Don't look now, but our entire planet is busily chasing after an asteroid a thousand feet across that's preceding the Earth in its orbit. It's the first known Earth-orbit example of what are called Trojans, and like other promiscuous planets (I'm looking at you, Jupiter), it now appears that Earth's got 'em.
Because of the way the gravitational field of the Earth interacts with the gravitational field of the sun, there are a couple places along the Earth's orbit where other (smaller) stuff can hang out without getting shoved around too much by any celestial bodies. These safe spots are called Lagrangian points, and while there are five of them, only two are stable, and they're perpetually located 60 degrees in front of the Earth and 60 degrees behind the Earth in its orbit.
Every planet in the solar system has these same stable points, and we've seen asteroids chilling there along the orbits of Mars, Jupiter, and Neptune. They're hard to see from Earth, though, since you can't look 60 degrees ahead (or behind) in our orbit without getting a telescope full of sunlight. We've theorized that there's probably stuff out there, but we weren't able to get a good look at the area until the Wide-Field Infrared Survey Explorer was launched in December of 2009. It now appears as though we do have company out here after all, in the form of a 1000 foot wide rock at Lagrangian point L4 that we're going to keep on chasing around the sun, well, forever.
Nature Paper (*.PDF), via Bad Astronomy
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