| Asteroid Threat
to Earth Diminished Jan
13, 2000, by RICK CALLAHAN, Associated Press Writer Earth
may be threatened by fewer killer asteroids than astronomers thought, researchers
reported today 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 risk
of asteroids or comets smashing into the Earth. 
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