# Would a Niven Ring in the Uranus atmosphere be stable?

For my science fiction universe, I need to mine a lot of deuterium and helium-3.

I have 10,000 intake fans each capable of 3,000 gigaliters an hour pushing atmosphere through 5 billion diffusers. This yields about 7 exagrams of deuterium and 2 exagrams of helium-3 a year. All the diffusers take up a lot of space, enough space that I could build a multilevel ring around Uranus. Obviously the ring would be in the atmosphere.

My question is, would the atmosphere make a Niven ring in this case, stable? You would have air friction and the atmosphere would increase in density as the ring gets closer to the center. I was also wondering what the radius of the ring would need to be to be stable, at what altitude or atmospheric density.

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• Can you provide more information about Instability of ring? Which kind of instability is used in your question? – MolbOrg Oct 19 '16 at 0:23
• You could clarify the question by defining a Niven ring as a megastructure similar to that in Larry Niven's Ringworld – Zxyrra Oct 19 '16 at 0:33
• Do you expect to be able to live on the underside of the ring due to centrifugal force? – John Meacham Oct 19 '16 at 1:41
• Why do you need to float your mining equipment above the planet? Why not build towers both to hold the extraction equipment which process the low altitude atmosphere and to hold up the ring where the big high altitude atmosphere extraction equipment resides? – Henry Taylor Oct 19 '16 at 1:58

I'll have to guess at what exactly you mean by "Niven Ring" and "stable"... but here's some "thinking out loud" to get the ball rolling:

I assume you mean a small Ringworld-like structure around Uranus, with some or all of it dipping into the atmosphere. Obviously, it will experience drag forces until/unless the parts in the atmosphere are moving at the same velocity as the atmosphere. But I assume that the structure needs to be moving relative to the atmosphere in order to perform its extraction function. So the structure will be rotating at one speed while the extraction operation is going on, but a different speed if the extraction ever stops. I can't guess which speed would be higher.

The classic Ringworld maintains its shape by virtue of the tension created in the base material ("scrith") by the fact that it is rotating at faster than orbital speed. Even then, it requires active control in order to keep it in position relative to its star. And at less than orbital speed, it would have to have enough compressive strength and rigidity to maintain its shape in order to be considered in any sense "stable".

So the question is, is either (or both) of the two speeds mentioned in the first paragraph large enough to keep the main part of the structure under tension — moving faster than its own orbital speed? I don't know enough about the atmosphere of Uranus or the orbital mechanics around it to calculate the answer.

Also, remember the active control required to keep the structure centered. If this ever fails (at either speed) and the ring goes off-center, the unbalanced drag forces will surely tear it apart. Do you consider this "stable"?

The simplest way to build this would be to create the ring to have a similar density to the air, this would then automatically stabalise it as if it sinks down density around it would increase and push it back out again.

You could then increase that stability by using the fact that air in different places is flowing at different speeds and the ring is not. Place ailerons, wings, or even sails on the surface of the machine and fairly basic automated control would be able to hold it in place.

You have a bigger problem though which is the massive amount of air you are sucking into and out of the ring in order to process it. That would effectively place massive engines all the way around the ring pulling it through the atmosphere. That would need to be factored into any design of the ring.

Honestly I know it doesn't have the same "coolness" impact factor as a ring circling a gas giant but you may be better off using an autonomous swarm approach. Just release millions of small ships that float around gathering resources and then hand the results off to collectors that then hand it off to bigger collectors etc. This is much easier to scale and avoids the problem of keeping the ring coherent.

Not only would it not be stable but putting it in atmosphere would increase the instability. The problem is that when one bit dips it goes into thicker air and experiences more drag, increasing the problem.

However, it doesn't matter. Remember how the Ringworld was kept stable? Your air processors are going to be pushing so much air that they can be used to stabilize it.

You still do have two other problems, though:

1) Scrith is an unobtainium. Nothing is that strong.

2) You're in atmosphere. Drag will slow it down, it's going to collapse unless it's so strong it can hold itself up against Uranus' gravity.

• Since a ring around Uranus would be much much smaller than Ringworld around a star, perhaps you wouldn't need something as strong as Scrith. – ohwilleke Oct 20 '16 at 20:31
• @ohwilleke Yeah, you don't need as much strength but you still need something beyond what chemical bonds can do. – Loren Pechtel Oct 20 '16 at 22:15
• If the ring had a smaller radius, it would be in a part of the atmosphere with higher pressure and more density. Couldn't you pick the "perfect" radius so that the ring is floating on the denser atmosphere, which would reduce the required material strength? – Blake Oct 20 '16 at 22:21
• @Blake At that point you have a huge balloon, not a Niven ring. – Loren Pechtel Oct 20 '16 at 22:22

One alternative to having a true rigid body ring around Uranus (which has a radius of about 25,000 km (a circumference of ca. 156,000 km) would be to put in place a dense ring system of asteroids/rocky stuff/ice a la Saturn. This wouldn't need to have nearly the extraordinary structural features of a rigid body ring but you could have almost a thousand largish (50-100 km diameter) asteroids hollowed out to hold habitable areas and hops from one asteroid to the next could be on the order of 1-2 km and could even be connected by wires with a fair amount of slack for communications and a guide lines for non-rocket powered transportation between them.

This would provide a lot of habitable living space and pretty managable connections between them while being far less of an engineering masterpiece. And, even if you eventually figured out how to build a rigid body ring, you'd probably need to start with something like this during the construction phase anyway.

Over time, you could combine nearby asteroids into rigid body chunks as large of the dynamics of the situation could support with ordinary materials, perhaps reducing the number of rigid bodies in the ring from a thousand or so to the low hundreds. When the instability of the chunks started to exceed your engineering capabilities, you'd stop building and settle in.