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  sci-tech > space > story pagecorner  

Scientists reduce odds of Earth-asteroid collision

July 28, 1999
Web posted at: 9:36 a.m. EDT (1336 GMT)

By Robin Lloyd
CNN Interactive Senior Writer

Artist's concept of a catastrophic asteroid impact with the Earth   

(CNN) -- The number of asteroids with the potential for the Earth-crushing scenario depicted in Hollywood's blockbuster "Armageddon" is half what was previously thought, a scientist said Tuesday.

Scientists with the Near Earth Asteroid Tracking project at NASA's Jet Propulsion Laboratory made the new estimates for the number of sizable asteroids with orbits that approach Earth.

"Our best estimate as of today is there are between 500 to 1,000 large near-Earth objects at 1 kilometer (diameter or more)," said David Rabinowitz, a scientist with the JPL project headed by Eleanor Helin.

If such an object collided with Earth, debris from the impact could cause worldwide clouding and cooling, with possibly disastrous effects on crops and animals.

Initial estimates made in the 1960s of the number of potentially dangerous asteroids were way off, Rabinowitz said at the seventh International Conference on Asteroids, Comets and Meteors, held this week at Cornell University in Ithaca, New York.

In the 1970s and 1980s, scientists came to agree on a count of about 1,000 to 2,000 objects that are 1 kilometer (0.6 miles) or more across.

The latest count is more accurate because it is based on more already existing asteroids than previous estimates. Currently, scientists at several tracking stations worldwide have identified 15 percent to 20 percent of the potentially Earth-crushing asteroids.

Scientists at major tracking stations in California, Massachusetts and Arizona are working to find 90 percent of the biggest near-Earth objects within a decade.

1-in-3 chance for impact

The bad news is there is a one in three chance for an impact that would cause local damage in the next 100 years, said Richard Binzel of the Massachusetts Institute of Technology.

Binzel recently created a 10-point scale for estimating the threat of a near-Earth asteroid, with zero indicating no chance of an Earth impact and 10 indicating a certain Earth impact with severe, global damage. The asteroid that could hit in the next 100 years is a class eight, he said.

Such asteroids come along perhaps once every 100,000 years or more, according to the scale, called the Torino Hazard Impact Scale.

A "doomsday" rock would rate a Torino 10, the highest rating. But the majority of asteroids rate a zero, "events having no likely consequences," according to the Torino scale.

The chance of a class 10 impact in the next century ranges from one in 1,000 to one in 10,000, Binzel said.

If such an object were detected, astronomers would have three days to confirm their calculations, under procedures currently being worked out by the International Astronomical Union. Then the matter would be turned over to governmental agencies, said JPL scientist Alan Harris.

In three recent cases, astronomers estimated -- and then revised their estimates to eliminate the threat -- that asteroids might strike Earth in the next century. JPL asteroid tracker Paul Chodas said it was entirely possible that a killer asteroid could emerge with far less advance warning.

"The smaller ones may not be seen until a week, or days before impact," Chodas said. "We only know of 20 percent (of the big NEOs) and one could hit with very little notice. That's why we need to search for them."

In fact, the status of the potential threat from such objects is so fluid that it is not tracked in publications, but on a Web site:

Reuters contributed to this report.

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Near Earth Object Dynamics Site
NASA Jet Propulsion Laboratory
   • Near-Earth Asteroid Tracking (NEAT)
Massachusetts Institute of Technology
Welcome to Cornell University
The International Astronomical Union
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