Oxygen (obviously useful for breathing) is also 86% the weight of both water and rocket fuel. The silicon will go into glass and solar cells. The metals are useful for structural materials. Aluminum and Titanium are valued by the aerospace industry for their combination of strength and light weight. Titanium, additionally, is a good high-temperature metal.
Research has also been conducted toward creating fiberglass, ceramics, and cement from lunar materials.
With the subsequent discovery by the Lunar Prospector space probe of ices frozen in permanently shadowed craters at the poles of the moon, we may not necessarily have to import Hydrogen, Nitrogen, and Carbon from the Earth as was originally thought. These elements, along with every other element needed, are also available in most asteroids.
This is a valid point; in terms of ore concentrations, lunar soils are little different from what's in your backyard. The economic value of extraterrestrial resources lies not in their concentration, but in their location outside of Earth's gravity well. A bucket of dirt in a high Earth orbit is worth its weight in gold. Why? Because that's how much money you'd have to spend to lift it there! Admittedly, this argument proceeds from the assumption that you want to build something in high Earth orbit.
Some have tried to make a case for this. One proposal is the retrieval of Helium-3 from the lunar surface for use as fusion fuel. But one must bear in mind that the commercial fusion reactors which would burn this fuel do not yet exist even as paper designs. One outside possibility is the importation of platinum group metals from asteroids. In this case, one must factor in the effect dumping large amounts of the metals into the marketplace would have on their prices.
One difficulty with the concept of retrieval of space resources to Earth is that the competition for those materials is Earth resources which will always have much cheaper transportation costs. When proposing use of space resources in space, the competition is Earth resources which have been rocketed up the steep gravity well of the Earth at considerable expense. O'Neill always felt that to bring space materials back to Earth would be to kill their one outstanding advantage: their location outside the steepest parts of Earth's gravity well.
There's no shortage of proposals, but right now the one with the most economic promise is the construction of Solar Power Satellites (SPS).
The SPS concept was invented in 1968 by Peter E. Glaser of Arthur D. Little, Inc. A SPS would be a satellite in Geosynchronous Earth Orbit (GEO). It would consist of a solar array several miles across, and a microwave transmitting antenna. In GEO, a satellite is in sunlight 24 hours a day 98% of the time. The SPS would convert solar energy to electricity, and transmit the power to Earth in the form of a low-density microwave beam. The beam would be intercepted on Earth by a receiving antenna (rectenna) 7 km (4.2 mi.) across, and converted back into electricity. The goal is to undersell electricity generated by fossil fuels or nuclear energy.
There is an unfortunate tendency that when we hear "microwaves" we think of what happens to our cheese-melt in the microwave oven. But the SPS microwave beam was studied extensively by the Department of Energy (DOE); they could find no harmful environmental effects. The actual Watts-per-square-meter is not terribly high: less than 1/2 that of sunlight. However, unlike sunlight, the beam is there 24 hours a day, rain or shine, and is convertible to electricity with an efficiency of around 80-90%.
There is no basis for comparison between microwave radiation and nuclear radiation. Despite the fact we use the word "radiation" for both, they are completely different physical phenomena. It takes several feet of concrete to stop atomic radiation; microwaves can be blocked by a sheet of aluminum foil. Induced radioactivity (and for that matter, Greenhouse Gases) can persist for centuries; microwaves cease the instant the switch is flipped off. Nuclear radiation is known to cause cancers and mutations; this has never been demonstrated for microwaves.
No. It would not make a useful "death-ray". Moving a SPS beam off of its rectenna would simply cause the beam to harmlessly defocus. This is an aspect of the laws of physics; not a safety feature which could conceivably be subverted.
We would be building solar power stations in a place which is dark half the time (for 14 days at a time). Near sunrise and sunset, lunar-based solar collectors would not be able to point at the sun optimally. A larger transmitting antenna would be needed to hold the beam spread down over >10x the distance. Also, the Moon is on only one side of the Earth at a time, so any given point on Earth could get power from a lunar power system only half the time (unless orbital microwave reflectors could be made to work).
For a lunar-based solar power station to be competitive, these disadvantages would have to be offset by the advantage of not having to lift resources (or components) off the lunar surface. Mass-driver technology makes the delivery of lunar ores into space a pennies-per-pound proposition. There are also advantages to concentrating the industrial activity in a place where solar energy is available full time and there is access to 0-G.
