Using a system designed to detect clandestine nuclear weapons tests, researchers at the Department of Energy's Los Alamos National Laboratory detected two large meteors that recently entered the atmosphere above the Pacific Ocean.

Using Los Alamos listening stations that can help alert international authorities to weapons tests by rogue groups or nations, Los Alamos researchers Rod Whitaker, Doug ReVelle and Peter Brown detected two meteors entering the atmosphere on April 23 of this year and on Aug. 25, 2000. Data from orbiting space platforms confirmed the objects. The Los Alamos team waited until the space-platform data were released publicly last week before releasing their data.

The meteors were very large, measuring about six and ten feet in diameter. They undoubtedly appeared as huge fireballs in the sky. Such large, fiery meteors are called bolides.

When a bolide enters the atmosphere - or when a large explosion is detonated - it creates a sound, or pressure wave, that at long-range is below the levels of human hearing. This infrasonic wave travels through the atmosphere and can be detected by special microphones that are set up in an array. Los Alamos operates four arrays located throughout the United States. By looking at the time of arrival of the sounds at different array stations and at the frequency of the infrasonic signal, researchers can pinpoint the location of the source and determine the amount of energy that created it.

The April 23 meteor plunged into the atmosphere above the Pacific Ocean several hundred miles west of the northern Baja California region of Mexico. The August 2000 meteor entered the atmosphere off the coast of Acapulco, Mexico.

The pressure wave of a bolide entering the atmosphere is akin to a pressure wave created by an explosion. Because of this, ReVelle and Whitaker often speak of meteor size in terms of explosive yield - the larger the yield, the greater the diameter of the meteor.

The August 2000 meteor had an explosive yield equivalent of 2,000 to 3,000 tons of TNT. The April meteor was much larger, with an equivalent explosive yield of 6,000 to 8,000 or more tons of TNT. Based on the energy and speed, ReVelle and Whitaker estimate the first meteor was six feet in diameter. The second meteor probably was at least twice as large.

"Had anyone seen the April 23 event, they would have seen quite a show," ReVelle said. "That meteor was one of the five brightest meteors that have ever been recorded, it was a very large bolide."

Each year a number of large meteors enter the atmosphere and are detected by the Los Alamos arrays. ReVelle said that on the average, 10 or more meteors that are six feet in diameter enter the atmosphere each year. Larger bolides entering the atmosphere occur less frequently, but they do occur nevertheless.

Bolides produce their brilliant light shows tens of miles above Earth's surface. Fortunately for people on the ground, most meteors explode into thousands of tiny pieces or burn up completely before they hit the surface. If the August and April meteors made it to the surface, they probably hit water - well away from any populated areas.

When they do hit the ground, their destructive power is unmistakable. The remains of a very large bolide collision with Earth can be seen at Meteor Crater, Ariz.

The Los Alamos infrasound arrays don't provide advanced warning of a meteor's approach because the infrasonic signature takes several minutes to hours to reach the stations. But the stations do have tremendous potential for detecting clandestine nuclear weapons tests, and for forecasting the rate of large bolide entry into the atmosphere.

The meteors of April and August played an important role in helping non-proliferation technology efforts.

"Because those two events were detected by our four arrays and by five other arrays operated by the International Monitoring System, we are able to use the space platform data to calibrate our instruments, and analyses, to make them better able to pinpoint the exact location where these events occurred," Whitaker said. "Every time we hear a bolide, we learn something about this technology and are better able to fine tune it."

But with other non-proliferation technologies available, why are infrasound arrays necessary?

"Infrasound is very simple, inexpensive and easy to operate as a backup system," Whitaker said. "Infrasound arrays are listening twenty-four hours a day, seven days a week. Sometimes other technologies miss events that infrasound arrays detect. Consequently, infrasound is inexpensive insurance for cost effective monitoring, and it is something that's available to the entire international community - which isn't the case with some other technologies."

Los Alamos National Laboratory is operated by the University of California for the U.S. Department of Energy's National Nuclear Security Administration.