A structure built to contain an environment suitable for living. Many forms exist, ring and cylindrical shapes built to rotate and simulate gravity, domes or buildings covering part of the surface of an inhospitable planet, hollowed out asteroids, tunnels carved into asteroids or mountains on a planet and tunnels running underground. If not meant to rotate, orbital habitats can have any shape. Most look black in colour, collecting light and distributing it via light tubes, usually to one half for a day cycle, while the other half experiences night, then reversing the cycle. Mirror arrays can collect the light and heat necessary. Some are powered by fusion energy. Those that rotate usually have a non rotating hub for ships to dock.