Nevertheless, Dr. David Criswell, Director of the Institute for Space Systems Operations at the University of Houston, maintains that lunar-based solar power stations are the way to go, and has written several detailed economic analyses of the concept. Which scenario is best makes little difference in the big picture, however. In both schemes, lunar resources are the key to space power, whether the collectors are built on the lunar surface as in Criswell's proposal, or in High Earth Orbit as in the High Frontier plan.
This would still not solve the problem of cloudy days. Plus, we have no way of predicting what this may do to the heat balance (and hence climate) of the Earth.
It's true that SPS involves the beaming of energy from space to the Earth, but since the major inefficiencies are in space, and since the microwave beam is convertible into electricity with over 80% efficiency, less than 20% of the energy beamed to Earth ends up as waste heat before use. By contrast, a nuclear or coal-fired power plant puts 1½ times as much heat energy into the environment as usable power. For a given amount of electrical capacity, SPS is the more benign energy source from a heat dissipation standpoint.
But in truth, the amount of heat directly contributed by energy generation is an insignificant part of the equation. The major factor is Greenhouse Warming due to CO2 emissions. Obviously, any fossil-fuel plant retired in favor of SPS energy is another victory in the battle against global warming. It's also possible that SPS energy could be used to synthesize liquid hydrocarbon fuels from the hydrogen in water and the carbon in atmospheric CO2. In that case, we would not be contributing to global warming since we would be removing the same amount of CO2 as we were putting back.
Large arrays of black solar cells on the surface would lower the albedo of the Earth, contributing to global warming. And we would need lots of arrays. Due to the sun's absence at night, interruptions due to weather, and photovoltaic conversion inefficiencies, you would have to give up 30 times as much land area to solar panels as to rectennas in the SPS concept. Since the solar arrays would be opaque to sunlight, the land underneath would not be very useful. A rectenna, by contrast, would be a fine wire mesh supported well off the ground. It would block the microwaves, but allow sunlight and rainfall through. The land underneath could be used for agriculture, or conceivably even for cattle grazing.
No. We will always need petrochemicals for plastics and fertilizers. In fact, it seems foolishness to light a match to it.
Most of the DOE and NASA studies centered on supporting SPS from the Earth with massive Heavy-Lift Launch Vehicles (HLLV's). The economics of this approach were judged viable, but marginal. The space-resources option is the key to reducing launch costs, but was viewed as somewhat riskier, and hence did not receive full consideration.
The Russians have continued to study power from space. The Japanese had a program called SPS 2000 which was intended to result in a small, low-orbital demonstrator SPS.
After a prolonged period of inactivity, in 1995-97 NASA funded a "Fresh Look" study of the SPS concept. While it did not deal with the use of space resources, the study did conclude that advances in technology have made SPS look even more attractive the second time around.
In October of 2007, the Pentagon's National Security Space Office released a study on Space-Based Solar Power (SBSP) for both military and civilian applications. In addition to having an interest in supplying power to military bases on remote battlefields, they also see SBSP as a way to avert future conflicts by reducing our dependence on oil from unstable regions. Lunar and asteroidal ores were cited as future long-term sources of raw building material.
Some Twenty-First Century SPS designs include:
Do we have to build large space habitats in order to establish an SPS industry?
Actually, no. All that's required is a Space Manufacturing Facility capable of taking in the lunar (or asteroidal) soils, smelting them into silicon, metals, and other pure elements, and fabricating the needed components. Workforce living quarters are apt to be spartan at first, but past a certain point (after SPS has begun to turn a profit) it's likely that some portion of the industrial output will be turned to the task of building better homes for the workers in order to reduce employee turnover. In the long term, the construction of additional settlements may become a chief occupation of the space settlers. Industrial productivity, in the form of additional living space, is calculated to exceed workforce population increase, so an industry can exist for the building of habitats for immigrants from Earth.
In truth, economic development is a better solution to rapid population growth than is space settlement.
There is a strong, inverse relationship between living standard and population growth. There is additionally a direct relationship between living standard and energy use. A cheap, clean, plentiful supply of energy (like SPS) could go a long way toward helping to improve the standard of living in many poorer nations, thus slowing their population growth rates. So space settlements may do more to reduce population pressures by helping to power economic development on Earth than by "siphoning off" excess population.
