A space habitat (also called an orbital colony, or a space colony, city, or settlement) is a space station intended as a permanent settlement rather than as a simple waystation or other specialized facility. No space habitats have yet been constructed, but many design proposals have been made with varying degrees of realism by both science fiction authors and engineers.
About 1970, near the end of Project Apollo, Gerard K. O'Neill, an experimental physicist, was looking for a topic to tempt his physics students, most of whom were freshmen in Engineering. He hit upon the creative idea of assigning them feasibility calculations for large space habitats. To his surprise, the habitats seemed to be feasible even in very large sizes: cylinders five miles (8 km) in diameter and twenty miles (34 km) long, even if made from ordinary materials such as steel and glass. Also, the students solved problems such as radiation protection from cosmic rays (almost free in the larger sizes), getting naturalistic sun angles, provision of power, realistic pest-free farming and orbital attitude control without reaction motors. He published an article about these colony proposals in Physics Today in 1974. (See the above illustration of such a colony, a classic "O'Neill Colony"). The article was expanded in the book High Frontier.
The result motivated NASA to sponsor a couple of summer workshops led by Dr. O'Neill. Several designs were studied, some in depth, with sizes ranging from 1,000 to 10,000,000 people. Attempts were made to make the habitats as self-supporting as possible, but all of the designs relied on regular shipments from Earth or the Moon, notably for raw materials and volatiles. Closed ecologies and aggressive recycling should dramatically reduce this reliance. Recent research has increased the probability of finding frozen water in deep craters on the moon's south pole, and found that certain asteroids contain significant amounts of volatiles such as water and ammonia. This suggests space habitats could rely less on Earth than these original studies indicated.
At the time, colonization was definitely seen as an end in itself. The basic proposal by O'Neill had an example of a payback scheme: construction of solar power satellites from lunar materials. O'Neill's intention was not to build solar power satellites as such, but rather to give an existence proof that orbital manufacturing from lunar materials could generate profits. He, and other participants, presumed that once such manufacturing facilities were on-line, many profitable uses for them would be found, and the colony would become self-supporting, and begin to build other colonies as well.
The proposals and studies generated a notable groundswell of public interest. One effect of this expansion was the founding of the L5 Society in the U.S., a group of enthusiasts that desired to build and live in such colonies. The group was named after the space-colony orbit which was then believed to be the most profitable, a kidney-shaped orbit around either of Earth's lunar Lagrange points 5 or 4.
In this era, Dr. O'Neill also founded the quieter, and more targeted Space Studies Institute, which initially funded and constructed prototypes of much of the radically new hardware needed for a space colonization effort, as well as number of paper studies of feasibility. One of the early projects, for instance, was a series of functional prototypes of a mass driver, the essential technology to be used to economically move ores from the moon to space colony orbits.
In 1986, the L5 Society later became the National Space Society, and former members started a number of related efforts, including the Artemis Project, and the Mars Society. As well, some former L5 Society members seem to be active in radical engineering groups such as the Extropian Institute. A number of prominent modern space engineers and rocket scientists trace their motivation to this era.
The space habitats have inspired a large number of fictional societies in Science Fiction. Some of the most popular and recognizable are the Japanese Gundam universe, and Babylon 5.
There are several reasons why we should build space colonies: survival, security, energy, raw materials and money.
Space habitats are immune to most of the natural disasters that plague the Earth, such as earthquakes, volcanoes, hurricanes, floods and tornadoes. Meteor strikes will still pose a risk to a space habitat as they do to the Earth but a space colony is much smaller than the Earth making the likelihood of a strike much less likely on a space habitat than on the Earth. A space habitat can be the passenger compartment of a large spacecraft for colonizing asteroids, moons, distant stars or other planets. (See: Space and survival) Spreading our population out into multiple space habitats across the solar system will increase our odds of survival; if one population is destroyed it will not doom the species. Leaving our entire population on Earth leaves us at risk for a global catastrophe extinguishing humanity.
Because the colony is contained within the space habitat, it will be protected from space radiation and from high energy cosmic rays to a limited degree (appropriately comparable to what we encounter on the surface of the Earth). Radar will sweep the space around each Habitat mapping the trajectory of debris and other man-made objects allowing corrective actions to be taken to further protect the Habitat.
