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Unformatted text preview: 1 Extraterrestrial Life: Spring 2008 Extraterrestrial Life: Lecture #26 Expected number of civilizations in the Galaxy is proportional to their mean lifetime What hazards are there to civilizations? Natural hazards: impacts from asteroids and comets: • how frequent? • how dangerous? Extraterrestrial Life: Spring 2008 Impacts by asteroids and comets are observed Damage from the Tunguska impact in Siberia: 1908 Witnessed by a handful of people, but no-one known to have been injured Extraterrestrial Life: Spring 2008 Smaller events (atmospheric explosions with yield of a few kT) are quite commonly detected by satellites 1972 event visible during daylight over Utah and Wyoming: appears to have been a ~10m diameter rock that grazed the Earth ’ s atmosphere Extraterrestrial Life: Spring 2008 Large events are much rarer, but have enormously greater consequences: ~10km asteroid created a crater ~180km in diameter off the Yucatan peninsula Parts of the crater can be traced out indirectly on the ground Extraterrestrial Life: Spring 2008 Impactors have speeds of 12-30 km/s, kinetic energy: E = 1 2 mv 2 A 1 km diameter asteroid with a velocity of 20 km/s has an impact energy of 3 x 10 20 Joules (~75,000 megatonnes TNT equivalent)...
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This note was uploaded on 08/31/2009 for the course GEOL 3300 taught by Professor Staff during the Spring '08 term at Colorado.
- Spring '08