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July 21, 2004 Ten years ago, the world watched with wonder and awe as the twenty-one fragments of Comet Shoemaker-Levy 9 slammed into Jupiter. The now- infamous string of pearls struck during a weeklong bombardment that left the Solar System’s largest planet battered and bruised. It was also a sobering reminder that our own planet is vulnerable to cosmic impacts as well. Movies such as “Deep Impact” and “Armageddon”, which appeared in theaters during the summer of 1998—four years after the Shoemaker-Levy impact— sensationalized the impact threat and exaggerated the resources we have at hand to deal with it. But we’ve had several spectacular near misses in the last ten years, often not discovered until after the potential impactor had passed by us. The most likely impact scenario would involve a stony asteroid the size of a football field. A body this size would be at the low end of the range detectable by optical telescopes, but would be easily picked up by radar systems used to track incoming ballistic missiles. Moving at 24 kilometers per second (15 miles per second), and with a density of 3,000 kilograms per cubic meter (187 pounds per cubic foot), it would strike with a force equivalent to 145 megatons (145,000,000 tons) of TNT. To put that in perspective, that would be equivalent to 9,667 Little Boys, the atomic bomb dropped on Hiroshima during World War II, which had an explosive yield of only 15 kilotons (15,000 tons of TNT). A land strike by a body this size would have serious regional implications. A medium-sized city like Charlotte would all but be wiped off the map, even if the impact occurred well away from the business district. Seismic shock, radiant heat and air blast would reduce a city to rubble and leave thousands dead or dying among the ruins. A strike at sea, though, would be more likely, simply because there’s more water on our planet than dry land. A hit in the Pacific Ocean would create tsunamis that would wreak havoc on the western United States and Hawaii or, if the hit happened farther west, Japan and Indonesia. At the present time, there is no active defense against a cosmic impact. The only early warning we have right now consists of amateur astronomers, who use their own telescopes, and a handful of dedicated professional astronomers, who use powerful university and government-owned facilities, tracking down potential threats to our safety. And even if a potential impactor is found, all that can be done now is to figure out, as best it is possible, to find out where it will hit and when. Until a viable defense can be devised, Earth will remain vulnerable to the threat of cosmic impact. It isn’t a question of if we’ll be hit; it’s a question of when. ------------ About the author: Ever since seeing the made-for-TV movie "A Fire In The Sky" in 1978, Claxton Graham has been fascinated by the threat of asteroid and cometary impacts. The calculations for this particular article were derived from the Earth Impact Effects Calculator, created by the Lunar and Planetary Laboratory at the University of Arizona. To visit the site and see how much damage an asteroid or comet would do to your home town, just go to: http://www.lpl.arizona.edu/impacteffects/ Email: scifiwriter8502@email.com Tell a friend about this site! ------------ |
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