Why do comets have tails?

Unlike the rocky and dusty asteroids, the comets are made up of a mixture of rock, dust, and ice. This is why astronomers also call it a 'dirty snowball'. When the comet gets close to the sun, the ice evaporates and forms a cover of gas and dust around the comet. This is blown away by the solar wind, a stream of electrically charged particles, so that it gives rise to a long tail. This is also known as the coma. A few comets can be seen regularly from the Earth, while the others never come near our planet.