Why Do Comet Tails Point Away from the Sun?

A comet’s tail is not simply a trail left behind along the direction in which the comet has been travelling. It points broadly away from the Sun. When a comet rounds the Sun and changes direction, its tail can even appear ahead of its motion. The controlling influence is sunlight and the flow of particles from the Sun, not the comet’s forward speed.

Far from the Sun, a comet’s nucleus is a cold mixture of ice, rock and dust. As it approaches the Sun, heating turns ice and other volatile materials into gas. Escaping gas carries dust away from the surface and makes a diffuse cloud around the nucleus called a coma. The tails grow when material in that cloud is pushed and accelerated by solar influences.

Most visible comets have two physically different tails. An ion tail contains gas that sunlight has ionized. The solar wind and the Sun’s magnetic field act strongly on those charged particles, producing a narrow tail that points almost directly away from the Sun. A dust tail contains heavier grains. Sunlight’s radiation pressure pushes them more gradually, so the dust tail is often broader and curved by the comet’s orbit and motion.

That difference helps explain many comet photographs. A fine bluish tail and a wider white or yellowish tail may appear together. Ionized gases can emit characteristic light, while dust is mainly visible because it reflects sunlight. Both components are directed away from the Sun, but they need not lie on precisely the same line.

Calling a comet tail a cosmic wind sock is therefore only partly right. The solar wind matters especially for charged gas, while light itself exerts pressure on dust. A tail is less a record of where the comet has been than a display of the forces acting on freshly released material around it at that moment.