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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
This work has astonishing figures and photographs. There is something about seeing illustrations of asteroids slamming into earth that is scarier than any horror movie.
Contending that the risk of an American being killed by a comet or asteroid collision during a lifetime is one in ten thousand, Duncan Steel argues that the government should spend at least one billion dollars a year protecting citizens from such an event.
That collision probability is higher than the probability of being killed by floods, tornados, and jet accidents combined, though it is much lower than the probabilities of dying from autos, fires, homicides, and accidental shootings. 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 rock collision deaths and you end up with about a billion dollars per year.
There is also 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 have only been around for a proverbial blink. There are perhaps billions of future generations that could exist. If a species-killing object is out there, deflecting it or avoiding it by some other method, may represent the greatest achievement in human history.
I am unclear, however, whether his death calculations represent only deaths from direct impact or include deaths from hunger, disease, tidal waves, climate disruption and other indirect factors.
Target 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 a speed of 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 even more dangerous. They create tidal waves that wipe out coastal cities. Coral at an elevation of 1,000 feet in Hawaii may have been deposited by an impact tidal wave. He claims that if the 1908 snowball that hit the forests of Tunguska, Russia exploded over the ocean, it would have produced more tidal wave damage than anyone can remember. He claims the energy of the Tunguska event was 20 megatons, which does not seem large enough to produce super tidal waves, but I'm only guessing.
Steel summarizes the history of asteroid and comet study, writing that there are a billion comets 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.