Meteoroids bombard Earth, military satellites show

January 25, 1994|By New York Times News Service

Secret data from military satellites in orbit thousands of miles above Earth show that the planet is regularly bombarded by big meteoroids that explode in blasts the size of atomic detonations.

The data, from spacecraft meant to watch for rocket firings and nuclear explosions, were declassified recently by the Defense Department and are to appear later this year in a book.

From 1975 to 1992, the satellites detected 136 explosions high in the atmosphere, an average of eight a year. The blasts are calculated to have intensities roughly equal to 500 to 15,000 tons of high explosive, or the power of small atomic bombs. Experts who have analyzed the data are publishing it in the book, "Hazards Due to Comets and Asteroids," say that the detection rate is probably low and that the actual bombardment rate might be 10 times higher, with 80 or so blasts occurring each year.

The disclosure of a new class of large meteoric impacts is seen as bolstering the idea that Earth is subjected to strikes from space in a wide range of severities, including an occasional doomsday rock perhaps once every 10 million years or so that causes mayhem and death on a planetary scale.

The new data are also being praised as a cold-war spinoff that can aid the cause of world peace by preventing false warnings of nuclear attack. Indeed, it turns out that federal analysts on several occasions have struggled quietly for months to determine if such explosions were natural or man-made.

Finally, impact specialists hope the release of formerly secret data will be repeated and promote a new alliance between astronomers and the keepers of military reconnaissance.

"It's important," Dr. Eugene M. Shoemaker, an astronomer at the Lowell Observatory in Flagstaff, Ariz., who helped found the field of Earth-impact studies, said of the new data in an interview. "It's a unique source of scientific information."

Sky and Telescope magazine, based in Cambridge, Mass., which discusses the data in its February issue, lauded the once-secret sightings as "an unprecedented body of spaceborne observations."

Sighted for ages but understood in detail only recently, meteoroids are rubble left over from the creation of the solar system. They are composed of ice, rock, iron and nickel in a variety of shapes and sizes.

Meteor showers and individual streaks of light that flash across the sky every night are generated when tiny flecks of celestial detritus, often no larger than grains of sand or pebbles, burn up while speeding through the atmosphere.

In contrast, the blasts seen by the military satellites are produced when speeding objects up to the size of large houses are heated to incandescence and then explode about 17 to 20 miles above Earth. They create vast fireballs and powerful shock waves that nonetheless leave few or no discernible traces on the ground, since they begin so high up.

If made of dense metal, meteoroids of this size have a good chance of punching through the atmosphere to hit the ground.

Scientists have suggested that once every 10 million years or so a truly colossal object from space cuts through the atmosphere and slams into Earth, sending up a global pall of dust that blots out the Sun, alters the climate and changes the course of evolution by killing off many plant and animal species.

Such a menacing rock is thought to have carved out a 185-mile-wide crater 65 million years ago that now lies buried in the Yucatan Peninsula of Mexico.

Its cataclysmic impact is believed by some scientists to have caused or assisted in the extinction of dinosaurs.

In general, the new observations are of a class of meteoroids too big to burn up harmlessly in the sky as shooting stars but too small to slice through Earth's atmosphere and strike the ground. They are middle children in the meteoroid family.

By definition, meteoroids are small bodies speeding through space that strike Earth's atmosphere. They can be comets made of ice or asteroids made of stone or metal. Meteors are streaks of light seen when meteoroids are heated while traveling through the atmosphere. Meteorites are the parts of relatively large meteoroids that survive passage through the atmosphere and fall to Earth as chunks of metal and stone.

The scientists who are publishing the new observations say the explosions of large meteoroids previously went largely unreported for many reasons. The vast majority occurred over the oceans or desolate parts of the continents. Many were obscured by clouds.

And even when occurring in broad daylight and rivaling the Sun in brightness, the explosions lasted only a second or so, limiting the opportunities for observation.

"There's many more of these objects impacting the Earth than we previously thought," said Dr. Edward Tagliaferri, a physicist who was the lead author of the satellite report. "Their discovery is a fascinating story." Dr. Tagliaferri is a consultant for the Aerospace Corp., a nonprofit engineering firm in El Segundo, Calif., that helps the Air Force run its numerous satellites.

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