In the 1970s, NASA held a series of summer schools to explore practical designs for future space colonies. Artists illustrated the concepts.

Each approximately the size of a Californian beach town, such colonies were imagined as completely self-contained habitats with artificial gravity, some with artificial weather, where people could live out their entire lives.

There were three main types of colony: toroidal (donut shaped), sphere and cylinder. All rotated in order to simulate Earth's gravity and were lit by huge mirrors that reflected the sun's rays into the interior. Construction materials would be mined from the moon and asteroids.

Because of the way Earth orbits the sun, there are five points in the Earth's orbit where an object can remain in a stable orbit, expending no energy to stay in position. These are Lagrangian (L) Points, named after their discoverer Joseph Louis Lagrange. L4 and L5 were thought ideal positions for such space stations.

Now, as then, the costs of building a colony remain prohibitive, but even though no organization has concrete plans to build a colony, the idea remains. One recent proposal is the Kalpana One, named after NASA astronaut Kalpana Chawla, one of the seven crew members killed in the Space Shuttle Columbia disaster.