Space is literally filled with light produced from the sun. In Earth orbit, this amounts to 1400 watts of power per square meter. This energy can be used to produce electricity from solar cells or heat engine based power stations, process ores, provide light for plants to grow and to warm space colonies, or to heat cold planets (Mars).
There are asteroids composed almost entirely of nickel-iron steel (with minable quantities of cobalt, platinum, silver and gold among other precious minerals) floating randomly through our solar system. There are other asteroids made of carbon rich materials with a mixture of silicates and metals throughout. Such bodies would be bountiful in precious volatiles for life support, fuel, and advanced material production. Other asteroids still are composed of pure Silicates that are suitable for building photovoltaic materials and insulation for space habitats. Most asteroids are a mixture of the aforementioned materials, virtually all stable elements on the periodic table can be found in the asteroids and comets and more importantly, because these bodies do not have substantial gravity wells, it is very easy to draw materials from them and haul them to a construction site.
There is estimated to be enough material in the main asteroid belt alone to build enough space habitats to equal the habitable surface area of 10,000 Earths.
Mining, manufacturing , construction, selling real estate and running entertainment facilities and resorts; all of these activities will occur in space and explode in popularity as more and more people begin to live off world. Those people will be paid large pay checks to work those jobs and those pay checks will be backed by selling resources to richer individuals, corporations, and nations. Much money will be made in space by those who successfully begin these activities and much money will be lost by those who prematurely attempt such activities without the capability to bring them through to a profitable state.
Space habitats orbiting Earth have a number of potential advantages over those on the surface of other planets:
1. Adjustable artificial gravity, via changing a colony's rotation speed. This attribute is important if humans born and raised on the colonies are to be able to return to Earth. It is expected that those born on low-gravity bodies (such as the Moon or Mars) would have insufficient skeletal strength to function effectively in Earth's higher gravity without significant habilitation.
Initial capital outlay
Even the smallest of the habitat designs mentioned below is more massive than the total mass of all items ever launched by mankind into earth orbit. Prerequisites to building habitats are either cheaper launch costs or a mining and manufacturing base on the Moon or other body having low delta-v from the desired station orbit.
Internal life support systems
Air pressure, with normal partial pressures of oxygen, carbon dioxide and nitrogen, is a basic requirement of any space habitat. Basically, most space colony designs propose large, thin-walled pressure vessels. The required oxygen could be obtained from lunar rock. Nitrogen is most easily available from the Earth, but is also recycled nearly perfectly. Also, nitrogen in the form of ammonia may be obtainable from comets and the moons of outer planets. Nitrogen may also be available in unknown quantities on certain other bodies in the outer solar system. The air of a colony could be recycled in a number of ways. The most obvious method is to use photosynthetic gardens, possibly via hydroponics or forest gardening. However, these do not remove certain industrial pollutants, such as volatile oils, and excess simple molecular gases. The standard method used on nuclear submarines, a similar form of closed environment, is to use a catalytic burner, which effectively removes most organics. Further protection might be provided by a small cryogenic distillation system which would gradually remove impurities such as mercury vapor, and noble gases that cannot be catalytically burned.
Organic materials for food production would also need to be provided. At first, most of these would have to be imported from the moon, asteroids, or the Earth. After that, recycling should reduce the need for imports. One proposed recycling method would start by burning the cryogenic distillate, plants, garbage and sewage with air in an electric arc, and distilling the result. The resulting carbon dioxide and water would be immediately usable in agriculture. The nitrates and salts in the ash could be dissolved in water and separated into pure minerals. Most of the nitrates, potassium and sodium salts would effectively recycle as fertilizers. Other minerals containing iron, nickel, and silicon could be chemically purified in batches and reused industrially. The small fraction of remaining materials, well below 0.01% by weight, could be processed into pure elements with zero-gravity mass spectrography, and added in appropriate amounts to the fertilizers and industrial stocks. This method's only current existence is a proof considered by NASA studies. It's likely that methods would be greatly refined as people began to actually live in space habitats.
