INSTANT EXPERT HOW ARE COMETS CLASSIFIED?

There are more than 5,000 known 'dirty snowballs' knocking about the Solar System, all of which fit into a selection of categories

Comets are known for their tails, and the name comes from the ancient Greek for ‘long-haired’. The largest ones are a bright presence in the night sky for a month or so as they make their closest approach to the Sun. They are icy bodies – dirty snowballs of water ice mixed with rock, dust and frozen gases like carbon dioxide. The tail appears when the comet approaches the Sun and the surface layers turn to gas. This is then pushed away by the solar wind; it’s mostly water and dust, though these can separate into two tails as the gas portion is more easily pushed outwards from the Sun than the dust.

Comets fall into three major categories: short period, long period and hyperbolic. While the vast majority do come from the Solar System, it’s likely that some have come from outside of it, but there’s no way to tell for sure. Short-period comets have orbits ranging up to 200 years and can be further divided into Encke-type, Halley-type and Jupiter-type.