Yes and no- it's all relative. For a satellite close to a large planet, you're right, Patrick. But for a large moon in a distant orbit, a small body could possibly maintain a more intimate orbit without fatal perturbations from the main planet. Jupiter orbits the sun, yet has moons in stable orbits. While the sun is huge, the mass ratio of this example can be scaled-down and remain stable mechanically. I think the lack of such objects might be due to the moon-forming process more than anything. It is accepted that our moon, for example, formed from the earth after a major impact by a Mars-size body. Extending the process would imply another impact, this time on the moon- and of just the right energy to tear a piece loose without setting the moon free, or destroying it outright. Too many "just-so" sequential events need occur to make the thing happen. Unlikely. I can't recall ever hearing or reading about such a case, so either none has been discovered yet or they probably don't exist in the classical solar system- but stay-tuned for the last word on KBO's. If moons of moons exist, they were probably captured long after the main parent-moon system formed. And another possibility is that such cases can exist, but as Patrick pointed out, they don't have incredibly long lifetimes as measured in cosmic timescales. --- Patrick Wiggins <paw@wirelessbeehive.com> wrote:
Just a guess but I'm guessing the gravitational interaction between the satellite and it's parent body would wreck havoc with the orbit of any body trying to orbit the satellite.
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