O'Neill cylinder

Artist's depiction of a pair of O'Neill cylinders

A diagram of an example of a pair of O'Neill cylinders (Island Three)

An O'Neill cylinder is a hypothetical space habitat type consisting of a giant cylindrical structure in space. The original concept proposed by NASA would be a pair of cylinders 8 km (about 5 miles) in diameter with a length of 32 km (20 miles) each, but other sizes would be possible. The entire internal surface of the cylinder (with the exception of the ends) could be usable land, or there could be strips of land alternating with strips of some transparent or translucent material to let Sun's light in.

Each cylinder of the pair would rotate in opposite directions to eliminate any gyroscopic effects. An outer agricultural ring, twenty miles (32 km) in diameter, would rotate at a different speed to support farming. In the middle of the cylinder, there would be an industrial manufacturing block, to allow for the low gravity to help for some industrial processes.

O'Neill cylinder, also known as Island Three, was designed in 1976 by American physicist Gerard K. O'Neill, who also designed Island One and Island Two space habitats, both of a much smaller size than O'Neill cylinder, and both of the Bernal sphere type of space habitat.