There are several arguments for space colonization that can be made: survival of human civilization and the biosphere from possible disasters (natural or man-made), and the huge resources in space for expansion of human society, being the two most common ones.
However, as of right now the building of a space colony would be a hugely difficult and massively expensive project. Space settlements would have to provide for all the material needs of hundreds or thousands of humans, in an environment out in space that is very hostile to human life. They would involve technologies, such as closed-loop life support systems, that have yet to be developed in any meaningful way. They would also have to deal with the as yet unknown issue of how humans would behave and thrive in such places long-term.
There have been no space colonies built so far, nor are there any governments or large-scale private organizations with a timetable for building any. However there have been many proposals, speculations and designs for space settlements that have been made, and there are a considerable number of space colonization advocates and groups. And several famous scientists, such as Freeman Dyson, have come out in favor of space settlement.
- 1 Reasons
- 2 Goals
- 3 Space colony types
- 4 Method
- 5 Location
- 5.1 Near-Earth space
- 5.2 The inner planets
- 5.3 The asteroid belt
- 5.4 Moons of outer planets
- 5.5 The Kuiper Belt and Oort Cloud
- 5.6 Other Solar System locations
- 5.7 Outside the Solar System
- 6 Funding
- 7 Terrestrial analogues to space colonies
- 8 History
- 9 Objections
- 10 Involved organizations
- 11 In fiction
- 12 See also
- 13 References
- 14 Further reading
Survival of human civilization
The primary argument that calls for space colonization as a first-order priority is as insurance of the survival of human civilization, by developing alternative locations off Earth where humankind could continue in the event of natural and man-made disasters.
Theoretical physicist and cosmologist Stephen Hawking has argued for space colonization as a means of saving humanity, in 2001 and 2006. In 2001 he predicted that the human race would become extinct within the next thousand years, unless colonies could be established in space. The more recent one in 2006 stated that mankind faces two options: Either we colonize space within the next two hundred years and build residential units on other planets or we will face the prospect of long-term extinction.
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. The physicist 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" 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.
J. Richard Gott has estimated, based on his Copernican principle, that the human race could survive for another 7.8 million years, but it isn't likely to ever colonize other planets. However, he expressed a hope to be proven wrong, because "colonizing other worlds is our best chance to hedge our bets and improve the survival prospects of our species".
Survival of the biosphere
Some of the more severe existential risks to humankind could also destroy parts or all of Earth's biosphere as well. An example would be a very large asteroid impact. And although many have speculated about life and intelligence existing in other parts of space, Earth is the only place in the universe currently known to harbor either of these (see: Fermi Paradox, and Rare Earth Hypothesis).
But even if these threats are averted, eventually Earth is to become uninhabitable. This is due to the Sun getting hotter and brighter over its lifetime (the Sun was only 70 percent as bright as it is now when it first formed 4.5 billion years ago). It has been suggested that approximately 800 million years from now, that Earth will cease to be able to sustain multi-cellular life. Later on in several billion years, the brightening Sun will cause a runaway greenhouse effect, extinguishing all life on Earth.
Vast resources in space
Resources in space are enormous, both in materials and energy. The Solar System alone has, according to different estimates, enough material and energy to support a human population anywhere from several thousand to over a billion times that of the current human population. Outside the Solar System in the Milky Way are anywhere up to several hundred billion other stellar systems.
Expansion with fewer negative consequences
Aside from Earth's, there are no currently known biospheres, nor indigenous people to be displaced by the encroachment of humanity.
Could help Earth
Another argument for space colonization is to mitigate the negative effects of overpopulation. If the resources of space were opened to use and viable life-supporting habitats were built, Earth would no longer define the limitations of growth.
Although Earth's resources do not grow, one more and more learns to exploit them effectively, and sometimes even almost completely, on the basis of nuclear engineering. In particular, progresses with the annihilation of matter could render spaceflight and colonization more efficient and affordable, to a revolutionary degree. Moreover, as extraterrestrial resources become available, demand on terrestrial ones would decline.
Additional goals cite the innate human drive to explore and discover, a quality recognized at the core of progress and thriving civilizations.
