HOW COMETS TRAVEL
Like the planets and their moons, comets travel about the Sun in paths known as orbits. The orbits of the major planets are almost circular, but the orbit of a comet such as Halley's is a much more elongated ellipse. A comet coming in from the Oort Cloud has an even more extended orbit: it speeds round the Sun and then heads out for a trip that either never ends or will not bring it back for millions of years. On rare occasions, a comet travels in on an elliptical orbit, but then encounters a large planet such as Jupiter, whose gravity puts it into a new, hyperbolic orbit that ejects it from the Solar System forever.
Comets that travel in elliptical orbits and return to the vicinity of the Sun within a period of 200 years are called short-period comets. The most famous of these is Halley's, whose return every 76 years allows each generation on Earth a chance to view it. Most short-period comets return much more often, with 6 years being the average. With a period of 3.3 years, Encke's Comet has the shortest period.
Comets Hyakutake and Hale-Bopp, the great comets of 1996 and 1997, are good examples of long-period comets. Returning for its first visit in 9,000 years, Comet Hyakutake passed close to Earth in March 1996. A year later, Comet Hale-Bopp rounded the Sun on its first visit in more than 4,000 years. An approach to Jupiter has shortened its orbit, so that its next visit will occur in only 2,400 years. It is possible that repeated encounters with Jupiter could shorten the period even more. In a hundred thousand years, Hale-Bopp might return as often as Halley's does now!