Comets are small and have a composition of ice, rock, and cosmic dust loosely packed together. Comets come from the cold, outer solar system. When a comet passes close enough to the sun, solar radiation heats the water ice so that the comet gives off gas and dust in the form of a long tail. This is how a comet gets its tail. Sometimes a comet can get two tails, an Ion tail or a dust tail. The ion tail is made up of electrically charged particles called ions. The solid center of the comet is called the nuclei. A comet's nuclei can range from less that a half a kilometer to more than 100 kilometers in diameter