This news, believe it or not, is seven years old.

"2003 YN107 arrived in 1999," said Paul Chodas of NASA's Near Earth Object Program at Jet Propulsion Laboratory in Pasadena, Calif., "and it's been corkscrewing around Earth ever since." Because the asteroid is so small and poses no threat, it has attracted little public attention, but he and other experts have been monitoring it.

"It's a very curious object," Chodas said.

Most near-Earth asteroids, when they approach Earth, simply fly by. They come and they go, occasionally making news around the date of closest approach. 2003 YN107 is different: It came and it stayed.

"We believe 2003 YN107 is one of a whole population of near-Earth asteroids that don't just fly by Earth," Chodas said. "They pause and corkscrew in our vicinity for years before moving along."

These asteroids are called Earth Coorbital Asteroids or "co-orbitals" for short. Essentially, they share Earth's orbit, going around the Sun in almost exactly one year. Occasionally a co-orbital 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," Chodas said, "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," Chodas said. He said he thinks the list will grow as asteroid surveys improve in sky coverage and sensitivity.

At the moment, only two co-orbitals are actually nearby: 2003 YN107 and 2004 GU9. The others are scattered around Earth's orbit.

2004 GU9 is perhaps the most interesting. It measures about 200 meters across, relatively large. And according to calculations just published in the Monthly Notices of the Royal Astronomical Society (S. Mikkola et al., 2006) it has been looping around Earth for 500 years - and may continue looping for another 500. It's in a remarkably stable "orbit."

Right now, however, researchers are paying more attention to 2003 YN107 for one simple reason: it's about to depart. The asteroid's corkscrew path is lopsided and on June 10th it will dip within 3.4 million kilometers ((2.1 million miles) from Earth, slightly closer than usual. Earth's gravity will then give the asteroid the nudge it needs to leave.

"This is a chance to observe one of these asteroids (on the way out)," Chodas explained.

It won't be gone forever, however. In about 60 years, 2003 YN107 will lap Earth again, resuming its role as a temporary, corkscrewing moonlet. In due course, other co-orbitals will do the same.

Each encounter is an opportunity for study - and possibly profit. Even the most powerful telescopes cannot see much of these tiny asteroids; they're just specks in the eyepiece. But one day, when the space program is more advanced, it might be possible to visit, explore the moonlets and tap their resources.

"For now, they're just a curiosity," Chodas said.

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