Comet tails
A Socratic walk-through of comet tails — reasoned out one step at a time, not lectured.
The question we started with
THE QUESTION #Why does a comet's tail point away from the Sun even while the comet is racing away from it?
The picture nearly everyone carries is of a tail streaming behind, the way smoke lies behind a locomotive or hair behind a runner. It is a reasonable picture, and it is wrong in a way worth chasing. On the outbound half of its orbit a comet is running away from the Sun at tens of kilometres a second, and its tail runs ahead of it, pointing back the way it came. Whatever builds that tail, it is not the comet's motion. What else is there?
Reasoning it through
REASONING #Ask first what the tail is made of and how it leaves. A comet nucleus is a few kilometres of ice and dust, inert for most of its orbit. Brought close to the Sun, its ices sublimate — pass straight from solid to gas — and the escaping gas drags dust off the surface with it. The raw material therefore departs slowly and in every direction. Everything that follows is done to it afterwards, from outside.
Now a detail the folk picture cannot accommodate: a bright comet does not grow one tail. Photograph it and you generally see two, at different angles and in different colours. Two tails means two agents. Ask what each is.
The straight, faintly blue one is gas. Solar ultraviolet strips electrons from the molecules, and the moment a molecule is charged it stops being a free particle and becomes something the solar wind can grip. That wind — a stream of charged particles leaving the Sun at typically around four hundred kilometres a second — carries a magnetic field that drapes around the coma and sweeps the new ions almost exactly anti-sunward. Set four hundred against the comet's own orbital tens, and you see why the ion tail barely bends: the comet's motion is a rounding error in that sum.
The reasoning once ran the other way, which is worth knowing. In 1951 Ludwig Biermann worked backwards from how sharply ion tails accelerated and concluded sunlight alone could not do it — there had to be a stream of particles. The solar wind was inferred from comet tails about a decade before Mariner 2 flew through it in 1962.
The second tail, broader and yellowish-white, is dust, and dust is neutral: the wind has no hold on it at all. What pushes dust is sunlight itself, which carries momentum. Put the two forces on a single grain side by side. Gravity pulls it sunward; radiation pressure pushes it outward. Both weaken as the inverse square of distance from the Sun — so their ratio is the same everywhere. It is a fixed property of the grain rather than of where the grain happens to be, and it settles that grain's fate once and for all.
What sets the ratio? Radiation pressure acts on the grain's cross-section, which grows as the square of its size; gravity acts on its mass, which grows as the cube. The ratio therefore scales as one over size. Small grains are blown out and large ones are not, with the crossover for ordinary cometary material somewhere near a micron, though it shifts with density and with how the grain scatters light. That is also why the dust tail curves: each grain leaves carrying the comet's orbital velocity and then coasts under a reduced effective gravity, so grains of different sizes released at different times fan out along different orbits. A dust tail is a family of orbits, not a plume.
And the grains too heavy to push? They stay near the comet's own path, spread in a sheet along it. Seen edge-on when Earth crosses the comet's orbital plane, that sheet can appear as a spike pointing toward the Sun — the anti-tail, a trick of projection rather than a rebellion.
The analogy
THE ANALOGY #Think of a chimney on a windy hillside. The smoke lies along the wind, not along anything the chimney is doing. Now suppose the plume carries both fine soot and heavy embers: the soot goes wherever the wind goes, while the embers barely deflect. One vent, two plumes, sorted by how much grip the wind can get.
A hillside wind pushes by collisions, and neither force here is drag — the ion tail is swept by a magnetic field frozen into the solar wind rather than blown along by it, and the dust tail is pushed by photon momentum in a vacuum where there is nothing to blow.
Clarifying the model
THE MODEL #Three refinements hold the reasoning together.
First, "points away from the Sun" is exactly true only of the ion tail, and even then only roughly — it lags the anti-solar line by a few degrees, and the dust tail can lag by tens. The rule is anti-solar, not anti-motion, and it is a statement about which way the forces point rather than a geometric law about tails.
Second, the tail is not something the comet has; it is material the comet is losing, and none of it returns. A tail stretching tens of millions of kilometres is fantastically thin, rebuilt continuously from the nucleus for as long as the ices hold out.
Third, the two tails run on different clocks. The ion tail is light and tightly coupled, so it twitches within hours: crossing a boundary in the solar wind's magnetic field, the whole tail can detach and drift off while a fresh one grows — a disconnection event. The dust tail carries weeks of accumulated emission in heavier grains and changes shape only gradually.
A picture of it
THE PICTURE #How to readStart at the nucleus box on the left: one source, releasing gas and dust in the same breath. The three arrows are the sorting, and each label is the criterion that decides which way a given particle goes — charge for the gas, size for the dust. Read each box's lines as that population's signature, and note that the top two differ in every row: different driver, different shape, different colour, different response time. The bottom box is the material neither agent can move, which is why it produces a feature that looks like it points the wrong way.
What became clearer
WHAT CLEARED #The tail is not something a comet drags behind it; it is something the Sun does to material the comet has already let go. Two agents act on two populations — a magnetised wind on charged gas, photon momentum on neutral dust — and both act radially outward from the Sun, so both results point away from it whichever way the comet is heading. The ion tail's straightness is the signature of a driver far faster than any orbital speed here; the dust tail's curve is the signature of grains keeping their orbital motion while gravity is partly cancelled, by an amount fixed by their size.
Where to go next
ONWARD #- Why the same released dust, spread around the whole orbit, becomes an annual meteor shower when Earth crosses it.
- How a comet stops: what makes a nucleus go dormant or disintegrate rather than simply run out of ice.
Key terms
TERMS #| Term | What it means |
|---|---|
| Sublimation | a solid passing directly to gas without melting, the process that switches a comet on near the Sun. |
| Solar wind | the continuous stream of charged particles and embedded magnetic field flowing outward from the Sun. |
| Radiation pressure | the force exerted by light's momentum, proportional to a grain's cross-section rather than its mass. |
| Anti-tail | an apparent sunward spike seen when Earth crosses the orbital plane and views a sheet of large, unpushed grains edge-on. |
Every term the collection defines is gathered in the glossary.