- Spread life and beauty throughout the universe
- Ensure the survival of our species
- Make money through new forms of space commercialization such as solar-power satellites, asteroid mining, and space manufacturing
- Save the environment of Earth by moving people and industry into space
- Provide entertainment value in order to distract from immediate surroundings, space tourism
- Ensure sufficient supply of rare materials, including from the outer Solar System—natural gas (in connection with expected worldwide hydrocarbons peak) and drinking water (in connection with expected worldwide water shortage)
Nick Bostrom has argued that from a utilitarian perspective, space colonization should be a chief goal as it would enable a very large population to live for a very long period of time (possibly billions of years) which would produce an enormous amount of utility (or happiness). He claims that it is more important to reduce existential risks to increase the probability of eventual colonization than to accelerate technological development so that space colonization could happen sooner. In his paper, he assumes that the created lives will have positive ethical value despite the problem of suffering, or that future technology could solve it.
There will be a very high initial investment costs of space colonies and permanent space infrastructure. However, proponents argue that the long-term vision of developing space infrastructure is that it will provide long-term benefits far in excess of the initial start-up costs. Therefore, such a development program should be viewed more as a long-term investment and not like current social spending programs that incur spending commitments but provide little or no return on that investment.
Because current space launch costs are so high (on the order of $4,000 to $40,000 per kilogram launched into orbit) any serious plan to develop space infrastructure at a reasonable cost must include developing the ability of that infrastructure to manufacture most or all of its requirements, plus those for permanent human habitation in space, through in-situ resource utilization. Therefore, the initial investments must be made in the development of the initial capacity to provide these necessities: materials, energy, transportation, communication, life support, radiation protection, self-replication, and population.
Although some items of the infrastructure requirements above can already be easily produced on the Earth and would therefore not be very valuable as trade items (oxygen, water, base metal ores, silicates, etc.), other high value items are more abundant, more easily produced, of higher quality, or can only be produced in space. These would provide (over the long-term) a very high return on the initial investment in space infrastructure.
|“||... the smallest Earth-crossing asteroid 3554 Amun ... is a mile-wide (2 km) lump of iron, nickel, cobalt, platinum, and other metals; it contains 30 times as much metal as Humans have mined throughout history, although it is only the smallest of dozens of known metallic asteroids and worth perhaps US$ 20 trillion if mined slowly to meet demand at 2001 market prices.||”|
Space colonization is seen as a long-term goal of some national space programs. Since the advent of the 21st century commercialization of space, which opened cooperation between NASA and the private sector, several private companies have announced plans toward the colonization of Mars. Among entrepreneurs leading the call for space colonization are Elon Musk, Dennis Tito and Bas Lansdorp.
Potential sites for space colonies include the Moon, Mars, asteroids and free-floating space habitats. Ample quantities of all the necessary materials, such as solar energy and water, are available from or on the Moon, Mars, near-Earth asteroids or other planetary bodies.
|“||... the goal isn't just scientific exploration ... it's also about extending the range of human habitat out from Earth into the solar system as we go forward in time ... In the long run a single-planet species will not survive ... If we humans want to survive for hundreds of thousands or millions of years, we must ultimately populate other planets. Now, today the technology is such that this is barely conceivable. We're in the infancy of it. ... I'm talking about that one day, I don't know when that day is, but there will be more human beings who live off the Earth than on it. We may well have people living on the Moon. We may have people living on the moons of Jupiter and other planets. We may have people making habitats on asteroids ... I know that humans will colonize the solar system and one day go beyond.||”|
|“||In the 2,900 km³ of Eros, there is more aluminium, gold, silver, zinc and other base and precious metals than have ever been excavated in history or indeed, could ever be excavated from the upper layers of the Earth's crust.||”|
The main impediments to commercial exploitation of these resources are the very high cost of initial investment, the very long period required for the expected return on those investments (The Eros Project plans a 50 year development.), and the fact that the thing has never been done before — the high-risk nature of the investment.
Space colony types
There are two main types of space colonies:
- Surface-based ones that would exist of the surfaces of planets, moons, etc.
- Space habitats — free-floating stations that would be in orbit around a planet, moon, etc. or in an independent orbit around the sun.
There is considerable debate among space settlement advocates as to which type (and associated locations for it) represents the better option for expanding humanity into space.
Locations in space would necessitate a space habitat, also called space colony and orbital colony, or a space station which would be 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. Many designs have been proposed with varying degrees of realism by both science fiction authors and scientists. Such a space habitat could be isolated from the rest of humanity but near enough to Earth for help. This would test if thousands of humans can survive on their own before sending them beyond the reach of help.
Building colonies in space would require access to water, food, space, people, construction materials, energy, transportation, communications, life support, simulated gravity, radiation protection and capital investment. It is likely the colonies would be located by proximity to the necessary physical resources. The practice of space architecture seeks to transform spaceflight from a heroic test of human endurance to a normality within the bounds of comfortable experience. As is true of other frontier opening endeavors, the capital investment necessary for space colonization would probably come from the state, an argument made by John Hickman and Neil deGrasse Tyson.
