There is a good reason for colonizing another planet, which is to avoid extinction if the Earth is hit by a 10km or larger asteroid, as has happened many times in the Earth's history. Colonization of Mercury appears to be a very real and practical possibility, whereas colonization of Mars or the other planets, moons or asteroids is really more in the realm of fantasy. The first thought about Mercury is that it would have very high temperatures and no water, because the equatorial surface temperature ranges between -183°C and 427°C as the planet rotates. But an analysis of temperature vs. latitude and depth shows that the temperature is nearly constant at room temperature (22+/-1°C) in underground rings circling the planet's poles, and deeper than .7 meter below the surface. Similar results are found using numerical techniques in an Icarus paper, Vol. 141, 179–193 (1999). Researchers at Arecibo observatory have found high radar-reflective areas near Mercury's poles in permanently shadowed crater bottoms, and this material was recently proven to be nearly pure water ice using neutron spectrometry data from the Mercury MESSENGER spacecraft, as described in Thermal Stability of Volatiles in the North Polar Region of Mercury and Evidence for Water Ice Near Mercury's North Pole from MESSENGER Neutron Spectrometer Measurements. Many of the craters are 1-2 km deep. The wavelength dependence of radar signals indicates a minimum of several meters of water ice over a total area of 10¹⁰m², resulting in 10¹⁴-10¹⁵kg of water. A more recent estimate using MESSENGER data is .5-20 meters of water ice over 5.65x10¹⁰m², resulting in 2.1x10¹³-1.4x10¹⁵kg of water. Laser altimeter reflections indicate that most of the ice is covered by a dark material which is thought to be a layer of hydrocarbons about 10cm thick (amounting to ~5.65x10⁹m³), as described in Bright and Dark Polar Deposits on Mercury: Evidence for Surface Volatiles.

      Agriculture would be possible with 2.1x10¹³-1.4x10¹⁵ kg of water covered by 5.65x10⁹m³ of carbon-rich hydrocarbons. Crops would provide food and oxygen, and consume the carbon dioxide we exhale. All human habitation and agriculture would be underground to avoid temperature extremes, ionizing radiation, and the loss of oxygen, water and carbon dioxide to the surface. Filtered light could be used for crops, but it is likely that rapidly growing crop varieties could be developed which would take advantage of the high light intensity and the long Mercury day, where sunrise to sunset lasts for 88 Earth days. X-ray spectrometry for Si, Mg, Al, S, Ca, Ti, Fe, Cl, Cr and Mn, gamma-ray spectrometry for K, Th and U and gamma-ray spectrometry for Al, Ca, S, Fe and Na from Mercury MESSENGER shows the following average composition of Mercury's soil compared to Earth:

Average Element Concentration (%weight)

Planet\Element	O	Si	Mg	Al	S	Ca	Ti	Fe	Cl	Cr	Mn	Na	K	Th	U	C	N	H
Mercury (tens of cm depth)	42.3	24.6	12.5	7.1	2.3	5.9	.2	1.9	< .2	< .5	< .5	2.9	.1	.00002	.00001	?	?	?
Earth (continental crust)	47	28	2.5	8	.04	4	.5	5	.02	.02	.1	2	2	.0007	.0002	.1	.002	.1

The consistency of the X-ray and gamma-ray spectrometry data indicates that the compositions above apply down to depths of tens of centimeters, and that the soil is homogeneous to this depth. It is not known for sure that there is enough of certain elements to grow crops, particularly the volatile elements C and N which cannot be measured by these methods. However, the fact that Mercury's K/Th ratio is higher than Earth strongly suggests that Mercury's volatile elements have not been boiled away at some point in the planet's history. The S abundance supports this conclusion. The K abundance also supports this conclusion, being similar to the abundance on Earth as a whole. However, K abundance is low compared to that in the Earth's continental crust, which might be a problem for plant growth. It is also possible that some of the elements are locked up in minerals which cannot be metabolized by plants. The MESSENGER probe is currently gathering ultraviolet and infrared spectrometry data and other data about the soil and the high radar-reflective areas, so some of these questions might be answered soon.

Several other aspects of Mercury make it a good prospect for a colony. One very important advantage is the high solar light intensity, which is stronger than on Earth by a factor of 10.6 at perihelion and 4.6 at aphelion. This strong light intensity would provide virtually unlimited power via electronic solar arrays, and the resulting vertical temperature gradients of ~200^oC/m would provide even more unlimited power via thermal solar arrays. With such an unlimited and inexpensive power source, almost anything needed for survival could be produced. The gravity on Mercury is 38% that of Earth, which is strong enough to avoid the reduction in bone mass that occurs in very low gravity and weightless environments. There are no temperature variations over periods longer than the Mercury day (like Earth's seasons), which avoids the need for heating/cooling equipment within the 22+/-1^oC underground rings mentioned above. This occurs because Mercury's orbit is synchronized with its rotation such that 0deg and 180deg longitudes always experience midnight and noon at perihelion whereas 90deg and 270deg longitudes always experience midnight and noon at aphelion. The rings would be about 5000km long, similar to the diameter of the planet. They would be only 20-60km wide because of horizontal temperature gradients of .035-.097^oC/km. This results in a total area of about 40x5000=200,000km² of 22+/-1^oC temperature around each pole. The rings could also be extended hundreds of floors downwards, essentially by making underground skyscrapers. And the entire area between the rings and the poles could also be populated (albeit more sparsely) simply by using abundant solar power. Now, an underground existence may sound undesirable to many people. However, that fact is that most people spend 95% of their lives indoors, and from a quality-of-life perspective there is little difference between indoors above ground and indoors below ground. And the colony could still have natural areas, trees, flowers, parks, lakes, wild animals, etcetera. In fact it would probably need all of these things to maintain the ecosystem. The only difference from Earth is that they would be in man-made underground greenhouses instead of on the planet surface.

Mars automatically comes to mind when discussing planetary colonization, and manned missions to Mars have been the long term focus of US space exploration plans since 2004. But despite all the hype, Mars is really a poor prospect for colonization. The solar light intensity on Mars is .43 that of Earth, which makes solar power and agriculture much less practical than on Mercury. The gravity of Mars is 38% of Earth, essentially equal to Mercury. The magnetic field of Mars is .1% of Earth, and its atmosphere density is 2% that of Earth, so protection from ionizing radiation would require underground habitation, the same as on Mercury. The average equatorial surface temperature of Mars is about -45^oC (-50^oF), which would be the constant temperature underground. And of course the temperature gets much lower away from the equator. Such low temperatures can be withstood by machines such as the Spirit, Opportunity and Curiosity Mars rovers, but not by people. Human habitation of Mars would be problematic because of the very low temperatures, limited solar power capacity, and a biological history which precludes oil, gas and coal deposits. Human habitation would probably be impossible without nuclear power, and uranium mining and nuclear plants would be very challenging in an airless, cold enviroment. Also, concentrated uranium deposits are probably less common than on Earth because they depend on sedimentary and hydrothermal processes which are more prevalent on Earth. The other planets, moons and asteroids have even worse drawbacks than Mars.

Asteroid impacts of 5km diameter or greater occur roughly once every 10 million years, and those of 10km or greater occur roughly once every 100 million years. In the past 540 million years there have been 5 extinction events where more than 50% of the Earth's species were killed off, including the Permian-Triassic extinction where 90% of the species were lost. Most scientists think that some of these were caused by asteriod impacts. A well proven example is the Chicxulub impact which resulted from a 10km asteroid impact at the Cretaceous-Tertiary boundary 65 million years ago and caused the extinction of 70% of the Earth's species, including the dinosaurs. Even larger impacts have occured at earlier times, of which only a few are known because their impact craters get erased by the Earth's geological processes over time. It is thought that a 20km or larger asteroid would cause the extinction of all higher order animals and plants, leaving only microorganisms. While the likelihood of such an event is very small in any given year, it could happen at any time, and it is almost guaranteed to happen eventually.

Given the facts above, it appears that the focus of US space exploration plans should be shifted from Mars to Mercury. In particular, the US has already had four successful Mars rovers, so how about a Mercury rover mission. Perhaps such a mission could focus on a detailed analysis of the water ice, the dark material covering the water ice, and the soil. Such an analysis would be critical for a Mercury colony, and it would also be of interest from a purely scientific standpoint. How deep are the water ice deposits? Would the water need to be purged of poisonous contaminants before it could be used for drinking or agriculture? Is the dark covering material made out of hydrocarbons as several scientists have suggested? How much of this material is there, and could it be used as a source of carbon for agriculture? What is the soil concentration of Carbon and Nitrogen, elements that could not be measured by MESSENGER's gamma-ray and X-ray spectrometers? What minerals are present, and are some of the elements critical for agriculture locked up in minerals which cannot be metabolized by plants? Perhaps it would be best to land the rover near a small crater with water deposits, so it could hide in the crater from the hot sun, but project solar cells or a mirror over the edge. Some good landing locations could probably be found among the large number of high resolution images taken by MESSENGER.

The main motivation for investigating Mercury is its potential for hosting a self-sustaining human colony, which would protect humanity from extinction in the event of a catastrophic asteroid impact. A second motivation is simply to increase our scientific understanding of the solar system. It is very unlikely that Mercury could ever be a practical source of minerals or energy to be transported back to Earth, or that Mercury would ever have any other Earth-serving economic value. But surely preservation of the human species and scientific curiosity are better motivations than economic benefit. Humans are part of a universe where time is measured in billions of years. We need to take a long term view, and consider the future of the human species in the next thousand, million and billion years, not just the next 10 or 100 years.

A Mercury colony would be a challenging and costly effort for sure. The voyage to Mercury might take 6.5 years like the MESSENGER trip because of the large velocity change involved, and the spacecraft would require heavy shielding against ionizing radiation. Much planning and preparation would be needed to ensure that the colony could get through the first weeks, months, and years, with little or no resupply from Earth. However, a Mercury colony appears to be a real possibilty using current technology, not a fantasy for the distant future. In fact, if we delay until the distant future, or even 50 years or so, such an effort will probably become impossible. This is because us humans will consume the Earth's non-renewable energy and mineral resources almost completely within the next several decades, severely reducing our discretionary income for costly activities such as space travel. We should be pursuing a Mercury colony now, before it is too late.

I don’t think any of us really trusted the Nerva-K under our landing craft.

Think it through. For long trips in space, you use an ion jet giving low thrust over long periods of time. The ion motor on our own craft had been decades in use. Where gravity is materially lower than Earth’s, you land on dependable chemical rockets. For landings on Earth and Venus, you use heat shields and the braking power of the atmosphere. For landing on the gas giants—but who would want to?

The Nerva-class fission rockets are used only for takeoff from Earth, where thrust and efficiency count. Responsiveness and maneuverability count for too much during a powered landing. And a heavy planet will always have an atmosphere for braking.

Pluto didn’t.

For Pluto, the chemical jets to take us down and bring us back up were too heavy to carry all that way. We needed a highly maneuverable Nerva-type atomic rocket motor using hydrogen for reaction mass.

And we had it. But we didn’t trust it.

(ed note: though as it turns out, Mercury has about six times the surface gravity of Pluto, and Mercury also does not have an atmosphere for braking. When this story was written it was thought that Pluto had a much higher gravity)

• 

For a long time nobody—not even Conrad Taylor—spoke a word. All the members of the Committee were marshalling their thoughts about those difficult people the Hermians, so ably represented here by their Ambassador.

To most people, Mercury was a fairly good approximation of Hell; at least, it would do until something worse came along. But the Hermians were proud of their bizarre planet, with its days longer than its years, its double sunrises and sunsets, its rivers of molten metal. By comparison, the Moon and Mars had been almost trivial challenges. Not until men landed on Venus (if they even did) would they encounter an environment more hostile than that of Mercury.

And yet this world had turned out to be, in many ways, the key to the solar system. This seemed obvious in retrospect, but the Space Age had been almost a century old before the fact was realized. Now the Hermians never let anyone forget it.

Long before men reached the planet, Mercury’s abnormal density hinted at the heavy elements it contained; even so, its wealth was still a source of astonishment, and had postponed for a thousand years any fears that the key metals of human civilization would be exhausted. And these treasures were in the best possible place, where the power of the Sun was ten times greater than on frigid Earth.

