Comets
A dirty snowball that grows two tails - and neither of them trails behind it.
Why comets grow tails
Junior level β plain language, no maths
A comet spends almost all of its life as a dark lump of ice and dust, a few kilometres across, drifting through the cold outer solar system. Nothing about it glows. It is, as astronomers like to say, a dirty snowball.
Then its long looping orbit brings it inwards, and around the distance of the asteroid belt the Sun becomes strong enough to matter. The ice does not melt into a puddle - in a vacuum it goes straight from solid to gas, a process called sublimation. Jets of gas burst off the sunlit side, dragging dust with them, and the little nucleus wraps itself in a glowing cloud called the coma that can grow larger than a planet.
Now the surprise. The tail does not stream out behind the comet like smoke behind a train. Sunlight and the solar wind push the escaping gas and dust away from the Sun, whichever way the comet happens to be travelling. So on the way in the tail trails behind, and on the way back out the comet chases its own tail, moving tail-first.
Look closely and there are two tails, not one. A whitish-yellow dust tail, which curves because the heavy dust grains keep some of the comet's orbital motion, and a straight blue ion tail of electrically charged gas, blown directly away from the Sun by the solar wind.
Things worth knowing
- Comet ice does not melt, it sublimates - goes straight from solid to gas - because there is no air pressure in space to hold a liquid together.
- A comet tail always points away from the Sun, so on the outbound leg the comet travels tail-first.
- Meteor showers are comet leftovers: the Orionids each October are dust shed by Halley Comet.
Sublimation, the frost line and two tails
Student level β the core equations
Activity switches on at a distance, not a date. Water ice starts sublimating strongly around 3 AU from the Sun, so most comets are inert rocks outside that line and only wake up when they cross it. Nearer in, more volatile ices - carbon dioxide, carbon monoxide - drive activity even earlier, which is why some comets show a coma while still far out.
The escaping gas drags dust off the surface and inflates the coma, which can reach \(10^5\) km across even though the nucleus is only a few km. Two separate forces then sort the escaped material into two tails:
The dust tail is pushed by radiation pressure - photons carrying momentum. Dust grains are heavy enough that they keep a memory of the comet's orbital velocity, so the tail lags behind the anti-solar direction and appears curved, broad and yellowish-white with reflected sunlight.
The ion tail is different. Solar ultraviolet ionises the gas, and the magnetised solar wind picks up those ions and sweeps them away at 400 km/s or more - far faster than the comet moves, so the tail points almost exactly anti-sunward, straight and narrow. It glows blue because ionised carbon monoxide emits strongly at 420 nm.
Every pass costs the comet material. A typical short-period comet loses a metre or so of surface per orbit, and the debris left along the track is exactly what Earth runs into as an annual meteor shower.
Key Formulas
| Sublimation onset | \(r \approx 3\ \text{AU}\) | water ice |
|---|---|---|
| Radiation pressure | \(F_{\text{rad}} = \dfrac{L_\odot A}{4\pi r^2 c}\) | curves the dust tail |
| Solar wind speed | \(v_{sw} \approx 400\ \text{km/s}\) | straightens the ion tail |
Things worth knowing
- The water frost line sits near 3 AU: outside it a comet is inert, inside it sublimation drives the coma and tails.
- The ion tail glows blue from COβΊ emission at 420 nm; the dust tail is yellowish because it simply reflects sunlight.
- Halley Comet loses several metres of surface per perihelion passage and has maybe 100,000 years of active life left.
Nucleus physics, Ξ² and the reservoirs
Scholar level β full mathematical depth
The dust tail's shape follows from a single dimensionless number, \(\beta\), the ratio of radiation pressure to solar gravity on a grain. Since both scale as \(1/r^2\), \(\beta\) is distance-independent and depends only on grain size and composition, roughly \(\beta \approx 0.6/(\rho a)\) in cgs. Grains with \(\beta > 1\) feel a net outward force and leave on hyperbolic paths; heavier grains follow modified Keplerian orbits. Because grains released at different times with different \(\beta\) end up in different places, the dust tail is a two-dimensional map of the emission history - the basis of Finson-Probstein syndyne and synchrone analysis.
Nucleus surfaces are among the darkest known, albedo around 0.04, and sublimation is concentrated in active regions covering a few per cent of the area. The rest is insulating dust mantle. This localisation makes outgassing anisotropic and gives a non-gravitational acceleration that must be included in orbit fits - the effect that revealed 1I/'Oumuamua's anomalous behaviour and that dominates the long-term evolution of short-period comets.
Two reservoirs supply them. The Kuiper belt and scattered disc feed Jupiter-family comets, low-inclination and short-period, while the roughly spherical Oort cloud at \(10^4\) to \(10^5\) AU supplies long-period comets on randomly oriented orbits, nudged inwards by galactic tides and passing stars. An object's orbital inclination distribution therefore fingerprints its origin.
Rosetta's two years at 67P/Churyumov-Gerasimenko rewrote much of the detail: a bilobed, extremely porous nucleus with a bulk density near 0.5 g/cmΒ³, a D/H ratio three times Earth's ocean value - evidence against Jupiter-family comets as the main source of terrestrial water - and abundant complex organics including glycine.
Key Formulas
| Radiation-pressure ratio | \(\beta = \dfrac{F_{\text{rad}}}{F_{\text{grav}}} \approx \dfrac{0.6}{\rho a}\) | independent of r |
|---|---|---|
| Non-gravitational force | \(\ddot{r} = A_1 g(r) + \ldots\) | anisotropic outgassing |
| Oort cloud | \(10^4 - 10^5\ \text{AU}\) | |
| 67P bulk density | \(\rho \approx 0.5\ \text{g/cm}^3\) | more void than rock |
Things worth knowing
- Ξ², the ratio of radiation pressure to solar gravity, is distance-independent, so dust-tail structure maps emission history directly (syndynes and synchrones).
- Comet nuclei are darker than charcoal, albedo ~0.04, with sublimation confined to active regions a few per cent of the surface.
- Rosetta found 67P has a D/H ratio three times Earth ocean water, arguing against Jupiter-family comets as the main delivery route for our oceans.