At 1:47 and 8 seconds Eastern Time today (yes, it's got to be that precise), NASA will launch Deep Impact -- a space probe designed specifically to ram a comet.

That exciting event is scheduled to take place this upcoming July 4th. Watch a webcast of the launch (and later, the impact) here.

Although the primary aim of this mission is to study comet structure (and provide a keen show for backyard astronomers), less well known is its role in developing a planetary defense against killer asteroids.

The last time we recall NASA purposely smashing a probe into something (not counting all the Lunar Modules and rocket bodies that impacted the moon during the Apollo missions), was during the Ranger moon missions of the '60's.

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UPDATE: Deep Impact was successfully lauched -- right on time.