Despite its scientifically ludicrous depictions, Hollywood has nevertheless served to educate people about the dangers of Earth-crossing asteroids and comets. The overall message of the late-twentieth-century movies Armageddon and Deep Impact was grounded in reality. Asteroid impact can be viewed as the most dangerous single threat to our species' existence.
The rate of collision of celestial objects with the Earth has been well established, and the destruction wreaked by such impacts is also well understood. Such events have occurred countless times throughout the history of our planet—and they will inevitably recur in the future. For example, it was a great planetary collision nearly 4 billion years ago that created the Earth-moon system, and in so doing may have made our planet unique as a womb for the gestation and diversification of life. A second great collision some 65 million years ago slayed the planet's dragons and set the stage for mammalian, and ultimately human, evolution. But of greatest relevance to our species are the future collisions, great and small, that will inevitably occur. For alone among the countless species that have populated this Earth, we have it in our power to defend the planet from these strikes.
Just how likely is it that a comet or asteroid collision might destroy our civilization? Is it more dangerous to arm ourselves with nuclear weapons larger and more destructive than any yet developed, ostensibly for planetary defense, than it is simply to pray?
While it is impossible to assign a precise number to the chance of an asteroid impact, we do know that significant hits have occurred as recently as 100 years ago. On June 30, 190', a relatively small meteor exploded in the lower atmosphere over remote Siberia, releasing the force of a hundred Hiroshima-sized atomic bombs. The blast flattened miles of forest, and 50 miles away, reindeer herders and their stock were blown into the air. Had that same explosion happened over a heavily populated area, it would have produced one of the greatest episodes of human carnage in recorded history. The small fragment that produced this explosion was only about 50 meters across. Just two years ago, an asteroid a hundred times as large barely missed the Earth, and was seen only after it went by. Today, several hundred Earth-crossing asteroids of various sizes have been detected, and that seems to be only a small sampling of the thousands that orbit in the vicinity of Earth, not to mention the estimated billion comets hovering far out in space. If an object larger than a mile in diameter were to strike the Earth, not only would our civilization be threatened, but so too would our species. And as we saw with the impact of Comet Shoemaker-Levy 9 on Jupiter, the time between detection and impact for even a giant comet—a species-killing comet, had it struck Earth—can be less than a year.
The reality is that this planet will continue to be hit by debris from space. Just as with earthquakes, the question is not if, but when—and how big. Very large impacts—such as those that might bring about mass extinctions—seem to occur at intervals of tens to hundreds of millions of years. But the frequencies and ages of craters found on the surface of the Earth demonstrate that asteroids of up to a kilometer in diameter seem to hit the Earth at million-year frequencies. Such collisions could be expected to disrupt human agriculture on a global scale for many years following the impact, and would certainly lead to a great slaughter of humanity. The complete extinction of humanity might occur through the impact of a comet or asteroid greater than about 15 kilometers in diameter—half again as large as the one that ended the Mesozoic era (and killed off the dinosaurs in the process) some 65 million years ago.
Although the impact of a very large asteroid or comet could completely eliminate the human race, a more likely scenario is that some significant proportion of our population would be removed by the direct effects of such an event, and the rest of the job would be done by the aftereffects. Just how easily that could occur was shown by astronomer John Lewis of the University of Arizona in his 1999 book, Comet and Asteroid Impact Hazards on a Populated Earth. Lewis not only wrote about the dangers of such events, but included with jus published book a software program that allows the reader to simulate such impacts.
Lewis's simulation program uses a statistical analysis to calculate the human deaths resulting from an impact. Michael Paine, a scientist at the Jet Propulsion Laboratory at Pasadena, ran the program to simulate the effects of likely asteroid impacts on human populations over the next million years. His analysis led to sobering results. Assuming a constant population of 5 billion people on the Earth at any time, the total death toll was 7.5 billion people over a million-year time span—or 7,500 fatalities per year. But the impacts were not evenly distributed. Paine's simulation yielded ten occurrences in which the impacting body was from one thousand yards to a mile in diameter, five in which it was a mile to 1.3 miles in diameter, and one in which the impacting body was greater than 1.3 miles in diameter. The latter event resulted in 2.5 billion deaths when a 1.3-mile comet hit the
American Midwest, releasing the energy of 60,000 H-bombs. Seven million people were killed instantly, and the rest died of starvation as sunlight was blocked and crops failed.
If our species survives for a long period of time—as I believe it will—then we will become used to occasional devastating global events that—at a minimum—knock humanity back into the Stone Age for long periods of time. One such event in conjunction with another human-killing factor could indeed cause our extinction. Of course, the impact of an even larger asteroid or comet could do the job all by itself.
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