A meteoroid is a small rocky or metallic object in orbit around the Sun (or another star). A meteoroid which strikes the Earth (or other large body) is called a meteorite. As a meteoroid encounters Earth's atmosphere frictional heating begins at an altitude of 100 to 120 km. What happens next depends on the speed, mass, and friability (tendency to break up) of the meteoroid. Micrometeoroids radiate heat so effectively that they survive unchanged to reach the surface as micrometeorites. Objects about the size of sugar grains burn up as meteors or 'shooting stars.' Friable meteoroids break up and are destroyed at altitudes of 80 to 90 kilometers. Those which are tougher survive longer and produce fireballs as their surface is melted and eaten away at temperatures of several thousand degrees. If they avoid destruction high up, they enter the lower, denser part of the atmosphere where they are rapidly decelerated. Finally, at subsonic speeds the fireball is extinguished and the residue falls to the ground as a meteorite. The last melted material on the surface of the object solidifies to form a thin, usually black, rind known as a fusion crust.