NASA TRACKING ASTEROID THREAT.

Byline: William J. Broad The New York Times

With little fanfare, the federal government has assumed a serious role in an activity seen until recently as the province of zealous astronomers and doomsday enthusiasts: scanning the skies for speeding rocks from outer space that could slam into the planet to cause widespread destruction and death.

It is the first such governmental search in the world. As such, it gives the issue a significant new degree of credibility.

The hunt involves an Air Force telescope usually used for the surveillance of orbiting spacecraft, including foreign spy satellites. The advanced telescope has been improved by the addition of a sensitive electronic camera developed by NASA, which finances and runs the search for faint celestial objects that might one day turn deadly.

To produce a planetary disaster, scientists say, an asteroid or comet would probably have to have a diameter of at least a kilometer, or six-tenths of a mile. It is estimated that, among the many thousands of comets and asteroids speeding through the solar system, up to 1,700 of those crossing Earth's path might be big enough to wreak global havoc.

The odds of a major collision with Earth are judged as fairly remote: perhaps once every 300,000 years or so. But scientists stress that such odds are basically guesses, given the dearth of observational data, and they generally agree that more monitoring is needed.

The government's aim is to refine what are so far very crude estimates of the odds of a major collision by taking a comprehensive census of the asteroids and comets whose orbits periodically cross the path of Earth as it circles the sun.

The project started scanning the heavens a little more than four months ago and has already discovered four previously unknown asteroids whose orbits intersect that of the Earth. The largest of these Earth-crossers, as such asteroids are known, is 1.8 miles across, big enough to cause a global catastrophe.

None of the four new Earth-crossing asteroids, and none of the 200 or so Earth-crossers already found in the private searches, are likely to hit the planet any time soon.

However, the new system's high rate of discovery implies that many more unknown objects zip through the void, scientists say, making the Earth an unwitting target in a cosmic shooting gallery.

``These discoveries certainly suggest that we could face a surprise encounter,'' said Dr. Eleanor Helin, the project's lead scientist, who is based at NASA's Jet Propulsion Laboratory in Pasadena.