Comet

Comet A small member of the solar system that becomes visible only when it reaches the vicinity of the Sun. It is composed of ice and dust, rather like a dirty snowball. When it nears the Sun, the pressure of the solar wind evaporates the ice and drives off the dust.

This usually produces a visible tail to the comet, pointing away from the Sun. Comets orbit the Sun and may have periods of a few years (like Encke's comet), of a few decades (like Halley's comet) or very long periods of tens of thousands of years.