Asteroids are rocky material left over from the formation of the solar system that orbit the Sun, but are too small to be

Asteroid 243 Ida and its moon, photograph. Corbis Corporation. Reproduced by permission.

viewed as planets. Most asteroids are composed of stone, iron, nickel, or a combination of the three ingredients, and resemble terrestrial rocks in appearance. Asteroids can range in size from pebble-sized rocks up to almost 1,000 km in diameter. Asteroids whose orbits will eventually cause them to collide with Earth are known as meteoroids. When the heat and friction of entering Earth's atmosphere at high velocity causes the meteoroid to burn brightly in its path across the sky, it is known as a meteor. Particles or chunks of the meteor that survive the atmospheric entry and fall to Earth are meteorites. Asteroids are classified according to their composition, size, or location. Although Near-Earth Asteroids (NEAs...