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BDO of the Day: Ringworld

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Today’s Big Dumb Object (BDO) is a Ringworld, the familiar creation of Larry Niven in his eponymous 1970 novel that later sprouted a series, not to mention inspiring many other big dumb rings of various sizes and kicking off a golden age of BDOs. (Rendezvouz with Rama came out in 1973, and the Orbitsville series began in 1975.)

Niven described the Ringworld again in his classic megastructures essay (Analog, March 1974, emphasis added):

I myself have dreamed up an intermediate step between Dyson spheres andplanets. Build a ring 93 million miles in radius—one Earth orbit—which would make it 600 million miles long. If we have the mass of Jupiter to work with, and if we make it a million miles wide, we get a thickness of about a thousand meters. The Ringworld would thus be much sturdier than a Dyson sphere.

There are other advantages. We can spin it for gravity. A rotation on its axis of 770 miles/second would give the Ringworld one gravity outward. We wouldn’t even have to roof it over. Put walls a thousand miles high at each rim, aimed inward at the sun, and very little of the air will leak over the edges.

Set up an inner ring of shadow squares-light orbiting structures to block out part of the sunlight-and we can have day-and-night cycles in whatever period we like. And we can see the stars, unlike the inhabitants of a Dyson sphere.

The thing is roomy enough; three million times the area of the Earth. It will be some time before anyone complains of the crowding. As with most of these structures, our landscape is optional, a challenge to engineer and artist alike. A look at the outer surface of a Ringworld or Dyson sphere would be most instructive. Seas would show as bulges, mountains as dents. River beds and river deltas would be sculpted in; there would be no room for erosion on something as thin as a Ringworld or a Dyson sphere.

Ringworld by Giuseppe Gerbino (CC BY-SA 3.0) Il Mondo ad Anello de “I burattinai” di Larry Niven by Hill

Niven goes on to say that the seas would be shallow qua purely cosmetic, but that doesn’t seem like the proper way to make an environment to me. He also explains how to move a Ringworld by moving its sun with “a jet of [solar] gas along the Ringworld axis of rotation”. But some things went unexplained until the sequel, The Ringworld Engineers (1980), like how the ringworld stays in its unstable orbit.

For more details and pictures, see Isaac Arthur’s video about ringworlds.