Astronomy/Comets

Comets are described by astronomers as "dirty snowballs." They have a nucleus of dust and ice surrounded by a coma (thin atmosphere of evaporated ice) [1]. The nucleus is mainly made up of H₂O (water), with some parts NH₃ (ammonia), CH₄ (methane), and CO₂ (carbon dioxide).

Comets have two tails: the ion tail (blue, composed of gas), and the dust tail (yellow). Meteor showers, like the Perseid or Leonids, occur when the Earth travels through a comet's dust tail.

Comets come from the Oort Cloud or the Kuiper Belt. [2] The Oort Cloud contains $10^{11}$ comets.

One of the most famous comets is Halley's Comet, which was discovered by Edmond Halley in 1705. The comet had last been sighted in 1682; Halley predicted it would return in 1758. Unfortunately, he died in 1742, so he was unable to see his prediction come true. The comet was subsequently named after him.

Halley's Comet has a period of about 76 years. It came by the Earth last in 1986, and will return in 2061.