Colonies on the Moon, Mars, or asteroids could extract local materials. The Moon is deficient in volatiles such as argon, helium and compounds of carbon, hydrogen and nitrogen. The LCROSS impacter was targeted at the Cabeus crater which was chosen as having a high concentration of water for the Moon. A plume of material erupted in which some water was detected. Anthony Colaprete estimated that the Cabeus crater contains material with 1% water or possibly more. Water ice should also be in other permanently shadowed craters near the lunar poles. Although helium is present only in low concentrations on the Moon, where it is deposited into regolith by the solar wind, an estimated million tons of He-3 exists over all. It also has industrially significant oxygen, silicon, and metals such as iron, aluminum, and titanium.
Launching materials from Earth is expensive, so bulk materials for colonies could come from the Moon, a near-Earth object, Phobos, or Deimos. The benefits of using such sources include: a lower gravitational force, there is no atmospheric drag on cargo vessels, and there is no biosphere to damage. Many NEOs contain substantial amounts of metals. Underneath a drier outer crust (much like oil shale), some other NEOs are inactive comets which include billions of tons of water ice and kerogen hydrocarbons, as well as some nitrogen compounds.
Recycling of some raw materials would almost certainly be necessary.
Solar energy in orbit is abundant, reliable, and is commonly used to power satellites today. There is no night in free space, and no clouds or atmosphere to block sunlight. The solar energy available at any distance, d, from the Sun can be calculated by the formula E = 1367/d² watts per square meter, where d is measured in astronomical units.
Particularly in the weightless conditions of space, sunlight can be used directly, using large solar ovens made of lightweight metallic foil so as to generate thousands of degrees of heat; or reflected onto crops to enable photosynthesis to proceed.
Large structures would be needed to convert sunlight into significant amounts of electrical power for settlers' use. In highly electrified nations on Earth, electrical consumption can average 1 kilowatt/person (or roughly 10 megawatt-hours per person per year.)
Energy may be an eventual export item for space settlements, perhaps using wireless power transmission e.g. via microwave beams to send power to Earth or the Moon. This method has zero emissions, so would have significant benefits such as elimination of greenhouse gases and nuclear waste. Ground area required per watt would be less than conventional solar panels.
The Moon has nights of two Earth weeks in duration and Mars has night, dust, and is farther from the Sun, reducing solar energy available by a factor of about ½-⅔, and possibly making nuclear power more attractive on these bodies. Alternatively, energy could be transmitted to the lunar and martian surfaces from solar power satellites.
For both solar thermal and nuclear power generation in airless environments, such as the Moon and space, and to a lesser extent the very thin Martian atmosphere, one of the main difficulties is dispersing the inevitable heat generated. This requires fairly large radiator areas.
- For velocity change requirements to get to different places in the solar system, see delta-v budget
- For cargo see Interplanetary Transport Network optimized for minimum energy
- For people see Interplanetary spaceflight optimized for minimum time
Transportation to orbit is often the limiting factor in space endeavours. To settle space, much cheaper launch vehicles are required, as well as a way to avoid serious damage to the atmosphere from the thousands, perhaps millions, of launches required. One possibility is the air-breathing hypersonic spaceplane under development by NASA and other organizations, both public and private. Other proposed projects include space elevators, mass drivers, launch loops, and StarTrams.
Cislunar and Solar-System travel
Transportation of large quantities of materials from the Moon, Phobos, Deimos, and near-Earth asteroids to orbital settlement construction sites is likely to be necessary.
Transportation using off-Earth resources for propellant in conventional rockets would be expected to massively reduce in-space transportation costs compared to the present day. Propellant launched from the Earth is likely to be prohibitively expensive for space colonization, even with improved space access costs.
Other technologies such as tether propulsion, VASIMR, ion drives, solar thermal rockets, solar sails, magnetic sails, and nuclear thermal propulsion can all potentially help solve the problems of high transport cost once in space.
Compared to the other requirements, communication is easy for orbit and the Moon. A great proportion of current terrestrial communications already passes through satellites. Yet, as colonies further from the Earth are considered, communication becomes more of a burden. Transmissions to and from Mars suffer from significant delays due to the speed of light and the greatly varying distance between conjunction and opposition—the lag will range between 7 and 44 minutes—making real-time communication impractical. Other means of communication that do not require live interaction such as e-mail and voice mail systems should pose no problem.
