Science Puzzle
Which Way Does a Comet’s Tail Point?
As a comet falls in towards the Sun, it grows a glowing tail millions of kilometres long, streaming out behind it like smoke from a speeding train. The comet swings round the Sun and heads back out into the cold depths of the solar system.
On its way out, which way does the comet's tail point?
The Answer
Away from the Sun. On its way out, the comet travels with its tail stretched out ahead of it.
A comet is a lump of ice, dust and rock a few kilometres across, often called a dirty snowball. Far from the Sun it has no tail at all. As it comes closer, sunlight warms it, and its ice turns straight into gas, carrying dust out with it into a huge glowing cloud. That material is then pushed away by the Sun itself.
Two things do the pushing. The solar wind, a stream of electrically charged particles pouring out from the Sun at hundreds of kilometres a second, sweeps the gas into a straight, bluish ion tail pointing directly away from the Sun. Sunlight itself gives a tiny push too, and that drives the dust into a broader, curving, yellowish tail. Neither has anything to do with which way the comet is moving: space has no air to stream past it, so nothing drags the tail behind.
On the way in, "away from the Sun" happens to be behind the comet, so it looks like a trail. On the way out, the same rule puts the tail out in front. A smoke trail is the wrong picture; a windsock pointing downwind of the Sun is a better one.
The principle: The solar wind and radiation pressure. Particles and light streaming from the Sun push a comet's gas and dust outwards, so its tails always point away from the Sun, whichever way the comet is travelling.