COSMIC ROCKS CONTINUALLY STRIKE THE EARTH : MILITARY REPORTS CONFIRM HITS.

Byline: William J. Broad The New York Times

Astronomers who scan the sky for big rocks on a collision course with Earth are getting new support for their pet nightmares from once-secret military data. The readings show that the planet is continually struck by speeding boulders that explode in blasts the size of atomic detonations, and that the rate of bombardment is higher than previously observed.

The blasts light the sky with brilliant fireballs but are seldom seen because they often occur over the sea or uninhabited lands. The rocky objects that cause them are anywhere from a few feet in diameter to 80 or so feet wide. They vanish in titanic explosions high in the atmosphere, since though small, their enormous energy of motion is converted almost instantly into vast amounts of heat. Unlike their much larger cousins, they usually leave few or no traces on the ground.

Scientists at the Los Alamos National Laboratory in New Mexico are now releasing old and contemporary observations of such blasts made with military sensors. The ground-based sensors work like sensitive ears to detect very low-frequency sound waves, which radiate outward around the globe over hundreds and thousands of miles.

The observed rate of bombardment is about 12 events a year. At a minimum, these cosmic intruders produce blasts as big as a nuclear warhead of one kiloton, equal to 1,000 tons of high explosive. The old rate, based on orbital data from early-warning satellites that watch for rocket firings and nuclear explosions with telescopes, was eight impacts a year.

``The theoreticians are thrilled,'' Dr. Douglas O. ReVelle, a meteorologist at Los Alamos who works on the detection project, said in an interview. ``People weren't aware that you get these acoustic responses from objects entering the atmosphere.''

For decades, astronomers, geologists and planetary scientists have hypothesized about the rate of terrestrial bombardment, with the Earth seen as an unwitting target in a cosmic shooting gallery. But their calculations have tended to be long on surmise and short on fact.

The disclosure of a high rate of impact from relatively small objects is seen as bolstering the idea that Earth is periodically subjected to strikes by even larger objects from space, including doomsday rocks a few miles wide. Objects this size are predicted to hit once every 10 million years or so, causing mayhem and death on a planetary scale.

``It helps refine the theory,'' Dr. David Morrison, an expert on doomsday rocks at the Ames Research Center of the National Aeronautics and Space Administration in Mountain View, Calif., said of the new data. ``It would be very interesting scientifically to pin down the contemporary rate, as you could do from atmospheric observations.''

For ages, people gazing at the night sky have been fascinated by, and at times frightened by, sudden streaks of bright light.

But scientists only recently have come to realize that the grain-of-sand-sized particles that regularly blaze as shooting stars and meteor showers are simply one end of a spectrum of cosmic debris that continually collides with the Earth as the planet sweeps though space. On occasion, the intruders are large enough to survive the fiery plunge through the atmosphere to strike the ground and dig large craters.

Rough weather and geologic activity mean Earth has relatively few cosmic scars. In contrast, the Moon's highlands are saturated with rocky wounds that overlap one another in dense profusion.

Despite the erosional forces on Earth, it has been clear for some time that the planet occasionally gets hit. In 1908 over Siberia, a speeding object exploded in the atmosphere with a force of some 20 hydrogen bombs, and the resulting shock wave flattened hundreds of square miles of forest and reverberated around the world.

It was the Siberian event and, later, in the space age, the observation of craters on the Moon, Mars, and distant planetary bodies that set scientists to pondering the possibility that the Earth coexists in space with a swarm of doomsday rocks.

Based on such evidence as telescopic surveys of planetary craters, and of comets and asteroids that occasionally come hurtling near Earth, scientists in the 1980s proposed that the planet faces a wide range of threats. Big intruders strike less often, they said, with the biggest ones striking once every 100 million years or so.

Relatively small objects that produce one-kiloton blasts in the atmosphere in theory should strike about once a month, the scientists calculated. But since civilians had no global observation network to watch for such fiery impacts, the estimates remained largely speculative.

After the cold war ended, however, the American military began to release data it had secretly gathered for decades to warn the national authorities of rocket attacks and nuclear blasts. By accident, such records also held clues to the rate of cosmic bombardment.

In 1993 and 1994, once-secret data from military satellites were released that showed Earth had suffered 136 explosions high in the atmosphere from 1975 to 1992, an average of eight a year. The blasts were calculated to have intensities roughly equal to 500 to 15,000 tons of high explosive, or the power of small atomic warheads. Fifteen kilotons is about the force of the nuclear bomb that leveled Hiroshima.

The observed rate from the satellites was below the predicted one. But experts explained this gap by saying that the spacecraft by nature missed many blasts, and that data on cosmic fireballs were not always saved by military archivists.