Space Colonization is the speculative practice of creating habitats and industries on various bodies of the Solar System or, even more speculatively, other star systems. Its purposes are permanent human habitation outside Earth and tapping the natural resources found on planetary bodies.
In-situ Resource Utilization
Since transporting raw materials from Earth is prohibitively expensive, space colonization is only possible if the local resources are exploited. The most basic resources needed for human habitation are oxygen, water, energy and food. Water can usuall be found on colder planetary bodies and carbonaceous asteroids in solid (ice) form. Electrolysis of water can provide oxygen; solar power can be used to provide electricity.
Other resources that can be extracted and utilized in space are construction materials and metals. Several technologies can be used to produce construction materials, such as lunarcrete or converting asteroids into Cole habitats.
Some scientists and engineers propose to focus on building habitats in open outer space rather than planetary bases. A space habitat, essentially a large space station, is easy to provide with spin gravity and solar power. The largest drawback of space habitats is that they require transportation of resources, since in-situ resource utilization is only possible on planetary bodies.
One of the most speculative and "sci-fi" of the proposed space technologies, terraforming involves massive engineering projects on other planets and aims to rebuild their environments to be habitable for humans.
Space Mining and Prospecting
Going beyond merely in-situ resource utilizations, humans can use advanced technology to make exploitation of space resources profitable and more preferable than exploitation of Earth resources. The bodies of the Solar system offer many unique resources that are much needed on Earth, such as helium-3, Titanian natural gas or platinum metals that can be found on M-class asteroids.
Space colonization faces a daunting 'chicken and egg' challenge of financing the establishment of an economically viable human community in the absence of previous human colonization or even a biosphere. Terrestrial colonization relied on both. In his influential essay The Political Economy of Very Large Space Projects political scientist John Hickman argues that solving the problem will require large public works projects.
The nearest possible target for space colonization, this region of space is inexpensive to reach and can be colonized with space habitats. The most convenient places to put such habitats are the Lagrange points on lunar orbit, namely L4 and L5.
The recent discovery of water ice on the Moon makes the prospect of colonizing it much more sound than previously considered. Terraforming, on the other hand, is out of the question and won't become feasible anytime soon, since it will require creating an atmosphere from scratch.
Mars is one of the more hospitable planets of the Solar system. Technologies required to create self-sustaining Martian bases or even terraforming Mars already exist; the obstacles are the enormous cost of transportation and lack of coordination between Earth countries.
Venus is a very inhospitable pressure cooker of a planet, but an atmospheric layer exists on this planet which is of quite Earthlike temperature and pressure. Earth air is significantly lighter than Cytherean air under normal pressure, so a balloon or torus filled with it will float at the desired altitude. Such an atmospheric base can be used as a scientific outpost or a mining base to extract carbon and sulfuric acid from the Cytherean atmosphere.
Mercury's unique property is its high density caused by its very large metal core. Thus, it is speculated to have rich deposits of heavy metal ores. Among the difficulties of extracting these ores are severe temperature variations caused by the day-night cycle unmitigated by Mercury's trace atmosphere, solar radiation and scarcity of water.
The greatest attraction in the main asteroid belt is the possibility of exploiting raw materials from asteroids. Minerals and volatiles could be mined from an asteroid or spent comet then taken back to Earth or used in space for space-construction materials. Materials that could be mined or extracted include iron, nickel, titanium for construction, water and oxygen to sustain the lives of prospector-astronauts on site, as well as hydrogen and oxygen for use as rocket fuel.
The main selling point of asteroid mining is immediate accessibility of the metals contained in asteroids, unlike the metal cores of planets which are buried to deep for any possibility of exploitation.