I happened to read this somewhat old (around 2003) essay the other day, A Comet's Tale. And reading it made me think about Fermi's paradox. The essay is a bit long and takes some time getting into the nature of the solar system. But, if the sun is a typical star, and if our solar system is typical, a civilization probably has less than 1 million years to get into space before a catastrophic end. The essay talks about the orbits of asteroids and our attempt to track them and predict any possible collisions. Toward the end of the essay he talks about comets. They are sort of wild cards and one could crash into the earth with just about a three month warning. Asteroids and comets are by-products of the formation of the solar system.
An excerpt:
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How did we get here? In 1178. a monk in everything else, for all time. Canterbury, England, recorded the testimony of two men who witnessed a "flaming torch" spring up off the face of the moon, which "writhed, as it were, in anxiety," then "took on a blackish appearance." What these men saw, some scientists believe (the issue is debated), was the formation by an asteroid collision of the moon's youngest known crater, Giordano Bruno, named in honor of a defrocked Italian philosopher-priest. The explosion had the estimated force of 120,000 megatons, equal to 120 billion tons of TNT. Hiroshima was a mere 15 kilotons. The greatest man-made explosion in history, a Soviet nuclear test on the Arctic island Novaya Zemlya in 1962, was 60 megatons. If every nuclear device on the planet were somehow to explode at the same moment, no more than 20,000 megatons would be unleashed. The formation of Giordano Bruno, if that is indeed what the two witnesses saw, marked perhaps the first time in recorded history that human beings observed what is now known as a large-body impact. The next would occur more than 800 years later, when two dozen fragments of a shattered comet would explode on the surface of Jupiter. Not even the intervening centuries of scientific advancement would allow us any true comprehension of the destructive potential of large-body impacts. Faced with the effects of 20 megatons of explosive energy for every man, woman, and child on Earth, the mind is quickly beaten into something misshapen and medieval.
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In its journey around the sun Earth passes through the orbits of twenty million asteroids. Many of these Earth-crossers are called near-Earth asteroids. NEAs much smaller than 100 meters wide are basically undetectable but for a fluke of stargazing luck; unfortunately, an object of only, say, 90 meters possesses the collisional capability of roughly 30 megatons of explosive energy, a figure that is dreadful but globally manageable. NEAs larger than 100 meters are thought to number 100,000, a fraction of which have been located; in the event of an impact these could effect serious global climate change. Around 20,000 NEAs are large enough, individually, to annul a country the size of the Czech Republic. The number of NEAs bigger than one kilometer in diameter is currently thought to be around 1,000. At astronomers' current rate of detection-roughly one a day-a survey of the entire population of one-kilometer NEAs will be complete within the next decade.
This one-kilometer threshold is important, for asteroids above it are known as "civilization- enders." They would do so first by the kinetic energy of their impact, striking with a velocity hitherto unknown in human history. The typical civilization-ender would be traveling roughly 20 kilometers a second, or 45,000 miles per hour - for visualization's sake, this is more than fifty times faster than your average bullet - producing an impact fireball several miles wide that, very briefly, would be as hot as the surface of the sun. If the asteroid hit land, a haze of dust and asteroidal sulfates would enshroud the entire stratosphere. This, combined with the soot from the worldwide forest fire the impact's thermal radiation would more or less instantaneously trigger, would plunge Earth into a cosmic winter lasting anywhere from three months to six years. Global agriculture would be terminated, and horrific greenhousing of the climate and mass starvation would quickly ensue, to say nothing of the likely event of world war - over the best caves, say. In the event of a 10-kilometer impact, every- thing within the ocean's photic zone, including food-chain-vital phytoplankton, would die, but this would hardly matter, as the deadly atmospheric production of nitrogen oxides, which would fall as acid rain, would for the next decade poison every viable body of water on Earth. Chances are, however, that the impact would be a water strike, as 72 percent of meteorite landings are thought to have been. This scenario is little better. A one-kilometer impact would, in seconds, evaporate as many as 700 cubic kilometers of water, shooting a tower of steam several miles high and thousands of degrees hot into the atmosphere, once again blotting out incoming solar radiation and triggering cosmic winter. The meteorite itself would most likely plunge straight to the ocean floor, opening up a crater five kilometers deep, its blast wave cracking open Earth's crust to uncertain seismic effect. The resultant tsunami, radiating outward in every direction from the point of impact, would begin as a wall of water as high as the ocean is deep. If a coastal dweller were to look up and see this wave coming he or she would be killed seconds later, as it would be traveling as fast as a 747. Of course, these are all projections based in physics, and can be scaled either slightly up or slightly down in their potential for global destruction. As the paleontologist David M. Raup puts it, "The bottom line is that collision with a. . . one-kilometer body would be most unpleasant."
Although one-kilometer impacts (at least several thousand megatons) are thought to occur once every 800,000 years, with 200-meter objects (1,000 megatons) striking once every 100,000 years and 40-meter objects (10 megatons) striking once every 1,000 years, only a handful of professional and amateur astronomers are currently watching the skies. Nearly half of the asteroids believed capable of destroying one quarter of humanity remain uninventoried. Not until 1998 did the U.S. Congress direct NASA to identify, by 2008, 90 percent of all asteroids and comets greater than one kilometer in diameter with orbits approaching Earth. Unfortunately, the government agency - of any government, anywhere - that would react to and be expected to deal with the likelihood of an asteroid impact does not currently exist. The impact threat is what Ostro calls "low probability and high consequence," and bureaucracies scatter like roaches from the kitchen-bright possibility of severe consequences. We need only to consider the disgraceful games of administrative duck-duck-goose played in the aftermath of comparatively smaller disasters, such as the terrorist attacks of September 2001, to recognize the federal unwillingness to counter its own congenital laxity.
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Just a thought.
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