This is one of the first pictures of what the inside of a comet looks like.
On schedule, the Deep Impact "impactor" probe smacked into the surface of comet Tempel 1, resulting in a blast of ejecta visible from Earth. The NASA website has a variety of interesting images and animations, including what the Hubble Telescope saw and a movie of the impact from the point-of-view of the impactor zooming in at over 23,000 miles per hour. Over at the ESA website for its Rosetta probe, more animations and data, including the (expected) detection of water in the ejecta.
The explosion from the impact was bigger and brighter than expected, which is a very good sign that this mission was entirely worthwhile.
The Earth has been hit by comets before; it's a rare event, but it will eventually happen again. And while known comets like Tempel 1 have orbits that can be plotted well in advance, not every object emerging from the cometary cloud surrounding our solar system has such a widely-recognized cycle. If we're going to be able to prevent a comet from hitting us, we need to know what we're dealing with. Comets don't have the same composition as asteroids, and it's possible that the plans for pushing an asteroid out of the way wouldn't work as well with a comet. The more we know about what they're made of, the better the chance we'll have of preventing planetary disaster.
The videos of the impact and the photos of the plume are amazing.
I'm surprised how intricate the surface of Tempel is. I was expecting frozen mush. Instead, you can see the place has a history.