Meteoroids are small pieces of rock or metal that originate in outer space. They are usually the size of a grain of sand to a few centimeters in diameter, and they can be composed of a variety of materials, including iron, nickel, and stony-iron. Meteoroids typically enter Earth's atmosphere at speeds of up to 70 km/s (45 miles/s) and burn up as meteors or fireballs. Large meteoroids may survive the journey through the atmosphere and impact the surface as meteorites.