A mile-wide asteroid capable of killing millions of people could strike the Earth without warning because threatening objects in space are not being cataloged, an expert says.

A mountain-size space rock "could hit tomorrow and we wouldn't even know it was coming," Clark R. Chapman, an asteroid expert with the Southwest Research Institute, told a congressional subcommittee Thursday.He said a mile-wide asteroid would send so much dust into the atmosphere that the sun could be blotted out for a year, destroying food crops, triggering starvation, and killing millions directly or indirectly.

Such a catastrophe, he said, would "threaten the future of modern civilization."

Testifying at a hearing of the House Science Committee's space and aeronautics subcommittee, Chapman said such an asteroid would gouge a crater bigger than Washington, D.C., and deeper than 20 Washington Monuments piled on top of each other.

He said the chances of such an asteroid striking the Earth next year are one in a few hundred thousand, but this "is more likely to happen than that the next poker hand you are dealt will be a royal flush." The odds for a such a poker hand are about 649,000 to one, he said.

The scientist said that an asteroid much smaller than a mile wide exploded over Tunguska, Siberia, in 1908 and the shock wave flattened trees across an area larger than New York City. Such a burst over a major city, he said, could kill millions instantly.

Chapman and other experts said that the Earth's only protection from such a space bombardment is to search the sky, find asteroids apt to hit the Earth and then fire rockets to divert the objects away from the planet.

With a 10-year warning, "we could probably save ourselves," said Chapman. "At the very least, we could evacuate ground-zero and save up food supplies to weather a global environmental catastrophe."

But he said that little effort is being put out to find Earth-threatening asteroids and only about 10 percent of an expected 2,000 near-Earth objects have been identified and tracked.

Rep. Dana Rohrabacher, R-Calif., the committee chairman, said that a committee led by the late asteroid expert Eugene Shoemaker recommended five years ago that the National Aeronautics and Space Administration start a systematic effort to search out, identify and plot all asteroids that pose a threat to the Earth. The report said the effort would cost about $5 million a year.

But Rohrabacher said the space agency has done little to follow up on the recommendation. Also, he said, an Air Force asteroid mission was canceled last year after President Clinton used his line-item veto against the project.

In response, Carl Pilcher, NASA's science director for solar system exploration, told the panel that his agency a year ago recognized it was not spending enough to complete a comprehensive survey of Earth-threatening asteroids.