Space colonization – Wikipedia

Space colonization (also called space settlement, or extraterrestrial colonization) is permanent human habitation off the planet Earth.

Many arguments have been made for and against space colonization.[1] The two most common in favor of colonization are survival of human civilization and the biosphere in case of a planetary-scale disaster (natural or man-made), and the vast resources in space for 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.[2][3]

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)[4] 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.[5]

On the technological front, there is ongoing progress in making access to space cheaper (reusable launch systems could reach $10 per-pound to orbit)[6] and in creating automated manufacturing and construction techniques.[7]

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.[8] 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.[9]

In 2005, then NASA Administrator Michael Griffin identified space colonization as the ultimate goal of current spaceflight programs, saying:

...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.[10]

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.[11] 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.[12]

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".[13]

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.[14][15][16] 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 the use of generation ships or revolutionary new methods of travel, such as faster-than-light (FTL) engines.

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.[17]

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 full-blown genocide. Because space has no known life, this need not be a consequence, as some space settlement advocates have pointed out.[18][19]

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.[20]

Additional goals cite the innate human drive to explore and discover, a quality recognized at the core of progress and thriving civilizations.[21][22]

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).[23] 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.[5] Their answers were:

There would be a very high initial investment cost for space colonies and any other permanent space infrastructure due to the high cost of getting into space. However, proponents argue that the long-term vision of developing space infrastructure will provide long-term benefits far in excess of the initial start-up costs.[citation needed]

Because current space launch costs are so high ($4,000 to $40,000 per kilogram), any serious plans for space colonization must include developing low-cost access to space followed by developing in-situ resource utilization. Therefore, the initial investments must be made in the development of low-cost access to space followed by an initial capacity to provide these necessities: materials, energy, propellant, communication, life support, radiation protection, self-replication, and population.[citation needed]

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.[24]

Some of these high-value trade goods include precious metals,[25][26] gemstones,[27] power,[28] solar cells,[29] ball bearings,[29] semi-conductors,[29] and pharmaceuticals.[29]

...the smallest Earth-crossing asteroid 3554 Amun... is a mile-wide (2km) 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.[25]

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.[30][31][32]

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 main impediments to commercial exploitation of these resources are the very high cost of initial investment,[33] the very long period required for the expected return on those investments (The Eros Project plans a 50-year development),[34] 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 miningthat create needs and capabilities for humans to be present in space.[35][36][37]

There are two main types of space colonies:

There is considerable debate among space settlement advocates as to which type (and associated locations) represents the better option for expanding humanity into space.[citation needed]

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 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 the state,[38] an argument made by John Hickman[39] and Neil deGrasse Tyson.[40]

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.[41] 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.[42] 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, 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.[43]

Farther out, Jupiter's Trojan asteroids are thought to be rich in water ice and other volatiles.[44]

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.[45]

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 nations on Earth, electrical consumption can average 1 kilowatt/person (or roughly 10 megawatt-hours per person per year.)[46] 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 elimination of greenhouse gases and nuclear waste from electricity generation.

However, the value of SPS power delivered wirelessly to other locations in space will typically be far higher than to locations on 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,[36] 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).[47]:132

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 - the solar energy of Earth orbit.[48] Another method would be transmitting energy wirelessly to the lunar or Martian colonies from solar power satellites (SPSs) as described abovenote again that 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.

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.

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.[citation needed] One possibility is the air-breathing hypersonic spaceplane under development by NASA and other organizations, both public and private. Other proposed projects include skyhooks, space elevators, mass drivers, launch loops, and StarTrams.

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, electric sails, and nuclear thermal propulsion can all potentially help solve the problems of high transport cost once in space.

For lunar materials, one well-studied possibility is to build mass drivers to launch bulk materials to waiting settlements. Alternatively, lunar space elevators might be employed.

Lunar rovers and Mars rovers are common features of proposed colonies for those bodies. Space suits would likely be needed for excursions, maintenance, and safety.

