A body of our Solar System. Comets are composed of rocks, dust and ices. This composition was first recognised by the astronomer Fred Whipple (Lawrence Frederick Whipple: November 5, 1906 – August 30, 2004) who described them as an ‘icy conglomerate’, translated for the press as being like a ‘dirty snowball’. As they approach the Sun, the ices begin to evaporate forming a coma and one or more tails. The particles frozen in with the ices are carried off as the ices blast their way off the surface and it is the Sun’s light reflecting off these that enable the tail to be seen. The dust tail is often curved whilst the second tail, if present, is straight  and usually much fainter; it it made up from plasma – charged particles of gas.

Long period comets are believed to come from the Oort cloud and possibly contain material from the earliest days of the Solar System.

Short period comets are believed to originate from the Kuiper Belt.

The first spacecraft to visit a comet was the European Space Agency’s firt deep space mission – Giotto. This was launched to take a closer look at Halley’s comet. It collected samples from the tail and managed to take some images of the nucleus, showing clearly that the gas and dust erupted from the surface of comets as jets. Despite being damaged, the craft survived to visit a second comet – Grigg-Skjellerup.


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