O'Neill was inspired by the papers written by his students. He began to work out the details of a program to build self-supporting space habitats in free space. Among the details was how to provide the inhabitants of a space colony with an Earth-like environment. His students had designed giant pressurized structures, spun up to approximate Earth gravity by centrifugal force. With the population of the colony living on the inner surface of a sphere or cylinder, these structures resembled "inside-out planets". He found that pairing counter-rotating cylinders would eliminate the need to spin them using rockets. This configuration has since been known as the "O'Neill cylinder".