What is the difference between asteroids and comets?
Comets are called ‘dirty ice-balls’, whereas asteroids, or minor planets, are ‘rocks in space’. The size of asteroids typically ranges from a metre to several hundred kilometres across. One of the main differences is that asteroids do not contain ‘volatiles’ (substances that ‘sublimate’ i.e. they pass directly from the solid to the gaseous state when heated) or other frozen material. Therefore asteroids do not develop a tail when they approach the Sun. The comet’s tail is due to the frozen gas vaporizing and dragging small dust grains with it into the surrounding space. In this way the comet’s atmosphere, the ‘coma’ is formed and evolves, and the ion and dust tails form. There is evidence that some asteroids are ‘dead comets’, comets that have lost their volatile materials after many approaches to the Sun.
