Threat of asteroid collision may be just a movie, after all

With improved detection, scientists now believe fewer big asteroids will hit the earth than previously thought.

If the astronomical fireworks of "Deep Impact" or "Armageddon" kept you up at night, you can rest a bit easier. Our cosmic neighborhood appears to hold far fewer objects capable of snuffing out life on Earth than previously thought.

New research announced this week at an international meeting on asteroids and comets at Cornell University in Ithaca, N.Y., does not mean Earthlings can stop scanning the skies for objects that might hit home, however.

Earth's geological record alone suggests that during the 21st century the planet stands a 1-in-3 chance of colliding with an object that could cause heavy local damage, such as the event that flattened a Siberian forest in 1908. And the planet faces anywhere from a 1-in-1,000 to a 1-in-10,000 chance of being hit by an object that could cause what Richard Binzel, a planetary scientist at the Massachusetts Institute of Technology in Cambridge, calls "instant global change."

These new, lower numbers do suggest that astronomers may be closer than they thought to reaching their goal of finding and tracking in the next 10 years at least 90 percent of potential Earth-crossing objects that measure over a half mile wide. This is the size range for asteroids that researchers say could cause a catastrophic change in global climate if one hits our planet.

Older approaches to spotting near-Earth objects (NEOs) led astronomers to estimate that from 1,000 to 2,000 asteroids with these diameters and higher had the potential to threaten Earth, according to David Rabinowitz, a member of CalTech's Jet Propulsion Laboratory in Pasadena, Calif.

"As of today, our best estimate is between 500 and 1,000," he says.

Improved search techniques that include new imaging detectors and telescopes specifically dedicated to finding asteroids have helped thicken the growing catalog of NEOs. These efforts scan over two-thirds of the sky and can spot objects 25,000 to 30,000 times fainter than objects humans can see with the naked eye.

Combined with more-powerful computer programs developed in the past year to improve long-term orbital predictions, they have enabled the asteroid-hazard community to act more quickly to confirm or knock down the notion that a new object has the potential to threaten Earth over a 50- to 100-year period.

Yet, though researchers have been increasingly successful at finding NEOs, keeping track of them has often presented problems.

"The problem of follow-up is one of the show-stoppers" in efforts to determine asteroid hazards, says Andrea Milani, an astronomer at the University of Pisa in Italy.

Roughly 10 percent of the NEOs astronomers find move in ways that mask them as more harmless asteroids that orbit in a belt between Mars and Jupiter. As a result, when astronomers find them, they write them off and fail to make the follow-up observations critical to determining their orbits and the likelihood of a collision with Earth.

In other cases, bad observing conditions prevent astronomers from keeping track of newly found asteroids. Last year, astronomers tried to keep their eyes on an object listed at 1998 OX4, estimated to have a 1 in 10 million chance of striking Earth in January 2046. They tracked it on and off for two weeks before losing it.

To boost the chances of recovering newly discovered, quickly lost asteroids, astronomers turn to computer simulations. Instead of scanning the skies frantically, they calculate the object's "killer" orbits, then look in those directions.

Dr. Milani likens it to looking both ways before crossing a street. "If a car is not there, you cross. You don't need to know where all the cars in the city are."

(c) Copyright 1999. The Christian Science Publishing Society