As comets pass near the Sun, some of their mass vaporizes, producing a long tail.a Comets also fragment frequently or crash into the Sunb or planets. Typical comets should disintegrate after several hundred orbits. For many comets, this is less than 10,000 years. There is no evidence for a distant shell of cometary material surrounding the solar system, and there is no known way to add comets to the solar system at rates that even remotely balance their destruction.c Actually, the gravity of planets tends to expel comets from the solar system, not capture them.d So, comets and the solar system appear to be less than 10,000 years old. [For more on comets, see “The Origin of Comets” on pages 310– 347.]