Close Encounters - Research Article from Macmillan Science Library: Space Sciences

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The Frequency and Energy of Impacts

Given that these cosmic bullets are flying around Earth, the expected frequency of impacts on Earth can be estimated. Any one NEA has a likelihood of hitting Earth in about 500 million years. Since there are about 1,000 NEAs larger than 1 kilometer, one impact about every 500,000 years can be expected. The energy of such an impact can also be estimated. A piece of rock 1 kilometer in diameter, traveling at 20 kilometers per second (12.4 miles per second) on impact, should release an energy equivalent to almost 100,000 megatons of TNT, or about the total energy of all the nuclear weapons in the world. Such a blast should make a crater nearly 20 kilometers (12.4 miles) in diameter.

Past Collisions

Evidence abounds of past collisions, on Earth as well as on the surfaces of almost all other solid-surface bodies in the solar system. Impact craters up to hundreds of kilometers...

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This section contains 1,374 words
(approx. 5 pages at 300 words per page)
Buy the Close Encounters Encyclopedia Article
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Macmillan Science Library: Space Sciences
Close Encounters from Macmillan Science Library: Space Sciences. Copyright © 2001-2006 by Macmillan Reference USA, an imprint of the Gale Group. All rights reserved.
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