Space habitat orbital colony, or a space colony, city, or settlement)



The O'Neill cylinder, also called an Island Three habitat, is a space habitat design proposed by physicist Gerard K. O'Neill in his book The High Frontier.&lt;ref name="ONeill-HighFrontier"&gt;&lt;/ref&gt; In the book O'Neill proposes the colonization of space for the 21st century, using materials from the Moon.

An O'Neill cylinder consists of two very large, counter-rotating cylinders, each 5 miles (8 km) in diameter and 20 miles (32 km) long, that are connected at each end by a rod via a bearing system. They rotate so as to provide artificial gravity via Centripetal force on their inner surfaces.&lt;ref&gt;ibid. High Frontier, chapter V&lt;/ref&gt;

Background
While teaching undergraduate physics at Princeton University, O'Neill had students design large structures in space, with the intent to show that living in space could be desirable. Several of the architectures were able to provide areas large enough to be suitable for human habitation. This cooperative result inspired the idea of the cylinder and was first published by O'Neill in a September 1974 article of Physics Today.&lt;ref name="ONeill-Colony"&gt;&lt;/ref&gt;

Islands One, Two and Three
O'Neill has created three reference designs:


 * Island One
 * A sphere measuring one mile in circumference (1,681 feet or 512.27 meters in diameter) which rotated, and people lived on the equatorial region. (See Bernal sphere.)
 * A later NASA/Ames study at Stanford University developed an alternate version of Island One: the Stanford torus geometry, a toroidal shape 1,600 meters (just under a mile) in diameter.&lt;ref&gt;Space Settlements, A Design Study, 1977, NASA SP-413, accessed June 4, 2009&lt;/ref&gt;
 * Island Two
 * Also a sphere, also 1,600 meters in diameter.
 * Island Three
 * Two counter-rotating cylinders each five miles (8 km) in diameter, and capable of scaling up to twenty miles (32 km) long.&lt;ref name="NSS-ONeill"&gt;&lt;/ref&gt; Each cylinder has six equal-area stripes that run the length of the cylinder; three are windows, three are "land".  Furthermore, an outer agriculture ring, as seen in the picture on the right, 10 miles (16 km) in radius, rotates at a different speed for farming.  The manufacturing block is located at the middle (behind the satellite dish assembly) to allow for minimized gravity for some manufacturing processes.

To save the huge cost of rocketing the materials from Earth, these habitats were to be built with materials launched into space from the moon with a magnetic catapult called a mass driver.&lt;ref&gt;ibid, O'Neil, High Frontier, p149&lt;/ref&gt;

Artificial gravity


The cylinders rotate to provide artificial gravity on their inner surface. Due to their very large radii, the habitats would rotate about forty times an hour, simulating a standard Earth gravity. Research on human factors in rotating reference frames &lt;ref&gt;Beauchamp, G.T.:Adverse Effects Due to Space Vehicle Rotation, Astronautical Sciences Review, vol. 3 no. 4 Oct-Dec. 1961, pp.9-11&lt;/ref&gt; &lt;ref&gt;Proceedings of the Symposium on the Role of the Vestibular Organs in Manned Spaceflight, NASA SP-77, 1965; Especially helpful: Thompson, Allen B.:Physiological Design Criteria for Artificial Gravity Environments in Manned Space Systems&lt;/ref&gt; &lt;ref&gt;Newsom, B.P.:Habitability Factors in a Rotating Space Station, Space Life Sciences, vol. 3, June 1972, pp192-197&lt;/ref&gt; &lt;ref&gt;Proceedings of the Fifth Symposium on the Role of Vestibular Organs in Space Exploration, Pensacola, Florida, August 19-21, 1970, NASA SP-314, 1973&lt;/ref&gt; &lt;ref&gt;Altman, F.:Some Aversive Effects of Centrifugally Generated Gravity, Aerospace Medicine, vol. 44, 1973, pp.418-421&lt;/ref&gt; indicate that almost no-one (at such low rotation speeds) would experience motion sickness due to coriolis forces acting on the inner ear. People would be able to detect spinward and antispinward directions by turning their heads, however, any dropped items would appear to be deflected by a few centimetres. &lt;ref&gt;ibid. NASA Study SP-413, pp22&lt;/ref&gt;

The central axis of the habitat would be a zero gravity region, and it was envisaged that it would be possible to have recreational facilities located there.

Atmosphere and radiation
The habitat was planned to have oxygen at partial pressures roughly like the Earth's air, 20% of the Earth's sea-level air pressure. Nitrogen would also be included to add a further 30% of the Earth's pressure. This half-pressure atmosphere would save gas and reduce the needed strength and thickness of the habitat walls. &lt;ref&gt;ibid. High Frontier, p117&lt;/ref&gt; &lt;ref&gt;ibid. NASA Study SP-413, p22-3&lt;/ref&gt;

At this scale, the air within the cylinder and the shell of the cylinder provide adequate shielding against cosmic rays. &lt;ref&gt;ibid. High Frontier, p113-116&lt;/ref&gt;

Sunlight
Large mirrors are hinged at the back of each stripe of window. The unhinged edge of the windows points toward the Sun. The purpose of the mirrors is to reflect sunlight into the cylinders through the windows. Night is simulated by opening the mirrors, letting the window view empty space; this also permits heat to radiate to space. During the day, the reflected Sun appears to move as the mirrors move, creating a natural progression of Sun angles. Although not visible to the naked eye, the Sun's image might be observed to rotate due to the cylinder's rotation. As an aside, the light reflected from the mirrors is polarized, which might confuse bees. &lt;ref&gt;ibid. High Frontier, p63..64&lt;/ref&gt;

To permit light to enter the habitat, large windows run the length of the cylinder. &lt;ref&gt;ibid. High Frontier, p63&lt;/ref&gt; These were not to be single panes, but would be made up of many small sections, to prevent catastrophic damage, and so the aluminum or steel window frames can take most of the stresses of the air pressure of the habitat. &lt;ref&gt;ibid. High Frontier, p112&lt;/ref&gt;

Occasionally a meteorite might break one of these panes. This would cause some loss of the atmosphere, but calculations showed that this would not be an emergency, due to the very large volume of the habitat. &lt;ref&gt;ibid. High Frontier, p112&lt;/ref&gt;

Attitude control


The habitat and its mirrors must be aimed at the sun. O'Neill and his students carefully worked out a method of continuously turning the colony 360 degrees per orbit without using rockets that discard reaction mass. &lt;ref&gt;ibid. High Frontier, p100&lt;/ref&gt; First, the pair of habitats can be rolled by operating the cylinders as momentum wheels. If one habitat's rotation is slightly retarded, 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.