In space settlements, a life support system must recycle or import all the nutrients without "crashing." The closest terrestrial analogue to space life support is possibly that of a nuclear submarine. Nuclear submarines use mechanical life support systems to support humans for months without surfacing, and this same basic technology could presumably be employed for space use. However, nuclear submarines run "open loop"—extracting oxygen from seawater, and typically dumping carbon dioxide overboard, although they recycle existing oxygen. Recycling of the carbon dioxide has been approached in the literature using the Sabatier process or the Bosch reaction.
Although a fully mechanistic life support system is conceivable, a closed ecological system is generally proposed for life support. The Biosphere 2 project in Arizona has shown that a complex, small, enclosed, man-made biosphere can support eight people for at least a year, although there were many problems. A year or so into the two-year mission oxygen had to be replenished, which strongly suggests that they achieved atmospheric closure.
The relationship between organisms, their habitat and the non-Earth environment can be:
- Organisms and their habitat fully isolated from the environment (examples include artificial biosphere, Biosphere 2, life support system)
- Changing the environment to become a life-friendly habitat, a process called terraforming.
- Changing organisms to become more compatible with the environment, (See genetic engineering, transhumanism, cyborg)
A combination of the above technologies is also possible.
Cosmic rays and solar flares create a lethal radiation environment in space. In Earth orbit, the Van Allen belts make living above the Earth's atmosphere difficult. To protect life, settlements must be surrounded by sufficient mass to absorb most incoming radiation, unless magnetic or plasma radiation shields were developed.
Passive mass shielding of four metric tons per square meter of surface area will reduce radiation dosage to several mSv or less annually, well below the rate of some populated high natural background areas on Earth. This can be leftover material (slag) from processing lunar soil and asteroids into oxygen, metals, and other useful materials. However, it represents a significant obstacle to maneuvering vessels with such massive bulk (mobile spacecraft being particularly likely to use less massive active shielding). Inertia would necessitate powerful thrusters to start or stop rotation, or electric motors to spin two massive portions of a vessel in opposite senses. Shielding material can be stationary around a rotating interior.
See also: Health threat from cosmic rays
Space manufacturing could enable self-replication. Some think it the ultimate goal because it allows a much more rapid increase in colonies, while eliminating costs to and dependence on Earth. It could be argued that the establishment of such a colony would be Earth's first act of self-replication (see Gaia spore). Intermediate goals include colonies that expect only information from Earth (science, engineering, entertainment) and colonies that just require periodic supply of light weight objects, such as integrated circuits, medicines, genetic material and tools.
The monotony and loneliness that comes from a prolonged space mission can leave astronauts susceptible to cabin fever or having a psychotic break. If this wasn't enough, lack of sleep, fatigue, and work overload can affect an astronaut's ability to perform well in an environment such as space where every action is critical.
A much smaller initial population of as little as two women 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.
Location is a frequent point of contention between space colonization advocates. The location of colonization can be on a physical body or free-flying:
- On a planet, natural satellite, or asteroid
- In orbit around the Earth, Sun, Lagrangian point or other object
Compared to other locations, Earth orbit has substantial advantages and one major, but solvable, problem. Orbits close to Earth can be reached in hours, whereas the Moon is days away and trips to Mars take months. There is ample continuous solar power in high Earth orbits. Weightlessness makes construction of large colonies considerably easier than in a gravity environment. Astronauts have demonstrated moving multi-ton satellites by hand. 0g recreation is available on orbital colonies, but not on the Moon or Mars. Finally, the level of (pseudo-) gravity is controlled at any desired level by rotating an orbital colony. Thus, the main living areas can be kept at 1 g, whereas the Moon has 1/6 g and Mars 1/3 g. It's not known what the minimum g-force is for ongoing health but 1 g is known to ensure that children grow up with strong bones and muscles.
The main disadvantage of orbital colonies is lack of materials. These may be expensively imported from the Earth, or more cheaply from extraterrestrial sources, such as the Moon (which has ample metals, silicon, and oxygen), near-Earth asteroids, comets, or elsewhere.
Due to its proximity and familiarity, Earth's Moon is discussed as a target for colonization. It has the benefits of proximity to Earth and lower escape velocity, allowing for easier exchange of goods and services. A drawback of the Moon is its low abundance of volatiles necessary for life such as hydrogen, nitrogen, and carbon. Water-ice deposits that exist in some polar craters could serve as a source for these elements. An alternative solution is to bring hydrogen from near-Earth asteroids and combine it with oxygen extracted from lunar rock.
