O'Neill cylinder

The O'Neill cylinder (also called an O'Neill colony) is a space settlement megastructure, built using materials extracted from the Moon and later from asteroids. It was first proposed by the physicist Gerard O'Neill in 1976.

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.

After a sufficient number of habitats have been erected in space - called Island One communities - the process of expansion would become self-sustaining. A group of Island One communities could pool their resources to form an economic cooperative, and engage in the construction of an even larger Island Two community. This may be 2 km in diameter and 6 km in length, housing a total of 140,000 people. The interior could be designed with a number of small villages separated by parkland or forest areas. An Island Three community would measure more than 6 km in diameter and 32 km in length, with a total land area of 1300 square kilometers and a human population of several million. Island Four communities would have land areas of 20,000 square kilometers and populations numbering in the tens of millions.