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Using the Roche Limit to Make Earth a Ringed Planet

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Are you sick of Saturn lording over the planets with its fancy rings and its Cassini Division? Do you want to give Earth a little style and pizzazz? Let's take a look at how, if we like it, we might be able to (and I apologize for this) put a ring on it.

Image by Damien Bouic.

Saturn is, without a doubt, the most striking planet in the solar system. Sure, the Earth has a wholesome blue-and-green prettiness, but you can't beat Saturn for those huge, ostentatious rings. They were noticed all the way back in the 1600s by Galileo, and even he was impressed. Since then they've been the subject of ever-more-spectacular photographs by whatever space agency wants to show off at the moment. There are certainly plenty of imitators in the solar system. All the gas giants have rings, although none of them are as easy to see. It's even thought that Mars has dust rings. It's enough to make an Earthling feel left out. Who wants to live on a planet that's just a bit more attractive than Mercury?

How do we remedy this situation? There are a couple of ways, and they both involve the Roche Limit.


The Roche Limit

Gravity, though a major contributor to many wonderful things — for example the smooth functioning of the human digestive system — can be a harsh mistress. In particular, it's rather tougher on planets than they let on. The Earth, under the influence of the Moon, has whole continents raised by about fifty centimeters. The Moon's surface shifts by about five meters in response to the pull from the Earth. And remember, these two bodies are both solid rock. A lot of heavenly bodies aren't that lucky.


Many comets are not solid objects so much as jumbled collections of stones, dust, and ice, that are pulled together by their own gravity. As long as they hurtle through space without any large gravitational influences on them, they stay pulled together. As they get closer and closer to larger bodies, their cohesion is tested by tidal forces. Once they get too close, they'll be ripped apart by the gravity of the larger object, and scatter across space.

In the late 1840s, French astronomer Edouard Roche determined the distance at which a given satellite would get ripped apart by the body it was orbiting.


Roche Limit = 2.4 x (Radius of Larger Object) x (Density of the Larger Object/Density of the Smaller Object)^1/3

In this we can see that, if the radius and the density of larger object are large, no self-respecting satellite will want to get near them, because the Roche distance will be huge. If, however, the density of the orbiting object is large — if it is packed tight — it can freely wander about the universe without any worries, since it's unlikely that it will be ripped apart. A dense orbiting object shrinks the Roche distance considerably.


What Can We Do To Get Rings?

There are two possible plans. The first has us looking for comets. Attracting a comet will be tough, but comets have the advantage of giving us a variety of densities to chose from. By grabbing comets that are loosely-packed, we can create far-flung rings with little chance of too many fragments colliding with Earth. Then, when we're feeling more confident and have developed a missile defense system or anti-gravity rays for those pesky loose pieces, we can play it a little fast-and-loose and grab more densely packed comets and create closer rings. But comets can be a pain to go out and lasso. What if we start with a material closer to home?


The Moon might seem very valuable right now, sure, but remember that it is slowly moving away from us. Every year it gets about 3.8 centimeters farther away. And when the planet is going to be around for billions more years, that adds up. Plus we have already been to the Moon. Not much to see. All anyone could do was bounce and play golf. The Earth-Moon Roche limit is 18,470 kilometers, about one twentieth the distance from the Earth to the Moon. But we don't just want one ring. While harnessing comets would have us work from the outside in, using the Moon would make us work from the inside out. We'd break off a chunk of the Moon and pull it in until it broke apart and formed rings. The leftover Moon would slowly be crushed into less and less dense parts, each with a further Roche limit, and each forming a wider ring.

The Problems

One of the observed tests of the Roche limit was the Shoemaker-Levy comet that crashed into Jupiter in 1994. Before it did so, it passed its particular Roche limit, and broke into twenty-one pieces. Each of these pieces collided with Jupiter. A comet, or fragments thereof, crashing into the planet is exactly the kind of thing that most members of the public would like space agencies to prevent from happening. As the rings on Saturn show, not everything within the Roche limit crashes into a planet, but flying debris is always a risk.


We might have to go the Mars route, and try for rings of shimmering dust, since flecks of space dust won't do much damage on impact. However, a whole bunch of dust hitting the Earth at once might kick up an atmospheric fuss that people would notice.

All in all, the likelihood that we'll be able to deck ourselves out in rings is unlikely. Perhaps if we discover sentient life on Saturn and it snubs us, popular support might be gathered into showing Saturn up. We've done loonier things during the Cold War. The possibility of our being able to Roche our way into a ringed planet isn't very high. Still, speaking as a child who was rather miffed to discover that Saturn outshone us in the solar system, I like to dream that we could make it happen.


Top Image: NASA/JPL/Space Science Institute

Second Image: NASA/ESA/JPL/SSI
Via NASA twice and UTK.