NASA's Wide-field Infrared Survey Explorer (WISE) has found the very first asteroid that (more or less) shares an orbit with Earth! Called 2010 TK7, this asteroid is about 300 meters (roughly 1000 feet) across, and is the first in an up-to-now theoretical class of objects called Earth Trojans. Here's a WISE image of the little bugger:
Doesn't look like much, does it? Of course, from 80 million km (50 million miles) from Earth it's amazing we can see it at all. Moreover, given its position in the sky, it's only up during the day as seen from Earth; it was only discovered because WISE orbits the Earth, so the sky is always dark. Also, WISE sees in the infrared, so warmer objects are easier to spot. This rock is probably around the freezing point of water or so, which, to an astronomer, is pretty warm. So what makes this asteroid ...
