A million asteroids populate the zone between Mars and Jupiter known as the Asteroid Belt. Asteroids are essentially chunks of rock but they range in size from dust particles to 1,000 km across. Darker asteroids are probably made of carbon materials and the brighter ones may contain such metals as iron and nickel. Astronomers know of some 2,200 asteroids. The largest of these are Ceres (about the width of France), Pallas, Vesta, and Juno.

We find out about the composition of asteroids by analyzing meteorites, which are essentially asteroids that have fallen out of orbit and survived the fiery passage through Earth's atmosphere. There are about 2,500 meteorites known. the largest is in Namibia, Africa and weighs 53.5 metric tons. The second largest, which landed in Greenland, weighs 53.5 metric tons and is on display at the Hayden Planetarium in New York City.

When these chunks of rock are still in space they are called meteoroids. they strike the Earth's atmosphere at speeds up to 40 km/second. At a height of about 128 km, friction with the atmosphere surrounds the object with hot, glowing air. This fireball streaks toward Earth, becoming visible as a bright meteor. If it does not break up, the meteoroid will strike the surface and become a meteorite.

Meteors from much smaller particles, about as large as grains of sand, can be seen almost any dark night. the best chance of seeing meteors occurs during meteor showers, when Earth passes through streams of dust left behind by comets.

Home Stars

Constellations Terrestrial Planets Gaseous Planets

Comets The Sun