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NASA Approves First Mission to Comets
David McAlary, Voice of America The U.S. space agency has approved its first mission to a comet. NASA will send a spacecraft to crash into a moving comet and bury itself deep inside to determine its composition. NASA calls the $279 million mission Deep Impact. But unlike the 1998 movie with that name, we are heading for a comet instead of comets heading for us. Comets are clumps of dusty ice that orbit the sun. The manager of the Deep Impact mission, University of Maryland astronomer Michael A'Hearn, says scientists are eager to study them because they are considered remnants of solar system formation. "The interest is because they are probably the most primitive accessible bodies," Mr. A'Hearn said. "They probably do provide more information about the early solar system what the disk was like that the planets formed out of than any of the other bodies we can easily get to." Another rationale for the mission is to teach researchers something about whether we can deflect comets that might threaten to hit Earth, as in the movie Deep Impact. "There are secondary reasons why they are of interest also - the impact hazard. Comets are a contributor to that, smaller than the near-Earth asteroids, as far as we know, but they do contribute to it," Mr. A'Hearn said. The Deep Impact team is completing a final design on a spacecraft to be launched in January 2004 to comet Tempel-I, which circles the sun every 5.5 years. The craft is to arrive 18 months later and split into two. The mothership will fly by and send radio images to Earth. It also will release a 350 kilogram computerized projectile, called the impacter, that will guide itself to a collision at a speed of 36,000 kilometers an hour. Astronomers will observe through telescopes. "The impacter does smart targeting. It basically determines where the center of brightness is of the nucleus and adjusts its trajectory so that it's always heading towards a point that is some fixed offset from the center of brightness," Mr. A'Hearn said. Mission scientists do not know the density of comets, so cannot predict how far Deep Impact will bury itself. Whatever depth it reaches will be revealing - and explosive. Mr. A'Hearn says the projectile could carve up to a 25-meter deep crater the size of a football field in the comet's five kilometer wide nucleus. He hopes it will go down far enough at least to determine if a comet's interior differs from its surface. A longer-term goal for some future project is to go even deeper than possible on this mission to get at primordial solar system material. "At some depth, we have preserved the actual molecules that were present in the early solar system and that's what will tell us what the composition was in the early solar system," he said. "For the time being, we can't explore the whole vertical profile through a comet, so we're just taking a first step and trying to understand, are there significant differences from what we see from most comets compared to what's at some depth on the inside? That depth depends on how deep our crater is." Another research goal is to learn if comets exhaust their supply of gas and ice as they close in on the sun or if they seal it into their interiors. Deep Impact is just one of several missions in preparation or already underway to study comets. The U.S. Stardust mission launched in 1999 is on its way to gather dust from comet Wild-2. NASA plans to launch a spacecraft in 2002 to fly close to three comets. And the European Space Agency is to send a spacecraft the following year on a more circuitous, eight-year journey to land on a comet.
And ye shall know the truth, and the truth shall make you free.
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