Unlimited energy, unlimited metal; that was Mercury. Its great magnetic launchers could catapult manufactured products to any point in the solar system. It could also export energy, in synthetic transuranium isotopes or pure radiation. It had even been proposed that Hermian lasers would one day thaw out gigantic Jupiter, but this idea had not been well received on the other worlds. A technology that could cook Jupiter had too many tempting possibilities for interplanetary blackmail.

That such a concern had ever been expressed said a good deal about the general attitude towards the Hermians. They were respected for their toughness and engineering skills, and admired for the way in which they had conquered so fearsome a world. But they were not liked, and still less were they completely trusted.

At the same time, it was possible to appreciate their point of view. The Hermians, it was often joked, sometimes behaved as if the Sun was their, personal property. They were bound to it in an intimate love-hate relationship—as the Vikings had once been linked to the sea, the Nepalese to the Himalayas, the Eskimos to the Tundra. They would be most unhappy if something came between them and the natural force that dominated and controlled their lives.

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Some psychologists had claimed that it was almost impossible to understand fully the mentality of anyone born and bred on Mercury. Forever exiled from Earth by its three-times-more-powerful gravity, Hermians could stand on the Moon and look across the narrow gap to the planet of their ancestors, even of their own parents, but they could never visit it. And so, inevitably, they claimed that they did not want to.

They pretended to despise the soft rains, the rolling fields, the lakes and seas, the blue skies—all the things that they could know only through recordings. Because their planet was drenched with such solar energy that the day time temperature often reached six hundred degrees, they affected a rather swaggering roughness that did not bear a moment’s serious examination. In fact, they tended to be physically weak, since they could only survive if they were totally insulated from their environment. Even if he could have tolerated the gravity, a Hermian would have been quickly incapacitated by a hot day in any equatorial country on Earth.

Yet in matters that really counted, they were tough. The psychological pressures of that ravening star so close at hand, the engineering problems of tearing into a stubborn planet and wrenching from it all the necessities of life—these had produced a spartan and in many ways highly admirable culture. You could rely on the Hermians; if they promised something, they would do it, though the bill might be considerable. It was their own joke that, if the sun ever showed signs of going nova, they would contract to get it under control—once the fee had been settled. It was a non-Hermian joke that any child who showed signs of interest in art, philosophy or abstract mathematics was ploughed straight back into the hydroponic farms. As far as criminals and psychopaths were concerned, this was not a joke at all. Crime was one of the luxuries that Mercury could not afford.

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Part of my series on countering misconceptions in space journalism.

From time to time, one reads of studies promoting the concept of crewed missions to Venus. Usually this crops up as part of the perennial debate about going to the Moon, or Mars, or an asteroid, or a deep space space station, or nowhere at all.

“What about Venus?” cry the sorts of people who would jump into a Mac vs PC argument advocating Solaris while fondling a beard that reaches their knees. Reader, this could be me, except I lack the necessary hair production talent.

The basic idea is that Venus requires slightly less delta-V than Mars to get to, has a much thicker atmosphere, more solar energy, and much more frequent launch windows. Capitalizing on all that was the Soviet Venera program, which launched dozens of orbiters, landers, and balloons to Venus during the 1960s, 1970s and 1980s.

While Mars’ surface is a frozen poisonous radioactive airless hellscape, Venus’ surface is rocky, volcanic, and a mere 462 C at 93 atmospheres, supercritical CO2 atmosphere with a side of sulfuric acid. This is so hot that future rovers will depend on mechanical computers/cameras/radios, because ordinary computers will melt. Obviously, sending humans to the surface of Venus is a non-starter.

That’s okay, say Venus exploration advocate(s), we’ll have a giant airship. Somehow we’ll perform EDL at 12km/s, deploy something a hundred times the size of the Hindenburg in a couple of minutes, float around for a few months, then drop a Earth Return Vehicle (ERV) the size of a Saturn V off the bottom, light the engines, and return home.

I love this stuff. Let’s dig in a bit.

I have a future blog planned on airships, because they’re awesome. Let’s rebuild the Hindenburg but with mylar, nylon, carbon fiber, and helium. We can put a thousand people on it. It will be slow, crowded, and a human petri dish, just like a cruise ship. It will also be perilous and too far above the ground, like a plane. And awesome!

How will this work on Venus? I have no idea. I think one could make an argument that a pressure-stabilized airship deployed in space could perform entry, descent, and landing (EDL) without substantial heat shielding due to its very low ballistic coefficient. Let’s say this is possible. After a 6 month trip to Venus, the crew would then hang out in a small capsule suspended from a giant balloon. Let’s hope they like confined spaces.

Inside this balloon, they can drift on Venus’ winds around the planet while they wait for the return launch window to line up. From their perch maybe 50 km off the ground, they’ll have a great view of the sulfuric acid clouds which, when the balloon gets blown around to the night side of Venus (its days are 117 Earth days long), will rise up and engulf the balloon.

While it is true that Earth-Venus requires slightly less delta-V than Earth-Mars, the reverse is not the case. Venus is nearly the size of Earth. With an escape velocity of 10.36km, the ERV will need to be comparable in size to a Saturn V. Fully fueled, it will weigh about 3000 T, which exceeds the working load of the Hindenburg by only a factor of 300. This monster rocket will also need to be launched somehow from a balloon about the size of Manhattan. Maybe it can be dropped and then fly to the side?

Bringing a 3000 T ERV from Earth, in addition to the gigaZeppelin, is a big resource drag. Perhaps its mass could be reduced somewhat by synthesizing its oxidizer from the ambient Venusian atmosphere. If the airship lacks the propulsion to remain on the day side of the planet, the chemical plant will require a substantial nuclear reactor to provide power. Further, as the ERV’s tanks are filled, the airship will somehow have to generate more lift (thousands of tonnes more) to remain buoyant. Lifting gasses can’t be synthesized from ambient CO2 atmosphere, so we’d better make sure we bring enough helium to last the entire mission.

I’ve spent years pondering the head-scratching difficulty of getting humans to and from Mars, building cities there, and solving problems. I think it’s possible.

A human exploration mission to Venus is in another category entirely, and all to get a crew of two from a monotonous space flight to a monotonous balloon flight to a monotonous space flight home. All it requires is about a thousand times the mass relative to a Mars mission, and a generous handful of technical miracles.

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Pilgrims with itchy feet and travelin' bones will head for the high frontier; wherever they find it, they'll push it back. These trail blazers, on their way to conquer new lands, will come to the Martian moons in an ever-growing stream. Like the boomers going to Saint Joe, Mo., these space settlers will outfit themselves at the 'rail-head' on Deimos. They will convert their hard-won grubstakes into tools and provisions, and strike out for the new territories. Most will head for the myriad micro-worlds of the asteroid belt. Other hardy souls will venture further afield, to the Trojan asteroids, and even the moons of Jupiter.

Many of these outbound immigrants will be moved by the simple desire to flee the cloying multitudes of the inner worlds. Others will be impelled by that unfailing motivator of mankind: greed. Settlement of the solar system will be greatly encouraged by the adoption of a Millennial Mining Law. The administrative code underpinning the Law can be complex in its particulars, but the essence of the Law itself should be utter simplicity: "First in time is first in right."

If you are the first person to reach a satellite, and stake and register your claim with the Solar Mining Office, it's yours! It is unlikely that an individual could mount a solo expedition to the asteroid belt; so you would probably have to sell shares in your venture to get the capital. At any rate, you and your partners are now the proud owners of a celestial mother lode.

Lets say you've staked your claim to an average carbonaceous chondrite in the outer asteroid belt. What's it worth? Much interesting speculation about asteroid mining has centered on those of the metallic variety.490 When one visits asteroids at the museum, it certainly appears that the nickel-iron types would be worth the most. After all, metals form the back-bone of industrial infrastructure, and their extraction and refinement is at the root of civilization. That is true enough today, but in the New Millennium, metals will be a side show. The real money is in water, carbon, nitrogen, sulfur, and the other light elements that make up the substance of animate matter. There will be claims staked on metallic asteroids to be sure, but in the first great "land rush", the savvy entrepreneurs will be after carbonaceous chondrites.

If you managed to be the first claimant on a typical smallish asteroid—10 km. in diameter—you would possess a trillion tons of the most valuable resources in the solar system. If, after paying for extraction and transportation costs, consumers in the realms of Asgard and Avallon paid an average of only one dollar per kilogram for light elements, your asteroid would be worth over one hundred trillion dollars.

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There are millions of asteroids that are more than a few kilometers in diameter, and billions and billions of smaller ones. Eventually, they will all be claimed, mined, and settled. As the human race grows to maturity, filling up the solar system, the market for asteroid organics will expand continually. Any asteroid prospector can stake his claim with full confidence that eventually there will be a demand for his minerals. It may take time. But asteroid settlers will be some of the only people in the solar system who can live with near perfect self-sufficiency. They can afford to wait.

Like all pioneers, the first settlers in the asteroid belt must be hardy souls with iron wills and rawhide constitutions. They must be capable of facing isolation and deprivation; they must be able to fend for themselves; and most importantly they should be endowed with infinite faith in the future. Like Europeans headed for the California gold fields in 1849, new immigrants to the asteroid belt will flood in from the Old World. They will book their passage on the trans-oceanic vessels of their time—orbital habitats cycling between Earth and Mars. When the pioneers arrive on Deimos, they will equip themselves for their perilous expedition to the new frontier. The planetary civilization on Mars will be able to provide all the technology needed to successfully homestead an asteroid.

In all likelihood, these space pilgrims will join up in "wagon trains" for the trip out. Each band of hardy trail blazers will form their own society, agreeing among themselves on its structure and systems of self-governance.

Each family buys a launch pod and a cargo pod, both tanked up with enough fuel for rendezvous maneuvers. In addition to housekeeping basics, each adult carries the obligatory space suit— the only major piece of equipment brought from Earth. The group pools its resources to buy the essentials: a small ecosphere habitat (with CELSS), a semi-autonomous robotic miner, a pair of multi-purpose utility vehicles, a universal fabricator, and some power bubbles with thermionic generators.

The colonists arrange financing for their expedition long before leaving Earth. All the adults in the group have special skills, and like most people in the Third Millennium, they customarily work as telecommuters. In the group there are engineers, designers, writers, counselors, researchers, and consultants. All of these people work by logging on to interactive telecommunications networks. Most of their jobs are insensitive to short time delays, so they can work without regard to their location in the solar system. The data professionals in such groups have all been selected as much for their ability to work through telepresence as anything else. As telecommuters, they can continue to work in their careers despite being on their way to the asteroid belt.

This has important economic ramifications for the colonists. First, they do not face a cut off in earnings during the two years or so it takes them to reach their destination. Second, it means that they can continue to work and generate revenue after their arrival, even though otherwise isolated on a barren asteroid. This ability to participate in the general economy while physically isolated, is one of the most important factors enabling our rapid expansion into space.

InVestament Bankers

Financial institutions in the Earth-Moon system have very little interest in bankrolling colonists headed for the asteroid belt; too remote, too risky. There is, however, a ready source of funding. The first big colonial push out to the asteroids targeted the giant asteroid Vesta. Vesta orbits close to Mars and is a big asteroid with a unique and valuable composition. The original Vestans flourished, building their tiny world into a major economic force in the outer solar system. Very quickly, the Vestans realized that the highest returns could be made by financing other colonists who desired to settle the wide frontier of the asteroid belt. Psychological barriers and remoteness from the inner worlds, plus very real risks, kept home-world banks out of the game. This allowed the Vestans to charge very high interest rates for colonization loans.

The inVestament bankers understood that loans made to colonists were doubly lucrative. First, the colonists invariably consist of highly motivated groups of mature professionals. (This Darwinian selection is self-enforcing; groups not meeting the rigorous standards are simply not approved for loans.) These hard­core groups inevitably hit the ground running, and very quickly transform their new habitats into productive resource mines. The telecommuting professionals give the groups an economic underpinning which allows them to carry heavy interest burdens. New productivity from asteroid resources enables them to pay back their loans very quickly. The asteroid bankers are consequently able to turn their capital over rapidly, so the investment pool doubles in size every five to ten years. Second, each new group of colonists adds a jolt of synergy to the whole economy of the region, bringing with them demands for goods and services which can be supplied by yet other colonists.

This double-barreled effect—rapid capital turn-over plus exploding demand—creates a very powerful positive feedback loop in the economy. As the Vestans make more and more money on their loans, the investment pool grows, making it possible to make more loans, generating greater profits, in turn increasing the investment pool available for loans, etc. The result is runaway growth of real wealth.

