Shooting stars, or 'meteors', are tiny fragments of space rubbish, which usually burn up when they hit the Earth's upper atmosphere, leaving a short bright trail of light behind them. Many meteors can be predicted - they happen in showers once a year, as the Earth crosses over the trail of dust and debris left behind by a comet. Large meteors that hit the Earth are called meteorites. They tend to be the tougher, asteroid-like material, rather than the flimsy iceballs which comets leave behind.
Meteorites fall into three main categories - iron, stone, and stony iron.
Just as many meteorites are stony types, often with little glassy spheres embedded in them. But these are much harder to find, as they blend in with normal rocks. The stony iron meteorites fall midway between these two groups. Meteorites are highly prized by astronomers interested in the origins of the Solar System, as they provide a chance to study original material which has been unaltered by any of the weathering that affects rock on Earth. There are even some meteorites which come from other planets - fragments of Moon and Martian rocks have recently been found scattered across the untouched ice of the South Pole.