Ancient people believed comets were omens. Sometimes they were thought to signal the fall of empires, other times the birth of kings. Even today, when we're all supposed to be much more intelligent, people do crazy things when comets pass by.
Though they're beautiful to look at, comets aren't supernatural. In fact, they are big, dirty clumps of ice, that can be a mile wide or more.
Who's throwing giant snowballs at us?
Well, if you lived in the time of the Roman Empire you might have said the gods were doing it. However, nowadays we know that comets come from the Oort cloud. The Oort cloud gets its name from Dutch astronomer Jan H. Oort, who hypothesized its existence around the middle of this century.
The Oort cloud is a spherical haze of comets that surrounds our entire solar system. We often get the false impression that the solar system ends with Pluto. In fact, beyond Pluto is the Oort cloud, encircling everything else like an enormous shell. It's so far out it's halfway to the next star, and almost beyond the reach of our sun's gravity.
If the Oort cloud is so far away, why do we see comets zooming past earth from time to time?
Passing stars do it. Another star passing through the Oort cloud, or just near it, will disturb lots of comets, sometimes causing them to go flying inward toward the planets.