Comet that orbits the Sun roughly every 75 years, named after English astronomer Edmond Halley, who calculated its orbit. It is the brightest and most conspicuous of the periodic comets, and recorded sightings go back over 2,000 years. The comet travels around the Sun in the opposite direction to the planets. Its orbit is inclined at almost 20° to the main plane of the Solar System and ranges between the orbits of Venus and Neptune. It will next reappear in 2061.
The comet was studied by space probes at its last appearance in 1986. The European probe Giotto showed that the nucleus of Halley's Comet is a tiny and irregularly shaped chunk of ice, measuring some 15 km/10 mi long by 8 km/5 mi wide, coated in a layer of very dark material, thought to be composed of carbon-rich compounds. This surface coating has a very low albedo
, reflecting just 4% of the light it receives from the Sun. Although the comet is one of the darkest objects known, it has a glowing head and tail produced by jets of gas from fissures in the outer dust layer. These vents cover 10% of the total surface area and become active only when exposed to the Sun. The force of these jets affects the speed of the comet's travel in its orbit.
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