Colonizing Outer Space - Living in Space
Living in Space 
Compared to other locations, 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 1g, whereas the Moon has 1/6g and Mars 1/3g. 1g is critical, at least for early colonies, to ensure that children grow up with strong bones and muscles.
Several design groups have examined orbital colony feasibility. 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, and that no new scientific breakthroughs are necessary, although a great deal of engineering would be required.
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.
A space habitat, also called space colony and orbital colony, is a space station which is intended as a permanent settlement rather than as a simple way-station or other specialized facility. They would be literal "cities" in space, where people would live and work and raise families. No space habitats have yet been constructed, we do not classify all space stations as a space habitat since they are not a replication of the natural environment necessary to sustain a species population, they are by definition artificially maintained and temporary, but many design proposals have been made with varying degrees of realism by both science fiction authors and engineers.
A space habitat could serve as a proving ground for how well a generation ship would function as a home for hundreds or thousands of people, this concept is also referred to as the Ark model. A colony ship would be similar to a space habitat, except with major propulsion capabilities and independent power generation. 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.
The Earth is an open system, it constantly gets input from external sources from energy to matter. In a generation ship (or a long term habitat) a subset of these functions need to be mimicked as a self sustained closed system (depending on the mission and location, on a solar system energy will be possible to be introduced at no cost during a long period). Much has been learned from attempts made on Earth to simulate isolated living systems (useful for the production of food, reprocessing or gases and water in space). The Biosphere 2, originally built to be an artificial, materially closed ecological system, now a center dedicated to research, outreach, teaching of living systems. There is also the BIOS-3 dedicated to the study of am algaculture based closed system.
The generation ship concepts is proposed in several hard science fiction works and it includes:
- Generation ship, hypothetical starship that would travel much slower than light between stars, with the crew going through multiple generations before the journey is complete
- Sleeper ship, hypothetical spaceship in which most or all of the crew spend the journey in some form of hibernation or suspended animation
- Embryo carrying Interstellar Starship (EIS), hypothetical starship much smaller than a generation ship or sleeper ship transporting human embryos in a frozen state to an exoplanet
The main disadvantage of orbital colonies in relation to a colony ship is the inability to seek them on their own, this is of course compensated by low costs (no engine, propellant) and reduction of risks. Building cities in space will require materials, energy, transportation, communications, life support, and radiation protection. These could be imported from the Moon, which has ample metals, silicon, and oxygen, or Near Earth Asteroids, which have all the materials needed with the possible exception of nitrogen.
Transportation is then the key to any space endeavor. Present launch costs are very high per kilogram from Earth to Low Earth Orbit (LEO). To settle space we need much better launch vehicles and must avoid serious damage to the atmosphere from the thousands, perhaps millions, of launches required. Transportation for millions of tons of materials from the Moon and asteroids to orbital settlement construction sites is also necessary. One well studied possibility is to build electric catapults on the Moon to launch bulk materials to waiting settlements, but then these type of solutions will get into the highest ground problem, since they can also be used as a weapon.
The issue with energy can be easily addressed by the use of solar energy, abundant, reliable and is commonly used to power satellites today. Massive structures will be needed to convert sunlight into large amounts of electrical power for settlement use. Energy may be even an export item for space settlements, using microwave beams to send power to Earth.
Lagrange Points 
Small Asteroids 
Some small asteroids have the advantage that they may pass closer than Earth or it'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.
A small asteroid could serve functions equal to space stations, with the benefit that some building material would already be present. Most of the disadvantages are similar to those of an artificially created space station. A lack of significant gravity, a population of more than ten and self sufficiency may be far in the future on/in very small asteroids. 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 their asteroid did not hit Earth or anything else of significant mass.
New Measuring Standards 
Life Off-Earth is going to be different enough that a number of "standards" that we take for granted on the Earth are also going to need modification. Even very basic physical measurements like time and distance will have to be adjusted to fit with experiences on Mars as those measurements are largely associated with physical aspects of the Earth.
- Units of Time - Even though target planets and the Earth may rotate at the same rate, there are can be some subtle differences that make measuring local time to be quite different from terrestrial experiences.
- Distance - While standard units of measure that were developed on the Earth can be used elsewhere, it is likely that some new measurement units will result from activities on orbitally static object (like a measurement to the sun and other locations important location on that solar system, this is important for travel time and estimating costs).
- Mass and Weight - The difference of gravity between the Earth and the target planet is going to have an impact on how things are built and how people live. Some things stay the same while there are some important differences as well.