Halley's Comet

• 1. Introduction

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  Halley's Comet is the only known short-period comet that returns to Earth's skies reliably enough to be seen with the naked eye, once in a human lifetime. In 837 AD, it passed so close that its tail stretched across half the sky, and Emperor Louis the Pious fell to his knees in prayer, certain that it announced the death of a prince. That single flyby came within 0.03 astronomical units of Earth - the closest approach in recorded history. What makes this comet so singular? Why did it take until 1705 for anyone to realise that all those terrifying, world-altering apparitions were the same object coming back around? And what did a fleet of spacecraft finally reveal when they caught up with it in 1986?

• 2. Edmond Halley's Revelation

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  In 1705, the English astronomer Edmond Halley published his Synopsis of the Astronomy of Comets, a work that changed how humanity understood the sky. He had assembled a list of 24 comet observations and, using Isaac Newton's new laws of gravity, began comparing their orbital elements. Three apparitions stood out: one in 1531, observed by Petrus Apianus; one in 1607, observed by Johannes Kepler; and one in 1682, which Halley had personally watched from Islington in September of that year. Their orbits were nearly identical. Halley concluded they were the same object, returning roughly every 76 years, and predicted it would come back in 1758. He died in 1742, sixteen years before he could be proven right.

  On the 25th of December 1758, a German farmer and amateur astronomer named Johann Georg Palitzsch spotted the returning comet. Confirmations flooded into Paris from observers across Europe. In Jamaica, an astronomer named Francis Williams also made an independent identification, though his observations never reached Europe. Williams had a portrait commissioned to mark the occasion; his hand rests on page 521 of the third edition of Newton's Principia, and a white smudge in the sky almost certainly depicts Halley's Comet near the constellations of March 1759. In 2024, X-ray imaging confirmed that the painting accurately shows the star field where the comet would have appeared.

  The comet did not pass perihelion until the 13th of March 1759, delayed by 618 days because of the gravitational pull of Jupiter and Saturn. A team of three French mathematicians - Alexis Clairaut, Joseph Lalande, and Nicole-Reine Lepaute - had calculated this delay in advance, missing the actual date by only one month. The confirmation was the first time any object other than a planet had been shown to orbit the Sun, and it was an early and decisive test of Newtonian physics. French astronomer Nicolas-Louis de Lacaille formally named the comet in Halley's honour that same year, 1759.

• 3. A History Written in Fear

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  The first certain sighting of Halley's Comet in the historical record appears in the Chinese chronicle known as the Records of the Grand Historian, describing a comet in 240 BC that appeared in the east and moved north. From that point forward, roughly one eighth of all comet sightings in historic records belong to this single object - a testament to how reliably bright it becomes.

  The 87 BC appearance was recorded in Babylonian tablets noting the comet was seen day after day for a month. Scholars Vahe Gurzadyan and R. Vardanyan have suggested that the Armenian king Tigranes the Great may have depicted it on his coins, showing a crown with a star bearing a curved tail. In 12 BC, Chinese astronomers of the Han dynasty tracked the comet from August through October; the Roman historian Cassius Dio recorded a comet hanging over Rome for days, interpreted as a portent of the death of Marcus Vipsanius Agrippa. The proximity of this apparition to the date of Jesus Christ's birth has led some to propose it as a candidate for the Star of Bethlehem.

  The 451 AD apparition was said to herald the defeat of Attila the Hun at the Battle of Chalons. In 684 AD, Chinese records called it the "broom star". In 760 AD, the Zuqnin Chronicle described "a white sign" that lingered for fifteen nights, shaped like a broom, visible before dawn in the northeast toward the sign of Aries.

  The 1066 apparition crystallised every pattern of awe and dread the comet had accumulated over centuries. Witnesses said it appeared four times the size of Venus and shone with a quarter of the Moon's light. Eilmer of Malmesbury, who may have seen the comet in 989 and again in 1066, reportedly addressed it directly: "You've come, have you? You've come, you source of tears to many mothers." Harold II of England died at the Battle of Hastings later that year; William the Conqueror took the throne. The comet was stitched into the Bayeux Tapestry as a star. The Armenian chronicler Matthew of Edessa noted its coincidence with a Turkic invasion that, he wrote, left armies "consuming all the faithful by the sword and enslavement."

  In 1222, according to legend, Genghis Khan read the comet's westward-seeming arc as a sign to turn his conquests toward Europe. The 1301 apparition was so spectacular that the Florentine artist Giotto di Bondone painted what he saw into the Nativity scene of his Arena Chapel cycle, completed in 1305, depicting the Star of Bethlehem as a fire-coloured comet. Art historian Roberta Olson judged Giotto's rendering far more accurate than any other contemporary description, unmatched in painting until the nineteenth century. It was Olson's identification that later inspired the European Space Agency to name their 1986 mission after the artist.

• 4. Mark Twain and the 1910 Pass

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  Mark Twain was born on the 30th of November 1835, exactly two weeks after the comet's perihelion that year. In his autobiography, published in 1909, he wrote: "I came in with Halley's comet in 1835. It is coming again next year, and I expect to go out with it. It will be the greatest disappointment of my life if I don't go out with Halley's comet." He died on the 21st of April 1910, the day after the comet's perihelion passage. The 1985 fantasy film The Adventures of Mark Twain was inspired by that quotation.

  The 1910 approach was the first for which photographs exist and the first from which spectroscopic data were gathered. Earth actually passed through the comet's tail on the 19th of May. Spectroscopic analysis detected cyanogen, a toxic gas, in the tail; the press speculated that life on Earth might be endangered. Despite scientific reassurances, panic buying of gas masks was recorded, and sellers moved "anti-comet pills" to a frightened public.

