[meteorite-list] Microphones Tell Asteroids From A-Bombs

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Apr 22 10:00:07 2004
Message-ID: <200207171702.KAA15696_at_zagami.jpl.nasa.gov>

http://www.nature.com/nsu/020715/020715-4.html

Microphones tell asteroids from A-bombs

Detecting low frequency rumbles could avert nuclear war.

TOM CLARKE
Nature Science Update
17 July 2002

Free data from a global array of microphones could spot nuclear false
alarms, averting disastrous retaliation, say scientists[1] and defence
experts.

The ground-based network will detect the faint, low-frequency rumbles of
meteor explosions high in the atmosphere that can look like nuclear
explosions to other sensors. Its primary purpose is to help police the
Comprehensive Test Ban Treaty (CTBT) that forbids the testing of nuclear
bombs.

The United States is the only nation with a satellite network and radar
sufficiently sophisticated to tell a nuclear blast from a meteorite
explosion. It sometimes makes these data available, but they can take months
to get hold of.

"This network will warn other nations that [a rumble] is just a meteor and
not a nuclear detonation," says Edward Tagliaferri, president of aerospace
consultants ET Space Systems in Camarillo, California, an expert in the
defence implications of asteroid explosions.

In with a bang

When meteors travelling at 15 to 20 kilometres per second hit the Earth's
atmosphere, they explode because of the increased pressure. A meteor 5
metres across can detonate with the force of 10,000 tonnes of TNT - like the
bomb that was dropped on Nagasaki, Japan, in 1945. Five or six asteroid
explosions of twice this size occur in the atmosphere each year.

Like atomic explosions, meteor blasts release electromagnetic pulses that
instruments on the ground can detect. "At a distance, an atmospheric nuclear
explosion looks like a large meteor," explains Peter Brown, a physicist at
the University of Western Ontario in Canada. That is cause for concern, in
the current political climate.

For example, in November 1990, as the United States prepared to invade Iraq,
the Iraqis threatened to fire Scud missiles at Israel, a nuclear power. A
meteorite blew up high over Micronesia with the force of 5,000 tonnes of
TNT. If it had arrived just a few hours later on the same trajectory, it
would have detonated over Tel Aviv. The Israelis might not have known it was
an asteroid, Tagliaferri points out.

A similar event, over India or Pakistan, say, could also have devastating
consequences, a US Senate round-table meeting on asteroids heard last week.
"Neither of those nations has the sophisticated sensors we do," Air Force
Brigadier General Simon Worden of US Space Command told the hearing. "The
resulting panic could have been the spark for nuclear war," he said.

Ground control

Ground-based groups of microphones, called infrasonic arrays, can
distinguish atomic blasts from exploding asteroids up to a few hundred
kilometres away, say Brown, Tagliaferri and colleagues1.

The arrays pick up the very-low-frequency sounds that penetrate hundreds of
kilometres of the Earth's atmosphere. Multiple arrays pinpoint the position
and size of a blast almost as accurately as the satellites used by US Space
Command, the researchers show.

Right now, there are 12 such arrays. Sixty will be built within the next 5
years as part of the CTBT International Monitoring Network. The rules of the
treaty dictate that their data must be available to all. A global array
should spot meteor explosions from most areas of the world, says Brown.

The infrasonic network will also be important for research. Meteorites
smaller than 10 metres across are hard to detect with telescopes, so
scientists have little idea of how often they breach our atmosphere.

An idea of how frequently small asteroids occur is important for estimating
the likelihood of larger ones, such as the one that devastated thousands of
square kilometres of Siberian forest in Tunguska in 1908. The microphone
array, says Matthew Genge of the Natural History Museum in London, UK, "will
help us tell just how many Tunguskas we can expect".

References

  1. Brown, P. G., Whitaker, R. W., ReVelle, D. O. & Tagliaferri, E.
     Multi-station infrasonic observations of two large bolides: signal
     interpretation and implications for monitoring of atmospheric
     explosions. Geophysical Research Letters (2002).
Received on Wed 17 Jul 2002 01:02:05 PM PDT