Space colonization, also called space settlement and space humanization, is the hypothetical permanent autonomous (self-sufficient) human habitation of locations outside Earth. It is a major theme in science fiction. Several design groups at NASA and elsewhere have examined the feasibility of various schemes.
For humans to live permanently outside Earth, the habitat must maintain variables within an appropriate range, ie. homeostasis. The habitat must contain non-human species--for example, microorganisms and crop plants.
The relationship between organisms, their habitat and the non-Earth environment can be:
A combination of the above is also possible.
Self-replication is an optional attribute, but highly desirable because it allows an exponential increase in colonies.
It could be argued that the establishment of a colony would be Earth's first act of self-replication.
See also: von Neumann probe, von Neumann machine, clanking replicator, self-replication
In 2002, the anthropologist Dr. John Moore estimated that a population of 150-180 would allow normal reproduction for 60-80 generations--equivalent to 2000 years.
A much smaller initial population of two female humans should be viable as long as human embryos are available from Earth. Use of a sperm bank from Earth also allows a smaller starting base with negligible inbreeding.
Researchers in conservation biology have tended to adopt the "50/500" rule of thumb initially advanced by Franklin and Soule. This rule says a short-term effective population size (Ne) of 50 is needed to prevent an unacceptable rate of inbreeding, while a long-term Ne of 500 is required to maintain overall genetic variability. The Ne=50 prescription corresponds to an inbreeding rate of 1% per generation, approximately half the maximum rate tolerated by domestic animal breeders. The Ne=500 value attempts to balance the rate of gain in genetic variation due to mutation with the rate of loss due to genetic drift.
Effective population size Ne depends on the number of males Nm and females Nf in the population according to the formula:
Location is a frequent point of contention between space colonization advocates.
The location of colonization can be:
Planet, natural satellite or asteroid
Small asteroids have the advantage that one passes closer than Earth's moon several times per decade. In between these close approaches to home, the asteroid may travel out to a furthest distance of some 350,000,000 kilometers from the Sun (its aphelion) and 500,000,000 kilometers from Earth.
Disadvantages are a lack of significant gravity, a population of more than ten and self sufficiency may be far in the future on/in very small asteroids. Unmanned supply craft should be practical with little technological advance even crossing 1/2 billion kilometers of cold vacuum. The colonists would have a strong interest in assuring their asteroid did not hit Earth or anything else of significant mass.
The most famous attempt to build an analogue colony is Biosphere 2, which attempted to duplicate Earth's biosphere.
Many space agencies build testbeds for advanced life support systems, but these are designed for long duration human spaceflight, not colonization.
Remote research stations in inhospitable climates, such as the Amundsen-Scott South Pole Station or Devon Island Mars Arctic Research Station, can also provide some practice for off-world outpost construction and operation.
Mars is a frequent topic of discussion. Its size is similar to Earth, it has large water reserves, and has carbon (locked as carbon dioxide in the atmosphere). It may have gone through similar geological and hydrological processes as Earth and contain valuable mineral ores, but this is debated. Equipment is available to extract in situ resources (water, air, etc.) from the Martian ground and atmosphere.
However, its atmosphere is very thin (averaging 800 Pa or about 0.8% of Earth sea-level atmospheric pressure) and the climate is colder. Its gravity is only around a third that of Earth. There is also the problem of native bacteria, which may live on Mars. Mars is often the topic of discussion regarding terraforming to make the entire planet or at least large portions of it habitable.
See also: Exploration of Mars
Due to its proximity and relative familiarity, Earth's Moon is also frequently discussed as a target for colonization. It has the benefits of close proximity to Earth and lower gravity, allowing for easier exchange of goods and services. A major drawback of the Moon is its low abundance of volatiles necessary for life such as hydrogen and carbon. Water ice deposits thought to exist in some polar craters could serve as significant sources for these elements.
See also: Colonization of the Moon.
The Artemis Project designed a plan to colonize Europa, one of Jupiter's moons. Scientists were to inhabit igloos and drill down into the Europan ice crust, exploring any sub-surface ocean. It also discusses use of "air pockets" for human inhabitation.
A space habitat, also called space colony and orbital colony, is a space station which is intended as a permanent settlement rather than as a simple waystation or other specialized facility. They would be literal "cities" in space, where people would live and work and raise families. 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.
A space habitat could serve as a proving ground for how well a generation ship would function as a home for hundreds or thousands of people. Such a space habitat could be isolated from the rest of humanity for a century, but near enough to Earth for help. This would test if thousands of humans can survive a century on their own before sending them beyond the reach of any help.
A colony ship would be similar to a space habitat, except with major propulsion capabilities and independent power generation.
Concepts proposed in hard science fiction include:
In 2001, the space news website SPACE.com asked Freeman Dyson, J. Richard Gott and Sid Goldstein for reasons why some humans should live in space. Their respective answers  (http://www.space.com/missionlaunches/colonize_why_011008-1.html) were:
- To Spread Life and Beautify the Universe
- To Ensure the Survival of Our Species
- To Make Money and Save the Environment
Louis J. Halle, formerly of the United States Department of State, wrote in Foreign Affairs (Summer 1980) that the colonization of space will protect humanity in the event of global nuclear warfare.  (http://www.foreignaffairs.org/19800601faessay8146/louis-j-halle/a-hopeful-future-for-mankind.html)
The scientist Paul Davies also supports the view that if a planetary catastrophe threatens the survival of the human species on Earth, a self-sufficient colony could "reverse-colonize" the Earth and restore human civilization.
The author and journalist William E. Burrows and the biochemist Robert Shapiro proposed a private project, the Alliance to Rescue Civilization, with the goal of establishing an off-Earth backup of human civilization.
