The O’neill cylinder, or O’Neill colony, is a hypothetical space colony proposed by American physicist Gerard K. O’neill. The concept of the structure first appeared in O’Neill’s 1976 book The High Frontier: Human Colonies in Space. The proposal focuses on materials from the Moon and asteroids to colonize space in the twenty-first century.
The proposal attempts to colonize space without harming life and without polluting any aspect of the Earth. The goal of the project is to move all industrial activity away from the Earth’s fragile biosphere and to create small-scale governing bodies and culturally diverse populations in space.
The O’neill cylinder is part of three reference designs known as “islands.” Islands one and two are both spherical, while the O’Neill cylinder consists of counter-rotating cylinders with three habitable land surfaces and an outer ring to support farming.
The cylinder's counter-rotation is necessary to eliminate gyroscopic effects and to keep the cylinders aligned with the sun. Each cylinder would be 20 miles long and 5 miles in diameter. The cylinders connect at each end via a bearing system that rotates to create artificial gravity through centrifugal force.
The O’neill cylinder was the result of a class project at Princeton University. While teaching undergraduate physics, O’neill asked his students to design large colonies in space that could efficiently support human life.