The U.S. space agency NASA says its program to track potentially dangerous large asteroids is finding more all the time, but is missing thousands of smaller ones that can still do widespread damage if they collide with Earth.

Space experts told a U.S. Senate science committee that the risk of a collision with a globally-destructive asteroid a kilometer wide or larger is extremely small, likely only less than once every 100,000 years. One is thought to have caused the extinction of the dinosaurs 65 million years ago.

Despite the rarity, NASA's goal has been to track 90 percent of these objects by the year 2008 because of the catastophic consequences of a collision. The chief of the agency's tracking program, Lindley Johnson, told the senators that after six years of effort, it has cataloged nearly 50 percent of the approximately 1,100 large asteroids thought to exist.

But he says the effort is not finding thousands of smaller objects about 100 meters or more wide that hit Earth every few hundred to a thousand years and that can do significant damage.

". . . devastation on the order of a continent if it would hit on land. However, if it were to hit in the ocean, the ocean wave that would be caused, the tsunami, would probably impact on both coasts. We're not tracking those yet. We're finding them when they come close enough to Earth for our sensors to see them, but we now there are a lot more out there," he said.

Last month, one such small asteroid, about 30 meters in diameter, passed just 40,000 kilometers away from Earth, our closest recorded encounter with a space rock.

Astronomer Wayne Van Citters of the U.S. National Science Foundation, the government agency that funds most of the country's non-medical research, says the nation could do more to extend its hunt for small near-Earth asteroids. He says his agency and NASA are considering how to do that with new telescope technology being developed.

One instrument being planned, the Large Synoptic Survey Telescope, would be 50 times more sensitive than any current Earth observatory.

"Things like the Large Synoptic Survey Telescope will quite naturally in the way they operate catalog tens of thousands of these objects," he said. "The estimates are that the surveys would be complete somewhere between seven years and 20 years of operation. The surveys would be 90 percent complete down to 140 meters [diameter], which is, of course, a substantial increase in our knowledge of the objects."

At the same Senate hearing, U.S. astronaut Ed Lu, President of an organization called the B-612 Foundation, argued that NASA should land a rocket on an asteroid by 2015 to test whether it could deflect the body just a few degrees. He says such a capability could save the planet if sky surveys warn of an Earth-bound asteroid many years ahead of time.

The astronaut says such a mission could also test nuclear and other propulsion technologies the space agency is developing to eventually send humans to Mars under President Bush's new space exploration program. Mr. Lu points out that such propulsion would have more power than current chemical systems to move a space rock weighing millions of tons.

"Human beings need to eventually take charge of our own destiny in this manner or we will someday go the way of the dinosaurs when the next great asteroid impact occurs," he said.