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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.

"They're doing very nice things," Dr. John Gibbons, the White House science adviser, said in an interview about the search. "It's a good example of how military assets and NASA technology can work together. They should have a full map of the most interesting objects in the next couple of decades."

The new hunt is different from its predecessors in that it is federal rather than private and employs better equipment than previous searches, which have been done principally by university astronomers.

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 Eleanor Helin, the project's lead scientist, who is based at NASA's Jet Propulsion Laboratory in Pasadena, Calif.

After a thorough census is completed, the warning time for a collision could vary from years to ages, depending on factors such as the object's orbit in relation to Earth's.

Comets and asteroids are rubble left over from the creation of the solar system. Comets are composed of ice, probably with rocky nuclei, and seem to visit Earth's neighborhood only when knocked loose by unknown forces from their home orbits at the edges of the solar system. That makes their arrival schedules somewhat unpredictable.

Asteroids, made of rock, fly mostly in a belt between Mars and Jupiter; once spotted, their orbits can be calculated with great accuracy.

Far from occupying a secure niche in space, Earth now seems to be subject to a steady bombardment of such cosmic debris, even though impacts may be hundreds of thousands of years apart. The idea is a novel one even for scientists; it has become generally accepted among them only recently, after a 15-year debate.

Both asteroids and comets of sufficient size can cause extraordinary damage on Earth because of their enormous speeds, typically many thousands of miles an hour. On impact, the kinetic energy of such objects is converted instantaneously to heat, making them explode with a force equivalent to that of millions of nuclear weapons.

A collision with a large asteroid would gouge out a huge crater and loft a global pall of dust that could block sunlight, disrupt the climate, kill plants and animals and perhaps end civilization.

Serious interest in the collision threat first arose in the 1970s as astronomers scanned the skies and geologists studied giant impact craters.