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 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.[1][2] Several designs were studied, some in depth, with sizes ranging from 1,000 to 10,000,000 people.[3]
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 the Japanese Gundam universe, and Babylon 5.
Several motivations for building space colonies have been proposed: 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. 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.[4]
Space is literally filled with light produced from the Sun. In Earth orbit, this amounts to 1400 watts of power per square meter.[5] 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).
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.[6]
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.[7]
Colonies would have constant access to solar energy up to very large distances from the Sun. Weightlessness allows the construction of large flimsy structures such as mirrors for concentrating sunlight.
Space habitats may be supplied with resources from extraterrestrial places like Mars, asteroids, or the Moon (in-situ resource utilization [ISRU];[4] see Asteroid mining). One could produce breathing oxygen, drinking water, and rocket fuel with the help of ISRU.[4] It may become possible to manufacture solar panels from Lunar materials.[4]
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.[8]
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.
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.[9]
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.
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.
Designs proposed in NASA studies included:
The Bubbleworld or Inside/Outside concept was originated in 1964 by Dandridge M. Cole and Donald W. Cox in a nonfiction book, Islands in Space: The Challenge of the Planetoids.[18]
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.
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.
The Bigelow Next-Generation Commercial Space Station was announced in mid-2010.[21] The initial build-out of the station is expected in 2014/2015, and will consist of two Sundancer modules and one BA-330 module.[22] 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[23] Bigelow began to publicly refer to the initial configuration—two Sundancer modules and one BA-330 module—as "Space Complex Alpha" in October 2010.[24]
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.[23]
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