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These asteroids are called Earth Coorbital Asteroids or "coorbitals" for short. Essentially, they share Earth's orbit, going around the Sun in almost exactly one year. Occasionally a coorbital catches up to Earth from behind, or vice versa, and the dance begins: The asteroid, while still orbiting the sun, slowly corkscrews around our planet.

"These asteroids are not truly captured by Earth's gravity," notes Chodas. "But from our point of view, it looks like we have a new moon."

Astronomers know of at least four small asteroids that can do this trick: 2003 YN107, 2002 AA29, 2004 GU9 and 2001 GO2. "There may be more," says Chodas. He believes the list will grow as asteroid surveys improve in sky coverage and sensitivity.