Definitions

# Space colonization

Space colonization (also called space settlement, space humanization, space habitation, etc.) is the concept of autonomous (self-sufficient) human habitation of locations outside Earth. It is a major theme in science fiction, as well as a long-term goal of various national space programs.

While many people think of space colonies on the Moon or Mars, others argue that the first colonies will be in orbit. They have determined that there are ample quantities of all the necessary materials on the Moon and Near Earth Asteroids, that solar energy is readily available in very large quantities.

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

As of 2008, the international space station provides a permanent, yet still non-autonomous, human presence in space. The NASA Lunar outpost, providing a permanent human presence on the moon, is at the planning stage. There is an ongoing development of technologies that may be used in future space colonization projects.

## Method

Building colonies in space will require access to food, space, people, construction materials, energy, transportation, communications, life support, simulated gravity, and radiation protection. Colonies will presumably be situated to help fulfill those requirements.

### Materials

Colonies on the Moon and Mars could use local materials, although the Moon is deficient in volatiles (principally hydrogen, and nitrogen) but possesses a great deal of oxygen, silicon, and metals such as iron, aluminum and titanium. Launching materials from Earth is very expensive, so bulk materials could come from the Moon, a Near-Earth Object (NEO — an asteroid or comet with an orbit near Earth), Phobos or Deimos where gravitational forces are much smaller, there is no atmosphere, and there is no biosphere to damage. Many NEOs contain substantial amounts of metals, oxygen, hydrogen and carbon. Certain NEOs may also contain some nitrogen.

Farther out, Jupiter's Trojan asteroids are thought to be high in water ice and probably other volatiles

### Energy

Solar energy in orbit is abundant, reliable, and is commonly used to power satellites today. There is no night in space, and no clouds or atmosphere to block sunlight. The solar energy available, in watts per square meter, at any distance, d, from the Sun can be calculated by the formula E = 1366/d², 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 at no cost; 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 has been suggested as 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, continuous energy could be beamed to the lunar surface from a solar power satellite at the Lagrange L1 location.

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. Alternatively, the waste heat can be used to melt ice on the poles of a planet like Mars.

### Transportation

#### Space Access

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 air-breathing hypersonic spaceplane under development by NASA and other organizations, both public and private. There are also proposed projects such as building a space elevator or a mass driver; or launch loops.

#### 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 relatively 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.

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

### Communication

Compared to the other requirements, communication is relatively 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.

### Life support

People need air, water, food, gravity and reasonable temperatures to survive for long periods. On Earth, a large complex biosphere provides these. In space settlements, a relatively small, closed ecological system must recycle or import all the nutrients without "crashing."

The closest terrestrial analogue to space life support is possibly that of the 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" and typically dump carbon dioxide overboard, although they recycle oxygen. Recycling of the carbon dioxide has been approached in the literature using the Sabatier process or the Bosch reaction.

Alternatively, and more attractive to many, 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:

97–99% of the light energy provided to the plant ends up as heat and needs to be dissipated somehow to avoid overheating the habitat.

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. Somewhere around 5–10 tons of material per square meter of surface area is required. This can be achieved cheaply with 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. Inertia would necessitate powerful thrusters to start or stop rotation.

### Self-replication

Self-replication is an optional attribute, but many 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, etc.) and colonies that just require periodic supply of light weight objects, such as integrated circuits, medicines, genetic material and perhaps some tools.

### Population size

In 2002, the anthropologist John H. Moore estimated that a population of 150–180 would allow normal reproduction for 60 to 80 generations — equivalent to 2000 years.

A much smaller initial population of as little as two female humans should be viable as long as human embryos are available from Earth. Use of a sperm bank from Earth also allows a smaller starting base with negligible inbreeding.

