History of planetary catastrophes...The Mysterious Tunguska Event - Carl Sagan

In the history of the solar system, and even in human history, there are clear records of extraordinary and devastating catastrophes… On the landscapes of other planets, where the records of the past are better preserved, there is abundant evidence of major catastrophes. It’s all a matter of time scales. An event which is improbable in a hundred years, may be inevitable in a hundred million years. Even on the Earth in this century there have been bizarre natural events.

Carl Sagan
Cosmos – A Personal Voyage (1980) [video]

The Tunguska Event, or Tunguska explosion, was a powerful explosion that occurred near the Podkamennaya (Lower Stony) Tunguska River in what is now Krasnoyarsk Krai of Russia, on June 30, 1908 (June 17 in the Julian calendar, in use locally at the time).

Although the cause of the explosion is the subject of debate, it is commonly believed to have been caused by the air burst of a large meteoroid or comet fragment at an altitude of 510 kilometres (36 miles) above the Earth's surface. Different studies have yielded varying estimates of the object's size, with general agreement that it was a few tens of metres across.

Although the meteor or comet burst in the air rather than directly hitting the surface, this event is still referred to as an impact. Estimates of the energy of the blast range from 5 megatons[4] to as high as 30 megatons of TNT, with 1015 megatons the most likely—roughly equal to the United States' Castle Bravo thermonuclear bomb tested in late February 1954, about 1,000 times as powerful as the atomic bomb dropped on Hiroshima, Japan and about one-third the power of the Tsar Bomba, the largest nuclear weapon ever detonated.

The explosion knocked over an estimated 80 million trees over 2,150 square kilometres (830 square miles). It is estimated that the shockwave from the blast would have measured 5.0 on the Richter scale. An explosion of this magnitude is capable of destroying a large metropolitan area. This possibility has helped to spark discussion of asteroid deflection strategies.

http://en.wikipedia.org/wiki/Tunguska_event

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Carl Edward Sagan, Ph.D. (1934-1996) was an American astronomer, astrochemist, author, and highly successful popularizer of astronomy, astrophysics and other natural sciences. He pioneered exobiology and promoted the Search for Extra-Terrestrial Intelligence (SETI).

He is world-famous for writing popular science books and for co-writing and presenting the award-winning 1980 television series "Cosmos: A Personal Voyage", which has been seen by more than 600 million people in over 60 countries, making it the most widely watched PBS program in history.

A book to accompany the program was also published. He also wrote the novel "Contact", the basis for the 1997 Robert Zemecki's film of the same name starring Jodie Foster.

During his lifetime, Sagan published more than 600 scientific papers and popular articles and was author, co-author, or editor of more than 20 books. In his works, he frequently advocated skeptical inquiry, secular humanism, and the scientific method.

http://www.carlsagan.com

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