Apart from the Sun, the nine planets and their many moons, the Solar System is full of small bodies called comets and asteroids. The first contain a lot of ice, the latter are rockier. On average, comets have lower bulk densities than asteroids.
However, due to fresh data from recent robotic explorers, the distinction between comets and asteroids is no longer very clear. Some asteroids are now suspected to be former comets on which all the ice has been evaporated by the Sun, leaving only the rocky core. Moreover, we have recently found out that rather than "dirty snowballs," as astronomers used to call them, comets are more asteroid-like "snowy dirtballs."
Fascinating as they are, asteroids and comets can be quite dangerous. When they hit planets they create havoc, as was impressively demonstrated when fragments of comet Shoemaker-Levy 9 crashed into Jupiter with the explosive force of several billions of atomic bombs. (Comets are named after the astronomers who discover them; this was the ninth comet found by Eugene and Carolyn Shoemaker and David Levy.) The geological history of the Earth is full of evidence of such collisions, like the famous Meteor Crater in Arizona.
We now think that an asteroid impact was at least partly responsible for the extinction of the dinosaurs. Some 65 million years ago, a large asteriod hit the Earth and filled the atmosphere with dust. The Sun's life-bringing light was blocked out for months. It became extremely cold and most of the plants died, breaking the food chain and causing the death of, first, the plant-eating dinosaurs and then the meat-eaters who preyed on them.
In this particular case the impact actually helped humanity, as the disappearance of the dinosaurs gave litde furry mammals a chance to prosper, develop further and eventually evolve to become humans. However, the next time a large comet or asteroid comes crashing down we will surely not be so happy about it (just have a look at the movie Deep Impact or, much more funny, Armageddon, and you'll get the picture). The geological and fossil record shows that life has probably been partly wiped out of existence by asteroid and comet impacts many times in the Earth's history.
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