Via io9:  ‘As recently as 1997, we discovered that another body, 3753 Cruithne, is a quasi-orbital satellite of Earth.

This simply means that Cruithne doesn’t loop around the Earth in a nice ellipse in the same way as the moon, or indeed the artificial satellites we loft into orbit. Instead, Cruithne scuttles around the inner solar system in what’s called a “horseshoe” orbit.’