A space habitat (also called an orbital colony, or a space colony, city, or settlement) is a space station intended as a permanent settlement rather than as a simple waystation or other specialized facility. No space habitats have yet been constructed, but many design proposals have been made with varying degrees of realism by both science fiction authors and engineers. About 1970, near the end of Project Apollo, Gerard K.

O'Neill, an experimental physicist, was looking for a topic to tempt his physics students, most of whom were freshmen in Engineering. He hit upon the creative idea of assigning them feasibility calculations for large space habitats. To his surprise, the habitats seemed to be feasible even in very large sizes: cylinders five miles (8 km) in diameter and twenty miles (34 km) long, even if made from ordinary materials such as steel and glass.

Also, the students solved problems such as radiation protection from cosmic rays (almost free in the larger sizes), getting naturalistic sun angles, provision of power, realistic pest-free farming and orbital attitude control without reaction motors. He published an article about these colony proposals in Physics Today in 1974. (See the above illustration of such a colony, a classic "O'Neill Colony").

The article was expanded in the book High Frontier. The result motivated NASA to sponsor a couple of summer workshops led by Dr. O'Neill.

Several designs were studied, some in depth, with sizes ranging from 1,000 to 10,000,000 people. Attempts were made to make the habitats as self-supporting as possible, but all of the designs relied on regular shipments from Earth or the Moon, notably for raw materials and volatiles. Closed ecologies and aggressive recycling should dramatically reduce this reliance.

Recent research has increased the probability of finding frozen water in deep craters on the moon's south pole, and found that certain asteroids contain significant amounts of volatiles such as water and ammonia. This suggests space habitats could rely less on Earth than these original studies indicated.

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