Comets

Abstract

Comets are the only objects in the solar system, outside the Earth, which the unaided eye perceives to have a non-spherical structure (figure 1). They are sometimes large enough to be resolved as more than a point of light and then can be seen to have a characteristic structure with a head and a long, slegder tail. The head is approximately spherical and is up to 10⁵ km in diameter and the tail, which is directed away from the Sun, has a visible length of up to 10⁸ km. The tail is not a single structure but consists of two tails with entirely different physical causes. One appears white from the sunlight scattered off the surface of the grains of dust which form it. The spectral distribution of the scattered light contains information about the size distribution of the grains but not about the chemical composition of the dust. The second tail is coloured because it emits light at the wavelengths characteristic of the molecules of gas of which it is composed The molecules are found to be ionised, consisting mainly of CO⁺, H₂O⁺, OH⁺. The dominant ion CO⁺ gives the tail its characteristic blue appearance in photographs. Since the particles are charged the second tail is known as the plasma tail.