Comets are mysterious, distant travelers originating from the depths of our solar system and orbiting the Sun in a highly elliptical path. Star gazers see comets as specters with luminous tails arching across the night sky for a month or two, and then disappearing from sight. Our ancestors considered comets to be portents of evil.
Some comets never come back, with orbital periods of tens of thousands of years.
Others return more often, with periodic orbits of less than 200 years long. For this reason, seeing comets like Halley's Comet, with its 76-year orbit, is often a once in a lifetime experience. Comets are scientifically valuable because they may be remnants of the material out of which the planets and moons formed. Comets may even contain clues about early life on Earth. They originate in either the Kuiper Belt, located outside Neptune's and Pluto's orbits, or in the Oort Cloud, hundreds of times farther away from Pluto extending halfway to the nearest star.