In September 1974 the Princeton physicist Gerard K. O’Neill published a paper in Physics Today called “The Colonization of Space”. It argued that people could live not on other planets but inside huge rotating structures built in space from material mined on the Moon. His largest design, now called the O’Neill cylinder, is still the classic picture of a space colony.
How it works
Each cylinder in O’Neill’s largest model is 32 kilometres long with a radius of 3 200 metres. It turns once every 114 seconds. That spin pushes everything inside outward against the floor with about the same force as gravity on Earth: people would walk on the inside of the curved wall, and looking up they would see the land on the far side overhead.
The inside is divided lengthways into six strips: three of land and three of windows. Large mirrors outside the windows reflect sunlight in. Opening and closing the mirrors makes day and night.
The cylinders come in pairs that spin in opposite directions. Because their spins cancel out, the pair can keep its axes pointed at the Sun without fighting gyroscopic forces. O’Neill proposed placing the colonies at the L4 or L5 Lagrange points, stable spots along the Moon’s orbit. His table put the population of a pair at anywhere from 200 000 to 20 million people.
Smaller steps
O’Neill did not expect to start with the largest model. His plans began with smaller habitats, and the idea grew in a 1975 NASA summer study held at Stanford University. That study produced the Stanford torus, a wheel about 1.8 kilometres across for 10 000 people. O’Neill’s own Island One was a version of the Bernal sphere, first imagined by J. D. Bernal in 1929.
In 1976 O’Neill set out the vision for a general audience in his book The High Frontier. The L5 Society, founded in 1975 to campaign for space colonies, took its name from his preferred location.
Why none exists
The physics of an O’Neill cylinder is ordinary: spin, sunlight and steel. The problem is scale. Building one would need enormous amounts of material, which O’Neill planned to mine on the Moon, and an industry in space that does not yet exist. It remains a design study, but it is the reference point for nearly every serious proposal for living in space.