The Moon's low surface gravity is also a concern, as it is unknown whether 1/6g is enough to maintain human health for long periods. As of 2013, the International Space Station provides a temporary, yet still non-autonomous, human presence in low Earth orbit.
Another near-Earth possibility are the five Earth-Moon Lagrange points. Although they would generally also take a few days to reach with current technology, many of these points would have near-continuous solar power capability since their distance from Earth would result in only brief and infrequent eclipses of light from the Sun. However, the fact that Earth-Moon Lagrange points L4 and L5 tend to collect dust and debris, while L1-L3 require active station-keeping measures to maintain a stable position, make them somewhat less suitable places for habitation than was originally believed. Additionally, the orbit of L2 - L5 takes them out of the protection of the Earth's magnetosphere for approximately two-thirds of the time, exposing them to the health threat from cosmic rays.
Many small asteroids in orbit around the Sun have the advantage that they pass 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.
The inner planets
The surface of Mars is about the same size as the dry land surface of Earth. The ice in Mars' south polar cap, if spread over the planet, would be a layer 12 meters (39 feet) thick and there is carbon (locked as carbon dioxide in the atmosphere).
Mars may have gone through similar geological and hydrological processes as Earth and therefore might contain valuable mineral ores. Equipment is available to extract in situ resources (e.g., water, air) from the Martian ground and atmosphere. There is interest in colonizing Mars in part because life could have existed on Mars at some point in its history, and may even still exist in some parts of the planet.
However, its atmosphere is very thin (averaging 800 Pa or about 0.8% of Earth sea-level atmospheric pressure); so the pressure vessels necessary to support life are very similar to deep-space structures. The climate of Mars is colder than Earth's. The dust storms block out most of the sun's light for a month or more at a time. Its gravity is only around a third that of Earth's; it is unknown whether this is sufficient to support human beings for extended periods (all long-term human experience to date has been at around Earth gravity, or one g).
The atmosphere is thin enough, when coupled with Mars' lack of magnetic field, that radiation is more intense on the surface, and protection from solar storms would require radiation shielding.
Terraforming Mars would make life outside of pressure vessels on the surface possible. There is some discussion of it actually being done.
Phobos and Deimos
The moons of Mars may be a target for space colonization. Low delta-v is needed to reach the Earth from Phobos and Deimos, allowing delivery of material to cislunar space, as well as transport around the Martian system. The moons themselves may be suitable for habitation, with methods similar to those for asteroids.
While the surface of Venus is far too hot and features atmospheric pressure at least 90 times that at sea level on Earth, its massive atmosphere offers a possible alternate location for colonization. At an altitude of approximately 50 km, the pressure is reduced to a few atmospheres, and the temperature would be between 40–100 °C, depending on the altitude. This part of the atmosphere is probably within dense clouds which contain some sulfuric acid. Even these may have a certain benefit to colonization, as they present a possible source for the extraction of water.
There is a suggestion that Mercury could be colonized using the same technology, approach and equipment that is used in colonizing the Moon. Such colonies would almost certainly be restricted to the polar regions due to the extreme daytime temperatures elsewhere on the planet.
Observations of Mercury's polar regions by radar from Earth and the on-going observations of the Messenger Probe have been consistent with water ice and/or other frozen volatiles being present in permanently shadowed areas of craters in Mercury's polar regions. Measurements of Mercury's exosphere, which is practically a vacuum, revealed more ions derived from water than scientists had expected. All of these observations are consistent with water ice and/or other volatiles being available to hypothetical future colonists of Mercury.
The asteroid belt
Colonization of asteroids would require space habitats. The asteroid belt has significant overall material available, the largest object being Ceres, although it is thinly distributed as it covers a vast region of space. 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 that their asteroid did not hit Earth or any other body of significant mass, but would have extreme difficulty in moving an asteroid of any size. The orbits of the Earth and most asteroids are very distant from each other in terms of delta-v and the asteroidal bodies have enormous momentum. Rockets or mass drivers can perhaps be installed on asteroids to direct their path into a safe course.
Ceres is a dwarf planet in the asteroid belt, comprising about one third the mass of the whole belt and being the sixth largest body in the inner Solar System by mass and volume. Ceres has a surface area somewhat larger than Argentina. Being the largest body in the asteroid belt, Ceres could become the main base and transport hub for future asteroid mining infrastructure, allowing mineral resources to be transported further to Mars, the Moon and Earth. See further: Main-Belt Asteroids. It may be possible to paraterraform Ceres, making life easier for the colonists. Given its low gravity and fast rotation, a space elevator would also be practical.
