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BDO of the Day: Banks Orbital

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Today’s Big Dumb Object (BDO) is a Banks Orbital, Orbital Ring, or simply Orbital, the little brother of Larry Niven’s more famous Ringworld. Introduced in 1987 by Iain M. Banks in Consider Phlebas, Orbitals solve the ringworld’s daylight and orbital stability problems by being the perfect size.

An Orbital orbits its sun in the usual fashion, not as a giant unstable ring with its center of gravity at the sun. By setting the Orbital at an angle and rotating it at the speed desired to maintain gravity, it can have a normal day-night cycle as well as seasons, although it will not have regional variations in insolation like our arctic or tropics. Like a ringworld’s, the Orbitals' atmosphere is held in by rimwalls, though of only half the height (300 miles) and transparent rather than disguised as mountains.

There is only one diameter of Orbital for any pair of gravity and day length requirements; for an Earth gravity and an Earth day, it’s a diameter of about 2.3 million miles. The width can vary; at a width of 3,000 miles, an Earth-like Orbital would have 22 billion square miles or so of surface, or about a hundred Earths' worth. (Culture Orbitals are a bit smaller due to their shorter day.) Though only a drop in the three-million-Earth bucket of a Ringworld, it still requires exotic materials to build.

Banks Orbital by Giuseppe Gerbino (CC BY-SA 3.0) Model of a Banks Orbital by Giuseppe Gerbino

Though Orbitals are big, they’re not dumb in the ancient abandoned BDO sense; they are an ongoing project of a living civilization. For more about the Culture and how they build their Orbitals, see Banks' essay “A Few Notes on The Culture.”