The settlers find it fairly easy, therefore, to borrow substantial amounts of money with no more collateral than their resumes. Colonists typically head out to the edge of the new frontier with an ample stock of capital equipment and a generous line of credit at the FIBV (First Interplanetary Bank of Vesta).

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Space Family Robinson

(Let's join a group of these pioneers, and go along to see what it's like to settle a virgin asteroid.)

Long before their expedition even started out, the Robinson group had targeted their bit of celestial real estate. They selected a destination only after detailed consultations with the solar mining office. They chose an asteroid of moderate size—10 km. in diameter—orbiting in the middle of the belt. This particular asteroid is already owned by its original claimant who is in residence there. The leaders of the expedition have been in radio negotiations with the old prospector and have come to terms. The new colonists have agreed to pay the owner an up-front cash bonus and a royalty of 8% on all exports.

There are still many millions of unclaimed asteroids; but this is a group of families and they don't want to face the extra risks of an unknown planetoid. The Solar mining office has conducted extensive surveys and has detailed spectroscopic information on almost every asteroid larger than a few kilometers in diameter. Little else is known about the unclaimed asteroids. By contrast, this asteroid—Sykes 1011—has been pretty thoroughly evaluated. In order to hold his claim, Sykes has been "proving up" his asteroid ever since he first staked it fifteen years before. He has done extensive exploratory drilling, made detailed assays of his cores, taken seismic readings, and otherwise scrutinized his private little world.

The old prospector's diligence has paid off. The rich data base has served its purpose and has attracted a well-heeled group of settlers, Sykes, who has lived for a decade and a half in almost complete isolation, now anticipates a rich pay day. He is going to be amply recompensed for his years of unrequited labor. The prospector's risky investment will pay rich dividends for the rest of his life.

The colonists—named for the largest family, the Robinsons— have been willing to pay a premium price for Sykes' asteroid. His certified assays show that S1011 has an especially high water content of 18%, and unusually high concentrations of tantalum, and cobalt. For extra bonus money and a higher royalty, Sykes has been willing to assign all future development rights to the Robinson group. The Robinsons have done detailed modeling of the economics of their risky venture; they have good reason to believe that they too will ultimately enjoy rich rewards.

The Robinsons launch from Deimos in a replay of their flight from Avallon. Families are catapulted off in their individual pods, and the wave riders skim across the top of Mars' thickening atmosphere, picking up a slingshot boost from the red planet's gravity well. The pods shoot off at precisely calculated trajectories to rendezvous with an orbital habitat cycling between Mars and the main asteroid belt. Unmanned cargo pods, bearing precious equipment follow. After a few days in cramped discomfort on the wave riders, the colonists rendezvous with a cycling habitat. They dock their riders and move into temporary but homey quarters in the outbound ecosphere. There they live comfortably for the 15 to 20 months it takes to reach the asteroid belt. During that time, educations, and careers carry on, without much interruption.

After the cycling habitat reaches an optimal point in its orbit, the colonists will jump off to their destination asteroid. The entire journey has been carefully timed to minimize the duration and energy requirements of this final leg. Everything has gone according to the schedule—arranged years in advance. Delays at any stage of their odyssey could have stranded the travelers at some intermediate point for years. As it is, the colonists are in a perfect position to rendezvous with their chosen planetoid. The Robinsons again board their wave riders and make the short hop to Sykes 1011, arriving at the asteroid with only a few kilograms of propellant left.

Touching down on their new world is more like docking with a giant ship than landing on a planet. The gravity is less than a thousandth of that on Earth. The little convoy sets -down in the midst of a scene of splendid desolation. The horizons of the little world crowd close, unbroken by anything but Syke's radio strobe and a hand-made banner with "Wellcum Robinsuns" spelled out in strips of gold foil.

Like the pioneers of another age—who on arrival in their new Canaans, unhitched the oxen from the wagon and immediately yoked them to the plow—this new breed of settlers also sets promptly to work. The Robinson's group leader, Dr. Zachariah Smith, meets with Sykes—a grizzled old varmint who looks four centuries out of place. Sykes has spent 15 years on the asteroid, living in an empty propellant tank buried under the regolith. He has been utterly alone except for the company of a holographic dog named Rimmer. From the musky smell inside the old prospector's digs, Dr. Smith judges that Sykes hasn't been out of his space suit in the past decade. True to form, Sykes conducts the entire closing over the upturned bottom of a supply canister, never-removing anything but his helmet. 'How in the world,' Dr. Smith wonders, looking at the old character's stained beard, 'can any one chew tobacco gum inside a space helmet?'

With legal formalities out of the way, the first order of business is to launch power bubbles. The group has brought three thermionic generators, each with enough capacity to provide all of the colony's power needs. Fuel is pooled for one of the wave riders, and the generators are towed into close polar orbits around the asteroid. This takes just a small amount of propellant as orbital velocity is only a few meters per second. Once the generators are in stable orbits outside the asteroid's shadow, the bubble reflectors are inflated around them. Properly focused and adjusted, the generators begin to crank out power. The electricity is converted to a narrow beam of microwaves and is transmitted directly to receiving antennas strung along the asteroid's equator.

Useful Materials Produced by Robotic Miner Every Day

Element	kg/day
Oxygen	1,152
Silicon	576
Water	518
Iron	430
Magnesium	403
Carbon	57
Calcium	52
Sulfur	52
Nickel	32
Aluminum	45
Sodium	23
Chromium	12
Cobalt	5
Manganese	5
Nitrogen	5
Titanium	4
Phosphorus	3
Potassium	2
Copper	0.3

With a good supply of power now at their disposal, the colonists begin to transform their Lilliputian planet. They set up and activate the mining station. The robotic augers immediately begin shunting loose regolith into the maw of the machine. Inside the miner, the soil is heated in a plasma arc furnace. As the temperature rises, steam hisses out of the hot ore, and the vapor is pulled off and condensed. When the first silver trickle of liquid water begins to dribble into the transparent collection tanks, a cheer bursts from the jubilant colonists. At this point they know they have succeeded. Their years of sacrifice and hard work have paid off. They have water. Where there is water there is life. Now they know they can survive on this orbital slag heap. They have everything they need to sustain life and build a new world.

The robotic miner is not a large machine; its throughput is only a couple of kilograms per minute. Watching powdered regolith pass through the holding bin is like watching sand trickle through a big hour glass. Even so, the machine produces over 500 kilograms of distilled water and half a ton of oxygen per day. In a highly efficient process, the crushed asteroid ore is melted and most of its usable elements extracted. The molten rock preheats the incoming ore stream and is then extruded from the miner in the form of cooled bricks, slabs, and other structural components.

The colonists garner everything they need from the asteroid. At this stage of development, the principal commodities required by the colony: power, oxygen, and water, are available in abundance.

The next pressing project is to erect a temporary habitat. The temporary structure is just a large fabric bubble with a cable net thrown over the top. The colonists hastily level the bottom of a small crater with their utility vehicles. The bubble is inflated and regolith is shoveled in to cover the whole thing three meters deep. Though palatial by Sykes' standards, it will be rough living for many months, until a proper ecosphere can be built. The colonists move into their temporary domicile, pitching their family tents inside the Spartan space. The temporary habitat is not roomy, and is harshly lit by yellow tritium bulbs. It provides a shirt-sleeve environment nonetheless. On the face of this hostile bit of real estate, adrift in the void, even this rough bubble is a welcoming haven.

Establishing a self-sustaining ecocycle to produce food and recycle wastes is now the colonist's top priority. Coils of algae tubing are deployed and an ecocycle centered on a super critical water oxidizer is organized. At this point, the colonists are dug in for the long haul.

Now they can turn their attention to raising the colony's standard of living. The first step in that direction is the construction of a proper ecosphere. The surface of the asteroid, closely resembles that of the Martian moon Deimos. It is pocked with craters of various sizes, up to a kilometer in diameter. A crater 200 meters in diameter is chosen, and work begins to transform it into a permanent ecosphere. The robotic miner forms a ring of fused regolith around the crater rim while the utility tractors terrace the inner slopes. While forming the foundation ring, the miner extrudes bulk materials and some simple finished goods like reinforcing cable and anchoring bolts.

Refined silicon and other elements are fed into the uni-fab (universal fabricator) which produces the silicone bubble membrane. The uni-fab is a remarkable piece of machinery and represents one of the colony's most expensive and valuable capital assets. The uni-fab is capable of producing virtually any material or machine component. All it requires is a supply of the appropriate raw materials and detailed design instructions for its computer.

The fabricator uses MBE (Molecular Beam Epitaxy) technology to produce parts and materials. MBE is extremely simple in concept: a beam of charged atoms is sprayed onto a substrate, not unlike painting a car. Successive layers of atoms are beamed on until the desired thickness is built up. The composition of the molecular beam can be varied at will, as can the shape and thickness of the final products. With the right materials and instructions, the uni-fab can produce anything, from ball bearings to saran wrap.

The only materials the uni-fab can't produce are living tissues. You could feed in the appropriate instructions and materials needed to form a frog, but all you would get is frog soup.

The uni-fab can, however, easily manufacture just about anything else. It could readily produce all the parts of a camera, for example. You would still have to assemble the components, but all the lenses, fittings, and tiny screws could be manufactured by the uni-fab. Depending on its design, it could be a very fine camera. All the parts would be made with a finish and precision accurate to a couple of angstroms—the width of a single atom. The uni-fab enables the colonists to be self-sufficient in virtually all manufactured products, from toys to computer chips.

While the uni-fab produces the ecosphere's dome material, work progresses on crater preparation. When the inner terraces and anchoring ring are completed, the bubble membrane is installed, and inflation of the ecosphere begins.

An ecosphere 200 meters in diameter will require about half a million kilograms of oxygen. It will take the robotic miner a little over a year to produce this much air. As the ecosphere is inflated, the crater is terraformed to provide a life-rich habitat. Trees, grass, and flowers are planted in profusion. An open stream meanders across the terraces and slowly cascades down the slopes to a small pond. A fountain sprays water in dramatic slow-motion arcs. The soft splashing of water gently falling through the micro gravity will fill the interior with the unmistakable sounds of Earth. Slowly, the robotic miner will produce enough water to form a water shield for the crater dome. When complete, the shield will allow the colonists to pitch pavilions in the gardens and live in the open, among the eight acres of grass and trees under the dome.

The Robinsons will construct the first ecosphere for somewhat the same reasons that Sykes did his core drilling—to attract new settlers. With the ecosphere completed, the otherwise barren little asteroid now beckons with the welcoming green glow of a miniature Eden.

New colonists, looking for opportunities, but less willing than Sykes or the Robinsons to face risk and hardship, will be attracted to the new habitat. This first permanent ecosphere is easily large enough for dozens of families. Several groups of immigrants join the growing colony. This third wave of settlers does not face the harsh wilderness that greeted old Sykes and the Robinsons.

These newcomers can move right in to a comfortable habitat, resuming their lives without much of an interruption. Since the hazards and discomforts are low, individual families, even single people, will be able to immigrate to the new colony.

Despite this lack of hardship, the new colonists will enjoy a large share of the asteroid's abundant wealth. The new comers will pay cash bonuses to the Robinson group. They will literally be buying a 'piece of the rock'. In return, they are supplied with dwelling space, food, water, energy, and amenities. The new arrivals also get a stake in the colony. They earn a position in the corporate identity of the colony and a share of future royalties and profits.

As more and more colonists arrive, the earlier waves grow wealthier. Sykes becomes rich as Midas, but he never does move out of his rickety fuel cylinder. The Robinson group took a big gamble and hit the jackpot. The Robinson's and the other foun­ding families become enormously wealthy, building their own private compounds in some of the asteroid's choicest craters. For generations to come, the descendants of these pioneers will enjoy the status and bank accounts that go with "old money".

New Bern

As the growing community accrues wealth, dramatic new projects can be undertaken. The first of these will be—as on most new worlds—the construction of a mass launcher. The launcher will be built almost entirely of local materials. For example, low-temperature superconducting electromagnets can be made of Alnico—an alloy of aluminum, nickel, iron, and cobalt. All these metals are available on the asteroid. Ultra-low temperatures can be maintained in the magnets by shielding them from radiant heat under a blanket of vacuum insulated regolith and by circulating liquid hydrogen at -252°C. Once the mass driver is complete, the asteroid colony can begin to export commodities profitably.

