That’s no moon…

…it’s a space station. Well, not really. But even this thing at a measly 300(ish) meters across has enough gravity to trap a moon (and keep it bound, even when it’s doing a flyby of MUCH bigger fish, like us).

Full story with an animated gif of the moon orbiting the asteroid here: http://www.jpl.nasa.gov/news/news.php?feature=4459