Participants at a NASA-sponsored conference on the hazards of comets and asteroids smashing into Earth estimated Friday that the planet probably would be hit about once each millennium by a space rock big enough to release about 10 megatons of explosive energy.
Such a rock, estimated at 180 feet across, scorched through the atmosphere over Tunguska in Siberia in 1908 and flattened trees across 800 square miles of forest. No crater was found and experts believe the damage came from atmospheric shock.
Bigger space rocks, which would cause considerably 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.
The planet-wide effects of such a catastrophe are unknown, he said, but debris thrown into the atmosphere could diminish sunlight and perhaps affect agriculture for months.
If such a rock should hit the ocean, it could trigger tsunamis, giant waves hundreds of feet high, to roll through and destroy coastal cities.
A planet-killer asteroid, big enough to destroy whole species, would be rarest of all. The last came 65 million years ago when a six-mile-wide rock wiped out the dinosaurs and about 70 percent of all species.
Although scientists can estimate the odds of an impact, they can't really pinpoint when it could happen.
"We don't know when these accidents will occur," said Duncan Steel of the University of Salford in England. "There could be one sometime in the next 100 years. We don't know."
Asphaug, the meeting's organizer, said scientists recognized the risk to the planet of asteroids and comets in the last few decades and only now are beginning to shape proposals to 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.
The National Aeronautics and Space Administration, under a congressional mandate, started an organized effort in 1998 to find and plot the orbital paths of every Near Earth Asteroid larger than three-fifths of a mile across. Six international observatories are scanning the skies. More than 600 such asteroids have been found, out of an expected 1,000. None represents a threat.
Astronomers now propose a special observatory, called the Large Synoptic Survey Telescope, that would be able to detect much smaller asteroids. It is estimated the proposed instrument, with a 27½-foot primary mirror, would be able to find and plot the path of space rocks down to a diameter of 820 feet.
While the search continues, experts are studying ways to prevent any object from hitting the planet.
It is believed that most asteroids that pose catastrophic danger would be spotted decades before they could endanger Earth. This makes it theoretically possible to deflect the speeding space rock and send it into a new, safer direction.
Unlike Hollywood films that have had crews blowing up such asteroids, the most promising method of deflection would be to change the path of the asteroid slowly, over decades, using small rockets or other devices, Asphaug and other experts believe. 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 Earth.
Before such engineering techniques can be developed, the experts said they need to know more about the asteroids.
Most of the space objects, astronomers believe, are aggregates of rock and dust, held together loosely by gravity. About one in six of the discovered large asteroids have moons, which complicates any effort to change their orbital paths.