Halley's Comet

Halley's Comet is often called the most famous comet because it was the first comet whose periodic return was understood by astronomers. It is officially designated 1P/Halley, where P denotes a periodic comet, and it takes its name from Edmond Halley, the English astronomer who predicted its return.

Halley's Comet travels backward around the Sun, opposite to the direction of Earth's own motion, in a plane tilted 18 degrees to Earth's orbit. Its greatest distance from the Sun, the aphelion, is about 35.25 AU, placing it well beyond the orbit of Neptune.

Perihelion during the 1986 apparition fell on February 9, 1986, at 0.5871 AU (87.8 million km).

The comet's orbital period averages about 76 Earth years, corresponding to an orbital circumference of about 7.6 billion miles (12.2 billion kilometers), though the period varies between appearances because of the gravitational effects of the planets.

Halley is a young, highly active comet, and with 30 recorded appearances its nature and orbit are perhaps the best understood of any comet.

Measured from one perihelion passage to the next, Halley's period has ranged from as short as 74.42 years between 1835 and 1910 to as long as 79.25 years between 451 and 530 CE.

Halley's Comet will next appear in the night sky in 2061, having previously been seen in 1758, 1835, 1910, and 1986, when it was last observed from Earth.

The comet's closest recorded approach to Earth occurred on April 10, 837, when it passed at 0.033 AU (4.94 million km) ^{11P/Halley · NASA Science ↗}. On that occasion it shone at about magnitude -3.5, nearly as bright as Venus at its greatest brilliance ^{11P/Halley · NASA Science ↗}.

Halley's nucleus measures roughly 9.3 by 5 miles (15 by 8 kilometers) and takes the form of a dark peanut-shaped body. It ranks among the darkest objects in the solar system, with an albedo of 0.03 that reflects only 3 percent of the light falling on it. The nucleus turns on its axis once in about 2.2 Earth days.

Its density was determined to be fairly low, at less than 0.6 grams per cubic centimeter.

A comet the size of Halley loses an estimated 1 to 3 meters (3 to 10 feet) of surface material with each orbit. An average periodic comet is calculated to survive about 1,000 trips around the Sun.

Only about 10 percent of the nucleus was active during the 1986 flybys.

Possible records of Halley's Comet from its 467 BCE apparition may be found in Greece and China, but the first positive observations date to 240 BCE in the Chinese chronicle known as the Shiji, or Records of the Grand Historian. Over the following centuries observations of the comet appear in records from Babylonia, China, Japan, India, the Middle East, Europe and possibly even petroglyphs in North America. A sighting of the comet in the years 684 to 693 is recorded much later, in an important incunable — the Nuremberg Chronicle, an illustrated encyclopaedia first published in 1493 and part of the earliest period of printing in Europe.

The spacecraft Giotto was named after the Italian painter Giotto di Bondone, who observed the comet in 1301.

• 467 BCE
  Possible early records
  Possible records of Halley's Comet from its 467 BCE apparition may be found in Greece and China ^{2955 Years Ago: Halley's Comet and the Battle of Hastings · NASA History ↗}. Some have speculated that a comet observed in Greece between 467 and 466 BCE was Halley ^{8Halley's Comet | Astronomy, Facts & History · Encyclopaedia Britannica ↗}.
• 240 BCE
  First positive observations
  The first positive observations date to 240 BCE in the Chinese chronicle known as the Shiji, or Records of the Grand Historian ^{2955 Years Ago: Halley's Comet and the Battle of Hastings · NASA History ↗}. The generally accepted earliest recorded appearance, witnessed by Chinese astronomers, dates to 240 BCE ^{8Halley's Comet | Astronomy, Facts & History · Encyclopaedia Britannica ↗}.
• 684 to 693
  Recorded in the Nuremberg Chronicle
  A sighting of the comet in the years 684 to 693 is recorded much later, in an important incunable — the Nuremberg Chronicle, an illustrated encyclopaedia first published in 1493 and part of the earliest period of printing in Europe ^{6Halley's Comet: Collisions with history · Science Museum Group, London ↗}.
• 1301
  Observed by Giotto di Bondone
  Giotto was named after the Italian painter Giotto di Bondone, who observed Halley's Comet in 1301 and depicted as the star of Bethlehem in his Adoration of the Magi in the Scrovegni Chapel ^{7Giotto overview · European Space Agency ↗}.

Edmond Halley was an English astronomer, born on November 8 of 1656, who died on January 14 of 1742 at Greenwich, and who was a supporter of Newton. As a young man he catalogued 341 southern hemisphere stars from St Helena. There, on November 7 of 1677, he made the first complete observation of a transit of Mercury, and he later proposed using such transits to determine the distance of the Sun.

