Why do comets have tails?
When a comet comes near the Sun, its ice turns to gas, releasing dust. Sunlight and the solar wind push the gas and dust away, making long tails.
Comets spend most of their lives far from the Sun, as cold, dark lumps of ice and dust. They only grow tails when they come close to the Sun.
Warming up. As a comet gets nearer the Sun, its surface warms. The ice does not melt into water; instead, it turns straight into gas. This gas bursts out, carrying dust with it, and forms a fuzzy cloud around the comet called the coma.
Pushed away. Two things push the gas and dust away from the Sun: - Sunlight gently pushes on tiny dust grains, sweeping them back into a curved, yellowish-white dust tail. - The solar wind, a stream of electrically charged particles from the Sun, pushes on charged gas, making a straight, bluish gas tail, also called an ion tail.
Pointing away from the Sun. A comet's tails always point away from the Sun, no matter which way the comet is moving. When a comet is heading away from the Sun, its tails even go in front of it!
Huge tails. Tails can stretch for tens of millions of kilometres, longer than the distance between Earth and the Sun in some cases.
Leftover dust. Some of the dust stays along the comet's path, forming a trail. When Earth passes through it, we see a meteor shower. So the tail of a comet can become the shooting stars of the future.