Yes. His motivations may come closest to the surface when he states, "This grandiose vision has attracted numerous adherents, including Congressman Dana Rohrabacher (R-CA), the chairman of the House Space Subcommittee, and his aide Mr. Jim Muncy". Congressman Rohrabacher has publicly stated his preference for NASA to build SPS, instead of doing "silly things like going to Mars".
When Zubrin factors in lack of cloudy atmosphere, and the ability to track the sun, together with conversion efficiencies, he calculates an orbital solar array would outperform a land-based one by a factor of 3. If the surface array is designed with sun-tracking panels, however, he estimates this factor would shrink to 1.5, and concludes this is too slim a margin to justify building the arrays in orbit. But this may be missing the point. The chief advantage of SPS over Earth-based arrays is not primarily that the orbital array can be smaller; although it is true (and Zubrin freely admits) that the SPS rectenna would take up less space than an equivalent average power photovoltaic array. The primary advantage of SPS over ground solar is that the energy is available 24 hours a day, 7 days a week, and regardless of the weather, which makes it much better suited for the generation of baseload electrical power.
Also, Zubrin seems to assume that billion-ton space habitats will be needed for building SPS from space resources (a Stanford Torus would mass 10 million tons, a Bernal Sphere would come in at under 4 million tons, and a simple Space Manufacturing Facility would mass much less still). Past a certain point, even O'Neill conceded that SPS would precede large, Earth-like habitats, and not the other way around.
Zubrin says even assuming a lunar mass-driver could deliver ores to GEO at 1/10,000th of current launch prices, launching the raw material for building an O'Neill habitat would cost $4 trillion. He then considers it a "reasonable guess" that factoring in the costs of refining, processing, manufacturing, and construction would justify multiplying this price tag 10 times over. But this latter calculation may gain unfair leverage from the current high costs of rocket launch into orbit, when the issues are the costs of refining ores in a region where solar energy is continuously available, and of construction in a region with access to zero gravity.
Launch costs 1/10,000th current prices certainly sounds like a generous assumption. But is there in fact any basis for comparison between rocketry and launch via electromagnetic forces? A M.I.T. study concluded that a lunar mass-driver could launch ore into space for a cost of around 10 cents/kilogram. For Zubrin to successfully dispute this, he must identify the calculation errors in these previous studies, and not merely throw out a number of his own, no matter how generous-sounding.
Zubrin says that, "...the size and complexity of the O'Neill operation...boggles the mind". Certainly all can agree on this point. But it seems inescapable that building self-sufficient settlements on the surface of Mars of comparable capacity would require at least an equivalent amount of infrastructure not only be launched into Earth orbit, but propelled the additional distance to Mars, and then soft-landed on the surface. Zubrin is well-known as an advocate of the position that this is within our capabilities.
But Zubrin's main bone to pick with the SPS-from-space-resources scenario is that in his opinion it has seriously misled space advocates into expecting that private enterprise, and not national programs, will open the space frontier for humanity. He says when we reach out into space, "...we will not be led, but be followed by the entrepreneurs... But the trail will have to be blazed by those who live for Hope and not for cash". In this he may be correct. It may very well be the case that the first SPS and the first independent space habitat are government-funded proof-of-concept prototypes. This FAQ will not presume to prescribe the correct mix of public and private financing.
Later in his book, Zubrin projects that a Type III (interstellar) civilization will build orbital habitats in asteroids and comets "with many of the features envisioned by O'Neill". So his position seems to merely be that space settlements will not be built in support of SPS, rather than that they are impossible or undesirable.
Another potential industry involving manned spaceflight which is receiving more and more attention (and even a small amount of real-world validation) is space tourism. One advantage space tourism has over SPS is that it can start out very small (returning profits at the earliest stages), and grow incrementally. Similarly, the kinds of space habitats which could result from this industry might evolve as a series of graduated improvements to orbital hotel designs rather than springing full-blown from a single project.
For example, one study of Closed-Ecology Life-Support Systems (CELSS) concluded that such systems become more economical than ongoing resupply for any facility used more than 8 years. No hotelier would consider constructing a new hotel without intending to use it well beyond 8 years. So even the earliest builders of space hotels for tourists would doubtless invest in CELSS, thus laying a key cornerstone of the orbital habitat concept.
