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Moons of Moons

  1. Sep 11, 2004 #1
    Greetings all :smile:

    This seems like a good place to ask a question that's been rolling around my head for a while......

    1. To my knowledge there are no "moons of moons" within the Solar Sytem - no natural satellites of any of the moons of Earth,Mars,Jupiter,Saturn,Uranus, Neptune or Pluto. Is this correct?

    2. If it is correct, is this because it is just unlikely - or would such an orbit be intrinsically unstable?

    Just wondering...........

  2. jcsd
  3. Sep 11, 2004 #2


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    There are moons of asteroids. Just thought it might should be mentioned. :)
  4. Sep 11, 2004 #3


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    This actually gets to be a bit complicated. Let's consider the stability of orbits around our own moon, which has been fairly well studied.

    If the moon acted like a point mass, therre would be a region of stable orbits around it known as the "hill sphere"


    Unfortunately, the moon is "lumpy" because of lunar mass concentrations (masscons), so that the analysis isn't this simple


    The above only analyzes the stability of low-altidude lunar orbits, I'm not sure what the analysis shows for the stability of high alititude lunar orbits.

    Maybe someone else can supply more info.
  5. Sep 11, 2004 #4


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    I would have to say that, if we can put something in orbit around a moon, this proves that things can orbit them. If things can orbit a moon, then this would hold true for natural as well as artificial sattelites. Therefore, I think it must be said that it is definitely possible (though it may be very improbable) more moons to have natural sattelites.

    EDIT TO ADD this link
    Last edited: Sep 11, 2004
  6. Sep 12, 2004 #5
    A moon orbits a planet due to its attraction, so if an asteroid or else passes very near of a moon, then the planet attraction is already higher than moon's one, so the asteroid would orbit the planet instead the moon. If that two bodies are a bit isolated, then there would not be any problem (asteroids can also have moons), as 243 Ida has.

    See here> [​IMG]
  7. Sep 12, 2004 #6
    Thanks so much for your thoughts! I guess I'm asking a 4-body problem here....complicated stuff (for me, anyway :smile: ). Thanks for link to Hill Sphere formula and also the orbit simulations - very interesting.

    I know some (or at least 1, Ida/Dactyl) asteroids have moons, but I was thinking of "3rd order" (for want of a better term) orbits - an object orbiting an object orbiting an object orbiting an object(sun).

    As noted above, we can put artificial satellites into orbit around the Moon so why not natural satellites. So my guess is that its just unlikely. It just struck me as interesting that out of the many (100+?) moons known in the Solar System, none seem to have their own smaller companions.

  8. Sep 15, 2004 #7


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    There are a whole bunch of things wrapped up here, in this seemingly simple question!

    First, AFAIK, the long term (read trillions of years) stability of a gravitationally bound system such as our solar system hasn't been solved analytically - at least not with the Newtonian paradigm (don't you just love that word!). In a GR world, an ideal solar system (just gravity, no other forces) is not stable; eventually everything will fall into the Sun, orbital KE being lost due to gravitational radiation (OK, maybe some small bodies will get slingshotted out too).

    Second, as some posters have already noted, there have been Earth-made satellites of the Moon ... but they all crashed onto the Moon (or maybe the Moon lost them, and they became new moons of the Earth, or new planets of the Sun) ... IOW, these 'moon of moon' orbits aren't stable even within a decade, let alone a few million years.

    Third, consider the Sun as a 'planet' of SagA* (the BH at the centre of the Milky Way). Of course, the Sun's orbit around SagA* isn't at all like that of the Earth around the Sun ... there are plenty of relatively nearby massive objects which affect our journey. However, suppose all the mass closer to SagA* were collapsed onto that SMBH, and all the mass further away were banished to the M31; then the Moon would be a 'moon of a moon', the Earth a 'moon', and the Sun a 'planet'! Would these orbits be stable? Certainly! as stable as the Moon's is today. :smile:

    Advanced homework question: what combinations of masses and orbital radii produce 'moons of moons' orbits stable for > 1 billion years? State your assumptions.
  9. Sep 15, 2004 #8


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    As far as I am aware, Sol is 'orbiting' the galactic core, so you can have your '3rd order' orbit by adding a layer on the other side of the sun.
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