There will always be an insatiable demand for water and other light elements back in the Earth-Moon system, but importing hydrogen from the asteroids will not be cheap. It requires a change of velocity (Δv) of 11 kilometers per second (kps) to move payloads from the inner asteroid belt to the Earth's orbit. That is the same Av required to lift payloads off the Earth. This fact will put a premium on the cost of living in the vicinity of the Earth. Escaping this cost penalty will be one of the motivations fueling immigration to the asteroid belt. In the self-sufficient colonies of the asteroids, the cost of living will be attractively low. There will always be people willing to pay the premium to live near the Earth, however; so the thirst for hydrogen from the asteroid belt is apt to remain unquenchable.

The colonists on Sykes 1011—now renamed New Bern— produce elemental hydrogen which they liquefy and hurl into space in vacuum insulated canisters of chromium cobalt alloy. The canisters need not be propelled to 11 kps—a very energy intensive proposition. Instead, they are impelled at a few tenths of a kps onto an orbital path that rendezvouses with the large asteroid Ceres. On Ceres there is a large interplanetary mass driver with huge cargo capacity. Hydrogen, metals, and other commodities from all over the asteroid belt are consolidated into bulk shipments. Big cargo carriers are flung off on journeys to the inner planets that can take years. The colonists' shipments of hydrogen, vitallium and other commodities are automatically credited to their accounts on Vesta.

The economy of New Bern thrives. Underpinned by the work of telecommuters, and supplemented by commodity exports, the colony grows richer. The colonists invest heavily in semi-autonomous factories. Highly specialized products are manufactured for niche markets all over the Solar system. The colonies on New Bern specialize in precision medical instruments and implants. Their vitallium endoskeletons come to be highly prized by people going through trans-geriatric metamorphosis.

New ecospheres spring up. All the best crater sites on the asteroid are domed over. As new immigrants arrive and children are born, the population burgeons. There is no shortage of room on the asteroid. It seems a tiny world, but it is big enough to accommodate a large city. The surface area of the asteroid is 314 million square meters. That is enough room for a population of three million people. It's hard to fathom, but even this many colonists wouldn't be crowded. Each person would have as much room as the marine colonists on Aquarius, where almost 40% of the area is dedicated to park land and open space. The same ratios would apply on the surface of the asteroid. Half the surface area could be dedicated to gardens, lakes, playing fields, and other open spaces, and there would still be ample room for a large population. All the industry, and much of the colony's supporting infrastructure, is put underground. The surface is left free for living.

Eventually, the whole surface of the asteroid is enclosed inside an ecosphere. With a membrane surrounding the asteroid, the whole surface can be terraformed and inhabited. Lush plant life will cover the asteroid's surface. People will live in their pavilions, set among the trees and flowers blanketing the once barren landscape.

A water shield for an ecosphere 12 kilometers in diameter would weigh 2.25 billion tons. This would require just over two percent of the asteroid's water supply. Inside the bubble membrane will be an oxygen atmosphere amounting to 86 million tons—requiring only a tiny fraction of the asteroid's oxygen.

The ecosphere will provide a tremendous volume of livable space. At some places, the asteroid's 'sky' will be two or three kilometers high. Trees will be able to grow thousands of meters tall, dwarfing even the Never-trees in the craters of the Moon.497 In the minuscule gravity, flight will be almost effortless. As in Lothlorien, many people will live in the branches and hollow trunks of the gigantic trees. Swiss Family Robinson will have come full circle.

As mining progresses, the asteroid's interior will become honeycombed with caverns and tunnels. Robotic miners will cut through the rich carbonaceous ore of the planetesimal like termites boring through fruit cake. Even after the asteroid is fully enclosed in an ecosphere, mining will continue deep in the interior. Exports will be spit out through rigid magnetic launch tubes that penetrate the outer shield membrane. Over time, the interior of the asteroid will be hollowed out and terraformed.

A large fraction of the asteroid's bulk will be converted into the living substance of people, plants, and animals. Every kilogram of water or metal ever removed from New Bern will be found somewhere in the solar system: bound up in the radiation shield of some other colony, rustling in the leaves of a tree, or coursing through the veins of a child. Nothing will be wasted, but everything will be transformed.

Over time, New Bern will mature into a vibrant miniature world. It will possess a unique history and culture all its own. The process that creates New Bern will be played out all over the asteroid belt at various times. The transformation of the belt will begin in the inner zones closest to the Sun and will proceed apace until the remotest asteroids have been settled, encapsulated, and terraformed. As the human population grows, each planetesimal will become the center of an expanding community, nourished and sustained by its asteroid's resources. As Solaria blossoms and ripens, the entire asteroid belt will become thickly sprinkled with free-floating ecospheres and terraformed asteroids.

Ah, the asteroid belt. There’s not a lot to see here; the view out of our windows as we arrive looks about as empty as everywhere else. And yet, this bleak void is the birthplace of humanity’s space colonization efforts. These boring space rocks were gold to the early spacefarers – well, platinum to be specific.

The Early Mining Colony

We set down in a hollow asteroid barely a thousand years after humanity first began to explore the void. Even now, we’re traipsing about in a tourist trap. The mineral resources and strategic location of this rock are a thing of the past; the cramped living quarters and maintenance tunnels are packed with floating tourists from Earth and Mars.

The tour guide tells us that “colonies” such as this weren’t that impressive at first; just temporary quarters for highly-paid remote-mining equipment operators. (A bit of a gender politics question: men are overwhelmingly overrepresented in very high-risk occupations, and suffer far more work-related injuries. In a (presumably) much more egalitarian future, will this still be the case? What will the gender breakdown of the early asteroid miners look like? Just one of those fun questions sci-fi writers get to answer.) Over time, the living space grew and the meager stations evolved into full-fledged communities. Yet even now, only a handful could be considered comfortable or urban – and only the largest bother producing their own centripetal “gravity.” We travel through the large common area on guide ropes strung between rooms. We could “free-fly” in this low-gravity environment, like the ancient miners did, but the last thing the station managers want is a bunch of landlubber tourists with no sense for gravity-free environments literally bouncing off the walls.

Traveling In Style: Inside an Asteroid

We’re done with this tourist trap; there’s more than one asteroid colony, and we’re sick of hearing tour-guides talk about the “ancient space miners” – we want to see some real mining in action. And if you want action these days, you head to Vesta.

How are we getting there? Well, we could just take our spacetimeship, but while we’re here in the asteroid belt, we might as well see what it’s like to travel in an actual asteroid. Eventually we find what we’re looking for: a colossal, spinning stone cylinder with a huge docking port on one end. We transmit credit information, fly through a series of airlocks, and find ourselves in a vast green-and-blue tube. We’re in a spinning paradise where travelers en route to Vesta can enjoy a quiet stroll through the garden, or gaze up at the “stars”: overhead streetlights and lanterns. The gravity is low-tech and the ride is slow, but nothing beats a hollowed-out asteroid for safety (putting half a mile of solid rock or ice between you and the radiation-rich void of space is a good idea for a lot of reasons), efficiency (carving out the middle of an asteroid and throwing an airlock on the top of the hole is a lot easier than constructing a starship from scratch – plus, you can sell all the platinum you just dug out"), and luxury (did I mention the view? The view of a tiny garden world curving all around the horizon is impressive, if a bit disorienting. Halo players may have a feel for what this looks like.).

Getting a Glimpse of Vesta

When we visited Mars, newswaves were abuzz about the Vesta (4 Vesta is the second-most massive asteroid in the Asteroid Belt, and only one of two with a differentiated metal core: it has an outer rocky layer covering a deliciously lucrative metal center.) project. And now, gazing at the camera monitors that pass for windows here, you see why. You’re witnessing the largest-scale mining operation in the history of mankind: the complete disassembly and processing of Vesta’s metal-rich core. It’s an unfathomably vast reservoir of raw metal, but considering the scale of some of humanity’s recent undertakings,(enormous solar panels near the Sun, colossal space stations orbiting Earth like extra moons, generation ships for traveling to a recently-discovered Earthlike planet many light-years away, etc.) it’s only just enough.

We dock at a half-asteroid space station with a good view of the action, and chat up some miners over dehydrated ice cream sandwiches. They’re really more like remote-control robotics operators and zero-G detonation experts than miners; actual humans almost never set foot on half-disassembled Vesta itself. Most of the personnel here are international observers, safety bureaucrats, heritage engineers extracting and safeguarding Vesta’s old colonies, and protesters from HACS: the Historical Asteroid Conservation Society. As we chat, we see bright flares in the distance. The miners fill us in: these are carefully positioned detonations for weakening and blasting away Vesta’s crust, and shuttles firing their engines to intercept any large pieces that drift past the perimeter control drones.

Skipping Ahead to Full Terraforming

It’s been a fun trip so far, but nothing in the current asteroid belt seems terraformed per se; some of the hollow asteroids are filled with little scraps of wilderness, but they’re more like terrariums (Terrarriums are exactly the name Kim Stanley Robinson gives these hollow-cylinder asteroids in his novel 2312. This novel wasn’t the first to feature mobile, Earth-like asteroid habitats, but it arguably discusses them with the most rigor and detail.) or zoos than whole worlds with ecosystems. (The lack of natural sunlight doesn’t help matters.) Even Ceres, largest and most planet-like of the belt asteroids, is too small for meaningful terraforming; there’s a large dome city and a few low-gravity skiing resorts, but nobody expects that it will ever resemble Earth in any meaningful way.

We’ve traveled space long enough – it’s time to travel, well, time. Let’s visit a small asteroid colony in the far-distant future. Skeptics, are you ready to be pedantic and annoying? Because we’re going to a blue-and green EARTH-LIKE asteroid. Sorry, but we’re going there. On the other hand, if you like The Little Prince or Super Mario Galaxy, this is the trip for you.

It’s the distant future’s distant future. The current biohacking trends make humanity nearly unrecognizable. Being entirely blue is in fashion now; last year it was scales and prehensile hair. Impossibly large solar collectors ring the sun… mostly as relics, since everyone uses direct matter-to-energy fusion cells now. We’ve passed a number of ships that look like ancient sailing vessels, complete with a full crew on deck. It’s hard to tell whether they’re air-agnostic androids or if the ship is surrounded by a pocket of air held in via an invisible shield. It looks like these “sailing” ships, the space whale, and dozens of other oddities that hardly appear spacefaring to our ancient eyes are headed to the same place we are. TerraTwin Corporation has invited the whole Solar System to witness the unveiling of their latest vanity project: the smallest-ever fully habitable Earth!

They have the resources, the talent, the power, and most importantly, the tech. Gravity is now bound to humanity’s leash. (A staple for most space-based sci-fi, even the near-future and semi-low-tech settings (for example, Firefly and Battlestar Galactica) have largely mastered gravity. The science is a bit iffy, but less impossible than FTL travel. Simple action-at-a-distance gravity control is possible if we combine a complete understanding of gravity with cheap fusion power; if we’re lucky, this could happen less than a century from now.) This has a number of bizarre implications, and we’re about to see the latest example. The company de-lenses the light just outside the port window, revealing… the Earth. Well, a version of the Earth that’s barely ten times the size of your ship.

It’s a legacy model; Greenland and Antarctica are white with what looks like real snow (the snowman on Greenland is a nice touch). Small cottages dot the tiny “continents” where the major cities would be: the vacation homes of ultra-rich celebrities and TerraTwin executives.

We’re allowed to land – because hey, who’s going to say no to time travelers (You have such nice, loving grandparents; it would be a shame if they’d never met.) – and we find the world to be both gorgeous and disorienting. This place is NOT for the agoraphobic; the horizon falls away much too quickly, giving the sensation of being atop a hill with nothing but space below. The gravity and the sky seem exactly Earth-like, but as you pick up the Rock of Gibraltar and give it a throw, you see it follow a bizarrely curved trajectory – it disappears behind the horizon before you can see where it landed.

More curious is the Sun. We’re in the far reaches of the Asteroid belt, yet the sun looks as big and bright as it does on Earth. Now the little moonlet we saw orbiting this miniature Earth makes sense: it’s an orbiting spotlight – an artificial sun. After all these years, humanity has decided to spite Galileo and prove Aristotle right. (Aristotle’s heliocentric theory of the solar system (whereby the sun and all planets orbited the earth) was so influential that it even became enshrined in Catholic doctrine. Galileo was put under house arrest until he died when he spoke out against this theory.) This fake sun orbits in such a way that it covers up the actual sun (two suns would break Earth canon, you see).

