The outermost members of the solar system occasionally pay a visit to the inner planets. As asteroids are the rocky and metallic remnants of the formation of the solar system, comets are the icy debris from that dim beginning and can survive only far from the Sun. Most comet nuclei reside in the Oort Cloud, a loose swarm of objects in a halo beyond the planets, reaching perhaps halfway to the nearest star. Comet nuclei orbit in this frozen abyss until they are gravitationally perturbed into new orbits that carry them close to the Sun. As a nucleus passes the orbits of the outer planets, the volatile elements of which it is made gradually warm; by the time the nucleus enters the region of the inner planets, these volatile elements are boiling. The nucleus itself is irregular and only a few miles across, and is made principally of water-ice with carbon monoxide, carbon dioxide, methane, ammonia, and dust -- materials very similar to those found in the moons of the giant planets. As these materials boil off of the nucleus, they form a coma or cloud-like "head" that can measure tens of thousands of kilometers across. The coma grows as the comet gets closer to the Sun. Charged particles from the Sun -- the Solar Wind -- push on gas molecules, and the pressure of sunlight pushes on the cloud of dust particles, blowing them back like flags in the wind and giving rise to the comet's "tails." Gases and ions are blown directly back from the nucleus, but dust particles are pushed more slowly. As the nucleus continues in its orbit, the dust particles are left behind in a curved arc. Both the gas and dust tails are blown away from the Sun; in effect, the comet chases its tails as it recedes from the Sun. The tails can reach 150 million kilometers (93 million miles) in length, but the total amount of material contained in this dramatic display would fit in an ordinary suitcase. Comets -- from the Latin cometa, meaning "long-haired" -- are essentially dramatic light shows. Some comets pass through the solar system only once, but others have their orbits gravitationally modified by a close encounter with one of the giant outer planets. These latter visitors can enter closed elliptical orbits and repeatedly return to the inner solar system.
