Multiple Moons orbiting terrestrial planets

[Guardsman Bass]

I remember that in some earlier threads about the Moon, it came up that a second Moon would likely have been unstable. After a few hundred million years, the interactions between its gravity, the gravity of the RL Moon, and the Earth would cause it to either collide with Earth, be ejected, or crash into the RL Moon.

So my question is, are there stable orbits for two or more moons around an Earth-like planet? Would both moons have to be smaller, or more widely spaced?

[CaptainChewbacca]

Smaller moons would be better. The other choice would be two worlds orbiting each-other and maybe one moon orbiting both at a distance.

[Purple]

Hm... Mind if I ask a few questions? How much smaller would these moons have to be? How elliptic would their orbits be? And how would they effect tidal pattens and stuff like that?

[Surlethe]

I submit Mars as an example.

[Samuel]

That isn't a great example though- Phobos's orbit is unstable.

[Guardsman Bass]

That's what I've read as well. Mars' moons are also pretty negligible in terms of mass compared to Mars itself.

[Purple]

And it has no liquid water for tidal pattens to form. Sort of a must if you (me) are engineering a twin mooned habitable world for SF.

[CaptainChewbacca]

Mars' moons are captured asteroids, and were likely not part of the original planetary accretion that Mars formed from.

[Xenophobe3691]

Well, you have the gas giants as an example, but they're so massive compared to their moons it might not apply.

Maybe make your story on a hot Jupiter?

[madd0ct0r]

download a solar system builder or grav sim and play about - two medium sized moons are ok, I think if you get more and more precise you can get more and more without sacrificing weight.

[Simon_Jester]

One large moon and one smaller, asteroid-sized one that got captured in its Trojan point and is large enough to be easily seen from planet-side would work, I think...

[Irbis]

How about 2 roughly identical moons orbiting the planet on the same orbit on the opposite sides of it? So each moon would be in the other's L3 point? And maybe smaller, captured bodies in other Lagrange points?

Or is it unstable, too, for a simple reason patently obvious to anyone familiar with astrophysics?

[starslayer]

That would be unstable, yes; perturbations from the rest of the solar system would move the two bodies out of each other's L3 points fairly quickly. Really, the best way to get multiple moons around a terrestrial planet is to have two relatively small ones orbiting each other and then orbiting the planet in turn far enough away that they essentially look to the planet like one moon. Such a system is pretty stable (in fact, we see things like this arising in stellar pairings not too infrequently, IIRC).

[Purple]

Hm... just how fast would these orbits need to be? Could we get something like a relatively large moon (still smaller than our own) and a relatively small moon (compared to the other as well) orbiting some arbitrary point betwen the two in such a way that the small moon is usually eclipsed by the large one and only appears on auspicious dates (not necessarily regular ones either, just rarely enough that it can be seen as an omen by the primitive medieval planet dwellers.)

[starslayer]

No. That system is very unstable and is going to result in an ejection or a collision very quickly. And you contradict yourself; if the smaller one is closer to the planet, it can't be occulted by the larger one, although it can partially eclipse the larger moon. Besides, if you have two bodies orbiting around something else, they cannot stay in-line with each other, since by simple Keplerian motion, the period is proportional to the 3/2 power of the semi-major axis.

[madd0ct0r]

he meant the two moons are orbiting each other, which is then going around the planet. like pluto/charon round the sun

[starslayer]

Even in that case, the answer is still no (although it could be stable); the orbital period of the two moons around each other would be something on the order of days, so both would be visible most of the time.