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Space colonization (also called space settlement, or extraterrestrial colonization) is permanent human habitation off the planet Earth.
This article is mainly about colonies on bodies apart from Earth. For free space colonies in micro-g see space habitat.
Many arguments have been made for and against space colonization. The two most common in favor of colonization are survival of human civilization and the biosphere in the event of a planetary-scale disaster (natural or man-made), and the availability of additional resources in space that could enable expansion of human society. The most common objections to colonization include concerns that the commodification of the cosmos may be likely to enhance the interests of the already powerful, including major economic and military institutions, and to exacerbate pre-existing detrimental processes such as wars, economic inequality, and environmental degradation.
No space colonies have been built so far. Currently, the building of a space colony would present a set of huge technological and economic challenges. Space settlements would have to provide for nearly all (or 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 controlled ecological 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. Because of the present cost of sending anything from the surface of the Earth into orbit (around $2,500 per-pound to orbit, expected to further decrease), a space colony would currently be a massively expensive project.
There are yet no plans for building space colonies by any large-scale organization, either government or private. However, many proposals, speculations, and designs for space settlements have been made through the years, and a considerable number of space colonization advocates and groups are active. Several famous scientists, such as Freeman Dyson, have come out in favor of space settlement.
On the technological front, there is ongoing progress in making access to space cheaper (reusable launch systems could reach $10 per-pound to orbit), and in creating automated manufacturing and construction techniques.
- 1 Reasons
- 2 Goals
- 3 Method
- 4 Location
- 4.1 Near-Earth space
- 4.2 Other micro-g orbits
- 4.3 The inner planets
- 4.4 Asteroid belt
- 4.5 Moons of outer planets
- 4.6 Trans-Neptunian region
- 4.7 Outside the Solar System
- 5 Funding
- 6 Terrestrial analogues to space colonies
- 7 History
- 8 Planetary protection
- 9 Objections
- 10 Involved organizations
- 11 In fiction
- 12 See also
- 13 References
- 14 Further reading
Survival of human civilization
The primary argument calling for space colonization is the long-term survival of human civilization. By developing alternative locations off Earth, the planet's species, including humans, could live on in the event of natural or man-made disasters on our own planet.
On two occasions, theoretical physicist and cosmologist Stephen Hawking has argued for space colonization as a means of saving humanity. In 2001, Hawking predicted that the human race would become extinct within the next thousand years, unless colonies could be established in space. In 2006, he stated that humanity 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.
... 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.
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.
Based on his Copernican principle, J. Richard Gott has estimated that the human race could survive for another 7.8 million years, but it is not 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".
Vast resources in space
Resources in space, both in materials and energy, are enormous. The Solar System alone has, according to different estimates, enough material and energy to support anywhere from several thousand to over a billion times that of the current Earth-based human population. Outside the Solar System, several hundred billion other stars in the observable universe provide opportunities for both colonization and resource collection, though travel to any of them is impossible on any practical time-scale without interstellar travel by use of generation ships or revolutionary new methods of travel, such as faster-than-light (FTL).
Asteroid mining will also be a key player in space colonization. Water and materials to make structures and shielding can be easily found in asteroids. Instead of resupplying on Earth, mining and fuel stations need to be established on asteroids to facilitate better space travel. Optical mining is the term NASA uses to describe extracting materials from asteroids. NASA believes by using propellant derived from asteroids for exploration to the moon, Mars, and beyond will save $100 billion. If funding and technology come sooner than estimated, asteroid mining might be possible within a decade.
All these planets and other bodies offer a virtually endless supply of resources providing limitless growth potential. Harnessing these resources can lead to much economic development.
Expansion with fewer negative consequences
Expansion of humans and technological progress has usually resulted in some form of environmental devastation, and destruction of ecosystems and their accompanying wildlife. In the past, expansion has often come at the expense of displacing many indigenous peoples, the resulting treatment of these peoples ranging anywhere from encroachment to genocide. Because space has no known life, this need not be a consequence, as some space settlement advocates have pointed out.
