Glossary term: Halley's Comet

Description: Halley's comet is, arguably, the most famous comet because it is the only short-cycle comet visible to the naked eye from Earth with a period short enough (about 75 years) to potentially allow people to see it twice in their lives. The comet's last visit was in 1986, and it is expected to return in 2061. It is named after the English astronomer Edmond Halley who was the first to calculate its periodicity and predict its next visit. Halley noticed that the comets that appeared in the years 1531, 1607, and 1682 all had very similar orbits and thus were all visits of the same comet to the inner Solar System. He correctly predicted the comet's return in 1758.

It has been visited by the Vega and Giotto space missions. These found that the comet's dust consists mainly of silicates, iron, and magnesium, in addition to carbon–hydrogen–oxygen–nitrogen compounds (CHON). The comet's nucleus is made mostly of ice.

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