Posted: Thu, Nov 21, 2002, 10:01 AM ET (1501 GMT)
A powerful impact like the one nearly a century ago in Siberia may take place on average just once a millennium, according to research published in the latest issue of the journal Nature. The study by Canadian and American scientists used data from US military satellites that detected 300 explosions in the upper atmosphere over eight years caused by asteroid collisions. They concluded that there is one impact each year that releases the equivalent energy of five kilotons of TNT. More powerful impacts, though, are less frequent: megaton-sized impacts will occur only once a century while ten megaton explosions, like the one that occurred over Tunguska, Siberia in 1908, may happen just once in 1,000 years. Scientists previously estimated that Tunguska-class impacts might be as frequent as once a century.

Related Links:

•  University of Western Ontario press release
•  SPACE.com article
•  New Scientist article
•  Spaceflight Now article