LOS ALAMOS, N.M., Nov. 22, 2002 — A system operated by the
U.S. Department of Energy’s Los Alamos National Laboratory
and used to “listen” for clandestine nuclear tests has
played a key role in helping scientists more accurately
determine how often Earth is hammered by giant meteors
like the one that flattened 1,200 square miles of forest
in Russia in 1908.

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.