See also: Space and survival
Space advocacy organizations:
- The Alliance to Rescue Civilization plans to establish backups of human civilization on the Moon and other locations away from Earth.  (http://www.arc-space.org/)
- The Artemis Project plans to set up a private lunar surface station.  (http://www.asi.org/)
- The British Interplanetary Society, founded in 1933, is the world's longest established space society.  (http://www.bis-spaceflight.com/)
- The Living Universe Foundation has a detailed plan in which the entire galaxy is colonized.  (http://www.luf.org/)
- The Mars Society promotes Robert Zubrin's Mars Direct plan and the settlement of Mars.  (http://www.marssociety.org/)
- The National Space Society is an organization with the vision of "people living and working in thriving communities beyond the Earth."  (http://www.nss.org/)
- The Planetary Society is the largest space interest group, but has an emphasis on robotic exploration and the search for extraterrestrial life.  (http://www.planetary.org/)
- The Space Frontier Foundation promotes strong free market, capitalist views about space development.  (http://www.space-frontier.org/)
- The Space Studies Institute was founded by Gerard K. O'Neill to fund the study of space habitats.  (http://www.ssi.org/)
- Students for the Exploration and Development of Space (SEDS) is a student organization founded in 1980 at MIT and Princeton.  (http://www.seds.org/)
There are many who object to the idea of colonizing space as being too expensive and a waste of time. There is nothing in space that we really need, they say, adding that moving beyond the solar system is totally impractical in any reasonable time scale.
The pragmatic argument to 'live together on the earth we have' is a powerful one, suggesting that if even half the money of space exploration were spent on social education many more people on earth would be much happier.
The anti-space arguments have gone so far as to suggest that space colonization is a remnant of historical colonization - it is (the idea at least) a lingering desire left over from a romanticized notion of the 'founding fathers', and the conquest of territory on earth. As such, the argument goes, space exploration wins the hearts and minds of voters but does little else. Worse still, it could be said that the objective of colonizing space adds fuel to the patriotic dogma of conquest, and thus reinforces national prejudice rather than unifying earth.
As an alternative for the future of the human race, many science fiction writers have instead focused on the realm of the 'inner-space', that is the (computer aided) exploration of the human mind and human consciousness. Perhaps one example of this trend is the popular movie The Matrix, where all the action takes place on (under the surface of) Earth, and in a computer generated reality in cyberspace.
Films and books that depict space colonies of Earth include:
- "IS THE SURFACE OF A PLANET REALLY THE RIGHT PLACE FOR AN EXPANDING TECHNOLOGICAL CIVILIZATION?" (http://www.nas.nasa.gov/Services/Education/SpaceSettlement/CoEvolutionBook/Interview.HTML) Interviewing Gerard O'Neill
- Biological Effects of Weightlessness (http://www.faqs.org/docs/Newtonian/Newtonian_91.htm)
- space colony - The Encyclopedia of Astrobiology, Astronomy, and Spaceflight (http://www.daviddarling.info/encyclopedia/S/spacecolony.html)
- HobbySpace: Life in Space: Section C: Colonies, Habitats, Space Industry, etc (http://www.hobbyspace.com/Links/spaceLife2.html) Extensive collection of links
- Orbital Space Settlements (http://lifesci3.arc.nasa.gov/SpaceSettlement/) Al Globus (NASA) advocates orbital space habitats, not planetary habitats.
- Space Development: The Case Against Mars (http://www.foresight.org/NanoRev/Mars.html) by K. Eric Drexler. A 1985 article with arguments against Martian colonization.
- Space Settlements: A Design Study (http://lifesci3.arc.nasa.gov/SpaceSettlement/75SummerStudy/Design.html) Authored by the participants of "The 1975 Summer Faculty Fellowship Program in Engineering Systems Design" under the sponsorship of NASA and American Society for Engineering Education. Proposal for a space habitat with 10,000 people.
- The Political Economy of Very Large Space Projects (http://www.jetpress.org/volume4/space.htm) John Hickman argues that only government can afford the high initial investment for very large space development projects.
- Space and Human Survival: My Views on the Importance of Colonizing Space (http://www.sylviaengdahl.com/space.htm) Sylvia Engdahl discusses the "critical stage" where a level of technology allows both space colonization and human extinction.
- Space Quotes to Ponder (http://www.spacequotes.com/) Quotes on why humankind must expand into space.
- Spaceflight or Extinction (http://www.spaext.com/) Academics and other leaders explain that we should colonize space to improve our chance of survival. Authors include Stephen Hawking and Carl Sagan.
- Space Settlement (http://members.aol.com/oscarcombs/settle.htm) discusses the orbital space settlement concepts of Gerard O'Neill.
- PERMANENT (http://www.permanent.com) (Projects to Employ Resources of the Moon and Asteroids Near Earth in the Near Term) is a guide to websites about asteroid mining and space settlement.
- Mars colonization:
- Warm-Blooded Plants and Freeze-Dried Fish (http://www.theatlantic.com/issues/97nov/space.htm) Freeman Dyson predicts that space colonization will only be affordable after a hundred years; and that biotechnology, not propulsion, will be the enabling factor. See Astrochicken.
- Testimony of Michael D. Griffin (http://www.spaceref.com/news/viewsr.html?pid=10683) Hearing on "The Future of Human Space Flight". Michael D. Griffin believes that the "human space flight program is in the long run possibly the most significant activity in which our nation is engaged".
- A Two-Planet Species? (http://www.theatlantic.com/issues/2004/01/langewiesche.htm) William Langewiesche writes that the American people should be asked "not whether there are immediate benefits to be gleaned from a human presence in space but, more fundamentally, whether we are to be a two-planet species".
- Island One Society (http://www.islandone.org/) - ideas about libertarian space colonization.