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Дата изменения: Fri Jun 8 23:31:38 2007
Дата индексирования: Sat Dec 22 06:13:47 2007
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Поисковые слова: comet
All kinds of death - 8

Cosmic catastrophe - 1

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Our future may lie outside our control
Cosmic catastrophes are rare, but violent, events which might overwhelm our civilisation.



Comet Hyakutake (Todd Lauer, KPNO)



The nucleus of Comet Halley

Cometary impacts

Comets are `dirty snowballs' - 1 to 10 km diameter lumps of ice (water-ice, methane-ice, CO2 ice) and dust - which originate from the outer Solar System.

  1. ~2 million comets are thought to lie in the Kuiper belt (beyond Neptune's orbit)
  2. as many as 10,000 billion (1013) may reside in the Oort Cloud, distances <50,000 Astronical units
A typical comet weighs ~100 billion tons and has a velocity of 1 to 10 km/sec relative to the Earth -
substantial energy released in planetary impacts - 10 million Megatons of TNT.
(The atomic bomb dropped on Hiroshima had an energy of 13 kilotons.)



Comet Shoemaker-Levy, after close passage to Jupiter


Fragment G hits Jupiter (Keck telescope)


Jupiter scarred by Shoemaker-Levy

We may already have experienced a (much smaller) cometary impact this century
the Tunguska event - Siberia, June 30, 1908



An artist's conception of the Tunguska bolide (W. Hartmann)



Tunguska impact site in 1926



Tunguska impact site now

The Tunguska fireball is estimated as having an original diameter of ~100 metres, mass of 50,000 tons, releasing an energy of ~100,000 Megatons. That type of impact is expected, on average, once or twice a century.
Impacts with 1-km diameter objects (releasing 1000 times more energy) are expected every 100,000 years, or so.
(Duncan Steel - Rogue Asteroids and Doomsday Comets)

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