In the 1970s, with the Apollo moon landing program completed and the space shuttle program under development, space planners realized that huge space colonies were feasible. Reasons for building such habitats include the following: to create new lands for population expansion, to ensure the survival of humanity in case of global disaster and to create wealth by exploiting space resources.
In the early 20th century, Austro-Hungarian rocket pioneer Herman Potočnik (also called Hermann Noordung) designed an early space habitat. His 1928 book "The Problem of Space Travel" describes a 100-foot (30 meters) ring-shaped station that would rotate to generate artificial gravity. A large parabolic mirror would focus sunlight on water pipes to heat them and generate steam power.
In a 1974 "Physics Today" article, Gerard K. O'Neill described a habitat in the form of two rotating 20-mile-long (32 kilometers) cylinders. Giant windows in the cylinders would allow sunlight to enter, reflected off of long, petal-shaped mirrors. Rocketing materials from Earth to build the cylinders would not be cost-effective, so O’Neill proposed building with lunar materials launched into space by an electromagnetic mass driver. An O’Neill-type colony would have 500 square miles (1,295 km) of living space and could support up to several million people.
In 1975, a Stanford University group designed a 10,000-person space habitat in the form of a torus. The station rotates once per minute to generate gravity. Lunar rubble coats the outer surface of the ring to provide radiation shielding. Space elevators carry visitors from a docking hub at the center to the outer habitat ring.
In 1929, J.D. Bernal proposed a spherical habitat 9.9 miles (16 km) in diameter that would house up to 30,000 people. In 1975, Gerard K. O’Neill refined the concept, proposing a sphere 1,640 feet (500 m) in diameter, rotating to provide Earth-type gravity at the equator. 10,000 people could live in this type of habitat. A larger version, 1.1 miles across (1.8 km), could house 140,000 space citizens.