From around 1695 Halley studied cometary orbits; where Newton favoured parabolic orbits, Halley believed that elliptical orbits might exist. Until his time comets were believed to make only one pass through the solar system. In 1705 he used Isaac Newton's theories of gravitation and planetary motions to compute the orbits of several comets.

Halley did not live to see his prediction come true, for he died in 1742. He had been dead for fifteen years when his comet was sighted on December 25 of 1758 — slightly later than he had expected — and the success made his name lasting. The comet was sighted late in 1758, passed perihelion in March of 1759, and was named in his honour.

He published in 1705 a prediction that it would return in 76 years and appear in December 1758, a difficult calculation because it had to account for perturbations produced by Jupiter.

In 1705 he published the first catalog of the orbits of 24 comets, drawing on Newton's theories of gravitation, planetary motions and calculus to identify the comets of 1531, 1607 and 1682 as one and the same object.

The apparition fell in the year of the Battle of Hastings, a bad omen for King Harold II in one reading and, for William the Conqueror, a sign from heaven of his upcoming victory.

It is an embroidery of wool yarn on woven linen made in the eleventh century, 70 metres long and 50 centimetres high, recounting the conquest of England in 1066 led by William the Conqueror. A comet appears in it between the death of King Edward the Confessor and the election of Harold Godwinson; the inscription calls it only a 'star,' but its hairy tail shows it is Halley's Comet. Scenes 32 and 33 show Harold II and his people cowering in fear as the comet passes.

William's army landed in England in late September and on October 14 in 1066 defeated Harold's army at the Battle of Hastings, killing the Anglo-Saxon king; William was crowned that December. The tapestry was most probably commissioned by Bishop Odon, William's half-brother, to decorate Bayeux Cathedral during its consecration on July 14 of 1077, and it recounts the conquest of England by William, Duke of Normandy, who defeated the English at Hastings. Its illustration unmistakably shows Halley's Comet, but its creators connected it to the sequence of Edward's death and Harold's coronation likely for political reasons and with hindsight, and it remains unknown when the tapestry was made, where, by whom or for what reason.

During its 1066 appearance the comet approached within about nine million miles (15 million kilometers) of Earth on April 23, after its closest approach to the Sun on March 23 of that year.

The apparition of 1910 produced the first photographs of Halley's Comet, and it also carried Earth through the comet's tail, a circumstance that gave rise to widespread popular unease. Barnard observed that people in the United States and around the world were full of anxiety before the sighting, stuffing their keyholes to prevent comet gases from entering their homes or taking anti-comet pills.

In a historical quirk, the American writer Mark Twain, who was born two weeks after the perihelion of 1835, died the day after the perihelion of 1910.

Giotto flew as part of an international armada — two Soviet, two Japanese and one NASA spacecraft — and went last, helped in its final targeting by information from the Soviet Vega probes. NASA's ISEE-3 spacecraft was renamed the International Cometary Explorer, or ICE, after a series of complex Moon flybys redirected it and it became the first spacecraft to fly past a comet.

Vega 1 returned the first images of the comet on March 4 in 1986 and passed within 5,525 miles of its nucleus two days later, while Vega 2 made its closest approach on March 9 at a distance of 4,990 miles. ICE also contributed to the Halley campaign by measuring the solar wind about 25 million miles (40.2 million kilometers) sunward of the comet on March 28 of 1986 while the fleet closed in on Halley.

Giotto encountered Halley on March 14 in 1986 while travelling at 68 kilometers per second relative to the nucleus, and the first of about 12,000 dust impacts was recorded 122 minutes before closest approach. In total 2112 images of Halley's nucleus were returned by Giotto, the last from a distance of 1372 kilometers about 18 seconds before closest approach.

Giotto passed only 596 kilometers (370 miles) from Halley's nucleus before dust disabled its camera.

Each time Halley returns to the inner solar system, its nucleus sprays ice and rock into space, and this debris stream produces two weak meteor showers every year: the Eta Aquarids in May and the Orionids in October. The dust grains shed by the nucleus eventually become the Eta Aquariids in May and the Orionids in October when they collide with Earth's atmosphere.

The Orionids originate from 1P/Halley and radiate from just north of the bright star Betelgeuse in Orion, yielding roughly 10 to 20 meteors per hour under dark skies. Orionid meteors are known for their brightness and for their speed, travelling at about 41 miles (66 kilometers) per second into Earth's atmosphere. The shower peaks each year in mid-October.

Eta Aquariid meteors are known for their speed, entering the atmosphere at about 40.7 miles (65.4 kilometers) per second, and the shower peaks during early May each year. During the peak of the Eta Aquariids about 50 meteors can be seen per hour, and the Southern Hemisphere is preferable for viewing the shower.