A space habitat, also called space colony, orbital colony, space city, or space 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 beyond Earth orbit, and no self-sufficient habitat has been constructed in Earth orbit either, but many design proposals have been made with varying degrees of realism by both science fiction authors and engineers. Space habitats orbiting Earth have a number of potential advantages over those on the surface of other planets: