http://news.nationalgeographic.com/news/2003/01/0128_030128_comets.html
Comets: How Big A Threat To Earth? John Roach National Geographic News January 28, 2003 Earth-bound asteroids grab newspaper headlines for good reason. Scientists say the fallout of an asteroid several city blocks wide smacking into the planet would be catastrophic. Mass extinctions, runaway infernos, erratic climate fluctuations, and devastating impacts on human civilization are just some of the scenarios imagined. Why, then, does the threat of a comet impact with Earth-potentially as dire if not worse than an asteroid-rarely leak onto the pages of the popular press? "Primarily because the rate of comet impacts on Earth is not as great as the rate of asteroid impacts," said Daniel Durda, a senior research scientist at the Southwest Research Institute in Boulder, Colorado. Most comets, and potentially some asteroids, have orbits that bring them close to Earth only once every 200 years or longer. Such bodies are known to astronomers as long-period objects. The rate of long-period comet impacts on Earth is on the order of one every 32 million years, whereas the rate of comparably-sized asteroid impacts is more like one per every 500,000 years. "When - note that I do not say if - we find a comet which has some potential to hit Earth, it might cause an even bigger sensation than potential asteroid impactors," said Robert Jedicke, an asteroid expert at the University of Arizona in Tucson. The Threat The consequences of comet and asteroid impacts on Earth are roughly comparable. Both would cause widespread destruction and loss of human life, said Jedicke. "Big chunks of rock with a little ice, an asteroid, or big chunks of ice with a little rock, a comet, create a lot of damage when they impact Earth," he said. "[It's] like getting hit on the head by a stone with an icy coating or an iceball with a lot of rock in it-it's going to hurt your head." A key difference is that long-period objects, like comets, will impact Earth with much greater speed than short-period objects, said Dan Mazanek, an engineer at NASA's Langley Research Center in Hampton, Virginia. "If we happen to come across a long-period object that is dense, it would not have to be large to produce the same kinetic energy of a one-kilometer [0.6-mile] near-Earth asteroid," he said. "To me, that seems like something worthwhile to investigate." Consider this example. An asteroid 0.6 mile (1 kilometer) wide with a density of 187 pounds per cubic foot (3,000 kilograms per cubic meter) traveling at 12 miles per second (20 kilometers per second) would impact Earth with a force approximately 15 times greater than the world's total nuclear arsenal. A comet of just over half the size and one-third the mass traveling at 37 miles (60 kilometers) per second could achieve an impact of similar force if it were to strike Earth. "Size matters," said Mazanek. "But so does density and speed." Protection Some astronomers are working to safeguard the Earth from potential impact by comets or other near-Earth objects in orbit around the Sun. The Near-Earth Object Program at NASA's Jet Propulsion Laboratory in Pasadena, California, coordinates the study of these objects. As near-Earth objects are detected, scientists perform calculations on their orbits to determine if or when they pose a threat to impact Earth. The hope is that astronomers can detect all near-Earth objects decades before they would potentially impact Earth. Meanwhile, other scientists are busy trying to figure out how to throw such threatening objects off course, thus mitigating the pending doom. Long-period objects like comets, however, are not easily detected until they enter the solar system. "A long-period object by definition may not have any records of sightings in written history," said Mazanek. "If it came back into the solar system and it was on [an Earth-bound trajectory], we would not have much warning." Mazanek leads NASA's Comet/Asteroid Protect System, a program that would expand on the Near-Earth Object Program to include the detection of long-period comets, as well as small asteroids and short-period comets that pose an Earth impact threat. The space-based system, not to be in place for at least 25 years, would provide constant monitoring and a system to divert and modify the orbits of threatening objects. Confirmation of a long-period object on an impact trajectory would be possible at least a year before impact, allowing more time to take defensive action than current detection systems allow. The problem is that not much could be done if a long-period object on an Earth-bound trajectory were detected today, said Durda. "The worst scenario I can think of is a multi-kilometer-diameter, long-period comet discovered several months out on an impact trajectory as it is entering the inner solar system," he said. "There is absolutely nothing we could do about it at this point in time. Nothing. ______________________________________________ Meteorite-list mailing list [EMAIL PROTECTED] http://www.pairlist.net/mailman/listinfo/meteorite-list
