Asteroids are rocky, metallic objects that orbit around the Sun, but are too small to be considered planets. The largest known asteroid, Ceres, has a diameter of about 1,000 kilometres. The smallest asteroids are the size of pebbles. Millions are the size of boulders. Most are irregularly shaped - only a few are large enough for gravity to have made them into spheres. About 250 asteroids in the solar system are 100 kilometres in diameter, and at least 16 have a diameter of 240 kilometres or greater. Their orbits lie in a range that stretches from Earth's orbit to beyond Saturn's orbit. Tens of thousands of asteroids exist in a belt between the orbits of Mars and Jupiter. An asteroid that hits Earth's atmosphere is called a meteor or shooting star, because it burns and gives off a bright flash of light. Whatever does not completely burn falls to Earth as a meteorite. Between 1,000 and 10,000 tons of this material fall to Earth daily. Much is in the form of small grains of dust, but about 1,000 metallic or rocky bits fall to Earth each year.

There has been much speculation about large meteors hitting the Earth. A large asteroid or comet is thought to have landed in Mexico about 65 million years ago. The impact may have led to the extinction of many species, including the dinosaurs, by throwing dust into the atmosphere, blocking the sunlight, and causing a climate change. The period of time between such a large meteor impacts is probably in the millions of years, but smaller meteors such as the one that caused the Metro's Carter in Arizona (about 1.2 kilometres in diameter), may hit the Earth every 50,000 to 100,000 years. There's no historical record of a person being killed by a meteorite. The only reported injury occurred on November 30, 1954, when an eight-pound meteorite that fell through the roof of her house bruised an Alabama woman.