FALLING SPACE ROCKS

Asteroid Trackers Build A Better Warning System

It may seem like the stuff of Hollywood disaster movies. But scientists say the asteroid threat is real.

In fact, it may have been a mammoth wayward space rock that brought the demise of dinosaurs. And some scientists now say the floods, fire, and destruction described in the Bible may have been caused by asteroids slamming into Earth.

Until now, the problem in avoiding a repeat of such calamities has been in tracking these cosmic rocks at a reasonable cost. But scientists at NASA's Jet Propulsion Laboratory - the same place that engineered America's recent low-budget trip to Mars - have developed a low-cost early warning system to chart incoming asteroids. JPL proposes mounting sophisticated electronic sensors on three Air Force satellite-tracking telescopes to scan the skies for asteroids.

A few years ago, when Congress mandated a study of an early warning system - dubbed "Safeguard" - the system was expected to cost some $50 million. But the NASA team now says it could map the sky for about $1 million with a Near Earth Asteroid Tracking (NEAT) system.

"We're offering a cheaper way," says asteroid tracker Eleanor Helin, who heads the NEAT team and outlined its plan during the meeting here of the American Astronomical Society's Division of Planetary Science last month.

NEAT may be cheaper and more effective than current systems. In early tests on an Air Force telescope on Mt. Haleakala in Hawaii, NEAT has identified 10,185 asteroids - 55 percent of which were previously unknown.

Its sensors are far more efficient than those now used. They would enable scientists to take a nearly complete inventory of dangerous asteroids over the next 10 years - as opposed to several decades with the original Safeguard system.

Mapping out the neighborhood

So far, NEAT and other early-warning systems have targeted 99 potentially dangerous objects - rocks that are one kilometer across or larger and will get within 5 million miles of Earth. But these are just a sample of what Ms. Helin and other planetary scientists believe is roaming our neighborhood.

Other efforts to track asteroids have been or are being conducted in the United States, Australia, and France. The most intensive continuous surveillance is by Tom Gehrels at the University of Arizona at Tucson. But none of these covers the whole sky, which NEAT would do. Helin is hopeful NEAT will get funding in NASA's new budget, although a final decision hasn't been made.

Deflecting a cosmic projectile

The goal of the asteroid-tracking system would be to detect earth-bound boulders decades - even centuries - before projected impact. That would give enough lead time to work out ways to deflect a cosmic "bullet."

Scientists and engineers concerned with this subject have no good strategy. They have discussed using explosives - including nuclear charges - or the thrust of "tug boat" rockets to nudge an asteroid aside. Some engineers have suggested trying to plant so-called solar sails on an asteroid so that pressure of solar radiation would deflect it.

Meanwhile, Helin warns that earthlings should be concerned about smaller objects too. Large asteroids hitting with the energy of many millions of average-size atomic bombs cause immense damage over a large area. They also can cause global catastrophe by filling the air with sun-darkening dust, which could kill off crops and many other plants globally.

But smaller objects of a few hundred meters in diameter can cause widespread local damage on land or devastating tsunamis if they hit the ocean.

Helin says more of these may have hit Earth in recent times than scientists realize. Although speculating, she explains that some tsunamis that have arisen without a detectable cause may have been caused by meteorites.

She and other scientists are even looking at old records and folk tales, including Bible stories and Chinese accounts, to see if they refer to actual impacts by medium-size asteroids that made fiery explosions and had the scent of sulfur.