O'Neill Cylinder
O'Neill Colony 2
Image from NASA Copyright free
O'Neill Colony (Island Three)

A pair of cylindrical orbital space colonies that rotate around their respective axis to produce simulated gravity (one rotates clockwise and the other counter clockwise to minimize torques). Usually between 10 to 100 km long, with mirrors providing sunlight and simulating the day/night cycle. The structure is large enough to have its own self-contained ecosystem, microclimate, etc. Named after the Atomic Age Old Earth visionary Gerard O'Neill.

Gerard O'Neill produced detailed plans for a large space colony, based on the cylinder. A cylindrical form gives the maximum possible habitable space in a rotating habitat, but long cylinders are prone to tumbling in orbit, so O'Neill proposed that cylinders could be linked together in counter-rotating pairs to stabilise their orientation.

By managing the rotation rates of each of the cylinders, the long axes could be oriented in any chosen direction. O'Neill proposed that the orientation of the cylinders could be managed so that they permanently pointed towards the Sun, and mirrors could then be used to illuminate the interior surface through long, strip like windows.

Each cylinder in a classic O'Neill colony is 3 kilometres in radius, and 30 kilometres long. Such a colony can support up to 10 million people. Each cylinder is divided into three sections by the window strips, with parkland and agricultural land providing a complete closed ecology. To simulate night the mirrors are adjusted to reduce the amount of sunlight coming in.

O'Neill Colony 1
Image from Steve Bowers
Conchobor Habitat in Tau Ceti orbit

Conchobor Habitat in Tau Ceti orbit; this modified O'Neill design is relatively remote from the local star, so uses larger (inflatable) mirror arrays to collect more sunlight. This type of colony is sometimes known as the Mirror Petal design.

O'Neill's original design included two rings of small agricultural modules, each module rotating just enough to allow the crops to grow under low gravity conditions. In lower light conditions such agricultural rings require larger sunlight collection surfaces, so often orbit separately from the inhabited cylinders.
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Development Notes
Text by M. Alan Kazlev
Initially published on 17 December 2001.