A space habitat (also called an orbital colony, or a space colony, city, or settlement) is a space station intended as a permanent settlement[not verified in body] rather than as a simple way station 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 engineers and science fiction authors.
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. O'Neill 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 his 1976 book The High Frontier: Human Colonies in Space.
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.[full citation needed]
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 move ores economically from the Moon to space colony orbits.
The space habitats have inspired a large number of fictional societies in science fiction. Some of the most popular and recognizable are in the Japanese Gundam universe, the space station Deep Space Nine and the space station Babylon 5.
Space habitats are immune to most of the natural disasters that plague the Earth, such as earthquakes, volcanoes, hurricanes, floods and tornadoes. 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 self-sufficient space habitats across the Solar System will increase our odds of survival, a possible ruin of the Earth's population as a whole not dooming all our species, any more.
Space has an abundance of 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.
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.[full citation needed]
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 3,000 Earths.
Power generation 
Colonies would have constant access to solar energy up to very large distances from the Sun. Weightlessness allows the construction of large flimsy structures. The ECHO-1 satellite was a balloon for reflecting microwaves that weighed 66kg and was 41.1 meters in diameter. It also reflected sunlight. Concentrating sunlight with mirrors is a source of power.
Space habitats may be supplied with resources from extraterrestrial places like Mars, asteroids, or the Moon (in-situ resource utilization [ISRU]; see Asteroid mining). One could produce breathing oxygen, drinking water, and rocket fuel with the help of ISRU. It may become possible to manufacture solar panels from Lunar materials.
Habitats may be constructed to give an immense total population capacity. Using the free-floating resources of the solar system, current estimates extend into the trillions.
Earth-to-space habitat trade would be easier than Earth-to-planetary colony trade, as colonies orbiting Earth will not have a gravity well to overcome to export to Earth, and a smaller gravity well to overcome to import from Earth.
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 habitat location.[full citation needed]
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 spectrometry, 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.
Zero g recreation 
Artificial gravity 
|This section does not cite any references or sources. (February 2011)|
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: Some very large space habitat designs could be effectively shielded from cosmic rays by their structure and air. 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.
- For instance, 4 metric tons per square meter of surface area could reduce radiation dosage to several mSv or less annually, below the rate of some populated high natural background areas on Earth. Alternative concepts based on active shielding are untested yet and more complex than such passive mass shielding, but usage of magnetic and/or electric fields to deflect particles could potentially greatly reduce mass requirements.
- If a 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.
- See 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.
- Foreign objects: The habitat would need to withstand potential impacts from space debris, meteoroids, dust, etc.
Radar will sweep the space around each habitat mapping the trajectory of debris and other man-made objects and allowing corrective actions to be taken to protect the habitat. Meteoroid strikes would pose a risk to a space habitat much stronger than they do to the Earth, unless there should be developed a method to avert them, because a space habitat does not possess a sheltering atmosphere.
Transportation and maneuvering 
- 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.
NASA designs 
Designs proposed in NASA studies included:
- Bernal sphere: "Island One", a spherical habitat for about 20,000 people.
- Stanford torus: A larger alternative to "Island One."
- O'Neill cylinder: "Island Three" (pictured), an even larger design.
- Bishop Ring (habitat): The largest design.
- McKendree cylinder: a scaled up O'Neill cylinder design
- Lewis One: A cylinder of radius 250m with a non rotating radiation shielding. The shielding protects the micro-gravity industrial space, too. The rotating part is 450m long and has several inner cylinders. Some of them are used for agriculture.
- Kalpana One, revised: A short cylinder with 250 m radius and 325 m length. The radiation shielding is 10 t/m2 and rotates. It has several inner cylinders for agriculture and recreaction.
- A "bola": a spacecraft or habitat connected by a cable to a counterweight or other habitat. This design has been proposed as a Mars ship, initial construction shack for a space habitat, and orbital hotel. It has a comfortably long and slow rotational radius for a relatively small station mass. Also, if some of the equipment can form the counter-weight, the equipment dedicated to artificial gravity is just a cable, and thus has a much smaller mass-fraction than in other designs. This makes it a tempting design for a deep-space ship. For a long-term habitation, however, radiation shielding must rotate with the habitat, and is extremely heavy, thus requiring a much stronger and heavier cable.
- "Beaded habitats": This speculative design was also considered by the NASA studies, and found to have a roughly equivalent mass fraction of structure[clarification needed] and therefore comparable[clarification needed] costs. Small habitats would be mass-produced to standards that allow the habitats to interconnect. A single habitat can operate alone as a bola. However, further habitats can be attached, to grow into a "dumbbell" then a "bow-tie," then a ring, then a cylinder of "beads," and finally a framed array of cylinders. Each stage of growth shares more radiation shielding and capital equipment, increasing redundancy and safety while reducing the cost per person. This design was originally proposed by a professional architect because it can grow much like Earth-bound cities, with incremental individual investments, unlike designs that require large start-up investments. The main disadvantage is that the smaller versions use a large amount of structure to support the radiation shielding, which rotates with them. In large sizes, the shielding becomes economical, because it grows roughly as the square of the colony radius. The number of people, their habitats and the radiators to cool them grow roughly as the cube of the colony radius.
