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In the past eight years, US Department of Defense
satellites scanning the Earth for evidence of nuclear explosions have
detected nearly 300 optical flashes caused by small (1-10 m) asteroids
exploding in the upper atmosphere. This has provided a new estimate of the flux of near-Earth objects colliding with the Earth, which P. Brown of the University of Western Ontario, Canada, and colleagues publish in this week's Nature.
The revised estimate suggests that Earth's upper atmosphere is hit
once a year by objects that release energy equivalent to five kilotons of TNT. The object that exploded above Tunguska in June 1908 was a 'small' asteroid, yet big enough to flatten 2,000 square kilometres of Siberian forest. Brown and colleagues calculate that Tunguska-like (ten-megaton) events are likely to occur about once every 1,000 years.
This is more encouraging than the previous estimate, from ground-based observations, of once every 200 to 300 years. This work "has linked the fields of meteor and comet/asteroid planetary astronomy in a manner which shows that they are not merely distant relatives," says Robert Jedicke of the University of Arizona, Tucson, in an accompanying News and Views article.
More info: Nature
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