按键盘上方向键 ← 或 → 可快速上下翻页，按键盘上的 Enter 键可回到本书目录页，按键盘上方向键 ↑ 可回到本页顶部！
————未阅读完？加入书签已便下次继续阅读！
the copestone is on； and the chips were carted off a million years ago。
—HERMAN MELVILLE； MOBY DICK； CHAPTER 2 （1851）
Camarina was a city in southern Sicily； founded by colonists from Syracuse in 598 B。C。 A generation or two later； it was threatened by a pestilence—festering； some said； in the adjacent marsh。 （While the germ theory of disease was certainly not widely accepted in the ancient world； there were hints…for example； Marcus Varro in the first century B。 C。 advised explicitly against building cities near swamps 〃because there are bred certain minute creatures which cannot be seen by the eyes； which float in the air and enter the body through the mouth and nose and there cause serious disease。〃） The danger to Camarina was great。 Plans were drawn to drain the marsh。 When the oracle was consulted； though； it forbade such a course of action； counseling patience instead。 But lives were at stake； the oracle was ignored； and the marsh was drained。 The pestilence was promptly halted。 Too late； it was recognized that the marsh had protected the city from its enemies—among whom there had now to be counted their cousins the Syracusans。 As in America 2；300 years later； the colonists had quarreled with the mother country。 In 552 B。C。； a Syracusan force crossed over the dry land where the marsh had been； slaughtered every man； woman； and child； and razed the city。 The marsh of Camarina became proverbial for eliminating a danger in such a way as to usher in another； much worse。
THE CRETACEOUS…TERTIARY COLLISION （or collisions—there may have been more than one） illuminates the peril from asteroids and ets。 In sequence； a world…immolating fire burned vegetation to a crisp all over the planet； a stratospheric dust cloud so darkened the sky that surviving plants had trouble making a living from photosynthesis； there were worldwide freezing temperatures； torrential rains of caustic acids； massive depletion of the ozone layer； and； to top it off； after the Earth healed itself from these assaults； a prolonged greenhouse warming （because the main impact seems to have volatilized a deep layer of sedimentary carbonates； pouring huge amounts of carbon dioxide into the air）。 It was not a single catastrophe； but a parade of them； a concatenation of terrors。 Organisms weakened by one disaster were finished off by the next。 It is quite uncertain whether our civilization would survive even a considerably less energetic collision。
Since there are many more small asteroids than large ones； run…of…the…mill collisions with the Earth will be made by the little guys。 But the longer you're prepared to wait； the more devastating the impact you can expect。 On average； once every few hundred years the Earth is hit by an object about 70 meters in diameter； the resulting energy released is equivalent to the largest nuclear weapons explosion ever detonated。 Every 10；000 years； we're hit by a 200…meter object that might induce serious regional climatic effects。 Every million years； an impact by a body over 2 kilometers in diameter occurs； equivalent to nearly a million megatons of TNT—an explosion that would work a global catastrophe； killing （unless unprecedented precautions were taken） a significant fraction of the human species。 A million megatons of TNT is 100 times the explosive yield of all the nuclear weapons on the planet； if simultaneously blown up。 Dwarfing even this； in a hundred million years or so； you can bet on something like the Cretaceous…Tertiary event； the impact of a world 10 kilometers across or bigger。 The destructive energy latent in a large near…Earth asteroid dwarfs anything else the human species can get its hands on。
As first shown by the American planetary scientist Christopher Chyba and his colleagues； little asteroids or ets； a few tens of meters across； break and burn up on entering our atmosphere。 They arrive paratively often but do no significant harm。 Some idea of how frequently they enter the Earth's atmosphere has been revealed by declassified Department of Defense data obtained from special satellites monitoring the Earth for clandestine nuclear explosions。 There seem to have been hundreds of small worldlets （and at least one larger body） impacting in the last 20 years。 They did no harm。 But； we need to be very sure we can distinguish a small colliding et or asteroid from an atmospheric nuclear explosion。
Civilization…threatening impacts require bodies several hundred meters across； or more。 （A meter is about a yard； 100 meters is roughly the length of a football field。） They arrive something like once every 200；000 years。 Our civilization is only about 10；000 years old； so we should have no institutional memory of the last such impact。 Nor do we。
et Shoemaker…Levy 9； in its succession of fiery explosions on Jupiter in July 1994； reminds us that such impacts really do occur in our time—and that the impact of a body a few kilometers across can spread debris over an area as big as the Earth。 It was a kind of portent。
In the very week of the Shoemaker…Levy impact； the Science and Space mittee of the U。S。 House of Representatives drafted legislation that requires NASA 〃in coordination with the Department of Defense and the space agencies of other countries〃 to identify and determine the orbital characteristics of all Earth…approaching 〃ets and asteroids that are greater than 1 kilometer in diameter。〃 The work is to be pleted by the year 2005。 Such a search program had been advocated by many planetary scientists。 But it took the death throes of a et to move it toward practical implementation。
Spread out over the waiting time； the dangers of asteroid collision do not seem very worrisome。 But if a big impact happens； it would be an unprecedented human catastrophe。 There's something like one chance in two thousand that such a collision will happen in the lifetime of a newborn baby。 Most of us would not fly in an airplane if the chance of crashing were one in two thousand。 （In fact for mercial flights the chance is one in two million。 Even so； many people consider this large enough to worry about； or even to take out insurance for。） When our lives are at stake； we often change our behavior to arr