A Bernal sphere is a type of space habitat intended as a long-term home for permanent residents, first proposed in 1929 by John Desmond Bernal. Bernal's original proposal described a hollow spherical shell 16 km (9.9 mi) in diameter, with a target population of 20,000 to 30,000 people. The Bernal sphere would be filled with air.
In a series of studies held at Stanford University in 1975 and 1976 with the purpose of speculating on designs for future space colonies, Dr. Gerard K. O'Neill proposed Island One, a modified Bernal sphere with a diameter of only 500 m (1,600 ft) rotating at 1.9 RPM to produce a full Earth artificial gravity at the sphere's equator. The result would be an interior landscape that would resemble a large valley running all the way around the equator of the sphere. Island One would be capable of providing living and recreation space for a population of approximately ten thousand people, with a "Crystal Palace" habitat used for agriculture. Sunlight was to be provided to the interior of the sphere using external mirrors to direct it in through large windows near the poles. The form of a sphere was chosen for its optimum ability to contain air pressure and its optimum mass-efficiency at providing radiation shielding.