'We have impact' — July 4, 2005

That's the day NASA's Deep Impact spacecraft is scheduled to release an 800–pound metal projectile aimed squarely at the heart of comet Tempel 1, cruising unawares out in deep space some 83 million miles from Earth at 65,000 miles an hour.

The wine–barrel size impactor will separate from the larger flyby portion of the spacecraft and accelerate to a speed of 22,800 mph at the time of impact.

See why it's called "Deep Impact?"

The impactor will hit with a force of 4.5 tons of TNT.

Earthlings are expected to be able to witness the blast, which will carve a crater up to seven stories deep.

During the 15 minutes of photographic and instrument exposure to the results before the flyby craft becomes hidden in the comet's tail, some 365 MB of data will be generated for return to Earth.

This past Monday, April 25, Deep Impact got its first picture of Tempel 1 (below, the brightest object),

from a distance of 39.7 million miles.

It's closing fast.

T minus 65 days and counting.

Ground control to Major Tom: begin preliminary preparations on my mark.

The Planetary Society is sponsoring a contest for anyone willing to guess the actual size of the crater–to–be.

Hint: it can't be larger than eight miles across: that's the size of the comet.

[via Noah Shachtman and Wired magazine]