Moons of outer planets
Jovian moons — Europa, Callisto and Ganymede
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. This plan discusses possible use of "air pockets" for human inhabitation. Europa is considered one of the more habitable bodies in the Solar System and so merits investigation as a possible abode for life.
Ganymede is the largest moon in the Solar System. It may be attractive as Ganymede is the only moon with a magnetosphere and so is less irradiated at the surface. The presence of magnetosphere, likely indicates a convecting molten core within Ganymede, which may in turn indicate a rich geologic history for the moon.
NASA performed a study called HOPE (Revolutionary Concepts for Human Outer Planet Exploration) regarding the future exploration of the Solar System. The target chosen was Callisto. It could be possible to build a surface base that would produce fuel for further exploration of the Solar System.
The three out of four largest moons of Jupiter (Europa, Ganymede and Callisto) have an abundance of volatiles making future colonization possible.
Moons of Saturn — Titan, Enceladus, and other
Titan is suggested as a target for colonization, because it is the only moon in the Solar System to have a dense atmosphere and is rich in carbon-bearing compounds. Robert Zubrin identified Titan as possessing an abundance of all the elements necessary to support life, making Titan perhaps the most advantageous locale in the outer Solar System for colonization, and saying "In certain ways, Titan is the most hospitable extraterrestrial world within our solar system for human colonization".
Enceladus is a small, icy moon orbiting close to Saturn, notable for its extremely bright surface and the geyser-like plumes of ice and water vapor that erupt from its southern polar region. If Enceladus has liquid water, it joins Mars and Jupiter's moon Europa as one of the prime places in the Solar System to look for extraterrestrial life and possible future settlements.
Moons of Uranus and Neptune
The five large moons of Uranus (Miranda, Ariel, Umbriel, Titania and Oberon) and Triton—Neptune's largest moon—, although very cold, have large amounts of frozen water and other volatiles and could potentially be settled, only they would require a lot of nuclear power to sustain the habitats. Triton's thin atmosphere also contains some nitrogen and even some frozen nitrogen on the surface (the surface temperature is 38 K or about -391° Fahrenheit). Pluto is estimated to have a very similar structure to Triton.
The Kuiper Belt and Oort Cloud
Pluto is estimated to have a very similar structure to Triton.
The Kuiper Belt is estimated to have 70,000 bodies of 100 km or larger.
The Oort Cloud is estimated to have up to a trillion comets.
Other Solar System locations
Statites or "static satellites" employ solar sails to position themselves in orbits that gravity alone could not accomplish. Such a solar sail colony would be free to ride solar radiation pressure and travel off the ecliptic plane. Navigational computers with an advanced understanding of flocking behavior could organize several statite colonies into the beginnings of the true "swarm" concept of a Dyson sphere.
Surfaces of gas giants
It may be possible to colonize the three farthest gas giants with floating cities in their atmospheres. By heating hydrogen balloons, large masses can be suspended underneath at roughly Earth gravity. A human colony on Jupiter would be less practical due to the planet's high gravity, escape velocity and radiation. Such colonies could export Helium-3 for use in fusion reactors if they ever become practical. Escape from the gas giants (especially Jupiter) seems well beyond current or near-term foreseeable chemical-rocket technology however, due to the combination of large velocity and high acceleration needed even to achieve low orbit.
Outside the Solar System
Looking beyond the Solar System, there are up to several hundred billion potential stars with possible colonization targets. The main difficulty is the vast distances to other stars: roughly a hundred thousand times further away than the planets in the Solar System. This means that some combination of very high speed (some percentage of the speed of light), or travel times lasting centuries or millennia, would be required. These speeds are far beyond what current spacecraft propulsion systems can provide.
Many scientific papers have been published about interstellar travel. Given sufficient travel time and engineering work, both unmanned and generational voyages seem possible, though representing a very considerable technological and economic challenge unlikely to be met for some time, particularly for manned probes.
Space colonization technology could in principle allow human expansion at high, but sub-relativistic speeds, substantially less than the speed of light, c. An interstellar colony ship would be similar to a space habitat, with the addition of major propulsion capabilities and independent energy generation.
- A generation ship would travel much slower than light, with consequent interstellar trip times of many decades or centuries. The crew would go through generations before the journey is complete, so that none of the initial crew would be expected to survive to arrive at the destination, assuming current human lifespans.
