Getting there

As we already discussed the most dangerous and costly side of space colonization is getting people there. We can supply a colony automatically and even with an acceptable fail rate, but the transport of humans will be not only more costly but also more technically challenging, the major issue will be the requirements of the colonist during this stage, considerations need to be made for keeping them healthy and fit to perform on arrival.

Mir and ISS have helped to understand most of the problems humans face in a prolonged stay on a space environment. The major issue continues to be the psychological stress of isolation and time of inactivity. But without the extreme risk of an actual interplanetary spaceflight the psychological data might be limited since the stress of such a risky and prolonged voyage will be impossible to fully replicate in a simulation.

Other physical factors still remain to be fully tested even if most seem that can be counteracted by technological solutions, for instance the problem with zero-gravity (or lower gravity in general) and the levels of radiation exposure.

This of course places a greater burden on the selection of candidates for such mission. Some could be experience astronauts, like the one in the Apollo missions, that had pilot experience but most will require to have a possess a greater set of skills as to enable the successful establishment of a colony.

Population

Regarding minimum size for establishing a self sufficient population, in 2002, the anthropologist John H. Moore estimated that a population of 150-180 would allow normal reproduction for 60-80 generations—equivalent to 2000 years.

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

Researchers in conservation biology have tended to adopt the "50/500" rule of thumb initially advanced by Franklin and Soule. This rule says a short-term effective population size (N_e) of 50 is needed to prevent an unacceptable rate of inbreeding, while a long-term N_e 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 N_e depends on the number of males N_m and females N_f in the population according to the formula:

${\displaystyle N_{e}={\frac {4\times N_{m}\times N_{f}}{N_{m}+N_{f}}}}$

Energy

Life Support

People need air, water, food and reasonable temperatures to survive. On Earth a large complex biosphere provides these. In space settlements, bio-regenerative approaches will be necessary, the use life forms to recycle air, water, and organic waste. Hydroponic gardens and algae tanks are proposed solutions for relatively small, closed system that must recycle all the nutrients without "crashing".

The famous attempt to build an analogue colony is Biosphere 2, which attempted to duplicate Earth's biosphere.

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

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.

On celestial bodies, oxygen and liquid water can be obtained from purified water ice. On the moon, oxygen can be made as a byproduct of regolith processing. On Mars, oxygen can be obtained from electrolysis of water (extracted from permafrost) or the Sabatier process (C02 extracted from the atmosphere).

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, ie. integrating the habitat into organisms (See also: genetic engineering, transhumanism, cyborg)

A combination of the above is also possible.

Agriculture

Hydroponics, an ancient technique for growing plants with nutrient-rich water instead of soil, has long been touted as the choice for growing plants in space. It is very space and water efficient and can also be part of a closed-life support loop (oxygen regeneration, water filtering and food production). For instance it has been suggested to incorporate plant respiration (of water vapor) into water purification and recovery systems. Research continues on automated hydroponic systems.

To save energy, it is possible to grow plants with colored light, eschewing frequencies they do not use.

Animals, given their low mass efficiency and high energy and space requirements, are unlikely to be raised in space any time soon. More likely sources of animal protein are insects and synthetic meat ("meat-in-a-dish").

Communication

Compared to the other requirements, communication is relatively easy for orbit and the Moon. Much of the current terrestrial communications already pass through satellites. Communications to Mars and further will suffer from significant delays making voice conversation impractical.

Communications is not only depended on infrastructure but energy, it requires also technical maintenance.

Manufacturing

On Earth, if one needs a new toothbrush and some toothpaste, it is a simple matter of driving to the corner drugstore to pick some up. On Mars or the Moon, the nearest drugstore will be millions of miles away! At least at first. And more to the point, the manufacturing capability of producing all these products is equally inaccessible, down at the bottom of Earth's gravity well.

Even for Earth orbit colonies, launching materials from Earth is very expensive, so bulk materials should come from the Moon or Near-Earth Objects (NEOs - asteroids and comets with orbits near Earth) where gravitational forces are much less, there is no atmosphere, and there is no biosphere to damage. Our Moon has large amounts of oxygen, silicon and metals, but little hydrogen, carbon, or nitrogen. NEOs contain substantial amounts of metals, oxygen, hydrogen and carbon. NEOs also contain some nitrogen, but not necessarily enough to avoid major supplies from Earth.

The Mars-500 experiment

The Mars-500 experiment ( http://www.esa.int/SPECIALS/Mars500 ), lasting from 2007 to 2011, divided into three stages. The final, 520-day stage of the experiment, which was intended to simulate a full-length manned mission, ended on 4 November 2011, where a team of six volunteers were locked into a cluster of hermetically sealed habitat modules for 520 days, to simulate a mission to Mars using the available technology. To add to their isolation, communications with mission control were artificially delayed to mimic the natural delays of a Mars flight. During the simulation the volunteers (the crew) performed several experiments, all linked to the problems of long duration missions in deep space. The simulation was a success all the volunteers managed to stay healthy in body and mind indicating that a voyage to Mars can be successfully in regards to the psychological aspect of such endeavor.

One way trip

Any one way trip to colonize outer space will be unlike anything else in human history. As the requirements to archive a way back will if not impossible due to lack of the necessary resources at the destination. They could take generations to archive if at all, making that type of voyage, to the colonist, a form of self-imposed exile from Earth. Of course there can be middle-steps, like using robots to create the necessary infrastructure before sending colonists or establishing a consistent resupply and support structure from Earth, but the costs and the added level of uncertain due to the great distances, possible windows for the lunches and time and economic commitment would be huge at today's level of technology and probably impossible on the political cycles most nations use.

Psychological issues

Needed qualifications

(In)dependence

One should think that while a colony is dependent of supplies from Earth or location nearby, it will be politically dependent and unless a more reasonable global governance is created, self sufficient colonies are more likely to become independent. A colony may revolt and pursue its independent self interest. In terrestrial civilizations, this frequently occurs on the order of 10 to 100 years. Even a loyal colony may decide that it is against its economic or political interests to aid a troubled Earth, or to recolonize Earth. Even then the goal is simply survival of the species will be assured and one should hope many independent colonies could be established as to collectively form a safeguard against one government amassing too much power over the fate of the whole human race.

Evolution

Natural selection occurs when a species is challenged, when there is a threat to its survival, a threat to the continued existence of fertile descendants. Simply sending humans to another location and waiting 100 000 years does not necessarily yield a different species.

If we colonize distant worlds, we are likely to settle more hostile places, like Mars or Europa, than Earth-like places, which seem to be rarer in the universe. It is expected that in colonizing space, humans will at first have restricted direct contact with the hostile worlds they will inhabit; instead, they will live inside domes and use space suits to work on the outside. Humans, humans' pets, humans' parasites and complex life in general are unlikely to be able to adapt for instance to the 95% carbon dioxide atmosphere of Mars or to the -160 °C temperature of Europa, a so we may even consider altering our genetic material and of other living specimens to permit a better adaptation to the new environments.

Architectural considerations