Meteoroids are the smallest particles orbiting the sun, and most are no larger than grains of sand. From years of studying the evolution of meteor streams, astronomers have concluded that clouds of meteoroids orbiting the sun were produced by comets. Meteoroids can not be observed moving through space because of their small size. Over the years numerous man-made satellites recovered by manned spacecraft have shown pits in their metal skins which were caused by the impact of meteoroids. Meteoroids become visible to observers on Earth when they enter Earth's atmosphere. They are then referred to as meteors. They become visible as a result of friction caused by air molecules slamming against the surface of the high-velocity particle. The friction typically causes meteors to glow blue or white, although other colors have been reported. Most meteors completely burn up in the atmosphere at altitudes of between 60 and 80 miles. They are rarely seen for periods of more than a few seconds.