Researchers in conservation biology have tended to adopt the "50/500" rule of thumb initially advanced by Franklin and Soule. This rule says a short-term effective population size (Ne) of 50 is needed to prevent an unacceptable rate of inbreeding, while a long‐term Ne of 500 is required to maintain overall genetic variability. The $N_e = 50$ prescription corresponds to an inbreeding rate of 1% per generation, approximately half the maximum rate tolerated by domestic animal breeders. The $N_e = 500$ value attempts to balance the rate of gain in genetic variation due to mutation with the rate of loss due to genetic drift.

Effective population size Ne depends on the number of males Nm and females Nf in the population according to the formula:

$N_e = frac\left\{4 times N_m times N_f\right\} \left\{N_m + N_f\right\}$

## Location

Location is a frequent point of contention between space colonization advocates.

The location of colonization can be:

### Planetary Locations

Some planetary colonization advocates cite the following potential locations:

#### Mars

Mars is a frequent topic of discussion. Its overall surface area is similar to the dry land surface of Earth, it may have large water reserves, and has carbon (locked as carbon dioxide in the atmosphere).

Mars may have gone through similar geological and hydrological processes as Earth and contain valuable mineral ores, but this is debated. Equipment is available to extract in situ resources (water, air, etc.) from the Martian ground and atmosphere. There is a strong scientific interest in colonizing Mars due to the possibility that 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. 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 of time (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.

Mars is often the topic of discussion regarding terraforming to make the entire planet or at least large portions of it habitable.

See also: Exploration of Mars, Martian terraforming

#### Mercury

There is a suggestion that Mercury could be colonized using the same technology, approach and equipment that is used in colonization of the Moon. Such colonies would almost certainly be restricted to the polar regions due to the extreme daytime temperatures elsewhere on the planet. The recent discovery of ionized water has astounded scientists. This discovery significantly improves the small planet's prospects as a future colony.

#### Venus

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 a height of approximately 50 km, the pressure is reduced to a few atmospheres, and the temperature would be between 40–100 °C, depending on the height. 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.

#### Gas Giants

It may also 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. Jupiter would be less suitable for habitation due to its high gravity, escape velocity and radiation. Such colonies could export Helium-3 for use in fusion reactors if they ever become practical.

### Satellite locations

#### The Moon

Due to its proximity and relative familiarity, Earth's Moon is also frequently 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 major drawback of the Moon is its low abundance of volatiles necessary for life such as hydrogen and carbon. Water ice deposits that may exist in some polar craters could serve as a source for these elements. An alternative solution is to bring hydrogen from NE asteroids and combine it with oxygen extracted from lunar rock.

The moon's low surface gravity is also a concern (it is unknown whether 1/6g is sufficient to support human habitation for long periods — see microgravity).

#### Europa, Callisto (or other Jovian moons)

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 also discusses possible use of "air pockets" for human inhabitation.

#### Phobos and Deimos

The moons of Mars may be an appealing 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 inhabited, with methods similar to those for asteroids. The original DOOM game featured a colony on Deimos and one on Phobos.

#### Titan

Titan has been suggested as an appealing target for colonization, because it is the only moon in our solar system to have a dense atmosphere and is rich in carbon-bearing compounds.

### Free space locations

#### Space habitats

Free 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 design proposals have been made with varying degrees of realism by both science fiction authors and engineers.

A space habitat would also serve as a proving ground for how well a generation ship could function as a long-term home for hundreds or thousands of people. Such a space habitat could be isolated from the rest of humanity for a century, but near enough to Earth for help. This would test if thousands of humans can survive a century on their own before sending them beyond the reach of any help.

#### Earth orbit

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, whereas all planets lose sunlight at least half the time. 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.

#### Lagrange points

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.

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

However, the fact that 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.

#### Asteroids

##### Near Earth Asteroids

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.

##### Main Belt Asteroids

Colonization of asteroids would require space habitats. The asteroid belt has significant overall material available, 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.

#### Statites

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.

## Outside the Solar system

Looking beyond our solar system, there are billions of potential suns with possible colonization targets.

