Scientists look for way to defend against asteroids

WASHINGTON -- It wouldn't take a big rock falling from space to cause widespread damage and death on the Earth. The chances of that happening are remote -- perhaps once in thousands of years -- but the destruction would be so extreme that experts say humanity needs to find ways to defend itself.

At a NASA-sponsored scientific conference on the hazards of comets and asteroids smashing into the Earth, experts last week estimated that the planet will probably be hit about once per 1,000 years by a space rock big enough to release about 10 megatons of explosive energy.

Such a rock, estimated at about 180 feet across, scorched through the atmosphere over Tunguska in Siberia in 1908 and flattened trees across 800 square miles of forest land. No crater was found and experts believe the damage came from atmospheric shock.

Bigger space rocks, that would cause more damage, would hit the Earth even more rarely.

An object of about 1,000 feet "would flatten everything in an area the size of New Jersey and kill everybody there," said Erik Asphaug of the University of California, Santa Cruz. Nobody knows, he said, what the planetwide effects would be, but debris thrown into the atmosphere could diminish sunlight, perhaps affecting agriculture for months.

If such a rock hit the ocean, it could trigger tsunamis, giant waves hundreds of feet high, that could roll through and destroy coastal cities.

A planet-killer asteroid, one big enough to destroy whole species on Earth, would be rarest of all. The last came 65 million years ago when a rock six miles wide wiped out the dinosaurs and about 70 percent of all species.

Although scientists can estimate the odds of an impact, they can't really estimate precisely when it could happen.

Asphaug, the organizer of the meeting, said that scientists recognized the risk to the planet of asteroids and comets in the last few decades and only now are beginning to shape proposals that would protect the planet.

"This is the only major natural hazard which can, in principle, be made predictable and even eliminated if we find the dangerous ones and learn how to modify their orbits over time," he said.

Looking for NEOs

NASA, under a congressional mandate, started an organized effort in 1998 to find and plot the orbital paths of every Near Earth Object (NEO) larger than six-tenths of a mile across. Six international observatories are now scanning the skies. More than 600 such NEOs have been found, out of an estimated 1,000. None represents a threat.

While the NEO search continues, experts are also studying ways to prevent any Earth-bound object from impacting the planet.

Unlike Hollywood films that have had crews blowing up such asteroids, Asphaug and other experts believe the most promising method of deflection will to slowly, over decades, change the path of the asteroid using small rockets or other devices. Some have suggested that solar concentrators placed precisely on an asteroid could heat and vaporize enough rocky material to provide a thrust that would reshape the object's orbit to spare the Earth.