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Early in the morning of June 30, 1908, in the Tunguska region of Siberia about 1,000 km (600 miles) north of Irkutsk, an asteroid about 60 meters (200 ft) in diameter entered the Earth's atmosphere, resulting in an immense explosion, centered about 8 km (5 miles) above the forest below. Trees were flattened over an area of 2000 sq km by the detonation of the object, however, no crater was formed because it was destroyed before it collided with the ground.
Estimating how often Tunguska like events happen is a difficult business because without a crater the damage they cause is quickly repaired by regrowth of vegetation. Although knowing the number of Tunguska-sized NEOs does allow you to estimate how often Tunguska events should be this itself is a problem because these are the most difficult to find with telescopes.
Dr. Alan Harris of the Space Science Institute in Boulder, U.S., has recently performed new calculations to estimate the number of small NEOs and the frequency of Tunguska-sized blasts. His research, based on new results published in the Journal Science, and the number of Tunguska-sized NEOs found by the LINEAR survey suggests that there are around half a million NEOs of this size. This would suggest that Tungska-like blasts would occur every 300 to 3,000 years (with every 1,000 years seeming the most likely) much less frequently than the 100 to 300 years of other estimates.
Despite the new study there is little agreement between scientists over exactly how often Tunguska-like impacts occur. The problem being the paucity of evidence for both the effects of the impacts and the small number of Tungska-sized objects discovered by telescope surveys. These smaller events, although having damaging local effects, are not as hazardous as those caused by the impact of NEOs larger than 1km that can change the global climate. It is these larger NEOs that scientists are currently aiming to discover and track.
More info: Our Tunguska page
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