A Space Colony is an artificial habitat built by humans, usually at one of the Lagrangian points, to allow human habitation in space. The interior of each space colony usually replicates the surface of the Earth and the colony simulates gravity by rotating. An O'Neill cylinder consists of two very large, counter-rotating cylinders, each five miles (8 km) in diameter and twenty miles (32 km) long, that are connected at each end by a rod via a bearing system. They rotate so as to provide artificial gravity via centrifugal force on their inner surfaces.[1]