# Can two moons orbit each other while orbiting a much larger planet?

Trying to work out mechanics and visuals from one of two moons orbiting a gas giant. My idea is that the moons are tidally locked so they can never see each other's backsides, but this might be achieved by a different type of orbit. I just want these twin moons to maintain close proximity and they circle a gas giant and need to figure out how Moon A could hide the construction of a space station from its sister moon, Moon B.

• Effectively, the Earth and the Moon already do this around the Sun. – RBarryYoung Jul 18 '17 at 21:01
• If the two moons are close to each other and far from the planet it is very likely to be stable. The differences in gravitational force on each moon from the planet will get averaged out by the moon's fast orbit. – Ross Millikan Jul 19 '17 at 5:23
• Yep. Build your station at the moons' "L2" Lagrange point. If the moons orbit close enough, the perturbation of the larger planetary primary will be insignificant. – imallett Jul 19 '17 at 8:02
• Possible duplicate: Moons of Moons of Moons – Frostfyre Jul 19 '17 at 12:58

# Orbital Mechanics Perspective: Yes, This is Possible

I can say with a fair amount of confidence that this is possible from an orbital mechanics perspective. Two moons could orbit each other while orbiting a larger body. From a moon-formation perspective, I'm not sure if this is particularly possible or not.

If you fool around with some orbit simulators- this one or this one, you could find a configuration that works. Don't worry about making a gas giant orbit around a sun, and then placing your two moons.

If you take the first simulator, change the number of bodies to 3, and make the mass of the third body 10, it can approximate the setup you want. (Alternatively: for body two, have a mass of 1 and a distance of 140; body 3 has a mass of 1 and distance of 145.)

Tidal locking in not hard to achieve- it just takes some time.

# Moon Formation Perspective: Also Possible

We have some ideas about how moons are formed. Given some theories of moon formation, this can happen, although it may not be very likely.

A Note: for most stories, the specifics of orbit are not as important, just that the configuration is possible.

This scenario is possible, but it relies heavily on how far away the moons are from the gas giant. A statelite cannot be tidally locked to two different bodies, whichever one has the greatest tidal forces acting on it will be the tidal lock "winner". Its why Pluto and Charon can be locked while still orbiting the sun. Since the tidal forces of the gas giant will decrease as we increase the distance, with enough space this setup can work.

Its hard to say, but I think this setup could work on a system that's a similar distance Iapetus is from Saturn. Though this relies on a lot of factors such as the mass of the three bodies in question and whether any other moons throw this delicate setup off balance.

What you describe is known as the 3 body problem. Stability in such a system is dreadfully difficult. The best bet you would have is with one of Euler's solutions, which require all 3 bodies to be co linear at every moment. Other than that, there's very few stable cases.

• If one body is of inconsequential mass (such as a spaceship in a system with a planet and the sun), there are stable solutions.
• A few dozen special cases have been discovered with special mass or orbit limitations.
• Considering that there is the star, around it the large planet, and around it the dual moon, it is already more like a 4-body problem. – Gray Sheep Jul 19 '17 at 0:29
• Btw, for example, the sun-earth-moon system is stable. The closed analytic solution for the problem doesn't exist, but there are a lot of partial results. – Gray Sheep Jul 19 '17 at 0:31