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BDO of the Day: Topopolis

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I mentioned today’s Big Dumb Object (BDO) in a recent post because I find it one of the most practical of the lot: a Topopolis is a toroidal or knottier O'Neill cylinder or McKendree cylinder stretched out lengthwise to big, dumb dimensions. Yet any spacefaring civilization could make one with materials at hand.

A topopolis rotates O'Neill-wise around its cylindrical axis, which is far more physically plausible than rotating around the sun at its donut center. At 1 AU long, compression of the topopolis as it rotates inward is negligible, well within the strength of non-magical materials like steel or carbon nanotubes. For similar reasons, only the short curve (around to the skylands) would be visible to someone standing on the inside of a topopolis; in the long direction the tube would appear to be perfectly straight.

In his classic megastructures essay, Larry Niven credited the Topopolis to Pat Gunkel under than name, but he also nicknamed them Cosmic Spaghetti. He didn’t seem to think they needed to be closed (others do); instead he envisioned them growing unchecked into other star systems:

With the interstellar links using power supplied by the inner coils, the tube city would expand through the galaxy. Eventually our aegagropilous galactotopopolis would look like all the stars in the heavens had been embedded in hair.

A topopolis’s orbit around a sun is unstable in the same way a ringworld’s is (as dramatized in Larry Niven’s The Ringworld Engineers), so some active provision must be made to keep it in place over the long haul. One notable feature you can build into a topopolis, mentioned at Orbital Vector and elsewhere, is a single river flowing all the way around the topopolis the long way—which is to say, for at least 1 AU or 93 million miles.

Orbital Vector has more details about topopolis construction, although they seem to assume that light coming directly in the windows can provide a day/night cycle. That isn’t practical at the spin rate of either type of cylinder; the original O'Neill or McKendree cylinder designs solve this problem by pointing an endcap at the sun and lighting the cylinder indirectly, but a topopolis has no such end. Even with carbon nanotubes, the maximum diameter of a McKendree-style topopolis is just under 1000 miles, so the slowest it could be turning to provide gravity is 48 times a day. An indirect lighting plan is required, such as artificial light, slow glass (i.e., magic), or a layer of shade material rotating more slowly than the inner tube.

Not subtracting for windows, at maximum diameter the McKendree style of topopolis provides about 300 billion square miles of surface area, or 1500 Earths, for a single loop around the sun. (A single topopolis can circle the sun more than once in a torus knot, and each loop in that case would be longer than 1AU.) The most modest O'Neill style is 10 miles in diameter (and can be constructed without carbon nanotubes), yielding only 3 billion square miles of surface area, or 15 Earths for one loop around the sun.

Although they’re big, dumb, and practical to build, topopoles are relatively rare in fiction, appearing in Matter (2009) by Iain M. Banks, White Light (1998) by William Barton and Michael Capobianco, in the background of Learning the World (2005) by Ken MacLeod, and, of course, at Orion’s Arm. You can’t go wrong with Iain Banks, but be warned: most readers complain about the sex in White Light. As an Amazon review put it, “it’s as if the cast of a bad porno movie was suddenly transported into what would have otherwise been a fascinating SF novel.”