Alleviating overpopulation and resource demand
Another argument for space colonization is to mitigate the negative effects of overpopulation.[clarification needed] 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 many of Earth's resources are non-renewable, off-planet colonies could satisfy the majority of the planet's resource requirements. With the availability of extraterrestrial resources, demand on terrestrial ones would decline.
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.
In a 2001 interview with Freeman Dyson, J. Richard Gott and Sid Goldstein, they were asked for reasons why some humans should live in space. Their answers were:
- 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
Although some items of the infrastructure requirements above can already be easily produced on 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 mining and extraction of metals from a small asteroid the size of 3554 Amun or (6178) 1986 DA, both small near-Earth asteroids, would be 30 times as much metal as humans have mined throughout history. A metal asteroid this size would be worth approximately US$20 trillion 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 saw greater 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.
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 venture has never been carried out before — the high-risk nature of the investment.
Major governments and well-funded corporations have announced plans for new categories of activities: space tourism and hotels, prototype space-based solar-power satellites, heavy-lift boosters and asteroid mining—that create needs and capabilities for humans to be present in space.
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 near 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 governments, 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. Mission chief scientist 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 (NEO), Phobos, or Deimos. The benefits of using such sources include: a lower gravitational force, no atmospheric drag on cargo vessels, and 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. Light intensity obeys an inverse-square law. So the solar energy available at distance d from the Sun is E = 1367/d2 W/m2, where d is measured in astronomical units (AU) and 1367 watts/m2 is the energy available at the distance of Earth's orbit from the Sun, 1 AU.
In the weightlessness and vacuum of space, high temperatures for industrial processes can easily be achieved in solar ovens with huge parabolic reflectors made of metallic foil with very lightweight support structures. Flat mirrors to reflect sunlight around radiation shields into living areas (to avoid line-of-sight access for cosmic rays, or to make the Sun's image appear to move across their "sky") or onto crops are even lighter and easier to build.
Large solar power photovoltaic cell arrays or thermal power plants would be needed to meet the electrical power needs of the settlers' use. In developed parts of Earth, electrical consumption can average 1 kilowatt/person (or roughly 10 megawatt-hours per person per year.) These power plants could be at a short distance from the main structures if wires are used to transmit the power, or much farther away with wireless power transmission.
A major export of the initial space settlement designs was anticipated to be large solar power satellites that would use wireless power transmission (phase-locked microwave beams or lasers emitting wavelengths that special solar cells convert with high efficiency) to send power to locations on Earth, or to colonies on the Moon or other locations in space. For locations on Earth, this method of getting power is extremely benign, with zero emissions and far less ground area required per watt than for conventional solar panels. Once these satellites are primarily built from lunar or asteroid-derived materials, the price of SPS electricity could be lower than energy from fossil fuel or nuclear energy; replacing these would have significant benefits such as the elimination of greenhouse gases and nuclear waste from electricity generation.
Transmitting solar energy wirelessly from the Earth to the Moon and back is also an idea proposed for the benefit of space colonization and energy resources. Physicist Dr. David Criswell, who worked for NASA during the Apollo missions, came up with the idea of using power beams to transfer energy from space. These beams, microwaves with a wavelength of about 12 cm, will be almost untouched as they travel through the atmosphere. They can also be aimed at more industrial areas to keep away from humans or animal activities. This will allow for safer and more reliable methods of transferring solar energy.
In 2008, scientists were able to send a 20 watt microwave signal from a mountain in Maui to the island of Hawaii. Since then JAXA and Mitsubishi has teamed up on a $21 billion project in order to place satellites in orbit which could generate up to 1 gigawatt of energy. These are the next advancements being done today in order to make energy be transmitted wirelessly for space-based solar energy.
