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 century.

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 years.

"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.

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