Press Release

Satellite Study Establishes Frequency of Megaton-sized Asteroid Impacts

By SpaceRef Editor
November 20, 2002
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In Hollywood films such as “Armageddon” and “Deep Impact”
Earth is threatened by enormous asteroids. New research at
The University of Western Ontario establishes a better
baseline for the frequency of large impacts that may cause
serious damage on the ground. Based on these new estimates
the average chances the Earth will be hit by an asteroid
impact capable of causing serious regional damage (roughly
one megaton TNT equivalent energy) is close to once per

The study, led by Peter Brown, Canada Research Chair in
Meteor Science and Assistant Professor in the Department
of Physics & Astronomy at Western, appears in the November
21 issue of the prestigious journal Nature.

United States Department of Defense and Department of
Energy satellites scanning the Earth for evidence of
nuclear explosions over the last eight years detected
nearly 300 optical flashes caused by small asteroids (one
to 10 metres in size) exploding in the upper atmosphere.
This provided Brown and his research team with a new
estimate of the flux of near-Earth objects colliding with
the Earth.

The revised estimate suggests Earth’s upper atmosphere is
hit about once a year by asteroids that release energy
equivalent to five kilotons of TNT. The object that
exploded above Tunguska, Siberia in 1908 was considered
‘small’ (30 to 50 metres across), yet its energy was big
enough to flatten 2,000 square kilometres of forest. It
would have completely destroyed a city the size of New
York. Brown and his colleagues calculate that Tunguska-
like events may occur as frequently as once every 400

“It is important to realize the impact estimates we have
measured are averages from the last eight and a half
years. Based on past observations, it seems likely there
is also a non-random component to the impact flux at
these smaller sizes which would suggest our estimates
are lower bounds to the true impact risk,” says Brown.

“We use Earth’s atmosphere as a detector of small asteroids
or comets by watching for the bright flashes produced as
they impact the upper layers of the atmosphere. This is an
ideal way to see smaller objects (one to 10 metres) too
small to be detected while still in space by ground-based
telescopic surveys, but too large to be detected after
they become bright fireballs by camera networks that
watch the skies,” says Brown. “Ultimately, this new method
of obtaining information redefines our range of knowledge
about how and when asteroids may hit the Earth. Eventually,
this will help us also better determine their origins,
effects, and orbits.”

Co-authors of the Nature paper are Richard E. Spalding,
Sandia National Laboratories in Albuquerque, New Mexico;
Douglas O. ReVelle, Los Alamos National Laboratory in
Los Alamos, New Mexico; Edward Tagliaferri, ET Space
Systems in Camarillo, California; and Brigadier General
Simon “Pete” Worden, formerly of the United States Space
Command in Colorado Springs, Colorado and now Director
of Transformation, Air Force Space Command.

Peter Brown will be available for interviews beginning
today at about 4:30 p.m. He can be reached at
(519) 661-2111 ext. 86458 (office), (519) 642-0924 (home)
or pbrown@u… For copies of the Nature paper please
contact Marcia Daniel, Communications & Public Affairs,
at (519) 661-2111 ext. 85468 or mdaniel@u… .

Note to broadcasters: Western has a VideoRoute service
and can arrange broadcast interviews until tomorrow,
November 20, at 10:30 a.m. Please call (519) 661-2111
ext. 85468 for more information.

SpaceRef staff editor.