What are comets made of?

Your average comet is a mixture of water and carbon dioxide ice, dust, rock, and a smattering of other molecules such as sodium. Sodium was responsible for the flaring tail of the Hale-Bopp comet, for example. Comets, having traveled here from the far corners of the solar system, are believed to be the remains of a vast interstellar gas cloud that formed our solar system in the first place.