The Solar System consists not only of the Sun, the planets and their satellites (“moons”) but also many other objects: particles of rock and dust, asteroids and comets, and lots of other things left behind when the Solar System was formed. These objects range in diameter from less than 1 mm to more than 100 km.
These objects are all orbiting the Sun, like the planets, but their orbits are affected by the gravity of all the planets, including the Earth’s. Sometimes one of these objects may come near the Earth. It may just be deflected by the Earth’s gravity but move back into space again, or it may go into orbit round the Earth, or it may spiral inwards and eventually either hit the Earth or burn up or explode in the Earth’s atmosphere.
These objects are moving very fast, and may be approaching the Earth at more than 40 000 km/hr. If one of them enters the Earth's atmosphere it will start to slow down. The very tiniest particles have very little kinetic energy and slow down so quickly that they do not get hot, and will eventually fall to the ground harmlessly and unnoticed.
But the larger particles have much more kinetic energy so travel through the Earth's atmosphere for far longer at high speed, and the air resistance on them heats them up to a very high temperature. They may leave a trail of glowing gas behind them, and at night we may see this as a streak of light across the night sky. We call this streak of light a shooting (or falling) star or a meteor. Correctly the term meteor refers to the streak of light, and the object causing it is called a meteoroid. Few meteors are bright enough to be seen during the day, and the trails they leave are seldom visible for more than a second, but on a clear night you should see at least one every fifteen minutes. But you have to be patient and wide awake!
About a hundred tonnes of space dust and meteoroids enters the Earth's atmosphere every day.
Most meteors cannot be predicted and can appear from anywhere in the sky. We call these sporadic meteors. But at certain times of the year the Earth passes through a cloud of particles, possibly the debris left behind by a comet, and then we have a meteor shower, with several meteors every minute, and all of them starting from the same point in the sky (the radiant). We name these showers after the name of the constellation containing the radiant. On the night of a meteor shower we just have to look in the direction of this constellation and we shall be sure to see lots of meteors.
In England the best meteor showers are
There is usually something about these showers on television or in the newspapers at the time.
You can find out more about these and other meteor showers by visiting a Meteor web site. To visit one good one click here 
Many meteoroids just skim the edge of the atmosphere before going back into space. In 1972 a meteoroid with an estimated diameter of about 10 m skimmed the atmosphere above the Grand Tetons in Wyoming. Although it was daytime the trail it left was bright enough to be clearly visible, and several tourists were able to photograph it. Here is a photograph taken by James M Baker.
Meteorites are important to scientists because they have come from outside the Earth and so they can tell us a lot about other planets and the origins of the solar system. There are three main types of meteorite: stony ones, made of rocks, iron ones, containing iron, and stony/iron ones, containing both rock and iron. Until Man learned how to extract iron from iron ore (about 2000 BCE but see the page on Metals and Alloys), iron meteorites were the only source of iron, and objects made of meteoric iron were more valuable than gold. There was a dagger made from meteoric iron in Tutankhamen's tomb. There was also a pectoral (a piece of jewellery worn on the breast) containing a piece of glass formed by an air burst over the Sahara Desert - air bursts are described later on this Page.
Very occasionally, every few thousand years, a large meteorite hits
the Earth. About 50 000 years ago a meteorite about 100 m in diameter landed in Arizona and left a crater more than a kilometre across. To see some pictures of this crater and to link to other sites about other meteor craters you can go to the Arizona Crater Web Site - do so click here
Usually however a large meteoroid gets so hot that instead of hitting the Earth’s surface as a meteorite it explodes several kilometres above it: this is called an air burst. Air bursts cause intense heating of the ground directly underneath but leave no crater, so after a few years very little evidence remains. For this reason until recently almost nothing was known about them. But we now think that almost all large meteoroids air burst rather than hit the surface.
In 1908 the air burst of a 50 m diameter meteoroid over Tunguska, in Siberia, destroyed 2000 km2 of forest. There is much more about this and other air bursts, including eye-witness accounts, on the Tunguska web site. Air bursts may be the origins of stories about a fire in the sky which form part of the traditions of Native American and other cultures.
Even more rarely, every few million years, a very large meteorite (a planetessimal) hits the Earth. Most scientists now believe that the Great Extinction of sixty five million years ago which wiped out all the dinosaurs was the result of a planetessimal about 10 km in diameter (the size of Mount Everest) hitting the Earth. This struck the Earth at a place called Chicxulub, in Mexico, and left a crater 180 km across, although of course after 65 million years none of this crater can be detected on the surface.
Planetessimals are asteroids which have been pulled out of their normal orbit between Mars and Jupiter. Astronomers plot the orbits of all asteroids, and have identified about 127 Near Earth Objects (NEOs) which might come very close to the Earth. A NEO with a diameter of about 370 m and with the unromantic name 2004 XP14 passed 340 000 km from the Earth (almost as close as the Moon) on 3rd July 2007. You can visit the Near Earth Object web site for more information about NEOs.
© Barry Gray June 2009