Earth-Comet Crash Less Likely

Earth may be threatened by fewer killer asteroids than astronomers thought, researchers reported Thursday in the journal Nature.

Scientists had estimated that 1,000 to 2,000 mountain-sized asteroids periodically cross Earth's orbit. That translated into about a 1 percent chance in the next 1,000 years of one of those asteroids slamming into Earth with catastrophic consequences.

But new research suggests that Earth shares the inner solar system with only half as many of these large, "near-Earth" asteroids - about 700.

Though the new findings cut in half the chances of a giant asteroid collision in the thousand-year timeframe, that doesn't mean humans can let their guard down when it comes to watching the skies for potentially life-snuffing asteroids, said David Rabinowitz of Yale University, the study's co-author.

"I'm not getting any more sleep knowing this. I'm just happy that we're well on our way to finding most of these asteroids," he said.

The asteroids being counted have diameters between two-thirds of a mile to six miles, or big enough to wreak global disaster if they smacked into the planet.

Rabinowitz and colleagues at NASA's Jet Propulsion Laboratory used a one-meter telescope in Hawaii that is equipped with a highly sensitive device that captures digital images of the night sky.

Photos snapped of the same section of the sky on different nights were scrutinized by computer software that quickly spots tiny blips of light from asteroids on the move. A few years ago, astronomers had to perform that task themselves by laboriously comparing photos - a time-consuming chore prone to error.

Rabinowitz said the new technology should allow astronomers to find 90 percent of large near-Earth asteroids within the next 20 years.

That's good news for anyone worried about a cataclysm like the one believed to have wiped out the dinosaurs 65 million years ago, said Scott Hudson, a professor of electrical engineering at Washington State University.

"It's a first step toward really knowing the magnitude of the task that lies ahead," said Hudson, who is involved in radar imaging of near-Earth asteroids. "Now the task is to go out and find all of these objects."

At the same time, there are still plenty of smaller asteroids out there that, while not posing a global threat to Earth, could still lay waste to entire cities, said Brian G. Marsden, director of the International Astronomical Union.

"I wouldn't want to say we're safe. I wouldn't be as rash as that because we've got all sizes of these things to watch out for," he said.

Asteroids are large lumps of rock, iron and other material believed left over from the formation of the solar system.

NASA has launched sky-watch programs aimed at finding most large near-Earth asteroids. Other countries also are keeping watch. Two weeks ago, Britain created a committee to assess the isk of asteroids or comets smashing into the Earth.

By Rick Callahan