Meteors
Interplanetary space is littered with rocks called meteoroids. When meteoroids strike the Earth's atmosphere, they become visible from the ground as shooting stars or meteors. A typical meteor has a mass that is only a fraction of a gram. When it hits the atmosphere, it is travelling at 10 to 40 kilometers per second, or roughly 50 times the speed of Concorde. At such high speed, a meteor's surface heats up because of friction. Small pieces of matter fall away, and atoms evaporate from the surface to form a hot, gaseous envelope around the tiny particle. This hot envelope, which may be a foot or more in diameter, hurtles through the atmosphere, making a streak of light in the sky. The meteor usually burns up in the mesosphere within 60km of the Earth's surface.
A very few meteors are bigger than the typical-sized particles. Known as fireballs, they are especially bright and can make explosive or hissing noises. If the meteor slows down enough, it will stop evaporating before it has been completely vaporised. Meteors that reach the ground are called meteorites.
On a typical clear night, you might expect to see about 5 meteors each hour. Several times a year, however, the Earth passes through a known cloud of space dirt, usually debris from a comet. At these times, it is possible to witness more intense meteor shows, with as many as 60 shooting stars visible every hour.
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