Occasionally, a comet is deflected into a new orbit that takes it close to the sun, and it becomes visible to observers on Earth.

The closer the comet gets to the sun, the brighter it becomes. The sun's heat vaporizes the comet's ice and the solar wind blows the vaporized gas and dust into tails that can stretch for hundreds of millions of kilometers. The longest tail yet discovered measured more than 500 million kilometers (300 million miles).

Comets have two tails—one made of gas, the other of dust. The gas tail is straight and points directly away from the sun, while the dust tail can be curved. The gas tail is usually longer.

Comets lose up to 1% of their ice on each trip around the sun. If the Earth's orbit passes through this cometary debris, a meteor shower takes place.