The asteroids may be barely-noticeable space grit in the 21st century, but their mineral wealth (mostly their abundance of platinum-group elements, which are exceptionally rare on Earth) and accessibility predestines them to undergo the Solar System’s most remarkable transformations. Over the centuries, they will be mining outposts, military bases, tourist traps, enormous hollow ships, quaint homesteads, impossible-looking cities, and even gardens.

Here's a quick analysis for planning and realism purposes for manned exploration of Jupiter's Galilean satellites:

Satellite	Daily radiation rate (Sv)	Yearly rate (Sv)	Single-year limit	5-year cumulative limit	Mild sickness	LD 10/30	LD 35/30	LD 50/30	LD 60/30	LD 100/14	LD 100/7	LD 100/2
Io	36	13149	2 min	4 min	20 min	40 min	1.33h	2 h	2.67h	4 h	6.67h	33.33h
Europa	5	1972	13.33 min	26.66 min	2.22h	4.44h	8.89h	13.33h	17.78h	26.67h	44.44h	-
Ganymede	0.08	29.22	15h	30h	6.25d	12.5d	25d	37.5d	50d	75d	-	-
Callisto	0.0001	0.037	1.37y	2.74y	-	-	-	-	-	-	-	-

The single year limit is 50 mSv, while the maximum 5-year cumulative exposure is 100 mSv (or 20 mSv per year). LD stands for Lethal Dose, LD x/y means "x" percent of individuals die within "y" days. LD 50/30 thus means half of people exposed at this level of radiation would die within 30 days. Some values have been omitted as the time required to reach the lethal dose level exceeds the time for lower lethal dose rates to be achieved or, in the case of Callisto, because it would simply be unreachable with the low radiation rate observed.

A few observations point to the obvious:

Astronauts on Io would experience fierce radiation flux, reaching their maximum cumulative radiation dose accepted by various safety regulations for a period of 5 years in only 4 minutes outside on the surface!!! Europa isn't much better, with less than half an hour of exposure. Ganymede is also a fierce environment, despite radiation being much less intense than at Io. With minimal shielding, Callisto would provide a safe working environment no worse than a nuclear power plant or research facility.

For shielding purposes to limit exposure to below regulatory levels for 5-year periods, the number of halving thickness of shielding material stands as follows:

Satellite	Number of shielding layers	Liquid water thickness
Io	20	3.6 m
Europa	17	3.06 m
Ganymede	11	1.98 m
Callisto	1	18 cm

As ice is less dense than water, about 10% more would be required.

• 

Interstellar Nomads

      But suppose all this is fantasy. Suppose that fastship (relativistic starship) travel is impractical even to the solar society our descendants might create in the next few centuries. Will humanity be forever bound to this one tiny corner of the Galaxy? We think not. There is another road to the stars, a road paved with comets, on which journeys between stars will take a few thousand generations. If there are no fastships, this slower human migration might fill the Galaxy in half a billion years.

     By far the most numerous objects in the Sun's family are comets. The Sun probably has several trillion, most circling our star far beyond the orbit of Pluto, forming a great spherical cloud, the Oort cloud, named after the Dutch astronomer who deduced its existence in 1950. From time to time a relatively near encounter with another star perturbs the orbits of a few comets. Some are sent spinning off into interstellar space. Others are nudged toward the inner Solar System where, a million years later, chance encounters with the giant planets may send the comets out of the Solar System like the Voyager spacecraft. During the last century the Solar System has definitely lost several dozen comets and probably very many more. If this process is common in the Galaxy, interstellar space is liberally strewn with comets. The average spacing is probably of the order of ten times the Earth-Sun distance, about 10 astronomical units (AU), or 80 light-minutes. As Freeman Dyson has suggested, if interstellar comets are common, “the galaxy is a much friendlier place for interstellar travelers than most people imagine.” We imagine then a human population living off cometary resources and diffusing slowly out into interstellar space.

     Before we describe how this migration of a people we might call interstellar nomads would begin, let us describe the resources available from the comets and how the scarcity of those resources might shape the nomad communities.

     A typical comet may not seem very hospitable, being an irregular chunk of ice and rock perhaps a kilometer or two across. In fact, we don’t imagine that people will actually live on the comets but, rather, use them as a source of raw materials to build habitats and supply the other physical necessities of life. Except for hydrogen, which will be underrepresented, and the chemically inert noble gases like helium, which will be very scarce indeed, the naturally occurring elements will be present in roughly cosmic abundances, a vast supply of material resources. The greatest obstacle facing potential nomads is energy, which is very scarce in the interstellar deep. Nonrenewable sources would include deuterium to power fusion generators and the kinetic energy (energy of motion) of the comet, which could be extracted through interactions with the galactic magnetic field. Renewable energy sources would include starlight collected with gigantic mirrors and possibly cosmic rays (if anyone can figure out a practical scheme for catching them). Elsewhere we have estimated that the aluminum in a typical comet would be sufficient to build mirrors to collect a few hundred megawatts of starlight. Other, more abundant substances may prove to be more applicable for the mirror surfaces. We expect that power levels of 1 megawatt (MW) per person are reasonable and therefore a typical comet could support a few hundred people with starlight. A megawatt is about fifty times the current per capita use in the United States.

     Energy is by far the scarcest resource in interstellar space; together with social and genetic factors it will determine the size and structure of the communities. There is energy to permit the growth of very large communities. Comets have random velocities of about 10-20 km/sec, mimicking the random velocities of the stars that eject them. In a sense the comets are the molecules of a gas stirred by random gravitational encounters with the stars embedded in that gas. Comets could be gathered into large clusters if velocity changes could be imparted, rather like maneuvering two spacecraft for a rendezvous in Earth-orbit. Only a few percent of the deuterium in the cometary ice would be needed to bring a new comet into a cluster. Such clustering would be necessary in the case of the relatively numerous small comets, which might not be able individually to support a self-sufficient community.

     Clusters could not grow without bounds for the simple reason that the gathering process would soon deplete the stock of nearby fresh comets. Imagine, for example, that the islands of the Pacific float and that before the Polynesians arrived the islands were distributed at random. Suppose then that the Polynesians wanted to live in island clusters and could drag the islands together. As the groups grew the average spacing between them would grow as well. In principle, all the islands could be clustered at mid-ocean but that would involve dragging a large number of islands long distances from the periphery of the ocean. A more efficient process (and one that would promote genetic and cultural diversity) would be to form a number of relatively small groups scattered over the ocean. The time and cost of gathering would decrease dramatically.

     Just how large the interstellar comet clusters would grow will depend on subtle factors; we offer only crude estimates. The starlight mirrors would be very large; each 1 MW mirror would be about the size of the continental United States. Although the mirrors would be tended by autonomous maintenance robots, the nomads would have to live nearby in case something went wrong. After all, the mirrors and the energy they supply will be vital to the well-being of the community. Although we could imagine that the several hundred people who could be supported by the resources of a single comet might live in a single habitat, the mirrors supporting that community would be spread across about 150,000 km. Trouble with a mirror or robot on the periphery of the mirror array would mean a long trip, several hours at least. It would make more sense if the community were dispersed in smaller groups so that trouble could be reached in a shorter time. There are also social reasons for expecting the nomad communities to be divided into smaller co-living groups. For reasons perhaps stemming from our distant past, we seem to function best in groups of about a dozen adults, be it a hunter-gatherer band, a bridge club, an army platoon, a political cell, or any other of numerous examples. We imagine then that people would live in bands of about twenty-five men, women, and children, each band occupying a spacious habitat constructed from cometary materials. They would tend (with considerable help from robots and computers) a farm of starlight mirrors stretching across perhaps 30,000 km of space (fig. 5.3).

• 

• 

• 

     We model our minimal cometary unit on the oldest of human examples: the hunting and gathering band, the social organization within which our ancestors lived for millions of years. As first pointed out by Joseph Birdsell in his studies on Australian aborigines and illustrated by Richard Lee’s sketch in this volume of the San people of the Kalahari, among surviving hunting and gathering people around the world these bands averaged roughly twenty-five men, women, and children. Within a band of twenty-five there are apparently enough men and women to effectively carry out the vital subsistence and childrearing functions, yet the band is not so large as to become unwieldy or fragmented when ranging over a hunting and gathering territory in search of scattered resources. However, although in everyday life these bands were essentially independent from one another, of necessity they formed into larger breeding communities averaging, on continent after continent, around 500 individuals. These tribal groupings, typically marked by dialect as well as gene-flow boundaries, with careful attention to marriage rules prescribing that mates must be sought among distantly related persons from outside the natal band, appear to be just large enough to avoid the genetic dangers of inbreeding and also to provide a seemingly necessary and enriching larger social environment. Thus, we expect that cometary bands would also cluster for purposes of healthy breeding and social enrichment in tribal groups of at least 500 citizens. We imagine clusters of some 20 cometary bands, the members of which would exchange marriageable youths and periodically meet to celebrate rites of passage, for calendrical observances, and for other social occasions.

     Because of the great size of the energy-gathering mirrors, the members of such a cometary tribe, living within the constituent bands, would spread across some 200,000 km of space (fig. 5.4). Assuming power levels of about 1 MW per person, a tribe could easily live off the resources of a single typical comet or a few smaller singles (the smaller comets being more numerous) gathered into a cluster. The habitats and mirrors would ‘fly in formation’ with the comet cluster. Although the bands would live in separate habitats, they would gather at frequent intervals for tribal ritual. Should someone want to leave the tribe entirely, there would be opportunities every decade or so to transfer to a passing tribe. Such transfers would be rather expensive, because they would involve large velocity changes, but would not be beyond the energy means of the group. Such inter-tribal transfers might be fairly common if they serve to strengthen social ties between groups that might want or need to exchange information or goods. Of course, the large relative velocities (10-20 km/sec = 2-4 AU/yr) between separate tribes would mean that they would be too far apart for physical exchanges after only a few years. However, close communications by electromagnetic means might persist for many generations.

     This proven band-tribe structure is the smallest grouping that seems practical for any human community. Although, as we have just mentioned, information exchange would always be possible with a far larger number of people by sophisticated electromagnetic communications channels, larger communities may also develop. Perhaps the diversity of skills needed to support an interstellar community will require larger groups. However, estimates of the energy cost of travel across a sizable community suggest that the largest practical population is of the order of 100,000 people. A one-day trip across such a “city” would cost about a megawatt-yr. If, as we have assumed, power generation is of the order of 1 MW, frequent trips across such a city would strain the resources of the community. We therefore suggest that communities of interstellar nomads, living off the resources of clustered comets, will range in size from 500 to no more than 100,000.

     How then might this comet-based human migration out of the Solar System proceed? We imagine that within a few hundred years exploitation of Solar System resources will have carried some adventuresome representatives of our species out to the Oort cloud. Because the Oort cloud comets are relatively close to the Sun, rather large communities could be supported by sunlight. O’Neill has estimated that the mass of solar collectors would have to equal the habitat mass at about 2,000 AU, well into the Oort cloud. There the interstellar comets may be sprinkled at a ratio of about one per several thousand solar comets. As the Oort cloud population grows, there will be a temptation to hitch a ride on an interstellar comet. The pioneers might well be a small group of young adults attracted by the adventure of it all. They would drift outward slowly, at about 2-4 AU per year. The group might grow rather quickly (as Birdsell suggests) and would soon be faced with the choice of group fissioning or gathering other comets. We suspect that both options would be exercised from time to time. Eventually, there would be an expanding sphere of nomads. At the frontier the gathering and fissioning options would both be possible, whereas closer to the Sun gathering and fissioning would end and some sort of steady state, a mix of large and small communities, would arise.

     After about 50,000 years something dramatic may happen.

     When the Vikings and, later, Columbus crossed the Atlantic, two branches of the human family that had been isolated for a similar period were reunited. It was not, of course, a trouble-free reunion. If, as we suspect, fastship travel is practical in some form, long before the interstellar nomads will have gotten very far out of the Solar System, fastship pioneers will have founded settlements in the nearest stellar oases. A thousand years later or so their descendants will have begun a cometary migration. About 50,000 years from now these two populations should meet roughly midway between the Sun and the nearest stars.

