The Wikipedia article on Natural Satellites doesn't really give an adequate distinction as to what distinguishes a moon from other orbiting bodies. What I am looking for is a classification that defines a moon, i.e. when can something be classified as a moon, or definitely not a moon?

Saturn, for example, has lots of orbiting space debris that form its rings. Certainly, there might be some cutoff point.


I suspect that this question will really only be answered by force of necessity. Pluto was considered a planet until a number of other similar objects were discovered that reclassified it as a new kind of object.

Similarly with moonlets; they will be followed and named as moons until it is clear that there is a separate class with a clear cut distinction. So far, nature seems to have solved this for us, since we have found no tiny natural objects orbiting Earth, Mars, Jupiter, or Saturn even though the environments of all these planets have been extensively explored by spacecraft. Likely, non-gravitational forces from sunlight and solar wind keep pushing them in the same direction until they eventually get into an unstable orbit or crash into their primaries.

Until a clear reason for a class separation is found, I see no reason to look for one. If people take the trouble to work out the orbit, then objects of any size can be counted as moons. Currently, 22,000 pieces of space junk are being tracked in orbit around the Earth, and the computers are certainly up to the task of keeping track of them and, presumably, giving all of them names or numbers. In this case, we know they are all artificial, but we could name and track them just the same if they were all natural.

Of course, there's a valid economic reason to track all the space junk in orbit around the Earth, so to predict and possibly prevent collisions. Around a planet such as Saturn, there would be less reason to track thousands of pieces of debris even if they were ever found. So the ultimate answer to your question is that they're moons unless and until there are too many to make it worthwhile to track them. Then they're just rubble or rocks or debris. Moonlets would be too fancy a word for such objects.

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As there wasn't a formal definition of a planet until recently, there still isn't one for a moon. But, a few guidelines:

  1. It should be in an orbit which is cleared of other objects, that is, not a bunch of objects in the same or very similar orbit.
  2. The typical minimum size for consideration is around a km. There is a smaller class known as moonlets that are between 200-400 m in diameter.
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  • $\begingroup$ Oh, good points. Do all of Saturn's inner moons clear the regions around them? (and thus form gaps?) $\endgroup$ – InquilineKea Aug 20 '11 at 17:18
  • $\begingroup$ More or less, except for some of the "Moonlets", which Wikipedia lists as moons, but I personally wouldn't count as moons. I think there's a few moons that share orbits, or at least are very close, but... $\endgroup$ – PearsonArtPhoto Aug 21 '11 at 12:15
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    $\begingroup$ Aren't some of Saturn's moons in each other's Trojan points (L4, L5)? $\endgroup$ – Keith Thompson Mar 20 '12 at 20:25

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