| Barringer Meteor Crater is a 0.8 mile diameter, 570 foot
deep hole in the desert located 18.6 miles west of Winslow, Arizona. Since
the 1890s, geologic studies here have played a leading role in developing
an understanding of impact processes on the Earth, the Moon and elsewhere
in the solar system.|
Two large meteors entered the atmosphere above the Pacific Ocean during the past nine months, said researchers at the Department of Energy's Los Alamos National Laboratory who, at the time, were monitoring an infrasound system set up to detect covert nuclear weapons tests.
Hundreds of miles from the entry points, Los Alamos researchers Rod Whitaker, Doug ReVelle and Peter Brown heard the two meteors entering the atmosphere - one on April 23 of this year and the other on August 25, 2000.
The meteors were very large, measuring about six and ten feet in diameter. They appeared as huge fireballs in the sky. Such large, fiery meteors are called bolides, or fireballs.
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.
Based on the energy and speed of the bolides, ReVelle and Whitaker estimate the first 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."
Bolides produce their brilliant light shows miles above Earth's surface. Most meteors explode into thousands of tiny pieces or burn up completely before they hit the surface.
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 the Barringer Meteor Crater in Arizona.
An enormous bolide fell to Earth about 35 million years ago on the Atlantic coast of North America near the Delmarva Peninsula. It carved a roughly circular crater twice the size of the state of Rhode Island, and nearly as deep as the Grand Canyon. Researchers believe the impact crater determined the present day location of Chesapeake Bay.
When a bolide enters the atmosphere - or when a large explosion such as a nuclear test 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 configured in an array. Los Alamos operates four arrays located throughout the United States. Sandia National Laboratory, another U.S. Department of Energy lab, monitors five arrays located in the Pacific Northwest and western Canada.
By looking at the arrival time 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 Los Alamos researchers were using listening stations designed to alert international authorities to clandestine nuclear weapons tests that may be conducted by rogue groups or nations that do not abide by international nuclear non-proliferation agreements.
Data from orbiting space platforms confirmed their observations. Infrared sensors aboard U.S. Department of Defense satellites detected the bolide's impact over the Pacific Ocean on April 23. The object was observed at an altitude of 17.6 miles above the Earth's surface.
Its impact was simultaneously detected by space-based visible wavelength sensors operated by the U.S. Department of Energy and by the Los Alamos researchers monitoring their infrasound system. A similar set of observations confirmed the entry of a meteor last August 25.
Each year a number of large meteors enter the atmosphere and are detected by the Los Alamos arrays which operate in addition to satellite detection systems. "Infrasound is very simple, inexpensive and easy to operate as a backup system," said Whitaker.
ReVelle said that at least 10 meteors that are six feet or greater in diameter enter the atmosphere each year. Larger bolides entering the atmosphere occur less frequently, but they do occur nevertheless.
The meteors of April and August played an important role in improving the accuracy of nuclear non-proliferation technology.
"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."
Whitaker said, "Infrasound arrays are listening 24 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. The Los Alamos team waited until the space platform data were released publicly last week before releasing their own data.
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