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Asteroid Danger to Earth Downgraded

Such objects hit far less frequently than scientists had thought, satellite data show.

November 21, 2002|Charles Piller | Times Staff Writer

An analysis of once-secret satellite data indicates that asteroids capable of causing catastrophic damage strike the Earth only about once every 1,000 years -- far less frequently than previously thought.

Using eight years of data collected for national security purposes by the departments of Defense and Energy, researchers disputed long-held assumptions that asteroids about 50 meters in diameter (about 55 yards across) enter the atmosphere once every 200 to 300 years.

The last 50-meter asteroid to hit Earth exploded above Tunguska River Valley in Siberia in 1908, flattening 700 square miles of remote forest land. Scientists blame a far larger asteroid, that struck the Yucatan Peninsula 65 million years ago, for profound climatic changes that led to the extinction of the dinosaurs.

Data from the new study suggest that if a Tunguska-sized rock hits every 1,000 years, a city is likely to be destroyed no more than every 30,000 years given that most of the Earth is ocean and much of the land mass is sparsely populated, said David Morrison, a space scientist at the NASA Ames Research Center.

The study, published in today's issue of the journal Nature, examined 300 small asteroids that exploded in the air.

"For the first time, this gives us an accurate estimate for the rate of impact of bodies hitting the Earth in the 1-to-10-meter-size range," said Peter Brown, an astronomer from the University of Western Ontario, and lead author of the paper.

By comparing that information with observations of larger space rocks viewed using ground-based telescopes, the researchers discovered a consistent way to calculate the rate of Earth collisions for asteroids of all sizes.

Until recently, most information on "near-Earth" asteroids and comets has been gathered by telescopes that can reliably find only objects 1 kilometer or larger in diameter.

A direct hit by such a gigantic rock would cause drastic environmental damage, if not global apocalypse.

About 650 of an estimated 1,000 such large objects have been tracked. None will hit the Earth in the foreseeable future, Morrison said. Most of the rest should be cataloged by 2008.

But more numerous medium-sized objects -- such as the Tunguska asteroid -- are usually too small to be detected using telescopes. The frequency of their collisions with Earth had previously been extrapolated from observations of larger objects.

The researchers improved on earlier estimates by charting collisions of relatively common asteroids that are too small to be seen using a telescope. This was accomplished by measuring the bright flash caused by their explosions in the atmosphere. The asteroid hunters extrapolated the data to fill in the gap between very large near-Earth asteroids and very small ones, Brown said.

Yet, knowing that catastrophic hits are rare provides cold comfort, because scientists cannot predict when or where a specific rock will land.

"When you cross a busy street, you don't care what the statistics are of a pedestrian being hit. You want to know if a car is headed for you," said Morrison.

But the study may help to provide a different kind of reassurance, said Clarke R. Chapman, a space expert at the Southwest Research Institute in Boulder, Colo.

"Unusual, bright exploding phenomena in the skies could be misinterpreted as a nuclear attack," he said, particularly during periods of tension between nuclear powers -- such as last summer's conflict between Pakistan and India. "So understanding how often they happen and what they are about could help calm nerves."

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