cubic centimeter dense. About a million times denser than frozen deuterium.

For our purposes the interesting point is it is about 150 times as dense as your average pellet of fusion fuel when laser-compressed to peak compression. Yes, this means do you not need metric-assloads of laser energy to crush the fuel pellet, a pellet just sitting on the table is already at 150 times the needed compression. It is pre-compressed. All you need is a miniscule 3 kilojoules worth of laser energy to ignite the stuff. That is pocket-change compared to what 200-odd compression lasers require. In fact it is so little that a single laser can handle the job. This results in a vast savings on laser mass and capacitor mass.

The laser pulse has to be quick, so the power rating is a scary 1 petawatt. But by the same token since the pulse is quick, it only require the aforesaid 3 kilojoules of energy.

Since you do not have to compress the fuel you can avoid all sorts of inconvienient hydrodynamic instabilities and plasma-laser interation problems.

You also have virtually unlimited "fusion gain". Meaning that with a conventional IC fusion engine there is a maximum fuel pellet size due to the hydrodynamic instabilities and the geometric increase in compression laser power. With UDD you can make the fuel pellet as large as you want (well, as large as the engine can handle without blowing up at any rate). With other laser intertial confinement fusion, if you make the pellets larger, you have to make the laser array larger as well. Not so with the UDD drive. The fusion gain depends solely on the size of the pellet, you do not have to make the lasers bigger.

An important safety tip: since UDD has such absurdly low ignition energy, there is a statistical change a large number of UDD atoms would undergo fusion spontaneously. This dangerous instability means the spacecraft will carry ordinary deuterium fuel and only convert it into UDD immediatly before use.

The cherry on top of the sundae is UDD fusion does not produce deadly neutron radiation. Instead it produces charged muons, which are not only easier to deal with, but also can be directly converted into electricity. Left alone, the muons quickly decay into ordinary electrons and similar particles.

And since deuterium is plentiful in ordinary seawater, you do not have to go strip mining Lunar regolith or set up atmospheric scoop operations around Jupiter were you to use a fusion reaction requiring Helium-3.

Sounds too good to be true, I hear you say. Well, there are a couple of drawbacks.

The minor drawback is that D-D fusion has a specific impulse (and exhaust velocity) which is about half of what you can get out of D-T fusion or D-He3 fusion. This drastically increases the mass ratio required for a given mission delta-V. Having said that it is still much better than what you'll get out of chemical or fission engines.

But the major drawback is UDD might not even have that magic ultra-density.

You see, the vast majority of the UDD-related papers has been published by a single scientific group at University of Gothenburg, Sweden, led by Dr. L. Holmlid. Currently there are no third-party confirmations about UDD observations and generally very few discussions about it in the scientific community. Until the density figure is confirmed, it might be all a pipe-dream.

The Mision

The spacecraft has two stages, kinda-sorta.

It accelerates for ten years using Stage One, reaching a velocity of 0.04c. Stage One is then jettisioned.

It accelerates for an additional two years using Stage Two, reaching a velocity of 0.04c. Stage Two stops burning, it still has fuel left. It jettisons about 68% of its heat radiator mass which is no longer needed.

The spacecraft proceeds to coast for the next seventy-five years.

At the end of the coast phase, the spacecraft is 0.378 light-years (0.368 + 0.010) away from the destination (Alpha Centauri). About 24,000 astronomical units. It then deploys a Magsail drag (with a mass of 238,000 kg). Over the next twelve years it decelerates the spacecraft to a velocity of 0.012c.

The spacecraft is now 0.01 light-years from destination. It jettisions the Magsail, bringing the spacecraft mass down to 612,000 kg. Stage Two's engine starts burning (in the diagram this is marked as "3rd Stage"). It burns for the next two years, bringing the spacecraft to halt at the destination. The spacecraft mass is now 320,000 kg, of which 150,000 kg is scientific payload.

Shepherd Generation Starship

This is from the paper Interstellar Flight JBIS Vol. 11 (1952) by the legendary Les Shepherd The Journal of the British Interplanetary Society (JBIS) proudly proclaims this to be the first technical paper on interstellar flight.

Lamentably I am still trying to obtain a copy. In the meantime I will make do with Mr. Shepherd's popularization of his paper which appeared in Science-Fiction Plus April 1953.

Mr. Shepherd points out that when it comes to interstellar colonization, the problem is not transporting a man across stellar distances, it is more a problem of transporting an adequate community. If the transport is not moving at relativistic velocities you are probably talking about a generation ship (I suspect the sleeper ship concept had not been conceived of as early as 1952). Shepherd opines that interstellar explorers or colonists, faced with the knowledge that they will not only never see Terra again but also never see their destination, should adopt a similar philosophy to that of a soldier setting out on a suicide raid. There will be no personal gain, but instead the dying knowledge that some will survive to benefit from their action. This is calling for the sacrifice of entire generations in the depths of space, which admittedly will require a revolution in society. But Shepherd says this may be necessary if we are ever to become a galactic people.