- A sleeper ship, in which most or all of the crew spend the journey in some form of hibernation or suspended animation, allowing some or all who undertake the journey to survive to the end.
- An Embryo-carrying Interstellar Starship (EIS), much smaller than a generation ship or sleeper ship, transporting human embryos or DNA in a frozen or dormant state to the destination. (Obvious biological and psychological problems in birthing, raising, and educating such voyagers, neglected here, may not be fundamental.)
- A nuclear fusion or fission powered ship (e.g., ion drive) of some kind, achieving velocities of up to perhaps 10% c permitting one-way trips to nearby stars with durations comparable to a human lifetime.
- A Project Orion-ship, a nuclear-powered concept proposed by Freeman Dyson which would use nuclear explosions to propel a starship. A special case of the preceding nuclear rocket concepts, with similar potential velocity capability, but possibly easier technology.
- Laser propulsion concepts, using some form of beaming of power from the Solar System might allow a light-sail or other ship to reach high speeds, comparable to those theoretically attainable by the fusion-powered electric rocket, above. These methods would need some means, such as supplementary nuclear propulsion, to stop at the destination, but a hybrid (light-sail for acceleration, fusion-electric for deceleration) system might be possible.
The above concepts all appear limited to high, but still sub-relativistic speeds, due to fundamental energy and reaction mass considerations, and all would entail trip times which might be enabled by space colonization technology, permitting self-contained habitats with lifetimes of decades to centuries. Yet human interstellar expansion at average speeds of even 0.1% of c would permit settlement of the entire Galaxy in less than one half of a galactic rotation period of ~250,000,000 years, which is comparable to the timescale of other galactic processes. Thus, even if interstellar travel at near relativistic speeds is never feasible (which cannot be clearly determined at this time), the development of space colonization could allow human expansion beyond the Solar System without requiring technological advances that cannot yet be reasonably foreseen. This could greatly improve the chances for the survival of intelligent life over cosmic timescales, given the many natural and human-related hazards that have been widely noted.
The star Tau Ceti, about twelve light years away, has an abundance of cometary and asteroidal material in orbit around it. These materials could be used for the construction of space habitats for human settlement.
If humanity does gain access to a large amount of energy, on the order of the mass-energy of entire planets, it may eventually become feasible to construct Alcubierre drives. These are one of the few methods of superluminal travel which may be possible under current physics.
Looking beyond the Milky Way, there are about 100 billion other galaxies in the observable universe. The distances between galaxies are on the order of a million times further than those between the stars. Because of the speed of light limit on how fast any material objects can travel in space, intergalactic travel would either have to involve voyages lasting millions of years, or a possible faster than light propulsion method based on speculative physics, such as the Alcubierre drive. There are, however, no scientific reasons for stating that intergalactic travel is impossible in principle.
Space colonization can roughly be said to be possible when the necessary methods of space colonization become cheap enough (such as space access by cheaper launch systems) to meet the cumulative funds that have been gathered for the purpose. However, there are no immediate prospects for the large amounts of money required for space colonization to be available.
The President's Commission on Implementation of United States Space Exploration Policy suggested that an inducement prize should be established, perhaps by government, for the achievement of space colonization, for example by offering the prize to the first organization to place humans on the Moon and sustain them for a fixed period before they return to Earth.
In addition, funding of research that aims to develop cheaper methods of space colonization also contributes to making it possible.
Terrestrial analogues to space colonies
The most famous attempt to build an analogue to a self-sufficient colony is Biosphere 2, which attempted to duplicate Earth's biosphere. BIOS-3 is another closed ecosystem, completed in 1972 in Krasnoyarsk, Siberia.
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. The Mars Desert Research Station has a habitat for similar reasons, but the surrounding climate is not strictly inhospitable.
Nuclear submarines provide an example of conditions encountered in artificial space environment. Crews of these vessels often spend long periods (6 months or more) submerged during their deployments. However, the submarine environment provides a somewhat open life support system since the vessel can replenish supplies of fresh water and oxygen from seawater.
Other examples of small groups in isolated living conditions are record long-distance flights, long-distance (single-handed) sails, oil platforms, prisons, bunkers, small islands and underground bases.
The study of terrestrial analogues is also a central focus in space architecture.
The Russian schoolmaster and physicist Konstantin Tsiolkovsky foresaw elements of the space community in his book Beyond Planet Earth written about 1900. Tsiolkovsky had his space travelers building greenhouses and raising crops in space. Tsiolkovsky believed that going into space would help perfect human beings, leading to immortality and peace.
