When a comet is in deep space, it’s not really a comet at all. It’s still a ball of ice and rock, but it doesn’t have the key feature that makes it a comet: a tail. That’s because the word “comet” comes from an ancient phrase that means “long-haired star” — a reference to the beautiful tail.
Most of the objects that become comets spend most of their lives far from the Sun. In fact, some of them pass close to the Sun only once, then leave the solar system forever.
When a comet does approach the Sun, the Sun’s heat vaporizes some of the frozen water and gas at its surface. The vapor streams out into space. Radiation from the Sun zaps many of the molecules of water and other substances. That gives them an electric charge. So they’re easily blown away from the comet by the solar wind, which also has an electric charge. The tail aims directly away from the Sun.
A comet also has a second tail, made of dust. The dust flies into space along with the gases. The pressure of the Sun’s radiation pushes some of the dust away from the comet, forming the second tail. It’s usually much wider and brighter than the vapor tail.
The length of a comet’s tail depends on many factors, including how close the comet passes to the Sun. A typical tail might stretch a few hundred thousand miles or longer. And the longest tail ever recorded stretched about 350 million miles — a long streamer that made the comet a comet.
More tomorrow.
Script by Damond Benningfield