Space Rock Misses Mars, Barely

An asteroid once thought to be on a collision course with Mars passed the Red Planet today without incident.

Astronomers first estimated that asteroid 2007 WD5 had as high as a 3.6 percent chance of striking the planet. Newer observations kept lowering the odds for the 164-foot space rock until Jan. 9, when NASA's Near-Earth Object (NEO) program office effectively ruled out chances of an impact.

"Mars sees these kinds of near-miss encounters every ten or twenty years, but the impact rate for asteroids this size is about once in a thousand years," said Steve Chesley, an astronomer at the Jet Propulsion Lab in Pasadena, California.

Astronomers had hoped the fleet of spacecraft orbiting Mars would get a chance to observe the asteroid plowing into the Martian surface. The subsequent crater would have roughly equaled the size of the Meteor Crater that formed in northern Arizona 50,000 years ago, with a 0.5-mile diameter. Such an impact would have also allowed scientists to study the dust cloud from the impact.

"We were hoping for a spectacular show to reveal a lot," Chesley said. "We've actually never seen a significant impact on a terrestrial planet."

Mars is a smaller and harder target for space rocks to hit when compared with Earth, but about five times as many asteroids cross the Martian orbit, according to Chesley. 2007 WD5's path around the sun ranges from just outside Earth's orbit to the outer edge of the asteroid belt between Mars and Jupiter, but will not impact with either Mars or Earth in the next century, JPL researchers said.

The asteroid missed Mars by a distance of approximately 6.5 Mars radii.

Similar near misses occur with Earth. And similarly, astronomers sometimes give odds on a possible impact and then, with further observations, reduce the odds to zero.

In fact, the Mars flyby occurred a day after a 500-foot asteroid flew by Earth at a distance somewhat greater than from the Earth to the Moon.

Chesley and other astronomers considered having one of the Martian rovers eyeball the passing 2007 WD5, but judged the task too difficult for the robotic explorers. None of the orbiting spacecraft turned their cameras or other equipment on the passing rock, either.

"After we knew it was going to miss, it's really a pretty ordinary asteroid cruising around the solar system," Chesley said.

Jeremy has written for publications such as Popular Science, Scientific American Mind and Reader's Digest Asia. He obtained his masters degree in science journalism from New York University, and completed his undergraduate education in the history and sociology of science at the University of Pennsylvania.