Long-term on-orbit studies have proven that zero gravity weakens bones and muscles, and upsets calcium metabolism and immune systems. Most people have a continual stuffy nose or sinus problems, and a few people have dramatic, incurable motion sickness. Most colony designs would rotate in order to use inertial forces to simulate gravity. NASA studies with chickens and plants have proven that this is an effective physiological substitute for gravity. Turning one's head rapidly in such an environment causes a "tilt" to be sensed as one's inner ears move at different rotational rates. Centrifuge studies show that people get motion-sick in habitats with a rotational radius of less than 100 metres, or with a rotation rate above 3 rotations per minute. However, the same studies and statistical inference indicate that almost all people should be able to live comfortably in habitats with a rotational radius larger than 500 meters and below 1 RPM. Experienced persons were not merely more resistant to motion sickness, but could also use the effect to determine "spinward" and "antispinward" directions in the centrifuges.
Protection from hostile external environment
* Radiation: Space radiation has two distinct problems. One is that cosmic rays expose one to 80 millisieverts of radiation per year, well above the maximum safe occupational threshold of 50 mSv, and well above the healthy population maximum of 3 mSv. Another, separate issue is that solar flares occasionally emit very large amounts of soft x-rays, and energetic particles. When these events occur, they can exceed 4 sieverts, the lethal dose for half the population. The most interesting result of the studies was the discovery that large space habitats are effectively shielded by their structure and air, which easily exceeds the two meters of steel needed. Smaller habitats could be shielded by stationary (nonrotating) bags of rock. Sunlight could be admitted indirectly via mirrors in radiation-proof louvres, which would function in the same manner as a periscope. If the space habitat is located at L4 or L5, then its orbit will take it outside of the protection of the Earth's magnetosphere for approximately two-thirds of the time (as happens with the Moon), putting residents at risk of proton exposure from the solar wind as well as the health threat from cosmic rays.
* Heat rejection: The colony is in a vacuum, and therefore resembles a giant thermos bottle. The sunlight to radiated energy ratio can be reduced and controlled with large venetian blinds. Habitats also need a radiator to eliminate heat from absorbed sunlight and organisms. Very small habitats might have a central vane that rotates with the colony. In this design, convection would raise hot air "up" (toward the center), and cool air would fall down into the outer habitat. Some other designs would distribute coolants, such as chilled water from a central radiator. Because blinds and radiators might be a major expense, inexpensive habitats might be very warm.
* Foreign objects: The habitat would need to withstand potential impacts from space debris, meteoroids, dust, etc.
* Orbital stationkeeping: The optimal habitat orbits are still debated, and so orbital stationkeeping is probably a commercial issue. The lunar L4 and L5 orbits are now thought to be too far away from the moon and Earth. A more modern proposal is to use a two-to-one resonance orbit that alternately has a close, low-energy (cheap) approach to the moon, and then to the Earth. This provides quick, inexpensive access to both raw materials and the major market. Most colony designs plan to use electromagnetic tether propulsion, or mass drivers used as rocket motors. The advantage of these is that they either use no reaction mass at all, or use cheap reaction mass.
* Attitude control: Most mirror geometries require something on the habitat to be aimed at the sun and so attitude control is necessary. The original O'Neill design used the two cylinders as momentum wheels to roll the colony, and pushed the sunward pivots together or apart to use precession to change their angle. Later designs rotated in the plane of their orbit, with their windows pointing at right angles to the sunlight, and used lightweight mirrors that could be steered with small electric motors to follow the sun.
Designs proposed in NASA studies included:
* Bernal sphere - "Island One", a spherical habitat for about 20,000 people.
* Bubbleworld; The Bubbleworld, or Inside/Outside concept, was originated in 1964 by Dandridge M. Cole and Donald W. Cox in a nonfiction book called Islands in Space: The Challenge of the Planetoids.
1. ^ "Space Settlements: A Design Study". 1975. http://www.nas.nasa.gov/About/Education/SpaceSettlement/75SummerStudy/Table_of_Contents1.html. Retrieved 2006-12-18.
* NASA's table of contents for the studies See the "online books" about half-way down the page.
* Lifeboat Foundation Space Habitats, a space habitat advocacy group.
* Health threat from cosmic rays