The O’Neill cylinder is named after an American physicist and space scientist who sought to engage his students by getting them to think about big problems—space settlement, in particular. He also led symposiums where the concepts behind large, permanent space habitats—including the cylinder that bears his name—were hashed out.
The basic principle is fairly simple. Construct a cylinder at least half a kilometer in diameter so that it can be rotated at low speed and provide 1 g of artificial gravity all along the interior wall. Vary the length to the intended population size, up to thirty-five kilometers. Divide that wall into six equal segments, three of which are gigantic windows. Cap both ends to enclose the volume. Install three large mirrors to reflect natural sunlight through the windows, as needed. Pressurize the interior to one atmosphere and outfit the three habitable walls as you see fit. Unfortunately, the resources, on-orbit manufacturing techniques, and low-cost access to space required to complete such an ambitious megaengineering project did not come until long after Prof. O’Neill’s death. To his credit, the O’Neill habitat remains the most iconic symbol of transhumanity’s colonization of space and is only now being superseded by such advances as the Hamilton cylinder and bioengineered habitats. There is absolutely no mistaking an O’Neill cylinder.
Those that follow the original specifications are divided into three districts: high-density residential, industrial and commercial, and parks and agriculture. O’Neill colonies are intended to grow their own food, clean their own air, provide local opportunities for recreation, and perform work of value in a space economy. The immense structures are fabricated from metals and composites mined from asteroids, though some of the first colonies at the Earth-Luna Lagrange points made use of lunar materials. Many increase their internal surface area by forgoing the long window sections and providing lighting through end-cap mirrors, suntubes, or artificial means.
Some habitats solve the problem of keeping their mirrors pointed at the sun by connecting two counter-rotating cylinders and using the system as a momentum wheel. A slight imbalance in the counter-rotation is enough to induce a precession in the habitat’s orbit and keep the sunward ends pointed that direction continuously, with no propulsion required. This has the added benefit of doubling a given habitat’s living area. Single-cylinder habitats use a variety of attitude control measures, including tethered counterweights and electric thrusters.
From a security operations standpoint, O’Neill cylinders have similar considerations as Bernal spheresand Cole bubbles, with the exception that interior surface gravity does not change (except on the end caps). O’Neill cylinders are usually much larger, with significantly more landscape and airspace, making internal operations quite similar to standard ground-based operations.

Reagan Cylinders

The Jovian Republic has focused on what they proudly call the Reagan cylinder (and the rest of the solar system derisively dubs “sarcophagus” habs) as an alternative. Rather than fabricate a pressure shell from scratch, the Jovians capture an asteroid or moonlet and put it in a parking orbit. Miners then dig out a cavern of cylindrical shape and use the slag as ballast to stabilize the spin generated for artificial gravity.
From an engineering perspective, this is actually a clever idea. The thick rock, metal, or composite walls provide highly effective radiation protection and armor against outside attack. Excavating a chamber and sealing it is arguably easier, as well. The problem is that Jovian investment in environmental systems is minimal and their suspicion of nanotechnology and biosystems precludes use of advanced life support that would make the Reagan cylinders a pleasant place to live. Only the Junta’s elite on Solano are usually free of the pollution and sepsis that plagues the other Jovian colonies. Usually.

Solarchive Search: McKendree Cylinder

Before the Fall, there was talk of building a McKendree cylinder from long-chain carbon nanotubes. This habitat would dwarf even the most massive O’Neill cylinders at a diameter of 920 kilometers and a length of 4,600 kilometers. Even with three windows running the entire length, the habitable surface area would be comparable to the nation of Russia. With the pressure to provide more physical bodies and living space for infugees, some outer system groups are considering resurrecting the McKendree cylinder on a much smaller scale.

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