Laboratory Helps Forecast Frequency Of Giant Meteors

Previously, scientists believed that meteors like the one that ravaged the Tanguska, Siberia, forest a century ago entered Earth's atmosphere every 200 to 300 years. Now, in a paper appearing this week in the journal Nature, Los Alamos researcher Doug ReVelle and his colleagues have collected evidence indicating that such catastrophic meteor strikes occur less frequently — about every thousand years.

ReVelle teamed up with researchers from Sandia National Laboratories, the University of Western Ontario, ET Space Systems and U.S. Space Command and looked at sound and light signatures from large meteors that had entered the atmosphere during the last eight years. When such meteors — ranging in size from three- to 30 feet in diameter — enter the atmosphere, they create a brilliant flash of light. Often they explode in the middle and upper atmosphere and leave no trace on the ground, but some have the potential to level cities if they reach the ground or explode just above it.

But because their arrival in the atmosphere is heralded by a brilliant flash of light and a burst of sound waves below the range of human hearing at long ranges, the meteors are easily detected by satellites that look for flashes from incoming missiles or nuclear blasts, or by Los Alamos' infrasonic arrays — a set of ears specially tuned to hear ultra-low-frequency rumbles. Satellite and infrasound systems were set up in part to provide a first-alert for clandestine nuclear weapons tests, but their ability to detect meteors is serendipitous.

Until recently, researchers hadn't integrated optical data from satellites and all-sky cameras with sonic data from infrasonic arrays to more precisely calculate the size and energy of incoming meteors.

The Nature paper describes the researchers' work in integrating spectral, photographic and acoustic data from more than a dozen large meteoric events. The result is a graph that allows researchers, astronomers, military officials and others to assign a meteor's energy with its spectral fingerprint — a feat that could not be accurately accomplished previously using either acoustic, photographic or spectral data alone. In addition, the graph allows researchers to assign a frequency for meteors whose destructive energy resides in the mid-range between pea-sized objects like those from the recent Leonid meteor shower and civilization-destroying asteroids.

"What is exciting about this paper for me is that without Los Alamos' infrasound array, this probably would not have been possible," said ReVelle. "Infrasound provided the key to unraveling the energy of 75 percent of these events. Also, the comparison between these Nature results and our earlier influx predictions using infrasound signals alone is very encouraging."

What's more, the ability to accurately distinguish between a meteor and a missile during tense times could be of comfort to leaders worldwide. Gen. Simon P. Worden of U.S. Space Command, and a co-author of the paper, testified before Congress this summer that a meteor could be mistaken for a missile strike without an accurate identification technology — something that satellites and infrasound could provide. As an example, Worden pointed to a large meteor that streaked over the Middle East in June, at a time when tensions between India and Pakistan were high. Fortunately, the very large "shooting star" didn't lead to shooting.

But detection capability is valuable from another point of view as well. A meteor that's 100 feet in diameter has the energy equivalency of a one-megaton explosion, roughly seven times greater than that of the weapon that leveled Hiroshima. According to ReVelle and his colleagues' work, such a meteor enters Earth's atmosphere about every 100 years. A Tanguska-scale meteor, with an explosive energy equivalent of 10 megatons could occur every 1,000 years, according to the researchers.

ReVelle's co-authors are Peter Brown, University of Western Ontario; Richard Spalding, Sandia National Laboratories; Edward Tagliaferri, ET Space Systems; and Simon "Pete" Worden of U.S. Space Command.

Los Alamos National Laboratory is operated by the University of California for the National Nuclear Security Administration (NNSA) of the U.S. Department of Energy and works in partnership with NNSA's Sandia and Lawrence Livermore national laboratories to support NNSA in its mission.

Los Alamos enhances global security by ensuring the safety and reliability of the U.S. nuclear stockpile, developing technologies to reduce threats from weapons of mass destruction, and solving problems related to energy, environment, infrastructure, health and national security concerns.