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 finitude of the speed of light and the greatly varying distance between conjunction and oppositionthe lag will range between 7 and 44 minutesmaking 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.[citation needed] 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:

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.[51]

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.[52] 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).[51] 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 the ultimate goal because it allows an exponential increase in colonies, while eliminating costs to and dependence on Earth.[53] It could be argued that the establishment of such a colony would be Earth's first act of self-replication.[54] 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.[55]

In 2002, the anthropologist John H. Moore estimated that a population of 150180 would permit a stable society to exist for 60 to 80 generations equivalent to 2000 years.

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 longterm 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:

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. The level of (pseudo-) gravity can be controlled at any desired level by rotating an orbital colony.

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. As of 2016[update], the International Space Station provides a temporary, yet still non-autonomous, human presence in low Earth orbit.

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.

Another near-Earth possibility are the five EarthMoon 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 EarthMoon 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 L2L5 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.

The five EarthSun Lagrange points would totally eliminate eclipses, but only L1 and L2 would be reachable in a few days' time. The other three EarthSun points would require months to reach.

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 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[56] 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.[citation needed]

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 pressure vessels on the surface possible. There is some discussion of it actually being done.[citation needed]

The moons of Mars may be a target for space colonization. Low delta-v is needed to reach 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 50km, the pressure is reduced to a few atmospheres, and the temperature would be between 40100C, 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.

Because of Mercury's extremely small axial tilt, there is a suggestion that Mercury's polar regions could be colonized using the same technology, approach, and equipment that is used in colonizing the Moon. Polar colonies on Mercury would avoid the extreme daytime temperatures elsewhere on the planetthe temperatures on the poles are consistently below 93C (135F). Moreover, "Mercurys very low axial tilt (0.034) means that its polar regions are permanently shaded and cold enough to contain water ice."[57]

Observations of Mercury's polar regions by radar from Earth and the MESSENGER spacecraft have been consistent with water ice and/or other frozen volatiles being present in permanently shadowed areas of craters in Mercury's polar regions.[58] Measurements of Mercury's exosphere, which is practically a vacuum, revealed more ions derived from water than scientists had expected.[59] These volatiles would be available to hypothetical future colonists of Mercury.[57]

Compared on the Moon, solar panels on Mercury would be exposed to far more energythe intensity ranges from approximately four and a half times to more than ten times the intensity at one astronomical unit. In addition, the solar energy available to a colony on Mercury would never be blocked by an eclipse. On the other hand, it would need to deal with the far greater variance of solar intensity, which is a product of the planet's highly elliptical orbit.[57]

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.

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.

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.[60] 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.

The three out of four largest moons of Jupiter (Europa, Ganymede and Callisto) have an abundance of volatiles making future colonization possible.

Titan is suggested as a target for colonization,[61] because it is the only moon in the Solar System to have a dense atmosphere and is rich in carbon-bearing compounds.[62]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.

Other large satellites: Rhea, Iapetus, Dione, Tethys, and Mimas, all have large quantities of volatiles, which can be used to support settlement.

Although they are very cold, the five large moons of Uranus (Miranda, Ariel, Umbriel, Titania and Oberon) and TritonNeptune's largest moonhave large amounts of frozen water and other volatiles and could potentially be settled. However, habitats there would require a lot of nuclear power to sustain a habitable temperature. Triton's thin atmosphere also contains some nitrogen and even some frozen nitrogen on the surface (the surface temperature is 38 K or about -391Fahrenheit).

The Kuiper belt is estimated to have 70,000 bodies of 100km or larger.

Freeman Dyson has suggested that within a few centuries human civilization will have relocated to the Kuiper belt.[63]

The Oort cloud is estimated to have up to a trillion comets.

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.

It may be possible to colonize the three farthest giant planets that is, Saturn, Uranus and Neptune with floating cities in their atmospheres. By heating hydrogen balloons, large masses can be suspended underneath at roughly Earth-like gravity. A human colony on Jupiter would be less practical due to its high gravity, escape velocity, and radiation. Such colonies could export helium-3 for use in fusion reactors if they ever become operational. Escape from the giant planets, especially Jupiter, seems well beyond current or near-term foreseeable chemical-rocket technology due to the combination of large velocity and high acceleration needed to even achieve low orbit.

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.[citation needed]

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.

Hypothetical starship concepts proposed both by scientists and in hard science fiction include:

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.

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,[64] 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.

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Space colonization - Wikipedia