Physicist Stephen Hawking has said:

The long-term survival of the human race is at risk as long as it is confined to a single planet. Sooner or later, disasters such as an asteroid collision or nuclear war could wipe us all out. But once we spread out into space and establish independent colonies, our future should be safe. There isn't anywhere like the Earth in the solar system, so we would have to go to another star.

### Starship

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:

• 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 (eg, 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 (assuming it is not inhabited already) 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.

### Example

The star Tau Ceti, about eleven 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.

## 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.

Many space agencies build testbeds for advanced life support systems, but these are designed for long duration human spaceflight, not permanent colonization.

Remote research stations in inhospitable climates, such as the Amundsen-Scott South Pole Station or Devon Island Mars Arctic Research Station, can also provide some practice for off-world outpost construction and operation. 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.

## Literature

The literature for space colonization began in 1869 when Edward Everett Hale wrote about an inhabited artificial satellite.

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.

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.

M. Dyson wrote Home on the Moon; Living on a Space Frontier in 2003; Peter Eckart wrote Lunar Base Handbook in 2006 and then Harrison Schmitt's Return to the Moon written in 2007.

## Justification

In 2001, the space news website Space.com asked Freeman Dyson, J. Richard Gott and Sid Goldstein for reasons why some humans should live in space. Their respective answers were:

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 scientist Paul Davies also supports the view that if a planetary catastrophe threatens the survival of the human species on Earth, a self-sufficient colony could "reverse-colonize" the Earth and restore human civilization.

The author and journalist William E. Burrows and the biochemist Robert Shapiro proposed a private project, the Alliance to Rescue Civilization, with the goal of establishing an off-Earth backup of human civilization.

Another important reason used to justify space is the effort to increase the knowledge and technological abilities of humanity.

## Objections

Colonizing space can be viewed as too expensive and a waste of time.

The argument to live together on the earth we have suggests that if even half the money of space exploration were spent for terrestrial betterment, there would be greater good for a greater number of people, at least in the short term. This assumes that money not spent on space would go toward socially beneficial projects. It also assumes that space colonization is not itself a sufficiently valuable goal (see Space and survival).

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 to space colonization to meet the psychological and social needs needs of people living in isolated colonies or generation ships. Although they are not being utilized yet, cultural anthropologists may have something to offer to the space programs.

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.

## Counter arguments

### The argument of need

The population of Earth continues to increase, while its carrying capacity and available resources do not. If the resources of space are opened to use and viable life-supporting habitats can be built, the Earth will no longer define the limitations of growth (see extraterrestrial population growth). On the other hand, extrapolations made using available figures for population growth, shows that the population of Earth will stop growing around 2070.

Furthermore, even if humanity manages to avoid devastating the Earth through war, pestilence, pollution, global cooling, global warming, and even cometary impacts, the Earth will ultimately become uninhabitable by the heating of the Sun as it ages. If humanity has not made permanent habitations in space by the time any one of these incidents occurs, it may very well go extinct.

"Maybe the reason civilizations don’t get around to colonizing other planets is that there’s a narrow window when they have the tools, population and will to do so, and the window usually closes on them."

--J. Richard Gott III

"If it’s true that civilizations normally go extinct because they get stuck on their home planets, then the odds are against us

--J. Richard Gott III

### The argument of cost

Very many people greatly overestimate how much money is spent on space, and underestimate how much money is spent on defense or social programs.

#### Military Spending

For example, as of 2008, over \$845 billion has been spent on the current war in Iraq. In comparison, it only cost \$2 billion to create the Hubble Space Telescope, and NASA's annual budget averages only about \$16 billion. In other words, the money that has been spent on the Iraq war could have theoretically funded NASA for approximately 52 years.

#### Social Spending Programs

The United States government spends ~\$581 billion on its Social Security program, an additional ~\$561 billion on Medicare, plus additional monies on other social programs whose budget’s lies within the bounds of the "Other Discretionary Spending" category of the Federal Budget. This means that the United States spends more than \$1.142 trillion on social programs per year (equal to more than \$3,807 per person per year). In comparison, the United States space program costs a mere \$53 per person per year.

