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NASA probe leaves crater, debris trail
PASADENA, Calif - A spectacular collision between a spacecraft and a comet
has freed a huge plume of primordial material from the comet's nucleus that
could unlock the secret of how life arrived on Earth, NASA scientists said on
Monday.
The impactor was vaporized upon slamming into the comet at 23,000 mph (37,000 kph) -- the speed it would take to fly from New York to Los Angeles in about six minutes. The collision, which occurred 83 million miles from Earth, marks the first time a spacecraft has come in contact with a comet. Observatories on the ground reported that the explosion brightened the comet by a factor of 5 within 15 minutes of impact, scientists at NASA's Jet Propulsion Laboratory in Pasadena said. Scientists could not immediately determine the size of the crater produced by the impact because of the large plume of ice, dust and gases streaming out and obscuring one end of the comet, which is half the size of Manhattan. "We are waiting for the outgassing to stop. It's clear it's was still coming out for several hours ... and could go on for weeks," principal scientist Mike A'Hearn told reporters at a Monday news conference. The Deep Impact team had estimated the washing machine-sized impactor would punch a hole anywhere from the size of a house to a football stadium, depending on the composition of the comet's surface. "We know that we created quite a crater. We believe it penetrated quite deeply so we know we'll get a good look at the interior," Project Manager Rick Grammier said on Monday. "We just have a wealth of scientific information to go through in the next few months." 'BIG CHANGES' The impact sent up twin plumes of debris, the first appearing as a narrow column that cast a long shadow across the comet. Another plume appeared seconds later on the heels of a brighter explosion, then fanned out in a star shape. Scientists said the plumes stretched for "at least thousands of kilometers" into space. Co-investigator Pete Schultz said the twin flashes showed that the impactor encountered softer, layered material on the comet's surface then hit a thick, hard crust. A spectrometer aboard the fly-by spacecraft captured "big changes" in the spectra of debris flying up from the crater, indicating a variety of materials were freed by the impact, A'Hearn said. Comets are made of gas, dust, organic material and ice from the solar system's farthest regions. Because they were not heated by the Sun during the formation of the solar system, comets retain the original chemical mixture from which the planets formed 4.6 billion years ago. Scientists think comets may be responsible for first bringing water and organic material to Earth by crashing into its surface during a period of heavy comet activity 3.9 billion years ago -- around the same time as the first signs of life. Tempel 1's rough surface, closely revealed for the first time in images snapped by the impactor up to 3 seconds before impact, differed markedly from the two other comets scientists have been able to observe up close, A'Hearn said. The surface showed what appeared to be layering, craters, small bright features and smooth areas that defy physics by stretching around two sides of the comet, he said. "There is something more going on here than we understand," he said. A'Hearn also said the Deep Impact team has been forced to reevaluate its ideas about Tempel 1's shape, which is more like a muffin or a loaf of bread than a pickle. The $333-million Deep Impact mission is the eighth in NASA's Discovery Program to produce relatively cheap missions, such as the 1997 Mars Pathfinder, to explore the solar system.
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