However, the value of SPS power delivered wirelessly to other locations in space will typically be far higher than to Earth. Otherwise, the means of generating the power would need to be included with these projects and pay the heavy penalty of Earth launch costs. Therefore, other than proposed demonstration projects for power delivered to Earth, the first priority for SPS electricity is likely to be locations in space, such as communications satellites, fuel depots or "orbital tugboat" boosters transferring cargo and passengers between low-Earth orbit (LEO) and other orbits such as geosynchronous orbit (GEO), lunar orbit or highly-eccentric Earth orbit (HEEO).:132 The system will also rely on satellites and receiving stations on Earth to convert the energy into electricity. Because of this energy can be transmitted easily from dayside to nightside meaning power is reliable 24/7.
Nuclear power is sometimes proposed for colonies located on the Moon or on Mars, as the supply of solar energy is too discontinuous in these locations; the Moon has nights of two Earth weeks in duration. Mars has nights, relatively high gravity, and an atmosphere featuring large dust storms to cover and degrade solar panels. Also, Mars' greater distance from the Sun (1.5 astronomical units, AU) translates into E/(1.52 = 2.25) only ½-⅔ the solar energy of Earth orbit. Another method would be transmitting energy wirelessly to the lunar or Martian colonies from solar power satellites (SPSs) as described above; the difficulties of generating power in these locations make the relative advantages of SPSs much greater there than for power beamed to locations on Earth. In order to also be able to fulfill the requirements of a moon base and energy to supply life support, maintenance, communications, and research, a combination of both nuclear and solar energy will be used in the first colonies.
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.
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.
Space manufacturing could enable self-replication. Some think it's the ultimate goal because it allows an exponential 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. 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. Moreover, 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, whereas 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.
Assuming a journey of 6,300 years, the astrophysicist Frédéric Marin and the particle physicist Camille Beluffi calculated that the minimum viable population for a generation ship to reach Proxima Centauri would be 98 settlers at the beginning of the mission (then the crew will breed until reaching a stable population of several hundred settlers within the ship) .
Location is a frequent point of contention between space colonization advocates. The location of colonization can be on a physical body planet, dwarf planet, natural satellite, or asteroid or orbiting one. For colonies not on a body see also space habitat.
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 lack of atmosphere provides no protection from space radiation or meteoroids. The early Moon colonies may shelter in ancient Lunar lava tubes to gain protection. The two-week day/night cycle makes use of solar power more difficult.
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 because their distance from Earth would result in only brief and infrequent eclipses of light from the Sun. However, the fact that the Earth–Moon Lagrange points L4 and L5 tend to collect dust and debris, whereas 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.
Other micro-g orbits
See space habitat
The inner planets
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Colonizing Mercury would involve similar challenges as the Moon as there are few volatile elements, no atmosphere and the surface gravity is lower than Earth's. However, the planet also receives almost seven times the solar flux as the Earth/Moon system.
Geologist Stephen Gillett suggested in 1996 that this could make Mercury an ideal place to build and launch solar sail spacecraft, which could launch as folded up "chunks" by mass driver from Mercury's surface. Once in space the solar sails would deploy. Since Mercury's solar constant is 6.5 times higher than Earth's, energy for the mass driver should be easy to come by, and solar sails near Mercury would have 6.5 times the thrust they do near Earth. This could make Mercury an ideal place to acquire materials useful in building hardware to send to (and terraform) Venus. Vast solar collectors could also be built on or near Mercury to produce power for large scale engineering activities such as laser-pushed lightsails to nearby star systems.
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 500 million kilometers of space. The colonists would have a strong interest in assuring 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.
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 habitation. Europa is considered one of the more habitable bodies in the Solar System and so merits investigation as a possible abode for life.
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 due to its distance from Jupiter, and thus the planet's harmful radiation. It could be possible to build a surface base that would produce fuel for further exploration of the Solar System.
Three of the Galilean moons (Europa, Ganymede, Callisto) have an abundance of volatiles that may support colonization efforts.
