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Target Earth: The Search for Rogue Asteroids and Doomsday Comets that Threaten Our Planet by Duncan Steel and Reader's Digest


Target Earth features astonishing figures and photographs. Seeing illustrations of asteroids slamming earth scares me more than horror movies.


Contending that the risk of an American being killed by a comet or asteroid collision during a lifetime equals one in ten thousand, Duncan Steel argues the government should spend at least one billion dollars a year protecting citizens from such an event.


Though lower than probabilities of death from autos, fires, homicides, and accidental shootings, that collision death probability exceeds the probability of death by floods, tornados, and jet accidents combined. The government, he claims, values human lives at "three or four million dollars each." I thought it was about two million each, but maybe I read the two million figure in an old book. Plugging in four million dollors times the mean number of space collision deaths, and you get about a billion dollars per year.


There also exists the tiny probability of a space object killing every human, which in addition to the harms done and huge opportunity losses to present humans, would represent astronomical opportunity losses for future humans. Humans lived for a proverbial blink. Billions of future generations could exist. If a species-killing object exists, deflecting or avoiding it by some other method may represent the greatest achievement in human history.


I am unclear, however, whether the author's death calculations represent only deaths from direct impact or include deaths from hunger, disease, tsunamis, climate disruption, and other indirect factors.


Target Earth is packed with intriguing info: About 40,000 tons of dust and meteors hit earth every year. Almost all are so small that the atmosphere prevents them from doing damage. The moon may have originated from a planet the size of Mars smashing into Earth over four billion years ago.


Calculations suggest that about once in 100,000 to 500,000 years an asteroid a half-mile across strikes the earth. If an asteroid with a one-mile diameter hit the earth traveling at 12 miles per second, the energy of the explosion would equal 240,000 megatons of TNT, about 20 million times the energy of the Hiroshima explosion.


Ocean collisions are worse, creating tsunamis that could wipe out coastal cities. Coral at an elevation of 1,000 feet in Hawaii resulted, perhaps, from an impact tidal wave. Steel asserts that if the 1908 snowball that hit the forests of Tunguska, Russia exploded over the ocean, it would have produced more tsunami damage than anyone can remember. The energy of the Tunguska event was 20 megatons, which seems far too small to produce large tidal waves, but I'm only guessing.


Steel summarizes the history of asteroid and comet study, writing that a billion comets reside in the Oort cloud well beyond Pluto and Neptune. Steel's writing is clear and engaging. Worth reading. Book review article by J.T. Fournier, last updated July 23, 2009.



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