Most asteroids are chunks of metallic rock that have virtually no atmospheres. Tens of thousands of asteroids circle the sun inside what's known as the main asteroid belt, a doughnut-like ring that lies between the orbits of Mars and Jupiter. (Explore an interactive solar system.)

By contrast, most comets are loose clumps of dirt and ice thought to originate in the Kuiper belt, far beyond the orbit of Neptune. When a comet's oval-shaped orbit brings it close to the sun, its ices vaporize and the comet develops its signature halo of gases and dust.

(Related: "Comet Is Cosmic