Tails from outer space

DURING my astronomy presentations as part of the recent Festival of Bright Ideas in Hobart, I was asked many great questions about all sorts of space and astronomy-related topics. One of them was a question about comets’ tails: Why do they point away from the sun?

It’s a fine question, because when we see a comet or a picture of a comet, it looks like the comet must be always moving through space headfirst, leaving a trail of material behind it. However, this is not the reason for the direction of the tail.

Comets are quite different from meteors, the latter often being called “shooting stars’’ or “falling stars’’. From a dark location, we can typically see several of these each hour, and they are a quite distinct phenomenon from both stars and comets. A meteor is a brief line of light in the sky caused by a (usually) tiny object being destroyed as it enters our atmosphere at high speed. Sometimes, we see a persistent line of light marking out its path for a short while. This light is called a meteor train, which is the glow of air molecules ionised by the passage of the meteoroid.

There is a connection between comets and meteors in that much of the material that collides with us originates from comets, especially as Earth annually intersects several of their orbits.

Comets are icy objects in orbit around the sun, and have been likened to “dirty snowballs’’. They are typically on quite elongated orbits. Some of these orbits are relatively small, such as that of Comet Halley, which passes through the inner part of the solar system about every 76 years but is currently beyond the orbit of Neptune. Others have extremely long orbits, so that in all of recorded human history there are many that have been seen only once.

Comets are famous for their tails. The reason for the tails’ existence is the interaction of the comet with energy and particles streaming outward from the sun.

When a comet moves into the inner solar system, the sun’s heat causes gas and dust to be dislodged from the “snowball’’ — the comet’s nucleus — forming a fuzzy surrounding called a coma. Ultraviolet light from the sun ionises the gas molecules — that is, it turns them into “charged particles’’.

There are two types of tails. The ion tail, sometimes called the gas tail, arises because the stream of charged particles from the sun interacts with the comet’s ionised gas molecules. Because the flow of solar particles is away from the sun, the ion tail leaves the nucleus in that direction.

The other kind of tail is a dust tail. The dust is composed of small grains of material, composed of various compounds including those involving magnesium, silicon and iron, and lighter elements including carbon, hydrogen and oxygen and nitrogen. The particles are mostly very much less than 1mm in diameter.

Unlike the ion tail, the dust tail is directed away from the comet by the light of the sun. This effect is called radiation pressure. The dust tail also points generally away from the sun, but the dust particles travel away from the nucleus much more slowly than the ions, resulting in that tail often having a curved shape that can be quite pronounced.

The dust tail is the one that is more easily visible with the unaided eye, and for some comets it can be quite prominent.

A spectacular example of a cometary tail was that of the dust tail of Comet McNaught in 2007, for which Tasmania was in the box seat for the best view of the comet. The streaks in McNaught’s tail are thought to have been caused by its interaction with the sun’s magnetic field. A comet famous for its two tails was Comet Hale-Bopp, seen at its best from the northern hemisphere in 1997.