Celestia Forums

I've heard that moons cannot have moons themselves, because the orbital dynamics are not stable. At least, as far as terrestrial systems are concerned. But, what about rings?

Earth may have had a ring caused by the KT impact (or possibly a later one, I forget at the moment). If this is true, could not a ring form around a moon, even a relatively large and close one such as Luna, and last at least for a few thousand years?

...John...

Rings are moons. Just lots and lots of little ones. And they have to follow the same orbital dynamics as a larger moon. The important thing here is the Hill Sphere, or the sphere of influence, must be stable over long time-scales. Our Apollo spacecraft had an easy time orbiting the Moon. A proposed spacecraft to orbit Europa isn't going to find this a problem either. But for a natural satellite to form in orbit around a moon and remain there for billions of years is another story. There are none in our solar system. But that's not to say that we can't find a solar system some day that has moons around moons.

But I'm not talking about a scale of billions of years. I'm talking about a few thousand. perhaps up to ten thousand years, but certainly no longer than that.

I guess what I am curious about is if a moon could support a ring system long enough for a civilization on the parent planet to believe that it has always been a part of the system (at least, until they develop the knowledge to realize how transient it would be).

...John...

Depends on your moon and your primary. There are formulae that'll let you predict the tidal spin-down time to synchrony for the moon, and then the time it will take a satellite at a given distance to tidally decay down to the moon's surface. What sort of system were you considering?

Grant

Dollan,

My best guess would be a moon could have rings if it were far enough from its parent planet...

We don't have a whole lot of options for a ring to form around a moon naturally. The only way for a ring to form - especially in the timescale proposed - would be if something (an asteroid or comet) got pulled into orbit around the moon and dipped below the moon's Roche Limit, tearing it apart. Let sit for a century and you have a nice little ring, albeit a shortlived one.

Of course, for any visitors or inhabitants of such a moon, life would be pretty much unpleasant as we'd constantly have fragments deorbiting and crashing into the surface for the next few millenia. Optimal real estate would be near the poles; land at the equator would probably sell real cheap

The chances of this scenario happening for a moon would probably be between "slim" and "none". The asteroid has to come in at just the right angle and speed to be captured. So I doubt we'll ever find something like this in nature - or at least discover it while the ring is still intact.

However, artificial rings are also possible. If one wanted to do so, one could shephard a suitable asteroid into orbit, and blast it apart, herding and breaking the smaller pieces until you have a ring. With the right level of organization and technology, a ring could be created without any risk to the moon's surface and any settlements there.

Andrew

I'm more optimistic about rings happening around moons naturally. In such a vast universe, there must be a moon with a ring that has formed naturally.

And it's only time before we find it...

Michael Kilderry