A space station is an inhabited structure designed for permanent use outside the atmosphere of any planet. Space stations are usually situated in a fixed orbit around a planet or star. Space stations differ from spaceships and starships in that they usually only have enough propulsion power to maintain their orbits, while ships are intended to move under their own power between planets or even star systems.
Several real space stations have been built, including Skylab, Mir, and the International Space Station. Also, a few space stations have been designed but exist only as concepts, among them the Stanford Torus, Bernal Sphere and O'Neill Cylinder.