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(meteorobs) Large Meteor/Atmospheric Collisions
Below is a clarification from Tony Beresford of something that I posted
recently. Just thought Meteorobs list would find it interesting as well.
George Zay
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WHEN IT COMES TO DETECTING METEORS, LOS ALAMOS RESEARCHER IS ALL EARS
SAN FRANCISCO, Dec. 18, 1996 - Chicken Little might have liked Los Alamos
National Laboratory researcher Doug ReVelle, a guy who keeps an "ear" to the
sky listening for falling objects that travel many times faster than the
speed of sound.
And each year at least one fairly large extraterrestrial object comes
rumbling into Earth's atmosphere, said ReVelle, who presented information
about using very low-frequency sound waves to detect meteors today at the
American Geophysical Union's Fall Meeting in San Francisco.
ReVelle and colleagues Rod Whitaker, Tom Armstrong and Paul Mutschlecner
work in the Comprehensive Test Ban Treaty International Monitoring System
infrasound program in Los Alamos' Earth and Environmental Sciences Division.
Using data from Los Alamos listening stations originally set up to monitor
underground nuclear explosions, ReVelle, a meteorologist in Los Alamos' new
Atmospheric and Climate Sciences Group, hears the infrasonic signature
created when meteors enter the atmosphere -- even if no one is around to see
them. The Los Alamos stations, around since 1983, still are enlisted in the
nation's nuclear non-proliferation efforts, but have provided a way for
scientists to gain insight into the proliferation of bolides,
larger-than-average space debris that slams into Earth's atmosphere and
creates brilliant fireballs in the sky.
"Each year, we see at least one object entering the atmosphere that's about
six meters in diameter," he said. "These make an infrasonic signal similar
to what you'd see from a 15-kiloton explosion, an explosion of 15,000 tons
of TNT, depending on the object's velocity and density. And each year we
see around 10 objects entering the atmosphere that are equivalent to a
one-kiloton blast -- or about two meters in diameter."
ReVelle often speaks of meteor size in terms of explosive yield because
meteors and nuclear tests have something in common: Each creates a
sound/pressure wave in the atmosphere that can be "heard."
"Infrasonic waves are very low frequency sounds that exist somewhere in the
realm between hearing and meteorology," ReVelle said. "These sounds are
well below the range of human hearing, which ends at about 30 Hertz, but
actually can be detected as small changes in atmospheric pressure. If you
had a barometer that was sensitive enough, you'd be able to see fluctuations
of several microbars when the waves arrive."
The United States Air Force operated a network of stations to listen for
nuclear weapons tests. The network was the nation's first line of warning
during the 1960s and early 1970s -- until the rise of the satellite era --
ReVelle said. With the array, scientists could determine the size and
origin of the infrasonic waves.
And in the early days of listening for nuclear weapons, the arrival of these
very low-frequency sound waves sometimes put the nation on very high alert.
"On Aug. 3, 1963, just before the Bay of Pigs, the stations detected a
one-megaton event south of Africa," ReVelle said. "As you can imagine, it
must have created quite a stir. It turned out to be a bolide that could
have been as large as 25 meters in diameter."
Since infrasound monitoring stations were set up, a number of large events
have been recorded, among them:
o On Sept. 26 and 27, 1962, two separate objects with an equivalent
explosive force of 20 kilotons and 30 kilotons (each at least six to eight
meters in diameter), respectively, entered the atmosphere above the Middle
East .
o On April 1, 1965, the network detected the Revelstoke Meteorite, an object
somewhere around six meters in diameter. The meteorite yielded enough
infrasonic and seismic data that researchers were able to plot a trajectory
and comb an area of Canadian wilderness in search of the crater. It was
never found, but scientists did find about two grams of the object on the
ground. The Revelstoke Meteorite was the smallest ever recovered and it was
comprised of a very soft material known as carbonaceous chrondrite, which
will crumble when lightly squeezed.
o On February 1, 1994, an object that was about 15 meters in diameter
slammed into the atmosphere over the Marshall Islands in the Pacific at a
velocity of about 25 kilometers a second. Luckily, the fireball, reported
by some witnesses as being brighter than the sun for about a second, most
likely came down in the ocean, ReVelle said.
Many large events have been recorded since the 1960s, but 1996 was a
particularly good year for fireballs, particularly the nights of Oct. 2
through 4, when nearly a dozen bolides were seen over the Earth.
"The Earth ran into a swarm of these things in October," ReVelle said. "Who
knows where they came from; perhaps they were the result of a near-Earth
asteroid that had collided with something, maybe the moon."
During that period, at least five separate fireballs were noticed and
recorded above California, as well as two above New Mexico and others above
the Pacific Northwest. A particularly bright fireball appeared near Little
Lake, Calif., on Oct. 3 at around 8:45 p.m. PDT, and could be seen above Los
Angeles and San Francisco; about 105 minutes earlier, a fireball had
appeared in the skies above New Mexico.
The California bolide -- estimated to be about three-quarters of a meter in
diameter and detected by three infrasound stations that were nearly 600
miles away and 31 California seismic stations -- was seen by more than 200
people. Many actually heard the object.
"Sometimes you'll actually hear a hissing or a buzzing noise and you'll turn
around, look and see a fireball," he said. "What you're hearing is more of
an electrical disturbance caused by the object interacting with Earth's
geomagnetic field. The perturbation travels at nearly the speed of light,
while the bolide itself only travels 50 to 100 times faster than the speed
of sound, and that's why people were able to turn around and see the thing
after they heard it."
The October fireballs above California and New Mexico were the subject of
plenty of publicity and speculation. Researchers originally believed that
one fireball had entered the atmosphere, skipped back into space, orbited
Earth once and re-entered the skies again.
ReVelle's infrasonic data and subsequent reports from ground observers
indicate, however, that the fireballs seen that night above New Mexico and
California came from two different objects -- trajectories indicate that the
first bolide didn't enter the atmosphere at an angle that would allow it to
skip back out into space.
Still, the events intrigue ReVelle and other researchers at Los Alamos,
Sandia National Laboratory, the University of California at Los Angeles and
the University of Western Ontario.
"There are a number of questions left to be answered about the Oct. 3
fireballs," he said, "and there are some things which don't quite add up.
You know, I'm not really sure what was happening in the sky that night."
The four arrays of listening stations operated by Los Alamos -- the only
such network left in regular operation in the world -- can detect meteors
that are as small as a few centimeters in diameter. The stations are useful
because they can help validate other non-proliferation and verification
techniques, and they cost very little to operate and maintain.
"In the realm of non-proliferation, it's a very inexpensive insurance
policy, and the array gives us a tremendous opportunity to learn about
meteors and atmospheric phenomena as well," ReVelle said.
Los Alamos National Laboratory is operated by the University of California
for the U.S. Department of Energy.
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