PhysOrg says our moon has lots of company – little “minimoons” are always stopping by for an orbital visit:
Mikael Granvik (formerly at UH Manoa and now at Helsinki), Jeremie Vaubaillon (Paris Observatory) and Robert Jedicke (UH Manoa)… concluded that at any given time there should be at least one asteroid with a diameter of at least one meter orbiting Earth. Of course, there may also be many smaller objects orbiting Earth, too.
According to the simulation, most asteroids that are captured by Earth’s gravity would not orbit Earth in neat circles. Instead, they would follow complicated, twisting paths. This is because a minimoon would not be tightly held by Earth’s gravity, so it would be tugged into a crazy path by the combined gravity of Earth, the Moon and the Sun. A minimoon would remain captured by Earth until one of those tugs breaks the pull of Earth’s gravity, and the Sun once again takes control of the object’s trajectory. While the typical minimoon would orbit Earth for about nine months, some of them could orbit our planet for decades.