At first, they were worried about being hit by de-orbiting
debris. That threat has passed, only to be replaced by a new one:
the possibility that highly-toxic fuel may leak out of the
splashed-down debris.
On July 4, NASA's Deep Impact spacecraft will
attempt an extraordinarily daring encounter with the far-flung
comet Tempel 1, which is hurtling through space at tens of
thousands of miles per hour. As if that is not challenging enough,
the comet's size, shape and other characteristics are not entirely
known.
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