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: http://www.stsci.edu/~inr/observ/dpics/d8.htm
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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 |
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.
We may already have experienced a (much smaller) cometary impact this century
the Tunguska event - Siberia, June 30, 1908
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)