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.

[28] 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.

In the Americas, John Winthrop lectured at Harvard University to explain the implications of the comet's reappearance for Newtonian mechanics and natural theology.

[33] This effect was computed before its return (with a one-month error to 13 April)[34] by a team of three French mathematicians, Alexis Clairaut, Joseph Lalande, and Nicole-Reine Lepaute.

[38] This theory notes a passage in the Babylonian Talmud, tractate Horayot[39] that refers to "a star which appears once in seventy years that makes the captains of the ships err".

[53] The non-gravitational effects can be crucial;[53] as Halley approaches the Sun, it expels jets of sublimating gas from its surface, which knock it very slightly off its orbital path.

[54] In 1989, Boris Chirikov and Vitold Vecheslavov performed an analysis of 46 apparitions of Halley's Comet taken from historical records and computer simulations, which showed that its dynamics were chaotic and unpredictable on long timescales.

[56][57] Based on records from the 1910 apparition, David Hughes calculated in 1985 that Halley's nucleus has been reduced in mass by 80 to 90% over the last 2,000 to 3,000 revolutions, and that it will most likely disappear completely after another 2,300 perihelion passages.

[60] Giotto provided the first evidence in support of Fred Whipple's "dirty snowball" hypothesis for comet construction; Whipple postulated that comets are icy objects warmed by the Sun as they approach the inner Solar System, causing ices on their surfaces to sublime (change directly from a solid to a gas), and jets of volatile material to burst outward, creating the coma.

[77] This appearance may be recalled in the representation of Tigranes the Great, an Armenian king who is depicted on coins with a crown that features, according to Vahe Gurzadyan and R. Vardanyan, "a star with a curved tail [that] may represent the passage of Halley's Comet in 87 BC."

[81] According to the Roman historian Cassius Dio, a comet appeared suspended over Rome for several days portending the death of Marcus Vipsanius Agrippa in that year.

[82] Halley's appearance in 12 BC, only a few years distant from the conventionally assigned date of the birth of Jesus Christ, has led some theologians and astronomers to suggest that it might explain the biblical story of the Star of Bethlehem.

[85] Another possible report comes from Jewish historian Josephus,[86] who wrote that in 66 AD "The signs ... were so evident, and did so plainly foretell their future desolation ... there was a star resembling a sword, which stood over the city, and a comet, that continued a whole year".

It was recorded by astronomers in China, Japan, Germany, the Byzantine Empire, and the Middle East;[80] Emperor Louis the Pious observed this appearance and devoted himself to prayer and penance, fearing that "by this token a change in the realm and the death of a prince are made known".

The Florentine chronicler Giovanni Villani wrote that the comet left "great trails of fumes behind", and that it remained visible from September 1301 until January 1302.

[b][93] It was seen by the artist Giotto di Bondone, who represented the Star of Bethlehem as a fire-coloured comet in the Nativity section of his Arena Chapel cycle, completed in 1305.

In 1470, the humanist scholar Bartolomeo Platina wrote in his Lives of the Popes [la] that,[109] A hairy and fiery star having then made its appearance for several days, the mathematicians declared that there would follow grievous pestilence, dearth and some great calamity.

Calixtus, to avert the wrath of God, ordered supplications that if evils were impending for the human race He would turn all upon the Turks, the enemies of the Christian name.

He likewise ordered, to move God by continual entreaty, that notice should be given by the bells to call the faithful at midday to aid by their prayers those engaged in battle with the Turk.Platina's account is not mentioned in official records.

[111] After witnessing a bright light in the sky which most historians have identified as Halley's Comet, Zara Yaqob, Emperor of Ethiopia from 1434 to 1468, founded the city of Debre Berhan (tr.

At Markree Observatory in Ireland, Edward Joshua Cooper used a Cauchoix of Paris lens telescope with an aperture of 340 millimetres (13.3 in) to sketch Halley's comet in 1835.

[119] Observations of streams of vapour prompted Bessel to propose that the jet forces of evaporating material could be great enough to significantly alter a comet's orbit.

[133] As James Hutson, a missionary in Sichuan Province at the time, recorded: "The people believe that it indicates calamity such as war, fire, pestilence, and a change of dynasty.

One that reached major newspapers claimed that the Sacred Followers, a supposed Oklahoma religious group, attempted to sacrifice a virgin to ward off the impending disaster, but were stopped by the police.

The approach of the comet was first detected by astronomers David C. Jewitt and G. Edward Danielson on 16 October 1982 using the 5.1 m Hale Telescope at Mount Palomar and a CCD camera.

[152] Based on data retrieved by the largest ultraviolet space telescope of the time, Astron, in December 1985, a group of Soviet scientists developed a model of the comet's coma.

[157] The mission to capture the ultraviolet spectrum of the comet ended in disaster when the Space Shuttle Challenger exploded in flight, killing all seven astronauts onboard.

[165] The most likely explanation is a combination of two effects, the polymerisation of hydrogen cyanide and a phase transition of amorphous water ice, which raised the temperature of the nucleus enough for some of the more volatile compounds on its surface to sublime.

The telescopes observed Halley, at the faintest and farthest any comet had ever been imaged, in order to verify a method for finding very faint trans-Neptunian objects.