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Basic Information:

This design of space colony was first proposed by Princeton University professor Gerard K. O'Neill in his 1977 book, The High Frontier: Human Colonies in Space (1977, Morrow Press/Bantam, ISBN 0-553-11016-0; reprinted 1989, SSI Press, ISBN 0-9622379-0-6; updated 2000, Apogee Books, ISBN 1-896522-67-X). It was the third and most ambitious of three colonies proposed in the book. Called "Island Three," the colony is a gigantic rotating cylinder 20 miles long and four miles in diameter, with hemispherical endcaps at both ends. Its interior is a landscaped semi-terrestrial environment divided into six alternating "land" and "sky" panels that run the length of the cylinder and terminate at large "mountains" at the endcaps. The land panels have one small city at either end, and a shared "wilderness" area in the middle; the sky panels are left empty, and are crossed by bridges to facilitate travel between the land panels. Sunlight is reflected in through the sky panels by large mirrors outside the main colony cylinder which can be rotated to create day and night - the appearance of these mirrors gives the colonies their nickname, "sunflower" style colonies.

The entire colony (with the exception of docking bays at the center of the endcaps) rotates at 1/2 RPM, using centrifugal force to produce 1G of simulated gravity on the inhabited "land" panels. Since centrifugal force is dependent on the radius of the cylinder, as one moves up the mountains at the ends of the colony, the simulated gravity drops proportionally to one's altitude, reaching zero at the colony's axis. The zero- and low-gravity areas of the endcaps are devoted to industry, leaving the rest of the colony available for habitation. The colony's agricultural needs are served by external agriculture units arranged in a non-rotating ring around the colony's sun-facing endcap. The colony supports an autonomous population of somewhere between three and ten million people. In the Mobile Suit Gundam anime, in which these colonies figure prominently, they are placed in groups of 100-150 called Sides. This entire group orbits the Earth at the same rate as the moon, in one of five gravitationally stable points near the Earth and the moon known as Lagrange points, after the mathematician who predicted them.

Construction Details:

The outer colony hull is a meter-thick plate of titanium-reinforced "mooncrete," a lunar-produced concrete containing anorthosite, ilmenite, and KREEP (potassium, rare earth elements, and phosphorus). The inner surface of the hull is covered in about five meters of topsoil, though some maintenance buildings may reach down to the outer hull. The sky panels are far more complex. Each panel is two miles wide and 16 miles long, composed of eight windows (2x4) of sixty-four sashes (8x8) of twenty-five panes (5x5) of one hundred frames (four-ply 5x5) of twenty-five prisms (5x5) of quartz glass. That's a grand total of 32 million prisms, each measuring 3.2 meters on a side (10.4 feet, or a thousandth of a mile) and weighing about 90 tons. Quartz glass is essential to the construction both for its strength and for its transparency to sunlight - if it weren't so transparent, the greenhouse effect would bake the colonists. This seems like an absurd amount of quartz - in fact, this amount is easily found in a single large asteroid. Electricity for the colony is provided by solar and geothermal power - conductive material running through the hull takes advantage of the temperature differential between the sun-facing endcap and the space-facing endcap to generate current. Though it's not depicted in the Gundam animation, Island Three colonies have to be lashed together in ballistically coupled pairs (turning in opposite directions) to ensure zero angular momentum, which is necessary to keep them pointing towards the sun.

Environmental Oddities:
(or, The Coriolis Force Hates You)

Because of the constant spinning of the external colony cylinder, and the constant heating of the sunside endcap compared to the starside endcap, the weather patterns within a Gundam-style Island Three space colony are bizarre. The weightless air at the center of the cylinder expands towards the southern endcap as it is heated. Once it approaches the ground it picks up the rotation of the cylinder and moves down the mountain and sunward in a helical downspin spiral, finally reaching the northern endcap and rising back up to repeat the process. The end result is a spiraling wind from upspin and starside (southwest), with variations caused by buildings, people, landscaping, et cetera. Coriolis forces make dust devils common, and as the clouds follow this spiral pattern as well, short but heavy showers occur roughly every six hours.

It's nearly impossible to play basketball. Due to the rotation of the cylinder, all freefalling objects veer upspin. Equally, heading downspin is fundamentally discouraged - it's considerably harder to walk downspin than up, as when moving downspin your acceleration is added to the acceleration of the rotating cylinder and increases your apparent weight. Furthermore, coriolis forces in the inner ear make complicated motion disorienting; space basketball is only for the strong of stomach.

Again due to coriolis forces, elevators can't go straight up (towards the axis of the cylinder) if they're going to rise any appreciable distance. The sum of the acceleration of the elevator and the acceleration of the rotating colony would cause the passengers inside to feel as if the floor had tilted on a 45-degree angle. To eliminate this effect, the elevator car would either have to rotate to compensate (putting the passengers on a 45-degree angle relative to the ground, but standing normally relative to the apparent direction of gravity) or it would have to run in a downspin spiral when going up, and an upspin spiral when going down.

WolfKeeper helpfully corrected my math regarding space basketball. On the inside of the colony hull, where most of the living gets done, the Coriolis force pales in comparison to the centrifugal force causing the artificial "gravity." Even the fastest athlete is going to incur only minor disruptions to the workings of his inner ear. Approaching the center, however, the centrifugal force drops off while the Coriolis force remains the same (so long as you're not in free-fall). So we can reinstate the SpaceNBA, but any plans for low-G Ultimate Frisbee will have to be called off.

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