Deep impact

By Jeff Hecht A $279 million pot shot aimed at blasting a hole in the surface of a comet has been given the go-ahead by NASA. The Deep Impact spacecraft will target Comet Tempel 1 and is scheduled to launch in January 2004. On 4 July 2005, it will hurl a 350 kilogram projectile toward the surface. “If it’s soft material like lunar regolith, we may get a very large crater”, about 125 metres wide and 25 metres deep, says Peter Schultz of Brown University in Providence, Rhode Island. Astronomers know that comets are mixtures of ice and dirt, with a dark layer at the surface, but they know little about what is inside. Blasting off part of the surface should expose interior ices. The heat of the impact and solar radiation should also vaporise some of the ice, so astronomers can study its composition. The after effects should brighten the faint comet by about a factor of 100, so it might become visible to the naked eye, says mission director Mike A’Hearn of the University of Maryland in College Park. Tempel 1 was chosen as it “was in the right place at the right time, and it’s big enough,” A’Hearn told New Scientist. Its orbit brings the five kilometre comet to within about 80 million kilometres of the Earth. It is well-known to astronomers, but light reflected from it is far too faint to see with the naked eye. The impactor is largely copper, a metal chosen so its spectral lines do not obstruct those of cometary material, and it will carry a camera. After releasing it, Deep Impact will zoom by the comet as the projectile hits and spend 15 minutes monitoring the blast with its cameras and spectrometers. The project will provide fundamental data on cometary structure, needed to assess the prospects of diverting a wayward comet that threatens the Earth. But A’Hearn said the impact will shift the comet’s closest approach to the Sun by only tens of meters – this distance is now about 230 million kilometres. Web links:
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