When you picture a comet, you probably imagine the tail streaming out behind it like smoke from a car. But that's not what happens at all. The Sun is constantly blowing out a wind of tiny particles and pouring out light, and both of them push the comet's dust and gas straight away from the Sun. So the tail always points away from the Sun, whichever way the comet is flying. On the way in toward the Sun the tail trails behind, but on the way back out the tail goes in front, leading the comet like a torch held out ahead. Open the simulator and follow the comet round its long loop — you'll see the tail swing around to always face away from the Sun.
Most people think a comet's tail streams out behind it like smoke, trailing its direction of travel. In fact the Sun blows the tail straight outward, so on the way back out the tail leads in front of the comet.
What's actually happening
It seems obvious that a comet's tail should stream out behind it, the way a runner's hair blows back or a jet leaves a trail. For centuries that's exactly what people assumed. But comets refuse to cooperate. Track one carefully across the sky and you find its tail doing something that makes no sense for exhaust: on the journey back out from the Sun, the tail points forward, out ahead of the comet, in the direction it's travelling. Whatever is shaping that tail clearly doesn't care which way the comet is moving.
The thing that shapes it is the Sun. Our star does two things to a comet that drifts close: it blasts out a steady wind of charged particles, the solar wind, and it shines with enough intensity that the light itself exerts a gentle pressure. Both of these push outward, directly away from the Sun, and both shove the loose dust and gas boiling off the comet's icy head in that same outward direction. The tail is simply that material being blown anti-sunward. So no matter where the comet is on its orbit, the tail lines up like a windsock in a one-way gale, always pointing away from the Sun. The Chinese astronomers who recorded comets two thousand years ago had already noticed this rule, long before anyone knew what the solar wind was.
There's a lovely consequence hiding in this. A comet only grows a tail when it's near the Sun, because it needs the Sun's heat to boil ice off its surface and the Sun's wind to blow that material out. Far from the Sun a comet is just a dark, frozen lump a few kilometres across, completely tailless. As it falls inward the tail erupts and can stretch tens of millions of kilometres — longer than the distance from here to the Sun. And the two tails part ways: the lightweight ionised gas gets blown into a dead-straight blue streak, while the heavier dust lags into a curved, yellowish-white fan. One comet, two tails, both obeying the same simple rule — get out of the Sun's way.
A comet's tail is steered by the Sun, not its motion, so it always points away from the Sun — proof you can see in the sky.
- 1Tie a few light paper streamers or strips of tissue to a small ball — that ball is your comet, the streamers its tail.
- 2Walk the comet in a big circle around a desk fan set on a table (the fan is the Sun, blowing the solar wind outward).
- 3Watch the streamers: they always blow away from the fan, never trailing behind your hand. Carry the comet 'outward' and the tail leads in front, exactly as a real comet's does.
Common questions
The solar wind sweeps ionised gas into a straight ion tail, while radiation pressure from sunlight pushes dust into a broader, gently curved dust tail. Both point away from the Sun, but the heavier dust lags into a curve.
Far from the Sun a comet is just a dark, frozen lump a few kilometres across, with no tail. Near perihelion the Sun's heat sublimates far more ice, so the tail erupts and can stretch tens of millions of kilometres.
Yes. Chinese astronomers recording comets two thousand years ago had already noticed that the tail always points away from the Sun, long before anyone understood what the solar wind was.