On a clear summer evening, lights shoot across the sky. If you are lucky, there will be more, like a real shower of fire. If you go out at night in the summer and autumn, you will have a good chance to see these shooting stars or meteorites.

They are pieces of cosmic debris that plunge into the atmosphere with immense speeds of maybe 20,000 kilometres per hour. They leave behind glowing trails of air heated by the friction. Most of them are just tiny dust particles that completely vaporize on their way across the atmosphere.

The cosmic dust comes from the tails of comets. Like the Earth and other planets, comets orbit around the Sun, but in much more stretched elliptic paths. They are big blocks of ice, rocks and dust (‘dirty snowballs’) with tails, thousands of kilometres long, facing away from the sun. Those tails contain numerous dust particles.

Every now and then, the Earth crosses the path of a comet and sweeps up its tail of debris. Thousands of dust particles enter the atmosphere and rain down in meteorite swarms or showers. These swarms appear to come from the same part of the sky. One of those, the Leonids, is named after the constellation Leo from where they seem to shoot. It occurs every year in mid November, when the Earth crosses the orbit of Comet Tempel-Tuttle. The Perseid meteor shower is another one, peaking on 12 August. The meteorites spread out from the constellation Perseids and are parts of Comet Swift-Tuttle.

Most meteorites are not big enough to reach the surface of the Earth. Even then, most of them are no bigger than a sand grain. The really big guys usually are asteroids, planet-like objects drifting between the orbits of Mars and Jupiter. Very rarely, a collision with another object knocks an asteroid out of its orbit, sending it to the Earth.

Read what happens when a big rock strikes the Earth: