A comet is an icy body that releases gas or dust. They are often compared to dirty snowballs, though recent research has led some scientists to call them snowy dirtballs. Comets contain dust, ice (frozen water), carbon dioxide, ammonia, and methane. As a comet gets closer to the sun, the ice on the surface of the nucleus begins turning into gas, forming a cloud known as the coma. Radiation from the sun pushes dust particles away from the coma, forming a dust tail, while charged particles from the sun convert some of the comet's gases into ions, forming an ion tail. Since comet tails are shaped by sunlight and the solar wind, they always point away from the sun. (See comet make up at right). 

The nuclei of most comets are thought to measure 10 miles (16 km) or less. Some comets have comas that can reach nearly 1 million miles (1.6 million kilometers) wide, and some have tails reaching 100 million miles (160 million kilometers) long. We can see a number of comets with the naked eye when they pass close to the sun because their comas and tails reflect sunlight or even glow because of energy they absorb from the sun. However, most comets are too small or too faint to be seen without a telescope. Comets leave a trail of debris behind them that can lead to meteor showers on Earth. For instance, the Perseid meteor shower occurs every year between August 9 and 13 when the Earth passes through the orbit of the Swift-Tuttle comet.