     There are probably good science fiction tales to be told about these meetings. The cultural and genetic diversity implicit in a large number of small communities suggests that the parties will find one another strange. However, we do not expect any surprise encounters. Because the comets drift at only a tiny fraction of lightspeed, the two flows of nomads should be well aware of each other’s progress. Communication between approaching communities would be possible long before physical contact. Perhaps our descendants will have learned something from the sometimes disastrous historical contacts that Crosby describes. We leave the implications to science fiction and to posterity.

     We have described in general terms how human beings might make a living as nomads wandering in the interstellar deep. We have tried to be cautious about extrapolations from present technology and have stayed close to admittedly sketchy modern concepts of interstellar resources. Just as the Solar System has proven to be a very different, but still wonderful, place from what we had suspected just a few decades ago, the interstellar deep will likely be stranger than we imagine—but perhaps more marvelous too and more hospitable to humankind.

Colonization of the Galaxy

     We began this essay with Clarke’s three laws of prophesy. The Master cautions against timidity. We could have indulged in flights of fancy, say, the warp drive that speeds Captain Kirk’s Enterprise from one adventure to the next. Perhaps such things are in the cards. Certainly, the universe has many surprises in store for us.

     However, our immediate purpose has been to explore the possibility that the very ancient human migration that began so long ago in East Africa might take our descendants beyond the terrestrial cradle through the Solar System to the stars. Very advanced technology (Clarke’s “magic”) might reduce the exercise to trivialities. We have tried to show that interstellar travel by one or both of two basic methods is feasible. We think that modest extrapolations of current trends in technology—in genetics, computers, and energy use—lead to the conclusion that interstellar migrations are a plausible expectation. Journeys to the stars and the establishment of viable human communities beyond the borders of the Solar System require no magic—only time, skill, patience, and continuity with the past.

      After spreading throughout the warm. compact inner core of the Solar System, the human race will face the daunting challenge of crossing the deep ocean of space that lies between Sol and even the nearest neighboring stars. But just off shore, at the outer fringe of Sol’s gravitational reach, countless small icy islands are scattered in a vast spherical archipelago reaching almost halfway to the Sun’s nearest neighbors—the myriad comet nuclei of the Oort Cloud.

     Human exploration and settlement of the Oort Cloud will create a new environment for a diverse array of social, economic, and biological experimentation, and that diversity may indicate the future course of the evolution of our species. The Oort Cloud will also be the proving ground for the technology of interstellar travel: these tiny, scattered islands on the periphery of the Solar System may be our first stepping stones to the stars.

     Although the notion of human settlements on comet nuclei dates back to the eighteenth century, it was not until the latter part of this century that the concept began to receive serious attention. In 1972, physicist Freeman Dyson offered this challenging vision:

It is generally considered that beyond the Sun’s family of planets there is absolute, emptiness extending for light-years until you come to another star. In fact it is likely that the space around the Solar System is populated by huge numbers of comets, small worlds a few miles in diameter, rich in water and other chemicals essential to life … comets, not planets, are the major potential habitat of life in space.

     More recently, astronomers, biologists, sociologists, historians and design engineers have begun to subject the idea to rigorous—if somewhat speculative —scientific and engineering analysis. They have begun to explore the very real possibility of establishing human settlements in the distant reaches of the Oort Cloud, and what life might be like there.

     The Oort Cloud is thought to form a vast, spherical collection of small icy bodies orbiting at enormous distances from the Sun. Although direct observational evidence is sketchy, there are sound theoretical reasons for postulating its existence.

     It is generally believed that the comet nuclei in the Oort Cloud formed along with the inner Solar System about 4.6 billion years ago. In the most widely accepted theories, the Solar System originated in the gravitational collapse of an immense cloud of interstellar gas and dust. As it collapsed the cloud flattened out into the classic solar nebula, a broad, fairly thin disk of material slowly rotating about the infant Sun.

     Within the nebula, the gas and dust particles began to condense and clump together into small aggregates that grew slowly into larger and larger bodies, forming planets, moons, asteroids—and comets.

     Most comets are thought to have formed on the outer fringe of the solar nebula, at about the same distance from Sol as Uranus and Neptune (roughly 20 to 30 Astronomical Units). The bulk composition of these two planets is close to what would be expected if a similar mass of comet nuclei were collected together into a single, planet-sized body.

     In another line of evidence, most comets seem to be significantly depleted in carbon relative to the average cosmic abundance of that element. This can be interpreted as an indication that comets formed in a region of the nebula warm enough to evaporate much of the methane (CH₄) present and prevent it from condensing with the remaining gasses and dust. As expected, comets are also badly depleted in hydrogen, as are all the other bodies in the Solar System except for Jupiter. Saturn, and the Sun.

     Most of these small bodies would be swept up into the growing planets, but the orbits of many others would be perturbed and altered by close encounters. An encounter with Jupiter or Saturn would most likely result in complete ejection from the Solar System into interstellar space. In the vicinity of less massive Uranus and Neptune, comet nuclei would also be perturbed into orbits far from the Sun—but not entirely beyond its gravitational reach.

     Initially,the comets would orbit more or less in the same plane as the inner system, but over time the faint tug from passing stars would randomize their orbital inclinations, creating a vast, dispersed, spherical cloud of small icy bodies—the Oort Cloud, named for Dutch astronomer Jan Oort, who first postulated its existence and structure in the 1950s.

     Estimates of the total number of comet nuclei in the Oort Cloud range from about l00 billion to several trillion—more than the total number of stars in the entire galaxy. Their total mass is difficult to estimate with any precision, but is probably at least equal to the mass of Earth, and may be much greater. It is thought that the original population of the Oort Cloud was about 2 trillion comets with a mass about 30 times that of Earth, but has since been depleted by escape into interstellar space and perturbation into orbits closer to the Sun, where the icy bodies evaporate after a time.

     The majority of the comet nuclei in the Oort Cloud are probably distributed throughout a broad spherical shell between 40,000 and 60,000 A.U. from the Sun (although some astronomers place the density peak a bit closer, at around 25,000 A.U.). This is just shy of one light-year (64,000 A.U.). It is possible that a substantial number orbit much closer to the Sun, between 100 and 10,000 A.U., forming an inner Oort Cloud. Within about 10,000 A.U. of the Sun, the orbits of the comets would lie more or less in the plane of the ecliptic. Beyond that distance, the influence of passing stars is strong enough to produce the random spherical distribution of the outer Cloud. The outer periphery of the main Cloud is probably at about 100,000 A.U.—close to two light-years. The maximum dynamic limit is about 200,000 A.U., where the gravitational pull of the galactic core begins to overcome that of the Sun.

     The typical separation between comet nuclei in the Oort Cloud is probably about 20 A.U.: very widely spaced indeed. In the main Cloud the orbital periods would range from one to ten million years.

     Since the close encounters of Halley’s Comet by Soviet and European spacecraft in 1986, a clearer picture of the nature of comet nuclei has begun to emerge. They are estimated to range in size from 0.1 to 10 kilometers in diameter and in mass from 10¹³ to l0¹⁹ grams—between 10 million and l0 trillion metric tons. Though small in comparison to the planets, comet nuclei still contain huge amounts of material.

     Comets are thought to be made up of an extremely light, porous aggregation of the ices of various volatile elements and compounds, with a scattering of embedded dust and complex organic molecules. The ratio of dust to gas is estimated to range from 0.7 to 0.85 by mass.

     The volatile component is thought to consist chiefly of water ice but also includes CO, CO₂, methane (CH₄), ammonia (NH₃) and a variety of other compounds containing oxygen, sulfur and nitrogen. The ices are thought to include a high proportion of clathrates, in which other molecules are trapped inside the lattices of the frozen gases. The grains of dust and organic material may be remnants of the interstellar cloud or the condensing solar nebula. A break- down of the estimated abundances of the chief elemental and chemical constituents of a typical comet is presented in table 1.

TABLE 1
Composition of Comet Nuclei: Estimated Abundances and Distributions

A. Elemental Abundances				B. Mass & Volume Distribution of Principal Components
Element	Number (%)	Mass (%)		Component	Mass (%)	Volume (%)
H	43.9	4.2		Silicates	21	8.6
C	6.7	7.7		Carbon (graphite)	6	3.4
N	2.7	3.8		Very Complex Organics	19	21.0
O	40.2	62.5		H2O	19	27.0
S	1.0	3.1		CO	10	13.0
Mg	1.9	4.5		Other	25	27.0
Si	1.8	4.9		Other Volatiles: CO2, CH4, N2, NH3, HCN, etc.
Fe	1.6	8.8
Ni + Cr	0.1	0.6
Al	—	0.4

     The average density of cometary material is estimated to be about l to 1.5 g/cm³—close to that of water (1.0 g/cm³). This material is thought to be a porous matrix of solid grains and larger particles, with the gaps between them filled with ice. Comet nuclei are likely to be irregular in shape, uncompacted and undifferentiated: for a body under about 10 km diameter, the force of gravity is simply too weak to affect shape or structure to any significant degree. The aggregate would have a light. very fragile structure resembling that of windblown snow. Both structure and composition are expected to be fairly homogeneous throughout, although the surface layers (to a depth of perhaps 10 m) may undergo some alteration due to sublimation of ices, irradiation by cosmic rays and other radiation, and occasional sand-blasting from interstellar gas and dust.

     This light fragile structure means that the resources present in the comet nuclei will be readily accessible to any human settlers. The porous mixture of dust and ice would offer little mechanical resistance, and the two components could easily be separated by the application of heat. Volatiles 'could be further refined through fractional distillation while the dust, which has a high content of iron and other ferrous metals, could easily be manipulated with magnetic fields. Little additional crushing or other mechanical processing of the dust would be necessary. and its fine, loose-grained structure would make it ideal for subsequent chemical processing and refining. Comet nuclei thus represent a vast reservoir of easily accessible materials: water, carbon dioxide. ammonia, methane. and a variety of metals and complex organics.

     Although comets offer a huge reserve of easily accessible resources for any would-be settlers, they will be useless without a readily available supply of energy. Conditions at those distances from the Sun will be bleak: the Sun will not even be the brightest star in the deep black sky, and the temperature is a bare 10 K. Making the cold, barren ice islands of the comet nuclei habitable will be a formidable task, and energy consumption is likely to be very great. Yet energy will be the scarcest resource in the Oort Cloud.

     The size of any comet-based communities will be closely constrained by two factors: the availability of energy sources and the rate of energy consumption. Access to a dependable, long-term energy source will be crucial to the establishment of a viable, self-sufficient settlement. Energy consumption levels will affect both the size and complexity of the economy, the standard of living, and longevity of the community as a whole.

     What are some of the potential energy sources likely to be available?

     Nuclear power—fission—is unlikely. Heavy elements such as uranium will be present in only minuscule amounts, certainly not enough to form the basis for a comfortable, long-term settlement. In addition, fission processes are “dirty” in the sense that they generate a relatively high flux of radioactivity that may be undesirable.

     Another potential energy source is the controlled combustion of hydrogen and oxygen, which both occur in abundance in the vast amounts of water ice that makes up about one-fifth of each comet’s mass. The catch, of course, is that water must first be hydrolyzed, an energy-consuming process. Settlements closer to the Sun, say within 1,000 A.U., or those with sufficient surplus energy from other sources, may employ H₂/O₂ reaction cells for some purposes. Hydrogen combustion will probably serve as the major propellant for long-distance travel, but is unsuitable for basic power generation.

     There is an alternate means of using hydrogen as an energy source—fusion. It seems reasonable to assume that the technology for fusion employing deuterium, a heavy isotope of hydrogen that fuses relatively easily, will be available by the time settlement of the Oort Court begins. Estimates of the ratio of deuterium to simple hydrogen in interstellar gases vary from about 1 in 10,000 to 1 in 100,000. Although rare, for an average-size comet this represents about 50,000 to 100,000 metric tons of deuterium.

     This enormous store represents enough energy to support a population of millions for several centuries—but when the supply is gone, those millions will perish. It seems much more likely that Oort Cloud communities will choose to support much smaller populations for much greater lengths of time, husbanding their precious supply of deuterium for perhaps thousands or tens of thousands of years. They may also employ a more efficient alternative fusion pathway involving a mixture of deuterium. tritium (another hydrogen isotope), and helium to extend the lifetime of the available supply.

     At some point humans may also learn how to initiate proton fusion and thereby assure a virtually unlimited supply of energy employing simple hydrogen, the most common element in the cosmos. But as yet, only the stars themselves have accomplished this on a large scale.

