Small asteroid plays cat-and-mouse game with Earth

Monday, January 6, 2003

SEATTLE -- In a space game of "catch me if you can," a small asteroid shares the same orbit with Earth -- sometimes ahead, sometimes behind, but never quite touching -- as the two race around the sun, astronomers say.

"This is one of the most interesting orbits for an asteroid we have ever seen," said Paul Chodas, a Jet Propulsion Laboratory researcher who studies asteroids and who first plotted the bizarre motion of the space rock.

The asteroid, called 2002 AA29, is in a precise circular orbit that follows the same general path as the Earth around the sun. But, like a mouse teasing a cat, the asteroid sometimes speeds up and precedes the Earth and then later slows to drop into a follow-the-leader approach.

Will never meet

But never will the two meet, Chodas says.

On Wednesday, the asteroid makes its closest approach to the Earth in almost a century, moving within 3.7 million miles.

"For a number of decades the asteroid has been going a little slower than the Earth and the Earth has been catching up," Chodas said in a telephone interview. "This week it makes its closest approach in 95 years."

Chodas said that during the close approach, the Earth's gravity will cause the asteroid to swing into a slightly lower orbit, which will make it move faster than the Earth.

The asteroid will continue moving ahead until, in 95 years, it approaches the Earth from behind. Gravity then will force the asteroid into a higher, slower orbit and the Earth will move ahead. In another 95 years, the Earth approaches from behind and the cycle is repeated.

"There's no possibility that this asteroid could hit Earth because Earth's gravity rebuffs its periodic advances and keeps it at bay," said Don Yeomans, head of a NASA asteroid program at JPL, in a statement. "The asteroid and Earth take turns sneaking up on each other, but they never get too close."

A computer simulation suggests that in about 600 years, the pattern will change slightly.

Chodas said the asteroid will loop about the Earth, but never will become a true satellite that actually orbits the planet. After about 40 years, it will drop back into its earlier pattern and the cat-and-mouse game will continue for many more centuries.

The asteroid is only about 200 feet across, too small to be easily seen. It was discovered last year by an Air Force telescope that is part of a NASA program to find and plot asteroids that orbit near the Earth.

Even if 2002 AA29 did hit the Earth it would not cause planetwide destruction as did the 6-mile-wide asteroid that hit and killed the dinosaurs some 23 million years ago.

Instead, said Chodas, the small asteroid would gouge out a crater about three-quarters of a mile across, similar to the Barringer meteor crater in Arizona.

Asteroid observations are among the 1,000 studies to be presented this week at the national meeting of the American Astronomical Society.

Respond to this story

Posting a comment requires free registration: