Rocky space debris created by collisions in the asteroid belt
between Mars and Jupiter or chunks that break away from comets rain
down on the Earth every day as meteroids, but most of the asteroid
or comet pieces are tinier than grains of rice and quickly burn in
the upper atmosphere as meteors.
In 1908, however, a meteor estimated to be up to 50 yards wide
nearly hit the ground before it burned up over Russia, causing an
explosion that flattened hundreds of square miles of forest in
Tunguska, Siberia. The blast was estimated to be the equivalent of
about 10 megatons of TNT — or 10 million tons.
By comparison, the nuclear bomb that exploded over Hiroshima in
World War II unleashed about 13 kilotons of explosive power — or
In the new study, satellite data taken over the past eight years
suggest that an intermediate-size asteroid like the one that struck
Siberia occur an average of only once every 1,000 years — not every
couple of centuries as previously believed, said Peter Brown, a
University of Western Ontario astrophysicist.
His study, to be published Thursday in the journal Nature, was
based on measurements of the flashes of light created when the
debris burns after hitting the upper atmosphere.
Even chunks larger than a yard wide are too small to be easily
detected with camera networks or telescopes on the ground, so Brown
and his fellow researchers — including Gen. Simon "Pete" Worden of
the Air Force Space Command — turned to military satellites used to
detect the flash of a nuclear explosion.
By measuring the intensity of the flash of light with highly
sensitive instruments, the researchers were able to estimate the
size of the asteroids and their explosive power.
They tracked about 300 meteor flashes caused by debris ranging 1
to 10 yards wide from February 1994 to last September. The incoming
debris typically packed an explosive punch of no greater than one
ton of TNT, leading Brown to conclude the chances of a
Tunguska-class asteroid damaging Earth are lower than previously
Scientists who did not participate in the study were impressed by
"It's a darned cool approach to this," said Timothy Spahr of the
Harvard-Smithsonian Center for Astrophysics, who specializes in
studying asteroids near the Earth.
"I'm sure the military has got other things to do but it's really
nice to see things that are used for other purposes help out in this
way," Spahr said.
Brown also compared his results to telescope data on larger
asteroids from the Lincoln Near Earth Asteroid Research project in
New Mexico, run by the Lincoln Laboratory at the Massachusetts
Institute of Technology (news
"When you draw a line from our data down to their data, they
intersect, which is remarkable because they're two completely
different techniques," Brown said. "And that gives us confidence
that numbers on both sides are reasonable."
However, other researchers said Brown's estimate may be subject
to unexpected changes, such as an uncharted comet moving closer to
Earth and showering the atmosphere with fragments of varying sizes.
"The study assumes the flux of asteroids and comets that we have
been observing over the last 20 to 30 years always remains the same,
a basic assumption that is regarded among some astronomers with some
skepticism," said Benny Peiser, an anthropologist at John Moores
University in Liverpool, England, who leads an international forum
on the threat posed by asteroids.
Peiser said the study should help reassure the public that
scientists are developing better ways to assess the asteroid threat,
leading to ways to prevent it or at least minimize it.
Peiser said the study was valuable in another way because it
helped show the U.S. military can detect the difference between a
nuclear explosion and a meteor that sets off a flash similar to it —
a capability he said could help governments avoid mistaking a meteor
blast for a nuclear weapon.
On the Net:
Nature magazine: http://rd.yahoo.com/dailynews/ap/ap_on_sc/inlinks/*http://www.nature.com
University of Western Ontario: http://rd.yahoo.com/dailynews/ap/ap_on_sc/inlinks/*http://www.uwo.ca
Harvard-Smithsonian Center for Astrophysics: http://rd.yahoo.com/dailynews/ap/ap_on_sc/inlinks/*http://cfa-www.harvard.edu
MIT Lincoln Laboratory asteroid project: http://rd.yahoo.com/dailynews/ap/ap_on_sc/inlinks/*http://www.ll.mit.edu/LINEAR