Published: August 2008

Earth Scars

Gosses Bluff Crater

Target Earth

Asteroids and comets in nearby space pose a constant threat to our planet. Can we avert catastrophe the next time around?

By Richard Stone
Photograph by Stephen Alvarez

The first sign of the threat was no more than a speck on a star-streaked telescope image. Just after 9 p.m. on June 18, 2004, as twilight faded over Kitt Peak National Observatory in Arizona, David Tholen was scanning for asteroids in an astronomical blind spot: right inside Earth’s orbit, where the sun’s glare can overwhelm telescopes. Tholen, an astronomer from the University of Hawaii, knew that objects lurking there could sometimes veer toward Earth. He had enlisted Roy Tucker, an engineer and friend, and Fabrizio Bernardi, a young colleague at Hawaii, to help. As they stared at a computer, three shots of the same swath of sky, made a few minutes apart, cycled onto the screen. “Here’s your guy,” said Tucker, pointing at a clump of white pixels that moved from frame to frame.

Tholen reported the sighting to the International Astronomical Union’s Minor Planet Center, a clearinghouse for data on asteroids and comets. He and Tucker hoped to take another look later that week, but they were rained out, and then the asteroid disappeared from view.

When astronomers got a fix on it again that December, they realized they had a problem. The rock, bigger than a sports arena, tumbles menacingly close to our planet every few years. As observations streamed into the Minor Planet Center, the asteroid, named Apophis after the Egyptian god of evil, looked increasingly sinister. “The impact hazard kept getting higher and higher,” says Tholen. By Christmas, models predicted 1-in-40 odds that Apophis would smash into Earth on April 13, 2029, and a ripple of alarm spread to the public. “One colleague called it the grinch that stole Christmas,” Tholen says.

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