Early on the morning of July Fourth, well before the patriotic pyrotechnics begin to light up the night sky, NASA expects to set off a fireworks display of truly astronomical proportions.

The unique mission, called "Deep Impact," will use a U.S. spacecraft to launch a washing machine-size "impact module" that will slam deep into the heart of a comet at a relative speed of 23,000 mph in an effort to learn what lies inside its core of ice and rock.