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Mean Orbital Separation Question

  1. Oct 7, 2007 #1
    So it was basic to figure out the masses and such, but I'm not exactly sure what the "orbital separation" really is? Can someone enlighten me? Here is the question for reference:

    A moon with a mass one quarter that of its parent planet orbits that
    planet with a period of 12 days. The mass of the planet is twice that of
    Pluto. What is the mean orbital separation of the planet and its moon?

    Thanks for any help.
  2. jcsd
  3. Oct 7, 2007 #2


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    Gold Member

    planet = 2Pluto
    moon = .25 * 2Pluto = 0.5Pluto
    total system mass = 2.5 Pluto

    Google for Pluto's mass

    Once you have it, you can algebraically re-write the Period formula P=2pi*sqrt(a3)/GM) to solve for a, the semi-major axis, which should equal to your orbital seperation in a circular orbit, or your average orbital distance when averaged over longitude rather than time. There's a couple of calculators on this page: http://orbitsimulator.com/formulas/ that will do it for you. The 3rd one is the Period formula, and the 4th one is this formula re-written to solve for "a".
  4. Oct 8, 2007 #3
    Great, thanks alot ...that link is very helpful
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