O'Neill's reference design, "Island Three", consists of two counter-rotating cylinders each two miles in radius, and twenty miles long. Each cylinder has six equal-area stripes that run the length of the cylinder. Three of the stripes are windows. Three are "land."
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.
Alexis A. Gilliland, a science fiction writer, proposed a "dragon scale mirror". This would consist of a cone of aimable mirrors around each habitat. These could be aimed into the habitat for illumination, or used for power generation, or weapons. The weapons proposed include direct heating of nearby spacecraft, and solar-pumped lasers to heat far spacecraft. Both sorts of weapons would cause damage by heating the spacecrafts' radiators, causing them to overheat.
He also proposed focusing the mirrors through much smaller windows to use more of the inner surface of the habitat. This would probably require an auxiliary radiator.
He also proposed structuring the habitat as a spiral wrapped on a central core. The advantage would be that much more area could be packed into the same volume, lowering the cost per hectare. A secondary advantage is that water would run downhill. Light would be distributed using light-pipes.
Ray Van De Walker proposed in a 1988 amateur publication that such habitats be constructed incrementally, from habitat balls about fifty meters in diameter. The basic idea is that first someone would swing a ball from a bag of rock. Next, they might substitute a ball for the bag of rock. Next, more neighbors add balls, removing bolted-on side-caps and sealing the balls into a cylindrical minimum-packing scheme. Finally, a ring is created. Then they add balls to the edge of the ring until a cylinder is completed. Last, they begin adding interior and exterior balls. The advantage of such a structure is that it has economies of scale, easy investment, and long sightlines. At every stage, adding more balls always results in a cheaper, more efficient structure.
In an incremental habitat designs, it may be most practical to substitute an internet connection for all existing communication, and an automated delivery (perhaps overhead monorail) system for piping and other transportation. In this way, a few simple systems-level connections can provide services similar or superior to those on the ground.Other Variants