O'Neill cylinder

Mega-structure
The O'Neill cylinder (also called an O'Neill colony) was a space settlement structure for the colonization of space for the 21st century, using materials extracted from the Moon and later from asteroids.

An O'Neill cylinder would consist of two counter-rotating cylinders. The cylinders would rotate in opposite directions in order to cancel out any gyroscopic effects that would otherwise make it difficult to keep them aimed toward the Sun. They are connected at each end by a rod via a bearing system. They would rotate so as to provide artificial gravity via centrifugal force on their inner surfaces.