Moons of Saturn – Titan, Enceladus, and others
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. Titan has ice water and large methane oceans. Robert Zubrin identified Titan as possessing an abundance of all the elements necessary to support life[where?], 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.
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.
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.
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. However it is probable that such a device could never exist, due to the fundamental challenges posed. For more on this see Difficulties of making and using an Alcubierre Drive.
Looking beyond the Milky Way, there are at least 2 trillion other galaxies in the observable universe. The distances between galaxies are on the order of a million times farther 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.
Although there are no immediate prospects for the large amounts of money required for space colonization to be available given traditional launch costs,[full citation needed] there is some prospect of a radical reduction to launch costs in the 2010s, which would consequently lessen the cost of any efforts in that direction. With a published price of US$56.5 million per launch of up to 13,150 kg (28,990 lb) payload to low Earth orbit, SpaceX Falcon 9 rockets are already the "cheapest in the industry". Advancements currently being developed as part of the SpaceX reusable launch system development program to enable reusable Falcon 9s "could drop the price by an order of magnitude, sparking more space-based enterprise, which in turn would drop the cost of access to space still further through economies of scale." If SpaceX is successful in developing the reusable technology, it would be expected to "have a major impact on the cost of access to space", and change the increasingly competitive market in space launch services.
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.
Terrestrial analogues to space colonies
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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.
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 has indicated that their first production model of the space habitat, the BA 330, could be launched by 2017.
Robotic spacecraft to Mars are required to be sterilized, to have at most 300,000 spores on the exterior of the craft—and more thoroughly sterilized if they contact "special regions" containing water, otherwise there is a risk of contaminating not only the life-detection experiments but possibly the planet itself.
It is impossible to sterilize human missions to this level, as humans are host to typically a hundred trillion microorganisms of thousands of species of the human microbiome, and these cannot be removed while preserving the life of the human. Containment seems the only option, but it is a major challenge in the event of a hard landing (i.e. crash). There have been several planetary workshops on this issue, but with no final guidelines for a way forward yet. Human explorers would also be vulnerable to back contamination to Earth if they become carriers of microorganisms.
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A corollary to the Fermi paradox—"nobody else is doing it"—is the argument that, because no evidence of alien colonization technology exists, it is statistically unlikely to even be possible to use that same level of technology ourselves.
Colonizing space would require massive amounts of financial, physical, and human capital devoted to research, development, production, and deployment. Earth'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.
Seen as a relief to the problem of overpopulation even as early as 1758, and listed as one of Stephen Hawking's reasons for pursuing space exploration, it has become apparent that space colonisation in response to overpopulation is unwarranted. Indeed, the birth rates of many developed countries, specifically spacefaring ones, are at or below replacement rates, thus negating the need to use colonisation as a means of population control.
Other objections include concerns that the forthcoming colonization and commodification of the cosmos may be likely to enhance the interests of the already powerful, including major economic and military institutions e.g. the large financial institutions, the major aerospace companies and the military–industrial complex, to lead to new wars, and to exacerbate pre-existing exploitation of workers and resources, economic inequality, poverty, social division and marginalization, environmental degradation, and other detrimental processes or institutions.
Additional concerns include 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—possibly en route developmentally to a Matrioshka Brain.
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.
Physical, mental and emotional health risks to colonizers
An additional concern is the health of the humans who may participate in a colonization venture, including a range of physical, mental and emotional health risks.
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 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.(registration required)
- 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.
- Asgardia (nation) - an organization searching to circumvent limitations placed by Outer Space Treaty
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.
- Domed city
- Extraterrestrial liquid water
- Extraterrestrial real estate
- Human outpost
- Mars One
- Mars to Stay
- Ocean colonization
- Colonization of Antarctica
- Planetary habitability
- Solar analog
- Space archaeology
- Space habitat, in orbit or free space (eg. Bernal Sphere, Stanford Torus, and O'Neill cylinders)
- Space law
- Timeline of Solar System exploration
- Underground city
- Criticism of the Space Shuttle program
- Mars analog habitat
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