NASA plans some big fireworks of its own for the Fourth of July.

The space agency announced Friday that it would crash a copper-clad "impactor" probe the size of a wine cask into a comet about half the size of Manhattan, all in the name of planetary science and at a cost of $333 million.

"We are really threading the needle with this one. In our quest of a great scientific payoff, we are attempting something never done before at speeds and distances that are truly out of this world," said project manager Rick Grammier of NASA's Jet Propulsion Laboratory in California.

Indeed. The cosmic crunch will occur at 1:52 a.m. EDT on July 4, about 83 million miles from Earth, with comet and impactor each clocking in at 23,000 mph.

Launched in early January, the bold mission is called "Deep Impact." But is it courting disaster, as told in the 1998 comet-hits-Earth film of the same name?

"The impact simply will not appreciably modify the comet's orbital path. Comet Tempel 1 poses no threat to the Earth," said Don Yeomins, a mission scientist.

Nevertheless, there's human culture onboard, and sci-fi drama, too. The little probe carries a compact disc bearing the names of 500,000 space enthusiasts, which will "melt, vaporize and essentially be obliterated -- along with everything else aboard the impactor" -- during the explosive collision, NASA says.

That explosion could yield some hubbub -- "19 gigajoules" worth, or the equivalent of 5 tons of TNT. Scientists estimate the resulting crater could be 20-feet to 140-feet deep and as large as a football field, with a blizzard of ice and dust ejected.

NASA is quick to point out that no comets were hurt in the making of this mission, comparing the collision to a 767 airliner hitting a mosquito. The comet is, after all, about 9 miles long and weighs a billion tons; the impactor is a yard across and 820 pounds...