Studies have indicated that the average person can tolerate 2-3 Revolutions Per Minute (RPM's). The smallest of the "Island" designs, Island One (Bernal Sphere), rotates at 2 RPM's. Island Three (O'Neill Cylinder) only rotates once every two minutes. The bigger a habitat, the lower the rotation rate which creates 1 G of centrifugal force.
Fearful concerns in this area may owe to a fundamental misconception of a habitat window as being a single window. They would instead be a great number of windows, each about 1 meter square, set in an aluminum or steel frame. It's this metal framework which bears the biggest part of the tensile load.
The artists who illustrated space habitats usually didn't show this grid because it would not be visible at a few miles distance. If someone consequently misperceives the window of a space habitat as a single window, they might expect it to have the dynamic characteristics of a window pane in their house. (Or perhaps more appropriately, the dynamic characteristics of a glass Christmas tree ornament.) But in actual fact, the window design proposed for space habitats would be quite flexible, with no reason to expect catastrophic spreading of cracks.
Space is not as densely populated with large meteors as "Lost In Space" has led us to believe. Even the largest model, Island Three, might have to wait a million years for a one-ton meteor to impact, and even that may not completely destroy the habitat. The odds of dying in this way would be 1/60th those of dying in an automobile accident. Island Three could expect to be hit by a meteor the weight of a tennis ball roughly every three years. Even if the hull were punctured, it would still take several years for the air to leak out; plenty of time to implement repairs.
This is a more serious concern. Anything beyond Earth's magnetic field will get bombarded by such radiation. There are a couple of solutions, but the simplest is probably the best: a shield consisting of around 2 meters (>6 ft) thickness of the slag left over from the ore-smelting operation. This would reduce radiation levels to those considered safe for everyone, including infants and pregnant women.
This is a cause for concern for the International Space Station, but only because it's located in Low Earth Orbit (LEO). That means it's still not above every last molecule of the Earth's atmosphere. The nearest space habitats will orbit far higher, at least halfway to lunar orbit. Orbital decay will be no issue at all at that distance from the Earth.
The kind of bomb most terrorists are capable of making would probably not be powerful enough to penetrate the metal hull, even if over-laying soil were dug up. If a terrorist could get to the windows, he could certainly blow out several panes. But like the meteor scenario, this would mean a routine repair, not a life-threatening emergency.
The life-support system of an orbital habitat would be every green plant growing in sunlight. So there would be no "Life-support Center" that a terrorist could hold hostage.
A terrorist armed with a nuclear weapon could no doubt completely destroy a space community, but consider that he could completely destroy an Earthly community just the same. Another point is that since constant sunlight would provide space settlers with all their energy needs, they would never be tempted to build nuclear reactors. Fissionable material for a terrorist's bomb would thus be much more difficult to come by in the space settlements than on Earth.
We tend to think of space as a dangerous place because rocket travel from the surface of the Earth into orbit is undeniably hazardous. But imagine a space settler watching the news and seeing reports from Earth of hurricanes, volcanoes, earthquakes, floods, and tornadoes (all absent from the space habitats). He would certainly conclude that a planet is a very dangerous place to live!
When energy is very cheap, and resources are comparatively more expensive (the opposite of the situation here on Earth), it pays to break every waste product down to its constituent elements for re-use.
This has been called a "disposable Earth" policy, and certainly wouldn't be supported by any individual of good conscience. Ideally, we should try to remove much of the burden humanity has placed on our planet into space before catastrophic damage is done, not afterwards.
Space habitats may make ideal refuges for endangered species.
If we eventually reach the point where more people are living in space than on Earth, the Earth's primary industry may then become tourism. In such a situation, there would be an economic incentive to restore much of the planet to a natural state.
Wouldn't lack of genetic diversity in a space habitat pose long-range health problems for the inhabitants? Wouldn't space settlers get sick of seeing the same scenery and people for their entire lifetimes?
These questions are grouped together because they both arise from the same common perception: That a space settler would have little or no opportunity to travel outside of their habitat.
A situation where there is only one or a very small number of space settlements is not likely to last very long. The second habitat can be built for a fraction of the price of the very first one because the in-space infrastructure will already be in place. Additional habitats will be built not by transient laborers from Earth, but by nearby permanent space residents. There is every reason to expect the number of self-reproducing space habitats to grow exponentially, like single-cell organisms in a rich growth medium.
