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NASA has given the go-ahead for a spacecraft to take a $279 million
potshot at a comet. The Deep Impact craft will hurl a 350-kilogram lump of
copper at comet Tempel 1 on 4 July 2005. Astronomers hope that the impact will
teach them something about a comet’s innards, as the blast will expose and
vaporise some of the interior ice. This will help them assess the prospects for
diverting wayward comets that threaten the Earth, says lead researcher Mike
A’Hearn of the University of Maryland in College Park.