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Tidal forces,moons & changing orbit in solar system.

  1. Nov 20, 2013 #1
    Ok I know our moon is getting farther away due to gravity (tidal force indirect), basically giving it kinetic energy and exchanging that for orbital energy.

    If the moon was in retrograde orbit, i.e. Triton around Neptune, it would work the opposite way and pull the moon closer to the planet.

    However something I just found out, Mars' moon Phobos (not in retrograde) is being pulled closer to the planet because it orbits faster than Mars spins.

    So my question is, assuming the moon doesn't pass any Roche radius and get disintegrated, could a moon in retrograde motion get pulled close enough to the planet, eventually orbit faster than the planet rotates (still in opposite direction) and actually gain altitude again? Kind of yo-yoing back and forth between being pulled in and pulled higher?
  2. jcsd
  3. Nov 20, 2013 #2


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    Staff: Mentor

    No, as in retrograde orbit the moon would always be decelerating under the tidal influence. Imagine our moon's perspective. It sees the Earth as spinning and as the Earth transfers momentum to it, the moon sees the Earth spin slower and slower until the two become tidally locked.

    Now imagine a moon in retrograde. It sees the planet as spinning, but as momentum is stolen from it, it falls and orbits even faster, making the planet appear to rotate faster and faster until eventually it is destroyed by tidal forces.
  4. Nov 21, 2013 #3
    Ah ok, didn't even think about it from the Moon's perspective.
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