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Halley's Comet or Comet Halley (officially designated 1P/Halley) is the most famous of the periodic comets and can currently be seen every 75–76 years. Many comets with long orbital periods may appear brighter and more spectacular, but Halley is the only short-period comet that is clearly visible to the naked eye, and thus, the only naked-eye comet certain to return within a human lifetime. During its returns to the inner solar system, it has been observed by astronomers since at least 240 BC, but it was not recognized as a periodic comet until the eighteenth century when its orbit was computed by English astronomer Edmond Halley, after whom the comet is now named. Halley's Comet last appeared in the inner Solar System in 1986, when it became only the second comet to be visited by spacecraft. It will next appear in mid-2061.

Though Halley is a periodic comet, its orbit suggests that it originated in the Oort cloud, the reservoir for long–period comets with orbits lasting thousands of years, but was dragged into the inner Solar System by the gravity of the planets. Halley's apparitions over the past thousand years have been interpreted as omens and portents for several historical events, including the Battle of Hastingsmarker in 1066 and the 1456 Siege of Belgrade.

Pronunciation

Halley is generally , rhyming with valley, or (especially in the US) "Hailey", but Edmond Halley himself probably pronounced his name "Hawley", as in the word "hall".

Computation of orbit

Halley's Comet was the first to be recognized as periodic. Although the longstanding belief, promoted by Aristotle, that comets were disturbances in the Earth's atmosphere had been disproved in 1577 by Tycho Brahe, who used parallax measurements to show that comets must lie above the Moon, many were still unconvinced that comets actually orbited the Sun, and assumed they must instead follow straight paths through the Solar System.

In 1687, Sir Isaac Newton published his Principia, in which he outlined his laws of gravity and motion. However, his work on comets was decidedly incomplete. Although he had suspected that two comets that had appeared in succession in 1680 and 1681 were in fact the same comet returning from behind the Sun (He was later found to be correct), he was unable to completely reconcile comets into his model. Ultimately, it was Newton's friend, editor and publisher, Edmund 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 the orbits of comets. This calculation enabled him, after examining historical records, to determine that the orbital elements of a second comet which had appeared in 1682 were nearly the same as those of two comets which had appeared in 1531 (observed by Petrus Apianus) and 1607 (observed by Johannes Kepler). Halley thus concluded that all three comets were in fact the same object returning every 76 years, a period that has since been amended to every 75–76 years. After a rough estimate of the perturbations the comet would sustain from the attraction of the planets, he predicted its return for 1758.

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) and did not pass through its perihelion until March 13, 1759, the attraction of Jupiter and Saturn having caused a retardation of 618 days, as was computed by a team of three Frenchmarker mathematicians, Alexis Clairault, Joseph Lalande, and Nicole-Reine Lepaute, previous to its return. Halley did not live to see the comet's return, having died in 1742. 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, and a clear demonstration of its explanatory power.

The possibility has been raised that 1st century Jewish astronomers had already recognized Halley's Comet as periodic. This theory notes a passage in the Talmud which refers to a "a star which appears once in seventy years that makes the captains of the ships err".

Orbit and origin

300 px


Halley's orbital period over the last three centuries has been between 75 and 76 years, though gravitational perturbations by the larger planets have made it vary from 68 and 79 years over the past 40 apparitions. Its orbit is highly elliptical, and focused on the Sun. Its perihelion, its closest distance to the Sun, is just 0.6 AU (between the orbits of Mercury and Venus), while its aphelion, or farthest distance from the Sun, is 35 AU, or roughly the distance of Pluto. Unusually for an object in the Solar System, Halley's orbit is retrograde; it orbits the Sun in the opposite direction to the planets, or clockwise from above the Sun's north pole. Its orbit is inclined by 18° to the ecliptic, with much of it lying south of the ecliptic. Due to Halley's highly eccentric orbit, it has one of the highest velocities relative to the Earth in the Solar System. The 1910 passage was at a relative velocity of 70.56 km/s (157,800 mph).

Halley is classified as a periodic or short–period comet (a descriptor for comets with orbits lasting 200 years or less). However, periodic comets on average have an inclination to the ecliptic of only ten degrees, and an orbital period of just 6.5 years, making Halley's orbit atypical. Halley's orbit suggests that it was originally a long period comet whose orbit was perturbed by the gravity of the giant planets and sent into the inner Solar System. It gives its name to the "Halley group" of comets, which share these orbital characteristics.

If Halley was once a long period comet, it is likely to have originated in the Oort Cloud, a sphere of cometary bodies that has its inner edge near 50,000 AU. This distinguishes it from most other periodic comets, which originate instead from the Kuiper belt, a flat disc of icy debris between 30 AU (Neptune's orbit) and 50 AU from the Sun.

Halley's Comet has probably been in its current orbit for 16,000 to 200,000 years, though it is not possible to numerically integrate its orbit for more than a few tens of apparitions, and close approaches prior to 837 can only be verified with prior observations. The non-gravitational effects can be crucial; as it approaches the Sun, it expels jets of sublimating gas from its surface, which knock it very slightly off its orbital path. These orbital changes can cause deviations in its perihelion of up to four days.

