Moons with moons

JV

Are there any moons in our solar system that have moons?
Or is this not possible, due to instability?

chroot

There are no such systems. I don't believe such a situation is theoretically impossible in an arbitrary planetary system, though it would probably be impossible in our own solar system. Most of the moons in the solar system are quite small compared to their parent planets, and are also quite close. For example, the first Lagrange point between the Earth and Earth's Moon is about 9/10 of the way from the Earth to the Moon. This is the point where the Earth's and Moon's gravity are equal in magnitude, and it's only 38,400 km from the Moon's center. Any satellite of the Moon that got further away than that would get tugged into an orbit around the Earth instead.

- Warren

tony873004

In the short-term you'll find many stable zones where moons can orbit moons, but in the long-term its a different story. And the solar system is old, so anything that once temporairly existed is long gone now.

Earth's Moon was formed much closer to the Earth than it is now. This would dramatically reduce its Hill Sphere and any chance it had of forming with its own moon. Even now, its Hill Sphere is not that large, and things such as the Kozai effect would work to destroy such an alignment.

Mars' moons can't have moons, not even artificial ones. Their Hill Spheres (sphere where something can be gravitationally bound to an object) are below their surfaces. Probably many of the solar system's moons share this feature.

But as we get better and better at finding solar systems beyond our, I would bet that its only a matter of time before we find something that orbits something that orbits something else (not counting galaxies)!

Labguy

I guess it depends on our definition of "Moon". There are two asteroids that have their own small moons which I think was considered unlikely until discovery. Ida is a biggie with its own moon. See: http://www.windows.ucar.edu/tour/lin...enia_moon.html
Asteroids aren't moons of planets, they aren't planets, so maybe they can be considered "moons" of the sun. (?)

JV

So it is no problem for a satellite to orbit the moon (stable), as long as it is close enough in the "Hill Sphere".
Apparently it is just a coincidence that there is no example of a moon orbitting a moon in our solar system. At least the chance of it happening was just small, but it is their. Am I right?

tony873004

Yes, if an object were in a moon's Hill sphere, it could orbit stabally. But the Hill spheres for moons are very small because of their close proximity to a planet. And Hill spheres can change size as moons' orbits change size and shape over long time periods. So over a long time period moons will likely lose their moons. So I don't think it's just a coincidence that our solar system's moons are moonless. Also, its unlikely that a moon of a moon could even form in such a small Hill sphere.

I'd bet that in a hundred years from now, when we have millions of other solar systems mapped out, that we will find a few examples.

Just my guesses...

Labguy

From a JPL news release today it seems that moons do have moons:
EDIT: Site here:
http://saturn.jpl.nasa.gov/news/pres...cfm?newsID=547

Garth

No. Polydeuces is orbiting Saturn roughly in Dione's orbit, in a 'teardrop orbit' (seen in the co-rotating frame) around the L₄ or L₅ Langrangian positions 60⁰ ahead or behind Dione, or even in a 'horse-shoe orbit' (seen in the co-rotating frame) around both and centred on the L₁ position 180⁰ from Dione.

Garth

Labguy

But at least it qualifies as a "companion" moon to a moon.

Garth

Well yes - all trojans are companions in orbits with the same period.