Ceres and Vesta Still Without Moons

Since the Galileo mission discovered tiny Dactyl circling Ida in 1993, quite a lot of asteroid systems have been found to be binary; there are even a few triples. (The most recent reference I could find on a five-minute search said that 195 companions of asteroids had been discovered, as of late 2010; it's got to be more than that by now.) So it's quite reasonable to guess that two of the biggest asteroids, Ceres and Vesta, might also have satellites.

A recent paper by Allyson Bieryla and a pile of coauthors reports on an attempt to find moons of Ceres using the Hubble Space Telescope and the Hale telescope at Palomar. The Hubble data allowed them to look very close to Ceres, so that they could spot moons down below the Roche limit, the altitude where tidal forces would tear them apart. The Palomar data extended their coverage to quite far away from Ceres, beyond its Hill sphere, the region of Ceres' gravitational influence. …