Others have also written about space colonies as Lasswitz in 1897 and Bernal, Oberth, Von Pirquet and Noordung in the 1920s. Wernher von Braun contributed his ideas in a 1952 Colliers article. In the 1950s and 1960s, Dandridge M. Cole published his ideas.
Another seminal book on the subject was the book The High Frontier: Human Colonies in Space by Gerard K. O'Neill in 1977 which was followed the same year by Colonies in Space by T. A. Heppenheimer.
As of 2013[update], Bigelow Aerospace is the only private commercial spaceflight company that has launched two experimental space station modules, Genesis I (2006) and Genesis II (2007), into Earth-orbit, and is planning to launch their BA 330 commercial production module into space by 2014 or 2015.
Colonizing space would require massive amounts of financial, physical and human capital devoted to research, development, production, and deployment. The planet's natural resources do not increase to a noteworthy extent (which is in keeping with the "only one Earth" position of environmentalists). Thus, considerable efforts in colonizing places outside Earth would appear as a hazardous waste of the Earth's limited resources for an aim without a clear end.
The fundamental problem of public things, needed for survival, such as space programs, is the free rider problem. Convincing the public to fund such programs would require additional self-interest arguments: If the objective of space colonization is to provide a "backup" in case everyone on Earth is killed, then why should someone on Earth pay for something that is only useful after they are dead? This assumes that space colonization is not widely acknowledged as a sufficiently valuable social goal.
Although seen as a relief to the problem of overpopulation, others have argued that space colonization is an impractical solution; in 1999, science fiction author Arthur C. Clarke said that "the population battle must be fought or won here on Earth".
Other objections include concern about creating a culture in which humans are no longer seen as human, but rather as material assets. The issues of human dignity, morality, philosophy, culture, bioethics, and the threat of megalomaniac leaders in these new "societies" would all have to be addressed in order for space colonization to meet the psychological and social needs of people living in isolated colonies.
As an alternative or addendum for the future of the human race, many science fiction writers have focused on the realm of the 'inner-space', that is the computer aided exploration of the human mind and human consciousness.
Robotic exploration is proposed as an alternative to gain many of the same scientific advantages without the limited mission duration and high cost of life support and return transportation involved in manned missions.
It could seem that nationalism might unfold ever bigger dangers, once one carries it up and out into space. The exploration of space stronger and stronger blocks up the practical possibility of a war, as it decisively strengthens the factor of deterrence.
Organizations that contribute to space colonization include:
- The Space Studies Institute funds the study of space habitats.
- The National Space Society is an organization with the vision of people living and working in thriving communities beyond the Earth. The NSS also maintains an extensive library of full-text articles and books on space settlement. 
- The Space Frontier Foundation performs space advocacy including strong free market, capitalist views about space development.
- The Living Universe Foundation has a detailed plan in which the entire galaxy is colonized.
- The Colonize the Cosmos site advocates orbital colonies.
- The Mars Society promotes Robert Zubrin's Mars Direct plan and the settlement of Mars.
- The Planetary Society is the largest space interest group, but has an emphasis on robotic exploration and the search for extraterrestrial life.
- The Space Settlement Institute is searching for ways to make space colonization happen in our lifetimes.
- Students for the Exploration and Development of Space (SEDS) is a student organization founded in 1980 at MIT and Princeton.
- Foresight Nanotechnology Institute – Guiding nanotechnology research to improve fuels, smart materials, uniforms and environments for the pursuit of space exploration and colonization.
- The Alliance to Rescue Civilization plans to establish backups of human civilization on the Moon and other locations away from Earth.
- The Artemis Project plans to set up a private lunar surface station.
- The British Interplanetary Society promotes ideas for the exploration and utilization of space, including a Mars colony, future propulsion systems (see Project Daedalus), terraforming, and locating other habitable worlds.
- In June 2013 the BIS began a project  to re-examine the space colony studies of the 1970s and revise them in view of advances made since then.
Although established space colonies are a stock element in science fiction stories, fictional works that explore the themes, social or practical, of the settlement and occupation of a habitable world are much rarer.
- Space habitat
- Domed city
- Extraterrestrial liquid water
- Extraterrestrial real estate
- Human outpost
- Mars One
- Mars to Stay
- Ocean colonization
- Planetary habitability
- Solar analog
- Space archaeology
- Space colonization of specific targets - see link panel at top-right of article
- Space law
- Space stations and habitats in popular culture
- Timeline of Solar System exploration
- Underground city
- Criticism of the Space Shuttle program
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