Shepherd does not mention the problem of generation born en route being angry at their forebearers presumptuously committing them to this role. He does point out that the society will have to be a bit regimented. There will be specific population goals (overpopulation or underpopulation is a problem) so procreation is strictly regulated. Civilization has to be preserved, knowledge and culture will have to be carefully handed from generation to generation. New developments in science and art will be needed since Shepherd is of the opinion that "stagnation is the first step to degradation."

Shepherd figures that generation ships should not be used until the state-of-the-art allows transit times less than one thousand years. However he apparently didn't think of the "jumping the gun" problem.

For the journey to Alpha Centauri Shepherd figures that a fission-powered generation ship with an amount of fission fuel equal to 2.4 times the ship's dry mass, plus enough hydrogen propellant to rase the total mass ratio to 5.0 would have an exhaust velocity of 6,000 km/s and a deltaV of 10,000 km/s (about 0.03c). It accelerates to 5,000 km/s, cruises, then brakes down to zero at Alpha C. The transit time should be about 250 years.

If the transit time was lengthened to 350 years, the acceleration and deceleration phases could be increased to 50 years each, with 250 years of coasting in the middle. This would reduce the required acceleration to 0.00327 m/s2 (about 1/3000 g). Which would reduce the required engine power output per short ton of wet mass (specific exhaust power) to "only" 10 megawatts. E.g., if the wet mass was 10,000 short tons the engine power would be 100,000 megawatts (100 gigawatts). Shepherd admits that designing engines which can crank out 100 gigawatts for fifty years will be a bit of a challenge.

Shepherd says the transit time can be cut to 140 years if "lithium-hydrogen" fusion is used, but I think Mr. Shepherd was unaware that there are much better fusion reactions that can be used. I'd be more sure if I could read his actual paper.

Shepherd tries to put a spin on matters, pointing out that while a thousand years sounds like a long time to us, it is actually a small interval in terms of geological time. Which will fool nobody with an I.Q. higher than room temperature. He also mentioned that it would be a real good idea if astronomers made quite sure that the target star indeed had a habitable planet. Otherwise it would be a most tragically ironic ending to a very long mission.

Shepherd also acknowledges that the biological problem of maintaining a life support system for thirty generations is a major engineering challenge, but that isn't his department. Conservation of resources is important since losses can really add up over a thousand years. E.g., a million ton vessel losing 100 milligrams of air per second doesn't sound too serious, but over a 1,000 years that adds up to about three thousand tons of atmosphere loss.

The ship should also transport an entire ecosystem to be transplanted to the new world. This would turn the ship into a veritable Noah's Ark, and might force the ship to be a hollowed-out asteroid in order to carry everything. An asteroid has such a large radius that it could be spun up for artificial gravity at a low enough rate to prevent spin nausea.

Torus World Ship

DesignPopulation SizeDry Mass
Propellant Mass
Cruise Velocity
Torus world ship [38]100,0001075×1071%

      More recently Hein et al. [38] have presented a world ship design with stacked Stanford Tori for population sizes on the order of 104 to 105, shown in Fig. 2. Similar to [58], this world ship design is based on the Daedalus fusion propulsion system and a habitat design borrowed from an O’Neill colony, in this case the Stanford Torus. Fig. 2 also shows the dust shield put on top of the Stanford Torus facing flight direction.

     The authors of [38] have subsequently further developed the design, in order to reduce the overall mass of the spacecraft, which is dominated by the shielding mass for the habitat (>90%).

     One possibility would be to use the deceleration propellant as the shielding material. The propellant mass mainly consists of Deuterium, which has similar shielding characteristics as hydrogen [69]. The propellant is used up during the last years of the trip for decelerating the spacecraft and would serve as a shielding up to this point. The two disadvantages of this approach are that the complexity of the spacecraft increases. The fuel needs to be transported from the shield to the fusion engine. The fuel pellets either need to be manufactured on-board or the shielding is already in the form of fuel pellets. In both cases additional equipment has to be installed.

[38] Andreas M Hein, Mikhail Pak, Daniel Pütz, Christian Bühler, and Philipp Reiss. World Ships Architectures & Feasibility Revisited. Journal of the British Interplanetary Society, 65(4):119–133, 2012.

[58] GL Matloff. Utilization of O’Neill’s Model I Lagrange Point colony as an interstellar arc. Journal of the British Interplanetary Society, 1976.

[69] M T Simnad. Nuclear Reactors: Shielding Materials. pages 6377–6384. Elsevier, Oxford, 2001.

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