- Nautilus-X Multi-Mission Space Exploration Vehicle (MMSEV): this 2011 NASA proposal for a long-duration crewed space transport vehicle included an artificial gravity space habitat intended to promote crew-health for a crew of up to six persons on missions of up to two years duration. The partial-g torus-ring centrifuge would utilize both standard metal-frame and inflatable spacecraft structures and would provide 0.11 to 0.69g if built with the 40 feet (12 m) diameter option. As of 2011[update], developing and assembling the NAUTILUS-X "would take at least five years and require two or three rocket launches. It would cost about $3.7 billion." NASA has released a short animation of NAUTILUS-X in space; the link is included in the External links section below.
- ISS Centrifuge Demo: Also proposed in 2011 as a demonstration project preparatory to the final design of the larger torus centrifuge space habitat for the Multi-Mission Space Exploration Vehicle. The structure would have an outside diameter of 30 feet (9.1 m) with a 30 inches (760 mm) ring interior cross-section diameter and would provide 0.08 to 0.51g partial gravity. This test and evaluation centrifuge would have the capability to become a Sleep Module for ISS crew.
The concept calls for drilling a tunnel through the longest axis of a large asteroid of iron or nickel-iron composition and filling it with a volatile substance, possibly water. A very large solar reflector would be constructed nearby, focusing solar heat onto the asteroid, first to weld and seal the tunnel ends, then more diffusely to slowly heat the entire outer surface. As the metal softens, the water inside expands and inflates the mass, while rotational forces help shape it into a cylindrical form. Once expanded and allowed to cool, it can be spun to produce artificial gravity, and the interior filled with soil, air and water. By creating a slight bulge in the middle of the cylinder, a ring-shaped lake can be made to form. Reflectors will allow sunlight to enter and to be directed where needed. Clearly, this method would require a significant human and industrial presence in space to be at all feasible.
The Bubbleworld concept was popularized by science fiction author Larry Niven in his fictional Known Space stories, describing such worlds as the primary habitats of the Belters, a civilization who had colonized the Asteroid Belt.
Asteroid Terrarium 
Hypothetical designs 
In the 1990s, as the potential usefulness of carbon nanotubes as structural material became apparent, proposals were advanced for much larger habitats taking advantage of this material. The technology to produce nanotubes of the required length is not available, so these designs remain speculative.
- Bishop ring: A torus 1000 km in radius, 500 km in width, and with atmosphere retention walls 200 km in height. The design would be large enough that it could be "roofless", open to space on the inner rim.
- McKendree cylinder: Paired cylinders in the same vein as the O'Neill cylinder/Island Three design, each 460 km in radius and 4600 km long (versus 3.2 km radius and 32 km long in the Island Three design).
Bigelow Commercial Space Station 
The Bigelow Next-Generation Commercial Space Station was announced in mid-2010. The initial build-out of the station is expected in 2014/2015, and will consist of two Sundancer modules and one BA-330 module. Bigelow has publicly shown space station design configurations with up to nine BA-300 modules containing 100,000 cu ft (2,800 m3) of habitable space Bigelow began to publicly refer to the initial configuration—two Sundancer modules and one BA-330 module—as "Space Complex Alpha" in October 2010.
Bigelow recently announced that it has agreements with six sovereign states to utilize on-orbit facilities of the commercial space station: United Kingdom, Netherlands, Australia, Singapore, Japan and Sweden.
See also 
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- Generation ship
- Human outpost (artificially created controlled human habitat)
- Inflatable space habitat
- Mars to Stay
- Space manufacturing
- Space stations and habitats in popular culture
- "The Brick Moon"
- "The Colonization of Space – Gerald K. O'Neill, Physics Today, Sept. 1974". 1974. Retrieved 2013-04-12.
- "Space Settlements: A Design Study". 1975. Retrieved 2006-12-18.
- "Ames Summer Study on Space Settlements and Industrialization Using Nonterrestial Materials". 1977. Retrieved 2006-05-28.
- Pournelle, Jerrold E., Dr. A Step Farther Out; O'Neill, Gerard K., Dr. The High Frontier: Human Colonies in Space (New York: William Morrow & Company, 1977); Heppenheimer, Fred, Dr. Habitats in Space.
- L. R. Shepherd described a generation starship in 1952 comparing it to a small planet with many people living in it. Encyclopedia of Science
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- http://www.astronautix.com/craft/echo.htm Encyclopedia Astronautca]
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- "Artificial-Gravity Swimming-Pool". spacefuture.com. 1997.
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- NASA's table of contents for the studies See the "online books" about halfway down the page.
- Lifeboat Foundation Space Habitats, a space habitat advocacy group.
- Paul Lucas (2005), Homesteading the High Frontier: The Shape of Space Stations to Come, Strange Horizons
- NASA animation of NAUTILUS-X in space
- NPR visualization of a large-colony space habitat, May 2011.
- NASA video about space habitats and space settlements construction as seen around 1970"s (5 min)