A Comet Has Two Tails Because Sunlight and the Solar Wind Push Different Things

A comet's tail isn't exhaust trailing behind it. It points away from the Sun, wherever the comet happens to be going. And look closely, and there are two of them, because the Sun is pushing on the comet in two quite different ways.

Picture one of those long-exposure photographs of a bright comet. There's a fuzzy glowing head, and streaming out of it, a broad, pale, slightly curved tail. Then, if you look again, a second one. Thinner, straighter, faintly blue, and pointing in a slightly different direction.

Two tails, from one object, not quite lined up. That's a clue. If a comet's tail were simply a trail of stuff left behind as it moved, like the smoke from a steam train, there'd be no reason for two of them, and no reason for them to disagree. Something else is going on.

A frozen lump that wakes up

Far from the Sun, a comet isn't much to look at. Its solid core, the nucleus, is a lump of ice and dust usually several kilometres across, about the size of a small town. The old description is a dirty snowball, and it's not a bad one: the nucleus is mostly ice.

Things change as it swings in towards the Sun. Sunlight warms the surface, the ice turns to gas, and that gas escapes into space, dragging dust out with it. The comet wraps itself in a hazy cloud called the coma. For a nucleus only several kilometres wide, the coma can get enormous; it can end up bigger than the Sun.

From that cloud, the material gets pulled out into tails. And here's where the two kinds of stuff in the coma part company, because the Sun treats them differently.

Tail one: dust, nudged by light

The first kind is dust. These grains are tiny, typically around the size of smoke particles.

What pushes them is light itself. It sounds strange that sunlight could push anything, but light carries a gentle pressure. You'd never notice it on your face. For a grain of dust the size of a smoke particle, floating in space, it's enough. The dust gets nudged outwards, away from the Sun.

But the push is gentle, and the dust was already moving along with the comet. So the grains don't shoot straight back. They get dropped along the comet's path and gradually eased outwards, spreading into a broad, fan-shaped tail that traces a gently curving path. Think of smoke drifting from a chimney in a light breeze. It leans away from the wind, but it also smears out along the way.

That dust tail looks whitish yellow, and the reason is simple: it's sunlight bouncing off the grains. The dust doesn't glow on its own. It reflects.

Tail two: gas, swept by the solar wind

The second kind of material is gas. And some of that gas gets electrically charged. Physicists call charged atoms and molecules ions, which is where this tail gets its name: the ion tail, sometimes called the plasma tail.

Charged gas responds to something else. The Sun doesn't only shine. It also blows out a constant stream of charged particles, called the solar wind, and that stream carries the Sun's magnetic field along with it. The comet's charged gas gets caught up in that field and swept away with it.

The result looks quite different from the dust. Where the dust tail fans and curves, the ion tail is narrow and straight, and it always points directly away from the Sun. It behaves less like drifting smoke and more like a windsock: lined up with the wind, wherever that wind is coming from.

The ion tail also does something the dust can't. It makes its own light. The charged gas absorbs sunlight and then gives that energy back out at a different wavelength, a different colour. Which colour depends on the molecule. The blue that ion tails typically show comes from carbon monoxide ions. Gas around a comet can glow other colours too: when a recent comet called ZTF glowed green, that pointed to a molecule made of two carbon atoms.

And the ion tail can be staggeringly long. Ion tails have been seen stretching about 3.8 times the distance from the Earth to the Sun. That's something like 570 million kilometres of glowing gas.

So that's the photograph explained. Two tails, because there are two kinds of material, pushed by two different things. Dust, nudged by the pressure of light, lagging and curving. Charged gas, carried off by the solar wind, ruler-straight. Both tails grow longer as the comet gets closer to the Sun, because the closer the comet gets, the more material the Sun's heat drives off into them.

Leading with its tail

Here's the bit that catches people out. Both tails point away from the Sun. Not behind the comet. Away from the Sun.

On the way in, those two directions are roughly the same, so the tail looks like it's streaming out behind, as you'd expect. But a comet doesn't stop at the Sun. It swings around and heads back out. And on the way out, the tail still points away from the Sun, which now means it's out in front. An outbound comet travels tail-first.

This is why the windsock is a better picture than the steam train. A windsock doesn't care which way the pole is being carried. It points wherever the wind is going.

And just to add to the confusion, comets sometimes appear to sprout a third tail pointing the other way, towards the Sun. It's called an anti-tail, and it's an optical illusion. The dust tail is really still pointing away from the Sun, but because of where the Earth happens to be, we see the wide dust fan from an angle that makes part of it look like it's sticking out on the sunward side. Nothing is actually pointing at the Sun. It's a trick of perspective, like a road that appears to bend back on itself when you see it from the wrong spot.

A tail you can see years later

The dust doesn't simply vanish once the comet has moved on, either.

Comets drop dust into their orbits as they go, and those swarms of particles stay out in space. Every year, the Earth's own orbit carries it through some of them. When that happens, the grains slam into our atmosphere and burn up, and we see them as shooting stars.

That's what a meteor shower is. There are around ten good, reliable ones every year, such as the Perseids in August and the Leonids in November. So if you've ever lain on your back and watched a meteor shower, you were, in a sense, watching a comet's tail. Just a very old one, arriving very fast.

Two forces, written in the sky

So the next time you see one of those photos, look for the two tails and the small angle between them. That gap is the visible signature of two different solar forces at work on two kinds of material. Light, pressing gently on dust. A wind of charged particles, sweeping away charged gas.

The comet isn't leaving a trail. It's showing you which way the Sun is pushing.

SOURCES

1. European Space Agency, "Structure of a comet" (November 2023). https://www.esa.int/ESA_Multimedia/Images/2023/11/Structure_of_a_comet (accessed 25 Oct 2026)

2. European Space Agency, "Comet ZTF and its apparent three tails" (February 2023). https://www.esa.int/ESA_Multimedia/Images/2023/02/Comet_ZTF_and_its_apparent_three_tails (accessed 25 Oct 2026)

3. European Space Agency, "Comets: an overview." https://esa.int/Science_Exploration/Space_Science/Rosetta/Comets_An_overview (accessed 25 Oct 2026)

4. NASA Space Place, "What Is a Comet?" https://spaceplace.nasa.gov/comets/en/ (accessed 25 Oct 2026)

5. Wikipedia, "Comet tail." https://en.wikipedia.org/wiki/Comet_tail (accessed 25 Oct 2026)