Co-orbital Moons
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Basic Information

Co-orbital Moons are two or more moons circling the same planet at roughly the same distance from that planet.

The only example of this in our solar system is Janus and Epimetheus, two of the moons of Saturn. At any given time, one tends to be in a slightly lower orbit than the other, and thus orbits faster. Every four years the lower moon catches up with the higher one, their mutual gravity shifts them both and they trade orbits, the lower and faster moon becoming the higher and slower moon.



Game and Story Use

  • Other co-orbital moon systems might exist in other solar systems.
    • In addition to the orbital transitions that Janus and Epimetheus undergo, there's also the possibility of stable non-swapping co-orbital moons. See Lagrange Point for an explanation of where and how that could work.
  • If something went wrong and the moons impacted (instead of staying in Lagrange accordance, or swapping like Janus and Epimetheus), you'd get one heck of a disaster, with the possibility of one or both being knocked into a different orbit. The impact could transfer kinetic energy from one to the other, resulting in one moon going further out, and the other dropping orbit or even crashing down to the planet.
  • Co-orbital moons would probably result in some really interesting tidal forces down on the planet. Probably far more complicated than our twice-a-day high tide here on earth.
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