O'Neill cylinder
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The O'Neill cylinder is a space habitat design proposed by Gerard K. O'Neill in his book The High Frontier. O'Neill was a physicist at Princeton. In 1969 he taught undergraduate physics. Dr. O'Neill decided to teach by having students design large structures in space. To the surprise of everyone, several designs appeared that used ordinary materials (steel and glass), and could provide large areas suitable for human habitation. This cooperative result was first published by O'Neill in a 1974 article in Physics Today.
O'Neill's reference design, "Island Three", consists of two counter-rotating cylinders each two miles (3 km) in radius, and twenty miles (30 km) long. Each cylinder has six equal-area stripes that run the length of the cylinder. Three of the stripes are windows. Three are "land."
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The cylinders rotate to provide simulated gravity on their inner surface. NASA experiments in rotating reference frames indicate that almost no-one would experience motion sickness because of coriolis effects in their inner ears. People would be able to detect spinward and antispinward directions by turning their heads, however. At this scale, the air within the cylinder and the steel shell of the cylinder provide adequate shielding against cosmic rays.
Large mirrors are hinged at the back of each stripe of window. The unhinged edge of the windows points toward the Sun. The intent 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 permits heat to radiate to space. During the day, the Sun moves as the mirrors move, creating a naturalistic progression of Sun angles. However, the Sun's image might be observed to rotate. The light reflected from the mirrors is polarized, which might confuse bees.
As they orbit, the gyroscopic effect of the rotating cylinders would naturally cause the habitat's mirrors to stop pointing at the sun. O'Neill and his students carefully worked out a method of aiming the habitats. First, the pair of habitats can be rolled by operating the cylinders as momentum wheels. If one 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.
O'Neill cylinders in science fiction
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In the Universal Century and Cosmic Era timelines of the Gundam anime science fiction series, O'Neill Cylinders are common—especially in the Universal Century, where nine billion human beings live in these colonies. In the Universal Century, the group of colonies are known as Sides.
Alexis A. Gilliland, a science fiction writer, proposed a "dragon scale mosaic mirror". This would consist of a cone of aimable mirrors around each habitat. These could provide illumination, generate power, and defend the habitat. The area of the array would be much larger than that of O'Neill's design, but wouldn't have to be built to take the stress of rotating with the habitat.
The science-fiction television series Babylon 5 starred an O'Neill style space station five miles long (sans windows). While the single cylinder Babylon 5 did not feature a counter-rotating section, its predecessor did.