Space Colonization Wiki

A pair of O'Neill Cylinders

The O'Neill Cylinder, or Island Three, is a space colony design proposed by Gerard K. O'Neill.

O'Neill envisioned that the colony would be built using materials from the Moon. The materials would be launched into space using a mass driver.

The habitats themselves are cylinder-shaped, and are always built in pairs. Each one is five miles (8 km) in diameter, is 20 miles (32 km) long, and can house several million permanent residents. Each cylinder is divided into six equal-area strips that run down its side. Three strips are "land", and the other three are windows. The two cylinders are counter-rotating and are connected at each end by a rod via a bearing system.

Artificial Gravity[]

The colonies rotate to provide artificial gravity on the inner surface. Because each cylinder has such a large radius, the colony rotates only 40 times per hour. At this low speed, no one would experience motion sickness. A person could detect spinward and antispinward directions by turning his or her head, and any dropped objects would appear to be deflected by a few centimeters. The central axis of the cylinder would be a zero-gravity region. Instagram: @lawsofthecosmos

You can experience this when you are on a merry go round. You feel a force acting on you when you are at the ends, whereas when you are at the center point you don't feel it.


An interior view of an O'Neill Cylinder

Each habitat would have half the air pressure of Earth's. 20% would be Oxygen, and 30% would be Nitrogen. This half-pressure atmosphere saves gas and reduces the strength and thickness required for the habitat walls. The cylinder shell and the air within provide shielding against cosmic rays.


Large mirrors are hinged to the back of each window. The unhinged edge points towards the sun so that the mirrors reflect sunlight into the habitat. The light reflected from the mirrors would be polarized, which could confuse bees. During the day, the Sun would appear to move as the mirrors move. The mirrors would open to simulate night as the window will view empty space. This also lets heat radiate into space. The windows would not be made of single panes, but instead would be made of many small sections to prevent catastrophic damage, and so the aluminum or steel window frames could take most of the stresses of the air pressure of the habitat.

If a meteorite broke one of the panes, some air would be lost but it would not be an emergency since the habitat is so large.

Other designs would do away with the complex arrangement of mirrors and glass. In lieu of that a 'sun-globe,' powered by solar cells on the exterior of the cylinder, would run through the central axis providing light in the same frequency as the sun. This sun-globe would dim and relight in order to simulate sunset, nighttime, morning, and day.

By removing the glass in the structure, interior surface area would double and be continuous.

Attitude Control[]

The habitat and its mirrors must be aimed at the sun. In order to continuously turn the colony 360 degrees per orbit without using rockets that discard reaction mass, the pair of habitats must first be rolled by operating the cylinders as momentum wheels. If one habitat's rotation is slightly off, the two cylinders will rotate about each other. Once the plane formed by the two axes of rotation is perpendicular (in the roll axis) to the orbit, then the pair of cylinders can be yawed to aim at the sun by exerting a force between the two sunward bearings: away from each other will cause both cylinders to gyroscopically precess, and the system will yaw in one direction, towards each other will cause yaw in the other direction. The counter-rotating habitats have no net gyroscopic effect, and so this slight precession can continue for the habitat's orbit, keeping it aimed at the sun.

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