Why Does a Comet have a Tail?

A comet is a crumbly body of rock and ice only a few kilometers across. When it passes near the Sun and becomes hot , the ice turns into gas, and dusty fragments are thrown out into space. Clouds of particles flying out from the Sun (the solar wind) push back this gas and dust to form the comet’s tail

The finest tails are formed by comets that have passed the sun only a few times. These can be millions of kilometers long , but are also very thin. In 1910, for example, the earth passed harmlessly through the tail of Halley’s Comet.