### The argument of benefits

Detractors of the development of permanent space colonies and infrastructure often cite the very high initial investement costs of space colonies and permanent space infrastructure yet they ignore all potential returns on that investment. The long-term vision of developing space infrastructure is that it will provide long-term benefits far in excess of the initial startup 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 / kg 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. 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.

Once the needs of the permanent settlements have been met, any additional production capacity could be use to either extend that initial infrastructure (a concept commonly called "bootstrapping" ) or traded back to Earth in payment of the initial investment or in exchange for goods more easily manufactured on the Earth.

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, silcates, 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.

Some of these high trade value goods include precious metals, gem stones, power , solar cells, ball bearings, semi-conductors, and pharmceuticals.

... the smallest Earth-crossing asteroid 3554 Amun (see orbit) is a mile-wide (2,000-meter) 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.

In the 2,900 cubic kms 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 (estimated to be 50 years or more by some ), and because it has never been done before - the high-risk nature of the investment.

### The argument of Nationalism

Space proponents counter this argument by pointing out that humanity as a whole has been exploring and expanding into new territory since long before Europe's colonial period, going back into prehistory (the nationalist argument also ignores multinational cooperative space efforts); that seeing the Earth as a single, discrete object instills a powerful sense of the unity, connectedness of the human environment, and of the immateriality of political borders; and that in practice, international collaboration in space has shown its value as a unifying and cooperative endeavor.

### The argument of 'Inner Space'

This form of exploration need not be exclusive to space colonization, as exemplified for example by Transhumanist philosophies.

## In fiction

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. The following list is restricted to works dealing primarily with the initial stages of colonization.

### Games

• Alien Legacy (1994), computer game. Player has to manage new colonies on the planets of Beta Caeli.
• Ascendancy (1995), computer game. Player tries to grow a colony into a spacefaring civilization.
• Halo (2001), computer game. Player fights for Earth's colonies against the alien Covenant.
• Outpost (1994), computer game. Player plans and manages a colony on another planet.
• Outpost 2 (1997), computer game. Player manages a colony on the fictional planet of New Terra.
• Sid Meier's Alpha Centauri (1999), computer game. Player tries to expand a human colony on Alpha Centauri.
• Starfarers of Catan (1999), tabletop game. Player manages trade and colonization in the fictional planetary system of Catan.
• Freelancer Players can explore several planets and systems colonized centuries prior to the start of the game.
• Starlancer Several colonies located on planets in the Sol systems are mentioned throughout the game.
• Marathon A early game by Bungie that show a human generation ship, fighting off alien slavers on their way to Tau Ceti.
• Gradius Gaiden The player uses the Vic Viper and 3 other craft to defend planet Gradius and it's colonies against unknown forces from the Dark Nebula.
• Mass Effect

### Television

• Battlestar Galactica (1978–1979) and (2003-present), a television franchise. In both series, mankind lives on the Twelve Colonies as well as the thirteenth colony of Earth, having evolved on the planet Kobol.
• Gundam (1979-present), anime television franchise beginning with Mobile Suit Gundam (1979–1980). The series as well as its sequels and spin-offs mostly revolve around the conflicts between the Earth and the space colonies.
• Macross (1982-present) or Robotech (1985), anime television franchise. The series and its sequels mostly revolve around space colony fleets known as "Macross".
• Earth 2 (1994–1995), television series. A refugee group travels to and tries to colonize a distant Earth-like planet.
• Firefly (2002–2003), a television series. The series, and the followup film Serenity, dealt with a mass exodus from an overcrowded Earth to a new solar system, involving the terraforming and colonisation of these new worlds.
• Planetes (2003–2004), anime series. Humanity has colonized parts of Earth's Moon and Mars.
• Doctor Who (2005–present), television series. The Great and Bountiful Human Empire is repeatedly mentioned to have sent colonists into different parts of space, sometimes interfering with or even destroying native species.