Comet

comet, a small body orbiting the Sun with a substantial fraction of its composition made up of volatile ices. When a comet comes close to the Sun, the ices sublimate (go directly from the solid to the gas phase) and form, along with entrained dust particles, a bright outflowing atmosphere around the comet nucleus known as a coma. As dust and gas in the coma flow freely into space, the comet forms two tails, one composed of ionized molecules and radicals and one of dust. The word comet comes from the Greek κομητης (kometes), which means “long-haired.” Indeed, it is the appearance of the bright coma that is the standard observational test for whether a newly discovered object is a comet or an asteroid.