Halley's Comet, a dusty ball of ice and frozen gases, spends most of its time in the chilly outland of the solar system. (Image: © By Karl Tate, Infographics Artist)

A comet is a mountain of ice and dust, tumbling through space. When a comet approaches the sun, its ices sublimate (turn from solid to gas). Vapor, dust and gases stream out to form a hazy tail behind the comet.