     Finally, there is one alternative renewable source of energy available: starlight. The concept of harvesting starlight to support comet-based communities was first put forward by astrophysicist Eric Jones and anthropologist Ben Finney:

In principle all one needs is a giant mirror to concentrate the thin photon stream. The mirrors would be truly gigantic … At half a parsec [about 1½ light- years] from a Sun-like star the minor would have to be about 1500 kilometers radius to collect a megawatt (roughly the size of the continental US). Practicality and common sense might well dictate a larger number of much smaller collectors.

     Jones and Finney estimate a typical medium-sized comet could supply enough aluminum for mirror surfaces to collect about 300 megawatts (MW) from starlight. If additional metals were used. this capacity could be enhanced somewhat. Jones and Finney estimated per capita power consumption at 0.6 MW. about 30 times the current US rate of 20 to 25 KW per capita; thus the typical population of a comet-based community would be about 500.

     The amount of aluminum and other reflective metals in a given comet will set a definite upper limit on the area of mirror surface that can be created, and thus on the population that can be supported by collecting starlight. Comet-based communities closer to the Sun in the inner Oort Cloud will therefore enjoy a higher photon flux and will have more energy available to them, and will be able to support larger populations.

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     Although the energy supply will be the most important limiting factor for any comet-based settlements, there are a host of other factors that will influence the nature of their economies. Most of these communities are likely to be small and isolated, with populations of only a few hundred. Such small economic systems are sure to face many limitations and constraints. Many types of tools, specialized pieces of equipment and expert skilled labor will all be in short supply. The economy will essentially be a closed system, especially for the more distant and isolated settlements. For many items and commodities, supply will not be related in any way to need or demand.

     The members of these communities will of necessity be very dependent upon one another and are also likely to develop a strong conservation ethic; they will recycle as much as possible. Their relative isolation will encourage self-reliance and cooperation, as well as the pooling of skills, labor, and resources. Small, isolated, tightly closed economies are thus likely to be more communal in their outlook, particularly in regard to resources, power consumption, and the allocation of labor. Larger communities will probably have a wider variety of economic pattens.

     Because populations are likely to be small, each individual will be of immense value to the community, representing a pool of knowledge, skills, and experiences as well as a heavy investment in training and education. It will be necessary for the settlers to become generalists: a small community will simply not have enough people to support a specialized expert in every field. Each person will have to develop a sound working knowledge in many overlapping fields, and consequently will have less time to devote to becoming expert in any one of them. Some specialties may have to be eliminated altogether in very small communities.

     Many of these limitations and constraints can likely be alleviated or even eliminated with the aid of sophisticated technology. The exploration and settlement of the Oort Cloud, the establishment of viable communities in the cold deeps of space, will demand heavy reliance on many different forms of high-technology equipment. A combination of the right tools and skills to use them will be vital.

     It seems unlikely that a small band of pioneers will be able to create a secure, comfortable niche in the Oort Cloud without the use of automated systems and artificial intelligence (AI). Such systems will eliminate the need for human intervention at many basic but routine levels of activity, freeing people to apply their skills where they are needed most. Computers and automated systems are skilled tools which can enhance and multiply human capabilities many-fold.

     The human residents of the Oort Cloud will control and monitor the operation of a complex array of self-directed systems. Most of the basic comet-mining and materials processing operations will probably be automated. They will produce stockpiles of refined materials and, at the highest level of complexity, produce many types of finished products.

     Artificially intelligent systems will also be essential to the viability of the communities, particularly for small settlements. They will rely heavily on expert systems—sophisticated computer programs that combine an extensive store of information with a powerful decision-making capability for interpreting complex data. They will operate at the level of a human expert that perhaps the community cannot support.

     The human members of the community, with their generalized skills and experiences, will be capable of handling all the normal operations and events; the expert systems would serve to supplement and complete the basic suite of skills and abilities required to keep the community healthy.

     Because of their dependence on AI systems, residents of the Oort communities will also need to be skilled in the manipulation and application of information. They will have access to the vast sum of human knowledge, but it will be useless to them without sophisticated information processing systems. They will have to become adept at working in cooperation with their AI systems to manage the masses of data available, to sift through it to extract what they need. The use of sophisticated computer-based systems will be a basic skill in every Oort Cloud community, and it seems likely that education will be based in large pan on AI systems.

     All these factors will have a number of important effects on the shape of the settlement’s economy. The distribution of activity will be very different for any Earth-bound economic system.

     The primary sector of the economy—the exploitation of natural resources—is likely to be small and almost completely automated. Human involvement will be minimal. The primary sector will consist of two basic activities: energy production and the harvesting of cometary resources. Once the appropriate systems are established, both will be relatively simply activities.

     The secondary sector—the transformation of the natural resources—will include refining and processing the raw cometary feedstock, manufacturing. construction and assembly operations. agriculture and food production, and recycling. Again, many of these activities will be highly automated. but closer human supervision will be necessary to tailor these activities to the current needs of the community.

     It is the tertiary sector of the economy, support services. that will engage the majority of human skill and attention. It will include operation of the utility and life support systems, maintenance and repair operations, transportation, health care, information processing, storage and retrieval. education and administration, business and commerce.

     The settlers will have to choose between many different possible paths for development. Will they allow the community to grow, or should they impose controls on themselves to maintain a small population? Will they want their community to last many years at a low rate of consumption, or will they want to trade communal longevity for a higher rate of resource consumption? The outcome of these decisions will depend primarily on two factors: the available supply of resources and the standard of living that the community desires and will defend.

     The average standard of living is likely to be quite comfortable, within the limits of the communities capabilities. Because of its extreme fragility, the surface of the comet itself will be unsuitable for permanent dwellings; the habitats will probably be free-flying. Physically, they may resemble the Bernal Spheres and O’Neill Cylinders already envisioned for space-based communities in the inner Solar System. The most significant difference will be the lack of large window surfaces, since there will be no need to admit sunlight for illumination, which will allow for an increase in the usable surface area inside the habitats.

     Communities lacking a large supply of deuterium or those choosing to conserve their supply would locate their habitats at the center of huge arrays of mirrors for the collection of starlight. A variety of social and economic factors suggest the members of such a community, which would number about 500, would be dispersed throughout the array in bands of about 25 persons—a dozen adult men and women and their children.

     Each band would tend a mirror farm stretching across perhaps 30,000 km of space, overseeing a complex network of robots and other automated systems responsible for the routine operation and maintenance of the mirror farm. In the event of an emergency requiring human intervention. the settlers would need to be able to reach even the edge of the array quickly, and so will gather the mirrors in tightly packed arrangements.

     The individual mirror farms of the separate bands would then be clustered into a vast array supporting the community’s entire population, spread out over l50,000 to 200,000 km of space: an immense surface area of ultra-thin metal film centered around the invisible point of the comet nucleus. gathering the faint light of the stars. Communication between bands within the community could easily be maintained by radio and laser.

• 

     Despite the limitations of the environment, Oort Cloud communities that choose to grow and develop large economies will have many options. One of the most obvious ways to enhance their resource base will be to create clusters of comet nuclei. Clustering would require the commitment of significant amounts of time, energy and resources, but the potential rewards will ensure that some communities will begin to move additional comets into more convenient orbits.

     Given the average separations and relative orbital velocities of comet nuclei in the Cloud. it would take between five and ten years to bring a new comet into a cluster. The energy required represents only about 3% to 5% of the available deuterium supply; clustering will be relatively economical once the necessary resources are committed to the project. Clustering will probably be common, if only because large comets will be rare.

     Clustering would allow relatively large populations to exist, especially in the inner Oort Cloud, where the photon flux from the Sun will be fairly high and consequent energy production levels will be capable of supporting much larger communities. Jones and Finney have estimated that the largest practical population for a cluster will be about 100,000, when the energy consumed in transport between the dispersed habitats of the cluster begins to strain the reserves of the community. Social factors may also limit the size of large cluster-based communities.

     In addition to clustering, there will be other means by which the settlements can enhance their economies. Their isolation in the depths of space will be a relative thing; they will be in constant communication with neighboring communities. The average separation between comets in the Cloud is thought to be a few light-hours; between settlements it will be perhaps a light-day. News, messages and other information will flow back and forth across the empty space between them.

     Some settlements, particularly the larger ones, may engage in some sort of trade, both with the inner system and among themselves. Initially, many communities will need to import those items that they cannot make for themselves. As they develop, they may consider more complex forms of trade; it is certain the decision to expend the time, energy and resources in long-distance commerce will be a complex cost/benefit calculation.

     Items for trade might include specialized technology, bioengineered species specifically tailored for conditions in the Oort Cloud, computer and AI programs, and information itself. Perhaps some communities will specialize within a larger dispersed cluster or bloc of trading communities.

     Because of the constraints and limitation imposed by the environment, one primary social concern of the Oort Cloud communities will be the careful regulation of the health and size of their populations.

     Until the settlement has developed the skills and resource base to support growth, some form of control will be necessary to limit the size of the population. In very small, isolated populations, even the timing of reproduction may need to be regulated, in order to maintain an appropriate distribution of age and sex.

     Unless communities adopt fairly stringent measures, the population will inevitably grow. The members of the settlement will then have to choose between a higher rate of resource consumption, making a commitment to enhancing their resource base, or to assisting their excess population in establishing daughter settlements around other comet nuclei.

     Both static and growing communities will also need to be concerned with the genetic health of their populations. Within the small, relatively isolated gene pools of the Oort Cloud settlements, population genetics will become a factor that will affect both everyday life and the long-term viability of the community.

     Very early on, when a new community is being planned or established, a major concern will be the “founder effect”—the skewing of the genetic composition of a population because its founding members are very few. The danger exists that the founding members of a community may share too high a proportion of deleterious genes that will soon become reinforced in subsequent interbreeding. This problem could be largely eliminated by careful screening during the planning and recruitment stages for a new settlement. New colonists would in part be chosen to maximize genetic variation in the new population, providing a sound base for healthy future growth.

     Founding populations can be very low—perhaps as low as l0 or 20 individuals—if the original breeders are genetically diverse and share few common deleterious genes. The negative effects of early inbreeding can also be avoided by social customs or other regulations that prevent bad crosses, particularly among close relatives.

     The potential negative effects of inbreeding will remain, however, as the settlement's population grows and becomes more uniform, but will be of major concern only to very small, isolated groups. Both natural and artificial selection will come into play before inbreeding could cause serious damage to the health of the community. In nature. inbreeding often results in a high level of spontaneous abortions and early deaths; natural selection operates to remove the bad crosses from the breeding population. In healthy communities. matings would be structured either by intent or by social custom to minimize the negative effects of inbreeding. It is also important to remember that genetic uniformity need not be a problem provided that any deleterious genes are eliminated from the founding population and that any new negative traits arising by mutation are culled or otherwise prevented from breeding. Genetic engineering techniques may also be employed to maintain a healthy gene pool.

     The human consequences of such requirements for the maintenance of the community's genetic health will be great. The social tensions generated by the practice of eugenics and genetic engineering may also be difficult to manage. However, there are other options for communities that wish to avoid negative inbreeding, including out-breeding with other settlements.

     Out-breeding has the obvious positive effect of increasing genetic variability. But its practice also sen/es as a mechanism for complex social interaction as well.

     Prior to the terrestrial agricultural revolution, people tended to live within small hunting and gathering bands of about 20 to 30 men, women. and children. It has been noted by various social anthropologists that groups of this size still seem to be the most congenial to humans worldwide in both their private and professional lives. This preference is no doubt part of our physical and social evolutionary heritage.

     But humans also tended to form larger social units, gathering into clans or tribes numbering about 500 or so. Normally dispersed as individual bands, the tribes maintained social cohesion through intermarriage. Indeed, the size of such groups is just about the minimum number needed to maintain a healthy diversity within the common gene pool and to avoid inbreeding.

     It is possible to anticipate, then, that the dispersed communities of the Oort Cloud may form similar social links. A single community tending an extensive mirror farm as described earlier could form a viable, self-contained breeding population by marriage exchanges between bands. In addition to preserving the genetic health of the community, such exchanges would serve to link the bands into a large, cohesive social unit. Just as dispersed tribes on Earth gathered periodically for important social and religious observances, so too might the dispersed bands of a comet-based settlement gather their entire population on occasion.

     Out-breeding would also allow individuals whose reproduction might otherwise be restricted or forbidden to find an alternative, providing an outlet for any tensions that might arise. And if such a place could not be found within the tribe, there is also the possibility of exchanges with other communities around other nearby comets. Such exchanges would be more expensive in terms of time and energy, but would serve to cement social ties between settlements.

