Alassë,
Please forgive me for giving the impression that the Deep Impact comet mission was smacking into Tempel 1 just for fun. I let my assumptions run away with me again. Sorry.
Many comets travel around the Sun in long, elliptical orbits, releasing dust and gas each orbit. Most of this dust and gas is released when the comet heats up while it is close to the sun – within the orbit of Mars. The solar wind blows the released dust and gas, the coma, away from the solid, icy core, the nucleus, which we on earth see as the comet’s tail.
Although astronomers believe that the material of the nucleus is unchanged since the Solar System was formed, very little is known about what this material is made of. In a sense, a comet is a time capsule containing information about the formation and early years of the Solar System. This is the first mission to probe beneath the surface of a comet and reveal its interior.
Hitting the comet at about 23,000 mph, the impactor will vaporize, releasing dust and gas from within the comet and exposing pristine material beneath the surface. The impactor will, in this way, function as a geologist’s rock hammer; NASA is chipping off the outer layer of Tempel 1 so that astronomers can get a peek at what is inside.
There are a couple of goals which can be achieved by doing this.
The first is straight up scientific exploration. The Rovers sent to Mars have done a bang-up job assessing the geology, both present and past. As I mentioned above, while astronomers believe that comets are “leftovers” from the time the Solar System was formed, they’ve never been able to look “inside” one. For a variety of reasons, the rover technique was inappropriate for use here. Deep Impact’s impactor will do that for them. For the exobiologists, this may/could provide a measurement of the ratio of water ice in the nucleus, helping to answer the conjecture that water was initially brought to earth by comet bombardment.
The second one is far more romantic – if far less likely. Over the eons the earth has been hit by a number of extraterrestrial objects. Everyone has heard about Cretaceous-Tertiary (KT) extinction event, the one which ended the age of the dinosaurs. Evidence has been accumulating over the last few years that a similar, but much larger impact brought on the Permian-Triassic (PT) extinction event, about 252 million years ago, which made the KT event look like a wet squib. It was the Earth's most severe extinction event, with about 90 percent of all marine species and 70 percent of terrestrial vertebrate species going extinct. No fooling! This was one bad ass happening! For some time after the PT event, fungal species were the dominant form of terrestrial life!
A few years ago Robert Duvall starred in a SF movie named “Deep Impact” about an attempt to divert a comet on a collision course with Earth. In the unlikely event that a comet is discovered with an obit dangerous to us, by knowing what a comet is made of, we’ll be more likely able to prevent an impact or dangerous flyby.