Asteroid Defense

The U.S. federal government is summoning the world's top scientists to an urgent conference this summer to plan defenses against an attack that could wipe out an American city or disrupt the whole country's infrastructure.

No, it's not global terrorism.

The scientists will map ways to combat an asteroid attack, a cosmic sucker punch like the collision that killed the dinosaurs 65 million years ago and flattened a Siberian forest in 1908.

While the world's attention is focused on the real threat of terrorism, the theoretical asteroid menace has been garnering a surprising amount of behind-the-scenes attention.

Britain's Royal Astronomical Society hosted an international meeting of experts on the asteroid impact threat in December. In January the world's astronomers petitioned Australia's government to fund a special asteroid-detecting telescope. In February NASA announced the "Workshop on Scientific Requirements for Mitigation of Hazardous Comets and Asteroids," which will be conducted in Washington in September. In March, NASA activated "Sentry," a new system to monitor near-Earth objects (NEOs) and assess their threat to Earth.

NEOs are small objects--asteroids and certain comets--that orbit in the solar system relatively close to Earth and could one day collide with Earth.

"We've had a couple of close shaves during the past few months," says Brian G. Marsden, with the Harvard

Smithsonian Center for Astrophysics in Cambridge, Massachusetts.

One asteroid caused public jitters when discovered March 12. Named 2002 EM7, it came from the direction of the sun--an astronomical blind spot where objects are hidden in the sun's glare.

Astronomers didn't detect 2002 EM7 until four days after it came within 288,000 miles (460,000 kilometers) of Earth, which they regarded as a close encounter. [The moon is about 239,000 miles, or 385,000 kilometers, from the Earth.]

The asteroid was about 200 feet (60 meters) in diameter--big enough to fill two-thirds of a football field--and could have flattened a city, unleashing the energy of a five-megaton nuclear bomb.

"I think