From a JPL press release on 2 Jan 2003:
http://www.jpl.nasa.gov/releases/20
The asteroid, named 2002 AA29, traces an unusual horseshoe pattern relative to Earth. The asteroid alternately leads and follows Earth around the Sun without ever passing it. "In some ways, the Earth and this asteroid are like two racecars on a circular track," said JPL's Dr. Paul Chodas, who discovered the object's unusual motion. "Right now the asteroid is on a slightly slower track just outside Earth's, and our planet is catching up.