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     The necessity for monitoring gene flow within a given population, along with all the other shaping influences of the deep space environment, will have profound social consequences for the Oort Cloud communities.

     Habitat residents will be able to set temperature, humidity, day length, the weather, atmospheric pressure, even the level of gravity according to their collective preference and the limitations of human physiology. They may even choose to live in zero gravity. Life style and social values, the entire “world view" of the Oort Cloud settlers will be very different from those of any terrestrial culture.

     Without diurnal or seasonal variations, their sense of time will differ. Their perceptions and use of space may also differ, and the ways in which private and public volumes are used and arranged thereby altered. In a tightly closed artificial environment, attitudes toward clothing and nudity are likely to change radically, as is the concept of privacy.

     In personal relationships, love, sex and mating will all be quite separate. Family structures are likely to be very different when marriage or cohabitation preferences may differ from permissible genetic crosses. Breeding will be much more of a social decision than it is on Earth. Given a basic band-and-tribe social structure, extended families are likely to be common. Communal or collective family arrangements may also be common.

     Social, political and administrative systems will also change, and new ones will arise. They will be shaped not only by the age-old needs and limitations of human social interaction, but also by wholly new factors unique to the deep space environment. Settlers in the Oort Cloud will face the exciting challenge of creating workable societies from whole cloth.

     As always, the basic social conflict within these communities will be between the needs and constraints of society as a whole and the personal freedom of individuals. Members of these small, isolated communities will be faced with a variety of constraints.

     Restrictions on breeding practices may be imposed by the community in order to maintain a balanced, genetically healthy population. In very small settlements,'even the timing of births may be regulated in order to preserve a balanced distribution of male and female, young, middle-aged and elderly. Communities aiming for a low or static population growth rate may impose strict absolute limits on breeding until the resource base has been developed to support larger numbers.

     Similarly, young people born and raised in an Oort Cloud settlement may face fairly tight constraints in their choice of career or vocation. As in all economies, the needs or demands of the community will determine the types of work available; career choices may often be a matter of timing and happenstance. In small communities, where age and gender distributions may have to be carefully controlled, career choices will be limited for the limited number of youths available for apprenticeship: they will be pressured to take the positions that need to be filled as they come of age.

     New settlements with undeveloped capabilities, and smaller settlements with few resources or a limited energy supply may also impose some restraints; small. isolated communities will probably be conservative in approving unnecessary expenditures of scarce resources, thus limiting the range of possible private and professional activity.

     Social values are likely to have an enormous shaping influence on the types of social systems that develop in the distant Oort Cloud communities. These settlements, perhaps comprised of highly interrelated clans, are likely to place a high value on cooperation, consensus decision making, and compromise. In the hostile and unrelenting deep space environment, where cooperation and mutual interdependence will be crucial to survival, they will feel a strong need to maintain social harmony.

     Members of such tightly knit communities will face a variety of social pressures: they will be part of a symbiotic network of duties and obligations. There will be ties to family and to tribe or clan, in addition to the community as a whole. Complex interrelationships will arise when emotional and sexual relations must be separated from breeding, as well as when family and friends emigrate to other habitats and communities. As in Earthbound societies, education, in addition to training each person for the complex task of deep space survival, will probably also serve as a primary means of inculcating the community’s social values.

     But there are other factors to counterbalance these pressures and constraints, to allow for the preservation of individual freedoms. In a community where every individual represents a heavy investment in education and training, and a valuable pool of badly needed skills and experience, rigid social controls such as ostracism, imprisonment, or capital punishment seem unlikely; a large range of social behavior will have to be tolerated. Coercion would be difficult at best, especially in very small communities with few resources or personnel to spare for “police actions”; nonconformists and dissidents must be allowed to find or to create an acceptable niche within the community. Larger settlements, in which any one individual plays a less vital role, may develop more rigid social structures.

     The Oort Cloud communities will need to develop social and political mechanisms to ensure the freedom to express diversity and individual choice, to avoid oppressive measures aimed at enforcing unnecessary conformity to arbitrary community standards, and to eliminate or at least alleviate the negative consequences of community decisions.

     It is likely a wide-variety of social systems will develop among the Oort Cloud settlements. In some, social pressures may win out. and a rigid, authoritarian social structure created. But the need for consensus and close cooperation among community residents may help prevent the development of oppressive. highly stratified societies, at least among the smaller settlements. Similarly, the necessities of cooperation and mutual dependence will temper any tendency toward extreme individualism. Individuals will be expected to take responsibility for their actions and their role in the community; both wild individualism and blind obedience to social convention would be frowned upon.

     In such a social context, within a culture that values diversity, mutualism, and social harmony, current Western-style democracy may be considered inappropriate. Despite constitutional guarantees, majority rule often enforces homogeneity and suppresses minority viewpoints. Democratic systems, in which individuals are expected to “vote in their own interests," might also foster a divisiveness that small, isolated communities could ill afford.

     The smaller band-and-tribe settlements may favor consensus decision making, in which minority and dissident viewpoints can be more easily accommodated. But such systems also have a drawback: because everyone must agree to the joint decision to reach a consensus, community members may feel pressured to “go along with the group." If the pressures became strong enough, they might foster a dangerous and oppressive homogeneity.

     Another communal decision-making system that often appears among communities of closely-related, tribal peoples is the “hardship-on-no-one" arrangement. In such a system, the emphasis is not so much on the final decision reached as on the consequences of each option. Decisions are often based on finding the option that places the least burden on the least number of individuals, and working to alleviate even those negative results of communal decisions. For example, youths guided toward a career specialty the community deems important might be allowed to pursue secondary specialties of their own choosing.

     A drawback to this system as well as for consensus decision-making systems is that they are often very time-consuming processes.

     As on Earth. the Oort Cloud communities may choose to deal with some social pressures by displacing them spatially. Nonconformists and dissidents in one band may find a comfortable and rewarding social niche in another habitat within their native tribe. Although a single clustered community would have a pool of shared communal values, each habitat within the settlement might have its own individual system, offering enough diversity to accommodate and alleviate most social frictions. As a last resort. migration between settlements would provide a social safety valve.

     It thus seems likely that a variety of social systems would arise among the Cloud settlements. They will have to respond to numerous needs and demands, seeking to find a balance between maintaining a stable, functional society and personal freedom for individuals.

     The size of the community will be an important factor in its social development. The larger Oort Cloud settlements will probably offer a wider range of social forms. They may be able to accommodate more competition and divisiveness. Large communities, however. are also difficult to administer by consensus, and may employ a form of electronic democracy. Clustering, trade, and the development of the settlement’s resource base and capabilities would create opportunities for the creation of more diverse and complex societies. A mosaic of many different social forms, distributed throughout the settlement. seems possible.

     But even the largest of the Oort Cloud communities will be quite small in comparison to the societies of billions that will develop in the inner Solar System. As the relatively compact inner core of the system is developed, it will become more tightly integrated socially and economically, more centralized, and its culture more homogeneous. Although it may take some centuries, it is likely that mass cultures with populations of billions will eventually develop.

     The small, dispersed communities of the Oort Cloud will remain largely independent of the inner core governments and societies. Their potential for diversity may offer a refuge for dissident groups seeking to leave the mass societies of the inner Solar System.

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     Just as the relative isolation of the Oort Cloud settlements from the inner system and from each other will foster social divergence and cultural diversity, so, too, will it create the conditions necessary for biological divergence and evolutionary change. As we know from terrestrial experience, reproductive isolation fosters such change—as Charles Darwin observed when he visited the Galapagos Islands as a young man. Diversity also fosters adaptive radiation—the opening and exploitation of new niches in the environment.

     Many factors are likely to encourage human evolutionary diversity in the Oort Cloud. Despite all efforts, the founder effect and random drift will cause some changes in the small isolated gene pools. Fortuitous beneficial mutations may be retained and encouraged to spread, and perhaps enhanced by artificial means. Eugenics and genetic engineering are likely to be well integrated into the social and economic life of the settlements, both as a means of maintaining a healthy gene pool and in the production of goods and trade items. Bioengineered products may be common.

     Social values will also be very different, and the distant Oort Cloud communities will feel much less keenly the social. cultural and governmental pressures that might prevent biological experimentation among the inner system cultures. Their environment and lives will be very different; they may choose to remake themselves in their own image. to adapt themselves to the needs of living in deep space.

     Estimates of the time required for natural selection to produce a new species of human being range upward toward a million years. However. in the small. isolated gene pools of the Oort Cloud settlements. perhaps encouraged by directed breeding practices and human genetic engineering. it would not take long for any chances to become widespread. Minor changes might take a few generations—perhaps a century —to become entrenched. Major structural or biochemical alterations would probably be attempted more slowly, on a scale of many hundreds or thousands of years. But it is clear that a new species of genus Homo could appear within about l0,000 years.

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     The exploration and settlement of the far reaches of the Oort Cloud will also have a profound effect on the course of human migration to other star systems. Simply reaching the Oort Cloud and establishing viable, long-term settlements there will spur the development of the technology required for interstellar flight. Although the propulsion systems needed for journeys of tens of hundreds of light-years will differ from those used to travel only one or two, the opening of the Oort Cloud will serve as the proving ground for such systems. as well as the development of closed artificial ecosystems. The social experiments of the Cloud communities may also play an important role in starship design.

     Though many interstellar missions may be launched by the large and wealthy inner Solar System cultures. others may depart from construction sites or launch points in the far reaches of Sol's Oort Cloud. which stretches out to perhaps two light-years—a significant head start. Such ships may travel directly across the deep oceans of space to reach other stars, but others may visit way stations during their journey: comets drifting in interstellar space. Estimates of the total amount of material ejected from the Solar System during its formation and subsequent evolution range from as low as 5 to 25 times the mass of the Earth up to l00 or even l,000 Earth masses. Presumably, other stars would produce roughly similar numbers of rogue comets. Interstellar space should be strewn with small. isolated islands.

     Distances between these drifting islands are difficult to estimate. They may be no more widely spaced that the comets of the Oort Cloud—about 20 A.U. Other estimates range up to hundreds of thousands of A.U. between these wandering interstellar comets.

     Some of these interlopers are bound to be found within Sol’s own Oort Cloud, working their way free of the Sun’s grip, at perhaps one in every several thousand. Relative velocities between Solar and interstellar comets will be low, and it will be tempting for Oort Cloud residents to hitch a ride outward—perhaps farther out into the permanent Cloud, perhaps out into interstellar space. Drifting outward at about 10 km/s or about 2 A.U. per year, they will make slow progress indeed. In 50,000 years they will be halfway to the nearest stars. But by then they will be wholly adapted to life in interstellar space and will perhaps not be too concerned with visiting other star systems.

     One could imagine the human species slowly spreading throughout the galaxy in this way, but it seems more likely humanity will move from star to star by fast ship, followed by slowly expanding waves of comet-based settlements meeting and overlapping in the voids between the settled stars.

     Other, more daring Oort Cloud communities may choose to take their home comet with them. If the slow outward drift seems too slow to them, and fast ships too expensive, they may attempt to launch themselves out of the Solar System via a gravity-assist maneuver close to the Sun.

     Large rocket engines burning hydrogen and oxygen, or perhaps a mass driver, could be used to dissipate the comet’s orbital velocity using only a fraction of the comet’s resources. During the long fall toward the Sun, the adventurers could use the rapidly increasing flux of solar energy to produce huge stores of hydrogen, to be consumed in fuel cells to provide power during the long interstellar cruise. The mirror farms or other solar power collectors would also serve to shield the comet nucleus from the Sun.

     After curving around Sol, the comet would leave the system with a velocity of 100 to 150 km/s, or about 1 light-year every 2,000 years. Travel times between star systems would be on the order of 10,000 years—far shorter than that of drifting “wild” comets. During the interstellar journey the settlers would live as they had in the Oort Cloud, relying on their comet’s stock of deuterium or on hydrogen produced when close to the sun. In the target system, a small band of colonists would be dropped off aboard habitats constructed during the long cruise, before rounding the new star and heading off toward a new destination.

     It thus seems likely that human expansion into the comet archipelago of the distant reaches of the Oort Cloud will only be the first step toward expansion to other star systems. Comet-based settlements will be humanity's stepping stones to the stars. across the far-flung islands in the deep ocean of interstellar space.