The Tails of a Comet

The gas which is blown away from the coma is ionised by solar radiation and becomes electrically charged. It is then affected strongly by the magnetic fields associated with the solar wind (a stream of charged particles expelled by the Sun). The gas tail is made visible by line-emission from the excitation of the gas by the Sun's radiation. This gives the gas tail its characteristic blue colour. The geometric shape of the tail is governed by the magnetic structures in the solar wind but predominantly the gas tail points directly away from the direction from the comet to the Sun.

The dust is blown away from the coma by radiation pressure from the sunlight absorbed by individual dust grains. It moves in a direction which is governed by the motion of the comet, by the size of the dust particles and by the speed of ejection from the coma. The dust tail can be complex, multiple and even curved but, in general, will point away from the Sun. Sometimes, due to projection effects, part of the dust tail can be seen pointing in a sunward direction. This is just due to the fact that the comet and the Earth are moving and that part of the tail has been ‘left behind’ in such a place as to appear to point towards the Sun. The dust tail is yellow because it reflects the Sun's light to us.

The gas tail can be about 100 million km long while the dust tail is around 10 million km long. The longest observed tail on record is the Great Comet of 1843, which had a tail that was 250 million km long (greater than the distance from the Sun to Mars!).

The Names of Comets

A comet takes the name of its discoverer, or discoverers. It also has a serial number consisting of the year and a letter designation. In this way all comets are named uniquely. Halley's comet is one of very few exceptions to the naming rule. Halley did not discover ‘his’ comet but has the honour of having his name attached to it because of his pioneering work in determining the orbits of comets and showing that this comet was periodic.

Prediction of Comets

Apart from the periodic comets, whose orbital periods are well known and hence whose returns can be predicted with great accuracy, it is impossible to predict when comets may be seen in the sky. Most of the brightest and most spectacular comets have been ones which have appeared only once and have never been seen again. When a comet is discovered, far from the Sun, it is very difficult to predict how bright it will appear when it comes close to the Earth and the Sun. Some comets seem to emit a lot of gas and dust and produce long and spectacular tails whereas others only produce a small amount of gas and dust and have almost no tail at all.