Meet our neighbours: Comet Halley

General information

Comets are small objects that orbit the Sun at very large distances, hundreds to thousands of times the Sun-Earth distance. They are believed to form a huge cloud, called the “Oort cloud”, around the solar system. Some of these comets may be dislodged from their distant location by the gravitational influence of Jupiter, and their new orbits take them on a long journey across the solar system.

Most of the comets that enter the solar system never come back for a second visit, but some become periodic comets, with periods that vary from less than 200 years (short-period comets) to thousands of years (long-period comets). In the case of Halley’s comet, it’s very elongated orbit takes it as far as 5250 million kilometres and as close as 87 million kilometres from the Sun over a period of 75.3 years.

A Comet seen from close

Physically, a comet nucleus is a solid body made of dust and ice . When it gets close to the Sun, the ice that makes up most of the nucleus begins to ‘vaporize’ and forms a “coma” (from the Latin word for “hair”) . Even closer to the Sun, solar radiation pushes gas and dust particles out of the coma; these particles form a dust tail that can extend millions of kilometres into space. This tail is often curved because it takes the shape of the comet’s orbit. The gas can also be ionized by solar radiation, and the charged molecules and atoms then develop an “ion tail” that is straighter than the dust tail and always directed in the direction opposite to the Sun.

Image: the different parts of a comet. Credits: ESA

Why are comets important?

Comets are leftovers from the formation of the Solar System, about 4.6 billion years ago. They have spent most of their lives far from the Sun, so their ices and dust have changed very little. Studying them is like opening a time capsule: they tell us what the Solar System was made of when the planets were born. Comets may also have helped bring water and organic molecules to the young Earth, although scientists are still debating how much. Finally, comets leave trails of dust along their orbits. When Earth crosses these trails, we see meteor showers. Halley’s Comet is the parent of two of them: the Eta Aquariids in May and the Orionids in October.

Halley's Comet story

Halley is the most famous comet: its passages have been recorded by astronomers since at least 240 BC, but they were not recognized as periodic visits by the same object. It was only in 1705 that the British astronomer Edmond Halley determined that the orbit of a comet that appeared in 1682 was the same as the orbit of two earlier comets that were observed in 1531 and 1607. He was the first to realize that these three comets were, in fact, three successive passages of the same comet, with a periodicity of about 76 years. He then predicted the return of the comet in 1758, a prediction that proved correct when the comet passed near the Sun in March 1759. In recognition of his work, the comet was given his name. Halley’s Comet last passed close to the Sun in February 1986. Its next return is expected in July 2061. In December 2023 it reached the farthest point of its orbit, beyond Neptune, and it is now on its way back towards us.

Image: Comet 1P/Halley as taken March 8, 1986 by W. Liller, Easter Island, part of the International Halley Watch (IHW) Large Scale Phenomena Network.