Moon of a moon?

[pharoah0110]

I'm not sure if anyone has asked this before, but a curious thought occured to me as I was transiting the Jovian moons.

Could it be possible, (and if so are there any known examples) for a planet which has a moon to have another moon orbiting the first one stabily?

I can only guess that the "primary" moon in question would have to be orbiting the planet at a large distance, to avoid pertubations in the "secondary" moon's orbit.

Any thoughts?
Cheers!

 

[insanity]

In one sense, our moon is the satellite of a satellite.

There are no planets in the solar system whose moons have a natural moon. If such a thing existed tidal forces would probably pull it out of orbit.

 

[Linguofreak]

It could happen, but would be unlikely. If it did happen, it likely wouldn't look very spectacular, because there would have to be rather wide separations relative to the size of the planet and moons involved to make the arrangement stable.

Probably something like a far moon of a small gas giant in the outer solar system.

 

[Hielor]

Not quite. Saturn does have at least one instance of that type of system. Although both of those moons are rather small compared to the other moons whos diameters can reach hundreds, or thousands of miles across.

Link?

 

[n72.75]

moon'

 

[PhantomCruiser]

TCR- Are you talking about Saturn's "shepard" moons? If so, that's a very loose consideration, while they do influence one another, they are both at the mercy of Saturn. They orbit Saturn, not each other.

 

[JamesG]

I can't think of any Jovian moons which have moons (moonlets?). I think they are far to gravitationally "busy" for any to persist for long.

About the only configuration I can think of where it would be stable would be a double world. Twin planets coorbiting each other with a smaller moon orbiting the pair at a good distance. Similar to how binary solar systems are predicted to be arranged. But then you get to the question of who's moon is it?

 

[Suzy]

Are there any online programs or sites that could calculate the orbits of multiple moons? Showing how fast each one orbited its planet.

 

[RisingFury]

Well, I think the real life Saturn has an example of that. The larger moon pulls a smaller moon into a slower orbit around Saturn, then shoots that moon into a higher orbit around Saturn. So the relationship between the two orbits can be considered a moon of a moon.

PS: It might be a different gas giant, but I'm pretty sure it's Saturn.

You're talking about resonant orbits... and you're off by quite a lot...

 

[JamesG]

Are there any online programs or sites that could calculate the orbits of multiple moons? Showing how fast each one orbited its planet.

Google: "Orbital period calculation" and you will find a couple, plus the info to do them pen and paper yourself.

 

[the.punk]

I also think a moon of a moon would be very unstable. Maybe the gas giants had something like that in the past. But then they crashed together and built a lager moon.

 

[RisingFury]

I think there is a chance that there could be a stable orbit such as this, but the orbital period of the secondary moon would have to be specific, so that the perturbations of the planet would cancel out over time.

I haven't done any calculations, but it might happen that the secondary moon's orbital period would have to be in resonance with the primary moon's orbital period...

 

[Urwumpe]

No, that should be extremely hard to achieve. You would need a VERY large moon, orbiting a planet almost alone, so other moons don't steal the moon of it.

The critical aspect is the size of the stable orbital region around the moon. Our own moon for example, is extremely large relative to us for a planet, but still, it is impossible to have a satellite there orbiting for years without propulsion.

 

[Jarvitä]

Our own moon for example, is extremely large relative to us for a planet, but still, it is impossible to have a satellite there orbiting for years without propulsion.

True, but that's because of Lunar masscons, not because of any inherent orbital anomaly. If the Moon had a more spherical gravity well, it would be perfectly capable of having another natural satellite within it's SOI, even if that happening by chance is extremely unlikely.

 

[Urwumpe]

If the Moon had a more spherical gravity well, it would be perfectly capable of having another natural satellite within it's SOI, even if that happening by chance is extremely unlikely.

No, then other effects would still play into it, for example the strong tidal forces of Earth on the moon.

 

[Linguofreak]

Link?

http://en.wikipedia.org/wiki/Co-orbital_moon

But as noted before, this is not a true "moon of a moon" arrangement.

 

[JamesG]

Earth would pull on it at its closest approach elongating the apogee until the moonlet hit the Moon or Earth stole it away.

This probably happened many times in the early days of the solar system.

 

[Linguofreak]

I think there is a chance that there could be a stable orbit such as this, but the orbital period of the secondary moon would have to be specific, so that the perturbations of the planet would cancel out over time.

I haven't done any calculations, but it might happen that the secondary moon's orbital period would have to be in resonance with the primary moon's orbital period...

More than likely this would actually lead to the moonlet leaving its parent body. Resonances tend to destabilize (there are gaps in the asteroid belt at resonances with Jupiter) more than they tend to stabilize (as with the orbits of Io, Europa, and Ganymede, which involves a resonance of three orbits, not two).

The biggest thing you would need would be for the planet and the primary moon to both have large hill spheres (which means both would have to be big and far out from the object they were orbiting). If the hill sphere of the planet is too small, then either the primary moon is close enough that tidal effects would strip the secondary moon out of orbit, or the primary moon is far enough out that it eventually will be stripped out of its orbit by the star and become a planet on its own.

If the hill sphere of the primary moon is too small, then it won't be able to have a moon of it's own for any significant length of time.

Think something like having the Earth/moon system orbit Neptune.

Neptune has two moons that are a third of an AU out (Semi Major Axis), but are quite small (one of them has quite an eccentric orbit, and reaches half an AU at apoposeidion. If you dropped an Earth-sized body into one of these orbits, it could certainly support its own moon. (Even a moon-sized body probably could).