SAN FRANCISCO, CALIFORNIA--Astronomers say they can now compute with great confidence which asteroids pose a threat to our planet. The problem, they told the annual meeting of the American Association for the Advancement of Science (which publishes ScienceNOW), is that preventing a collision would require mounting a not-yet-planned space mission by a not-yet-identified governmental body.

The next known close encounter with an asteroid will occur, somewhat ominously, on Friday the 13th of April 2029. Then, Near-Earth Object 99942--also known as Apophis (Greek for "The Demon of Darkness")--is expected to miss the planet by a mere 30,000 kilometers. The real sweating begins soon after, when astronomers must determine whether Earth's gravity has steered Apophis onto a course for impact seven years later. Current calculations place the chance of that happening at about one in 30,000.

At 250 meters wide, Apophis is five times larger than the object that hit Earth 50,000 years ago and blew out the 1200-meter-wide Meteor Crater in Arizona. It's also six times larger than the Tunguska object, which grazed Earth's atmosphere before exploding over Siberia in 1908, flattening 2100 square kilometers of forest.

Apophis isn't the only asteroid of concern. Since 1998, NASA's Spaceguard Survey has been identifying and tracking all near-Earth objects (NEOs) larger than 1 kilometer in diameter. The agency expects to complete the survey next year, logging more than 1000 objects, says astronomer Steven Chesley of NASA's Jet Propulsion Laboratory in Pasadena, California. That's a start, but astronomers say there may be another 1 million NEOs larger than 40 meters wide, big enough to penetrate Earth's atmosphere.

So far, there is no formal plan for defending the planet against these objects. Former NASA astronaut Russell Schweickart thinks the world needs a new global body that is authorized to take action, such as launching a spacecraft that could push the intruder safely out of the way. "In 20 years," says Schweickart, who heads the Association of Space Explorers, "we will have identified half a million of these objects, so everybody is going to have to be involved." He says his group, composed of former astronauts and Russian cosmonauts, plans to petition the United Nations soon to establish an international protocol to deal with NEO threats.

NASA astrophysicist Ed Lu of Johnson Space Center in Houston, Texas, doesn't think ramming or blasting an asteroid would be a good idea. He says that would simply convert a single threat into many. Lu says the best way to deflect an asteroid is with a "gravity tractor"--a spacecraft displacing about 1 metric ton that simply hovers near an asteroid and gently tweaks its orbit.

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