Comets are small astronomical bodies composed of rock, ice and frozen gases; they range in size from 1000 meters to 30 miles or more in diameter. Almost 5000 comets are known to science though they likely represent a tiny fraction of the actual number. Short period comets, defined as those with a period of less than 200 years, are thought to originate in the Kuiper Belt, beyond the orbit of Neptune, which contains countless icy objects and planetoids (including Pluto). Long period comets, which have periods exceeding 200 years (up to many thousands of years) are assumed to arise from the Oort Cloud, at the outer edge of our solar system.
In either case, the comet is likely drawn from its original orbit by gravitational effects from the large gaseous planets, sending it toward the sun. As it approaches the sun, ice, gases and dust from the comet are vaporized by the solar wind, producing a "coma" envelope around the rock nucleus and a tail that points away from the sun. The rock surface of comets is known to have low reflectivity and the light that we see is solar reflection from the vaporized elements as well as ionization within the volatile gases (similar to what occurs in our aurora displays). Over many periods, the volatile components are exhausted and the comet becomes an inert, invisible asteroid; in some cases, as we observed in 1994 when the fragmented Comet Shoemaker-Levy 9 collided with the surface of Jupiter, comets may break-up in response to gravitational effects from the Sun or from the large planets. Finally, some comets are thought to become "hyperbolic," slung into space after they loop around the sun (never to return) while some long period comets are converted to short period comets by the gravitational effect of Jupiter.
Our best known comet, Halley's Comet, has a known period of just over 75 years; following its appearance in 1986, it will next return in 2061. Many of us also remember the spectacular Hale-Bopp Comet, which graced the night sky in 1997 and will return in about 4200 years. This month, Comet Pan-STARRS appears low in the western sky just after sunset and, on March 12-13, will be found adjacent to the moon; this relatively faint comet is best viewed with binoculars, away from city lights. In November, Comet ISON, discovered last year, is expected to provide a more spectacular display.