Still, there is a persistent perception that travel from one habitat to another will be uncommon. Why? Because it will involve space travel, and, as everyone knows, space travel is expensive, difficult, complicated, and dangerous. But we must remember that the kind of space travel we typically think about here on Earth is travel from the Earth's surface into orbit, which is undeniably all four. But travel from one space habitat to another in a nearby (or perhaps even the same) orbit will be a much simpler kind of space travel.
By contrast with an Earth-to-space transportation system, an "inter-colonial transport":
These relaxed requirements make low-thrust engines, with efficiencies far higher than any chemical rocket, likely candidates for propulsion.
This type of inter-orbital space travel is not only much easier than the kinds we typically think about; it is, by some criteria, even easier and safer than air travel. In the vacuum of space, there can be no inclement weather (fog, ice accumulation, microbursts, etc.), nor air turbulence. Although radar and radio transponders are likely to be used, one could conceivably get by without them, and navigate by the stars. And consider that on an airliner, failure of all engines would mean a serious problem for everyone involved. On a space-to-space transporter, you would continue to coast in the general direction of your destination. Such an incident would merely mean a delay in your schedule as the engineers worked on the problem, and a course correction after the engines came back on-line.
Not having to constantly push your way through the atmosphere means less energy expended. Speeds well above the speed of sound (relative to destination) will be easily achievable.
It's even conceivable that co-orbiting space habitats built in interconnected arcs might be linked by maglev-like transportation systems. Such systems needn't constantly expel reaction mass (which would have to be replaced). One would be hard-pressed to imagine a more energy-efficient mode of transportation than magnetic flight through vacuum.
Thus there's every reason to believe that routine travel among many different habitats, some with widely varying climates, ecologies, and lifestyles, will be very common.
This is a very popular question. The interesting thing is that anybody's answer tells you more about their own personal ideologies than about space settlements. Space habitats seem to be a kind of political Rorschach Test.
Regardless of anybody's opinion now, the future reality will probably be that we will see every conceivable kind of political/economic system tried out in space habitats, plus a few surprising new ones we haven't even thought of yet. In this sense, space settlements may become ideal political experimentation laboratories. If one's society ultimately fails, it would have to be a result of its underlying philosophy. In a space habitat, one could hardly shift blame to resource depletion, an energy crisis, population pressures, a crop failure, or inconvenient location.
(A zero-sum game situation is where "A" cannot gain wealth without taking it from what's available to "B".) When you reflect on it, that's actually the situation we have here right now. In fact, it was Gerard O'Neill's concern for the zero-sum game situation we find ourselves in here on Earth which originally led him to think in terms of trying to break free of it. While the resources of the Earth are probably not as limited as some doomsayers have insisted, they are still inarguably finite. But for as long as mineable bodies continue to exist in the solar system, space settlers can continue to gather space resources, and (in combination with the constant power of the sun and the productivity of their work force) use them to generate new wealth for their populations.
In the near term: Some Earth orbit above the Van Allen radiation belts, but probably no further away than the moon. After initial consideration of the 5th Lagrange point (L-5), the most recent thinking was a circular, 2-week orbit about half-way to the moon.
In the long term: Anywhere you like, as long as you're not frequently eclipsed by a planet. You can locate farther from the sun as long as you make your mirrors bigger, and curve them to concentrate the sunlight to make up the difference. How far out can you go? Let's assume no one would want to build a habitat where the mirrors weighed more than the rest of the habitat (a totally arbitrary cut-off point). Earth-like conditions could still be sustained approximately 4 light-days from the sun. That's 10 times the distance of Pluto!
Interstellar travel is likely to be done in habitats very similar to what has been described here, and by people who have already been living in space for generations. The major difference is that instead of relying on the sun, they would have to take their power supply with them (probably in the form of antimatter).
Most significantly, space habitats make any solar system a candidate for settlement, not just the ones with Earth-like planets (or indeed any planets at all).
Almost anything is possible, but the post-human concept (the prediction that technology will eventually allow us to trade in our flesh-and-blood bodies for newer models) is predicated on the capability of "reading out" the consciousness from a human brain and "uploading" it into an artificial one. At present, we have so little understanding of how our own brains work. It's always possible that dramatic advances in this area lie ahead, but equally dramatic advances in space transportation technology are also possible, and might even be derived from present knowledge.
