PASADENA, Calif. -- It's a space mission straight out of Hollywood -- launch a spacecraft 268 million miles so it can aim a barrel-sized probe toward a speeding comet half the size of Manhattan and smash a hole in it.
But that's what NASA expects its Deep Impact mission to do this weekend, with a goal of viewing the icy core of a comet that may hold cosmic clues to how the sun and planets formed.
It's not without challenges. To ensure a bull's-eye hit -- and a spectacular fireworks display in space -- several things must happen just right. Around 2 a.m. EDT today, the Deep Impact spacecraft was to release the 820-pound copper ''impactor" on course for a collision expected 24 hours later with the comet Tempel 1. Scientists are confident they will be able to position the probe in the comet's path.