100 Years Since Tunguska

[Richard Fiero]

http://www.fas.org/blog/secrecy/2008/06/tunguska.html
Monday, June 30 marks the 100th anniversary of the Tunguska incident 
in 1908, in which a meteor or comet fragment entered the atmosphere 
over Tunguska in Siberia producing an enormous explosion.
We know that a rather massive body flew into the atmosphere of our 
planet, said Boris Shustov of the Russian Academy of Sciences.
It measured 40 to 60 meters in diameter. Clearly, it did not consist 
of iron, otherwise it would have certainly reached the earth. The 
body decelerated in the atmosphere, the deceleration being very 
abrupt, so the whole energy of this body flying with a velocity of 
more than 20 meters per second [probably should be: kilometers per 
second] was released, which resulted in a mid-air explosion, very 
similar to a thermonuclear blast, he told Tass news agency yesterday.
. . .
Impacts such as the Tunguska incident are thought to occur about once 
in one hundred years based on the density of impact craters on the 
Moon, according to a White Paper on Planetary Defense attached to the 
1994 U.S. Air Force report Spacecast 2020.
A 2007 NASA summary report to Congress on planetary defense is here 
[pdf]. A longer account is here [pdf].