But even if a post-human future does come first, most of O'Neill's arguments regarding 24 hour/day solar power, the building of large yet flimsy structures in 0-G, not being at the bottom of a gravity well, environmental control, mobile territories, and long-term limits to growth would be just as compelling to a machine race as to ours. In fact, it's likely that a machine race would have even less of an attachment to the planetary concept than do we planetary-evolved biological organisms. The only difference would be that we might expect the space platforms they would build to be much less Earth-like than present artist's conceptions of space habitats designed to support biological humans.
Most utopian schemes revolve around "improving" humanity. This is usually done with tight control over people in rigid, highly-disciplined communities. Space communities will be too independent and too widely scattered for any authority to prevent everybody from going off and doing their own thing. Space settlement promises only to improve the standard of living, and nothing more.
There were two main factors which went against immediate implementation of O'Neill's High Frontier plan:
This is the best question of all. Even our science-fiction is dominated by concepts involving the settlement of the surfaces of other planets. Isaac Asimov dubbed it "planetary chauvinism". We also tend to visualize extraterrestrial civilizations as living on planets, when a planetary existence may only represent a very early stage in the development of technological cultures. But living on a planet is all we have ever known, so it may be little more than a failure of imagination. This may be best illustrated by an analogy:
Imagine that you can intelligently communicate with a fetus in his eighth month of development. You inform him that in 30 days, he will be leaving his womb. What is his reaction?
In all likelihood, his response would be along the lines of, "Well, I sure hope my new womb is bigger, this one has been getting a little crowded here lately. I also wonder if the next womb is going to wiggle around as much as this one". The concept of life outside a womb is beyond his experience and comprehension.
We are born on a planet, and live our entire lives on a planet. When we struggle to visualize our future in space, we imagine leaving Earth and going to live on...another planet. We then pat ourselves on the back for this brilliant leap of the imagination.
In imagining ourselves climbing upward away from the Earth and crossing the sunlit vastness of space, only to head back down a gravity well to the surface of another planet, we may be manifesting the same lack of experience as that fetus.
It could be that humanity is on the verge of being born out into the universe.
And it may not be just more of the same.
Space Studies Institute: Founded by Gerard O'Neill, this non-profit
organization funds research into space manufacturing. Features the SSI slideshow,
the Don Davis High Frontier Art Show, and several good articles.
Space Settlement: Web page maintained by Al Globus, features pictures of space habitats.
The Living Universe Foundation: Founded by Marshall Savage, author of "The Millennial Project". Dedicated to expanding life into space.
The PERMANENT Website: PERMANENT is an acronym for Program to Employ Resources of the Moon and Asteroids Near Earth in the Near Term. This website goes into deeper detail on many of the subjects of this FAQ.
Island One Society: Group emphasizing the political freedom which may be possible in space habitats.
National Space Settlement Design Competition: Academic contest for students to design their own space habitats.
The Artemis Society: Devoted to a return to the moon with an emphasis on commercial development.
Moon Miners' Manifesto: Required reading for all lunar prospectors.
The Space Frontier Foundation: Pushing for the opening of the high frontier to the average citizen, and Cheap Access To Space.
The High Frontier by Gerard K. O'Neill, 1976, Bantam Books/SSI Press, ISBN: 0-688-03133-1, 0-9622379-0-6 & 1-896522-67-X
Colonies in Space by T. A. Heppenheimer, 1977, Warner Books, ISBN: 0-446-81-581-0
Toward Distant Suns by T. A. Heppenheimer, 1979, Stackpole Books, ISBN: 0-449-90035-5
The Millennial Project by Marshall T. Savage, 1992, Little, Brown & Company, ISBN: 0-316-77163-1 & 0-316-77163-5
Space Colonies edited by Stewart Brand, 1977, Penguin Books, ISBN: 0 14 00.4805 7
2081: A Hopeful View of the Human Future by Gerard K. O'Neill, 1981, Simon & Schuster, ISBN: 0-671-24257-1
Mining the Sky by John S. Lewis, 1996, Addison-Wesley, ISBN: 0-201-47959-1
Space Trek: The Endless Migration by Jerome Clayton Glenn & George S. Robinson, 1978, Warner Books, ISBN: 0-446-91122-4
The High Road by Ben Bova, 1981, Pocket Books, ISBN: 0-671-45805-1
A Step Further Out by Jerry Pournelle, 1980, Ace Books
The Illustrated Encyclopedia of Space Technology, 2nd ed. by Kenneth Gatland, 1989, Orion Books, ISBN: 0-517-57427-8