Astronomers have accumulated detailed orbital information on more than 1,500 individual comets. Of those, only about 10 percent complete an orbit around the Sun in less than 200 years. A typical short-period comet travels once around the Sun every 7 years in an orbit inclined to Earth's by some 13 degrees, passing no closer to the Sun than about 1.5 AU, or just within the mean distance of Mars. Halley's Comet is the brightest and most active member of this group. The remaining population consists of long-period comets, those that take at least 200 years to return to the inner solar system. So comet aficionados pin their hopes to the unanticipated arrival of an as-yet-unknown long-period comet.