Aleisha Cruse, of Balclutha School, asks :-

How do you tell a comet has fire?

Duncan Steel, a space physicist and author of the book "Rogue Asteroids and Doomsday Comets", responded.

Comets do not 'have fire', if by that you mean that they have flames. But I can see how one might imagine this. Looking at a comet through a telescope you see a long tail behind it, and that looks like smoke. But it isn't.

In fact comets are really cold, rather than hot, and they are made mostly of ice. The icy lump is what we call the nucleus, and it is typically just a kilometre or two across. Some comets are bigger and brighter, like Comet Halley which passed by in 1986 (it won't be back again until 2062), and Comet Hale-Bopp, a vivid sight in 1997.

This ice starts to evaporate as the comet comes closer to the Sun, forming a huge cloud of vapour that we call the coma. This cloud may be 100,000 km across, so that it reflects a great deal of sunlight, making the comet bright and obvious.

The molecules in the coma also tend to be broken apart by the solar ultraviolet light, and left electrically charged. As a result they are swept outwards away from the Sun, forming the familiar tail of a comet. In fact the word 'comet' is derived from the Greek for a 'hairy star' because of this.

Although very bright comets visible to the naked eye are rare, using large telescopes there are typically several dozen comets that can be detected at any time.