  The comet also stirred unrest in China on the eve of the Xinhai Revolution, which would end the Qing dynasty in 1911. A missionary named James Hutson recorded what he heard in Sichuan: people believed the comet announced war, fire, pestilence, and the fall of dynasties. On certain days doors stayed shut for half a day, water was not carried, and many refused even to drink, fearing that pestilential vapour was pouring down from the sky.

  In Japan, Emperor Hirohito observed the 1986 return at age 84. He had first seen the comet in 1910, when he was eight years old - a span of 76 years between the same comet and the same pair of eyes.

• 5. The Halley Armada

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  David C. Jewitt and G. Edward Danielson first detected the returning comet on the 16th of October 1982, using the 5.1-metre Hale Telescope at Mount Palomar and a CCD camera. The first visual confirmation came from amateur astronomer Stephen James O'Meara on the 24th of January 1985, using a home-built 24-inch telescope atop Mauna Kea to spot an object of magnitude 19.6.

  The 1986 apparition was the least favourable on record for Earth-based observers: the comet and Earth sat on opposite sides of the Sun, the closest approach was 0.42 astronomical units, and light pollution from cities blocked views for much of the world. Yet it was also the most scientifically productive in history. The Soviet Vega 1 probe returned its first images of Halley on the 4th of March 1986, captured the first photograph ever taken of a comet nucleus, and made its closest flyby on the 6th of March, passing within 8,890 kilometres. Vega 2 followed on the 9th of March, coming within 8,030 kilometres. On the 14th of March, the European Space Agency's Giotto spacecraft made the closest pass of all, approaching the nucleus directly.

  The fleet also included two Japanese probes, Suisei and Sakigake, and the repurposed International Cometary Explorer, which flew through the comet's tail on the 28th of March 1986, coming within about 40.2 million kilometres of the nucleus. Together, these missions were nicknamed the Halley Armada. A planned American contribution never arrived: the Space Shuttle Challenger, carrying the Halley's Comet Experiment Deployable satellite, disintegrated in flight and killed all seven astronauts aboard. The Columbia mission planned for March 1986 was cancelled in the aftermath.

• 6. What the Spacecraft Found

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  Giotto's close pass transformed the understanding of comets. Before 1986, Fred Whipple's "dirty snowball" model had been the leading hypothesis: comets were icy bodies whose surfaces sublimated in sunlight, sending jets of volatile gas outward to form the coma and tail. The Giotto and Vega missions confirmed the broad outline but sharpened the picture in unexpected ways.

  Halley's nucleus turned out to be barely 15 kilometres long and 8 kilometres wide, with a shape variously compared to a peanut, a potato, or an avocado. Its mass is roughly 2.2 trillion kilograms, with an average density of about 0.55 grams per cubic centimetre - low enough to suggest the interior is a loosely held rubble pile rather than solid ice. The surface revealed hills, mountains, ridges, depressions, and at least one crater.

  Most surprisingly, the nucleus was nearly black. Its albedo is about 0.04, meaning it reflects only 4% of the sunlight striking it - roughly the reflectivity of coal. Astronomers had predicted an albedo closer to 0.17. Surface temperatures measured by Vega 1 ran between 300 and 400 Kelvin, far warmer than sublimating ice would allow. Only about 10% of the surface was actually active, with the rest coated in dark, non-volatile dust that trapped heat. The missions suggested that Halley was less a dirty snowball than what some researchers called a "snowy dirtball".

  Gas ejected from the nucleus was found to be roughly 80% water vapour and 17% carbon monoxide, with carbon dioxide and traces of methane and ammonia making up most of the rest. Dust particles were primarily carbon-hydrogen-oxygen-nitrogen compounds and silicates similar to those found in Earth's rocks. One early finding - that Halley's ratio of deuterium to hydrogen resembled that of Earth's ocean water, raising the possibility that such comets seeded Earth with water - was later overturned: subsequent observations showed Halley's deuterium ratio to be far higher than Earth's oceans, making it an unlikely source.

• 7. Orbit and Future

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  Halley's orbit is unusual among short-period comets in almost every respect. Its path around the Sun is highly elliptical, with an eccentricity of 0.967: at its closest, the perihelion, it passes between the orbits of Mercury and Venus at 0.59 astronomical units from the Sun. At its farthest, the aphelion, it reaches roughly 35 astronomical units - about the orbital distance of Pluto. It travels that path in the opposite direction to the planets, a retrograde orbit inclined at 18 degrees to the ecliptic. The 1910 flyby recorded a relative velocity of 70.56 kilometres per second.

  Because its orbit crosses Earth's in two places, Halley is responsible for two annual meteor showers: the Eta Aquariids in early May, and the Orionids in late October.

  In 1989, Boris Chirikov and Vitold Vecheslavov analysed 46 apparitions drawn from historical records and computer simulations, showing that the comet's dynamics are chaotic and cannot be reliably predicted over long timescales. David Hughes calculated in 1985, based on data from the 1910 apparition, that Halley's nucleus has shed between 80 and 90% of its mass over the last 2,000 to 3,000 revolutions, and that the comet will likely disappear entirely after roughly 2,300 more perihelion passages. More recent modelling suggests it may instead evaporate, split in two, or be ejected from the Solar System within a few hundred thousand years.

  On the 9th of December 2023, the comet reached aphelion, the farthest and slowest point in its orbit, travelling at just 0.91 kilometres per second relative to the Sun. It was last observed in 2003 by three of the Very Large Telescopes at Paranal, Chile, when its magnitude was 28.2 - the faintest and farthest any comet had ever been imaged. The next perihelion is predicted for the 28th of July 2061, when the comet will be on the same side of the Sun as Earth and is expected to reach an apparent magnitude of -0.3, far brighter than its 1986 showing.