Ring World

A kind of MegaStructure.

Habitable structure described by LarryNiven consisting of a giant ring with a radius comparable to that of the Earth's orbit around the sun which completely girdles a star. A larger version of a FullerRingBridge.

Think of it as a thin slice through a DysonSphere. It has several advantages over a Sphere; eg it's easier to build and you can spin it to fake gravity.

I myself have dreamed up a structure intermediate between Dyson spheres and planets. Build a ring 93 million miles in radius - one Earth orbit - around the sun. If we have the mass of Jupiter to work with, and if we make it a thousand miles wide, we get a thickness of about a thousand feet for the base.

And it has advantages. The Ringworld will be much sturdier than a Dyson sphere. We can spin it on its axis for gravity. A rotation speed of 770 m/s will give us a gravity of one Earth normal. We wouldn't even need to roof it over. Place walls one thousand miles high at each edge, facing the sun. Very little air will leak over the edges.

Lord knows the thing is roomy enough. With three million times the surface area of the Earth, it will be some time before anyone complains of the crowding. 

 - Larry Niven, "Ringworld"

I doubt if there is a suitably strong material for building atmosphere retaining walls. Where do the materials come from that are needed to build the ring and make it rotate?

Can someone explain Niven's 770 m/s math? With a radius of 93 million miles centered on the sun, a rotational velocity of 30000 m/s (the Earth's orbital velocity) corresponds to a net force towards/away from the sun of zero. At 770 m/s everything on the ring should fall off the ring and into the sun. -- AndyPierce

I think Niven is being misquoted here; in my copy of Ringworld, the rotational velocity of the ringworld is given as 770 miles per second, about 1240 kps. This agrees rather nicely with the calculation given by Alan Eisinger at http://www.macalester.edu/~aeisinger/niven-aze.htm -- MikeKrajnak

Hmm, what really might be interesting is the force to throw you off RingWorld if you were standing on the outside of the ring. I'd be first in line for that ride...

93 million miles and only a thousand feet thick? I can see some massive oscillations building up!

Indeed so. As the old filk goes:

Oh, the Ringworld is unstable / the Ringworld is unstable / did the best that they were able / and it's good enough for me!

Unfortunately, the structure as described by Niven is unstable. That is, whenever a part comes ever so slightly closer to the sun, gravitation will pull that part closer and closer until the ring hits the sun.

You could, theoretically, address this problem with thrusters sticking out of the outside.

Niven acknowledged this in a later book of the series, and added thrusters. (Our heroes discovered them).

Isn't that instability only a problem at a zero-gravity spin rate - the 30kps that AndyPierce mentions? In that case a little closer to the sun the gravity could be -0.001g, and you have runaway positive feedback. But at the 1240kps spin rate for 1g (thanks MikeKrajnak) that would be +0.999g and so the ring would tend to expand outward to the limits of its tension. Of course there sure would be other instability issues: http://en.wikipedia.org/wiki/Tacoma_Narrows_Bridge Could the solar wind create Solarelastic Flutter? -- BobStein

Could you spin a Dyson sphere for gravity? Lots of gravity at the equator, very little near the poles?

That should lead to some rather violent (though interesting) weather patterns.

I don't think so; the atmosphere would fall to the equator and produce RingWorldlike weather.

Like architects, those who design worlds don't worry about them being habitable :-)

On that note, just fill the entire thing with air

The recent 'Halo' game has an interesting perspective on the construction of ringworlds. Instead of making it massive enough to encircle a star, make it the same diameter as earth. Here is my idea:

Tilt the ring at a 67.5 degree angle towards the sun to provide day/night as the ring rotates, while allowing the sun to be high enough in the sky to provide adequate warmth.

With 1g of gravity present, atmospheric retention is accomplished by a 150km high barrier, set at a 45 degree angle. If it is any shorter, it would not retain enough atmospheric pressure. Earlier comments about a 1000 mile high barrier would give too much atmospheric pressure at the surface. The 45 degree angle is set so that the residents of the ringworld do not feel overly boxed in.

Another way to retain atmosphere could be accomplished by surrounding it with a 75km deep glass-like enclosure. It would be pressurized, but due to the depth of the atmosphere, the amount of artificial pressurization would not have to be too great, and thus the structure would be able to contain it without being overly stressed.

The biggest problem with having a complete enclosure around the atmosphere is that travel to and from the ringworld would have to be through airlocks. Either the airlocks would have to be massive, or the size of aerospace vehicles would have to be limited.

Of course, it'd be easier to launch via airlocks anyway, because of the extra kick provided by slingshotting off the back side of the ring (instead of blasting rocket engines in your nice clean atmosphere). I might be more concerned with avoiding the massive greenhouse effect and getting a transparent material strong enough to withstand impacts (or patchable enough quickly enough to keep your pressurized atmosphere from rushing out). For both these reasons, I prefer the open top approach.

Niven's Ringworld had anti-asteroid defenses. However, they neglected to cover the direct underside of the ring. One of the mountains in the Ringworld didn't show up on any of the "historical" maps because it was caused by an asteroid strike on the underside of the ring that punched all the way through to the top.

IainBanks' TheCulture series has "ring planets" called Orbitals. Instead of a planet spinning on its axis as it rotates a sun, it's a toroid (more or less) spinning on its axis. The toroid is slightly inclined, so you get day and night cycles without needing shadow plates.

Apparently a page on Niven's site (http://www.larryniven.net/ringworld_movie_news.shtml) has been tracking any news of a movie for some years.

There were major RingWorld movie rumors in e.g. 1997 and 2001, but they turned out to be false.

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