Sunday, April 17, 2011
A recent paper here has concluded based on data from Hubble and Palomar that Ceres does not have any asteroids, at least no asteroids of a size 1-2 km in diameter or greater. It isn't that surprising to have not found any moons yet, as many bodies do not have them, and the tendency towards small asteroids to have moons is generally due to the rotation of asteroids themselves: their gravity is so low that a fairly rapid rate of spin will cause the outer edges of an irregular asteroid to eventually break off, sending it into orbit around its parent body. Our moon was almost certainly formed from a massive impact, which is a bit similar to the forming of a moon around a tiny asteroid in the sense that the moon formed is one that its parent body couldn't hope to gravitationally capture, and so some sort of external event is assumed to have happened.
It is easily possible that Ceres could have one or more tiny asteroids orbiting it, however, and so we won't know for sure until Dawn arrives. Very few of the asteroids that fly by us are of a size greater than 1 or 2 km, and so none of them would have shown up on this survey either. One of Jupiter's moons (S/2003 J12) is actually smaller than this too, at a mere 500 m in diameter, so that one would have been overlooked too.