Halley's Comet

Lore & Background

Halley was the first comet to be recognised as periodic. Until the Renaissance, the philosophical consensus on the nature of comets, promoted by Aristotle, was that they were disturbances in Earth's atmosphere. This idea was disproven in 1577 by Tycho Brahe, who used parallax measurements to show that comets must lie beyond the Moon. In 1687, Sir Isaac Newton published his Philosophiæ Naturalis Principia Mathematica, outlining his laws of gravity and motion, but his work on comets was decidedly incomplete. It was Newton's friend, editor and publisher, Edmond Halley, who in his 1705 Synopsis of the Astronomy of Comets used Newton's new laws to calculate the gravitational effects of Jupiter and Saturn on cometary orbits. Having compiled a list of 24 comet observations, he calculated that the orbital elements of a second comet that had appeared in 1682 were nearly the same as those of two comets that had appeared in 1531 (observed by Petrus Apianus) and 1607 (observed by Johannes Kepler). Halley thus concluded that all three comets were the same object returning about every 76 years, a period that has since been found to vary between 72 and 80 years. After a rough estimate of the perturbations the comet would sustain from the gravitational attraction of the planets, he predicted its return for 1758. He personally observed the comet around perihelion in September 1682, but died in 1742 before he could observe its predicted return.

Reader's Guide

Halley's prediction of the comet's return proved to be correct, although it was not seen until 25 December 1758, by Johann Georg Palitzsch, a German farmer and amateur astronomer. The comet did not pass through its perihelion until 13 March 1759, the attraction of Jupiter and Saturn having caused a delay of 618 days. This effect was computed before its return (with a one-month error to 13 April) by a team of three French mathematicians, Alexis Clairaut, Joseph Lalande, and Nicole-Reine Lepaute. The confirmation of the comet's return was the first time anything other than planets had been shown to orbit the Sun. It was also one of the earliest successful tests of Newtonian physics. Another independent recognition that the comet had returned was made by the Jamaican astronomer Francis Williams, but his observations did not reach Europe. A unique portrait commissioned by Williams demonstrates the impact of the comet's return on period astronomers; in 2024, using X-ray imaging, the painting was shown to depict the field of stars in which the comet would have been visible in 1759. In March 1986, during its latest visit to the inner Solar System, Halley's Comet became the first comet to be observed in detail by a spacecraft, the European Space Agency's Giotto mission, providing the first observational data on the structure of a comet nucleus and the mechanism of coma and tail formation. These observations supported several longstanding hypotheses about comet construction, particularly Fred Whipple's 'dirty snowball' model, which correctly predicted that Halley would be composed of a mixture of volatile ices—such as water, carbon dioxide, ammonia—and dust. The missions also provided data that substantially reformed and reconfigured these ideas; for instance, it is now understood that the surface of Halley is largely composed of dusty, non-volatile materials, and that only a small portion of it is icy. It was also visited by the two spacecraft of the Vega program, Vega 1 & 2, on 6 and 9 March, respectively, going as close as 8,890 km (5,520 mi), and 8,030 km (4,990 mi), providing data on Halley's dimensions, shape, temperature, and surface properties. Halley's orbital period has varied between 74 and 80 years since 240 BC. Its orbit around the Sun is highly elliptical, with an orbital eccentricity of 0.967. The perihelion is 0.59 au (88 million km), between the orbits of Mercury and Venus. Its aphelion is 35 au (5.2 billion km), roughly the orbital distance of Pluto. Halley's orbit is retrograde, inclined by 18° to the ecliptic. Because its orbit comes close to Earth's in two places, Halley is associated with two meteor showers: the Eta Aquariids in early May, and the Orionids in late October. Halley is classified as a periodic or short-period comet: one with an orbit lasting 200 years or less. Most short-period comets are called Jupiter-family comets; those resembling Halley, with orbital periods of between 20 and 200 years and inclinations extending from zero to more than 90 degrees, are called Halley-type comets. As of 2024, 105 Halley-type comets have been observed, compared with 816 identified Jupiter-family comets.

Did You Know?

• Halley's Comet was the first comet to be observed in detail by a spacecraft, the European Space Agency's Giotto mission, in March 1986.
• The comet's orbit is retrograde, meaning it orbits the Sun in the opposite direction to the planets.
• Halley's predicted return in 1758 was confirmed on 25 December 1758 by German farmer and amateur astronomer Johann Georg Palitzsch.
• The comet's surface is largely composed of dusty, non-volatile materials, with only a small portion being icy.
• Halley is associated with two meteor showers: the Eta Aquariids in early May and the Orionids in late October.