Why are we interested in collisions?
Why do collisions occur?
Comet Shoemaker-Levy: A classic example.
In July of 1994, at least 21 fragmented pieces of comet Shoemaker-Levy collided with Jupitier, providing Scientists with a once-in-a-millenium opportunity to study an actual collision.
Image taken by NASA's Infrared Telescope Facility, Io is crossing the planet in the upper right corner, and the great red spot is in the lower left corner. Fragments Q and R are seen in the lower right corner.
What are the effects of a collision?
An enourmous amount of energy was released when Shoemaker-Levy collided with Jupitier. In fact, The average impact energy from each 1-km-diameter comet fragment is equivalent to the simultaneous explosion of about a quarter million moderately large (1 magaton) hydrogen bombs.
Another important effect of the collsion was it's impact on Jupiter's weather. Computer simulations conducted at MIT show waves traveling outward from the impact sites and propagating around the planet in the days following each impact.
In general, the impact of the comet fragments did not effect Jupiter as a whole very much. It was analagous to sticking 21 needles into and apple inasmuch as each needle does significant damage locally, but the apple as a whole isn't really modified that much. The energy deposited by the comet fragments, although tremendous, fell way short of the energy required to set off sustained thermonuclear fusion. Jupiter would have to be 10 times more massive to sustain a fusion reaction.
What if a comet collided with Earth?
It is widely believed that large comets have, in fact, collided with earth throughout its exsistence. The greatest evidence we have to support this belief are the presence of impact craters caused by either comets or asteroids such as the one that was created 200 million years ago in Chad, Africa.
An impact by an 8km object is in the mass extinction category. Furthermore, there are many comets in the 1km to 10km class, 15 of them in short-period orbits that pass inside the Earth's orbit, and an unknown number of long-period comets. Virtually any short-period comet among the 100 or so not currently coming near Earth could become dangerous after a close pass by Jupiter. In general, though, the chances of a collision with a very large comet are considered extremely small, but recent discoveries suggest Earth is bombarded daily by hundreds of comet-like 20-40 ton "snowballs" on a daily basis.
With a final reference to Shoemaker-Levy, Eugene Shoemaker said that if a similar comet crashed on Earth it would be catasrophic: "...we're talking about a million megatons of kinetic energy. We're talking about the kind of event that is associated with mass extinction of species on Earth; really truly a global catastrophe. It might not take out the human race, but it would certainly be very bad times."