In 1989, Boris Chirikov and Vitaly Vecheslavov

performed an analysis of 46 apparitions of Halley's Comet taken from historical records and computer simulations. These studies showed that the comet's dynamics follow a simple area-preserving map similar to the standard map. Its dynamics were shown to be chaotic and unpredictable on long timescales. Halley's projected lifetime, as determined by differential escape, could be as high as 10 million years. More recent work suggests that Halley will have evaporated or split in two within the next few tens of thousands of years, or have been ejected from the Solar System within a few hundred thousand years.

Structure and composition

The Giotto mission gave planetary scientists their first view of Halley's surface and structure. Like all comets as they approach the Sun, Halley produces a coma which may extend about 100 million kilometres into space, caused by molecular ions being trapped by the Sun's magnetic field. Pressure from the solar wind, a stream of charged particles emitted by the Sun, pulls the coma out into a long tail, which always faces away from the Sun. Despite the vast size of its coma, Halley's nucleus is relatively small; barely 15 kilometres long, 8 kilometres wide and perhaps 8 kilometres thick. (JPL lists its average diameter as only 11 km) Its shape vaguely resembles that of a peanut. Its mass is extremely low; roughly 2.2×1014 kg.Its average density is about 0.6 g/cm³, indicating that it is very loosely constructed. Its albedo is about 4 percent, meaning that only 4 percent of the sunlight hitting it is reflected; about what one would expect for coal. Thus, despite appearing brilliant white to observers on Earth, Halley's Comet is in fact pitch black. Giotto also confirmed 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 their surfaces to sublimate (change directly from a solid to a gas), and jets of volatile material to burst outward, creating the jets and tails. Halley showed that this model was broadly correct. There is some debate as to the nucleus's rotation period. Some claim that it rotates every 52 hours, and others that it rotates every 7.4 days; however, given the irregular shape of the nucleus, its rotation is likely to be complex. Its day side is far more active than its night side.

The gases ejected from the nucleus were 80 percent water vapour, 17 percent carbon monoxide and 3–4 percent carbon dioxide with traces of hydrocarbons. The dust particles were primarily a mixture of carbon-hydrogen-oxygen-nitrogen (CHON) compounds common in the outer Solar System, and silicates, such as are found in terrestrial rocks. The ratio of deuterium to hydrogen in the water released by Halley was initially thought to be similar to that found in Earth's ocean water, suggesting that Halley-type comets may have delivered water to Earth in the distant past; however, subsequent observations showed that Halley's deuterium ratio to be far higher than that in Earth's oceans, making Halley-type comets unlikely sources for Earth's water.

The nucleus is covered with a layer of dust, which retains heat. Each large dust grain is thought to consist of many tiny particles with spaces in between. Some of these spaces are filled with ice, and others are empty. When Halley's Comet is closest to the Sun, temperatures can rise to about 77 °C. Near the Sun, several tons of gas and dust are emitted each second in the jets. Halley has several shallow craters which are about 1 km in diameter.

Meteor showers

Because its orbit comes close to Earth's orbit in two places, Comet Halley is the parent body of two meteor showers: the Eta Aquariids in early May, and the Orionids in late October. The Eta Aquariids show orbital similarities approaching Earth as they do of Mars and so a meteor shower at Mars is anticipated there as well but this time appearing to come from Lambda Gemini.

Apparitions

Halley's calculations enabled the comet's earlier appearances to be found in the historical record. The comet may have been recorded in China as early as 467 BC, but this is uncertain. The first certain observation dates from 240 BC, and subsequent appearances were recorded by Chinese, Babylonianmarker, Persianmarker, and other Mesopotamian texts.The following list gives the dates of Halley's apparitions since its first recorded appearance in 240 BC.

The astronomical designations for those apparitions are also given. The designations consist of Halley's designation ('1P') with the year and half-month of perihelion; for example, "(1P/1982 U1, 1986 III, 1982i" indicates that for the perihelion in 1986, this apparition was the first seen in "half-month" U (the first half of November) in 1982 (giving 1P/1982 U1); it was the third comet past perihelion in 1986 (1986 III); and it was the ninth comet spotted in 1982 (provisional designation 1982i). The perihelion dates of each apparition are shown. The perihelion dates farther from the present are approximate, mainly because of uncertainties in the modeling of non-gravitational effects.

Note that perihelion dates 1607 and later are in the Gregorian calendar, while perihelion dates of 1531 and earlier are in the Julian calendar.

25 May 240 BC (1P/−239 K1, −239)

The first certain appearance of Halley's Comet in the historical record is a description from 240 BC, recorded in the Chinesemarker chronicle Records of the Grand Historian or Shiji, which describes a comet that appeared in the east and moved north.

12 November 164 BC (1P/−163 U1, −163, −162a)

The only surviving record of the 164 BC apparition is found on two fragmentary Babylonianmarker tablets, now owned by the British Museummarker.

6 August 87 BC (1P/−86 Q1, −86)

The apparition of 87 BC was recorded in Babylonian tablets which state that the comet was seen "day beyond day" for a month.

This appearance may be recalled in the representation of Tigranes the Great, an Armenianmarker king who is depicted on coins with a crown that features, according to V.G. Gurzadyan and R. Vardanyan, "a star with a curved tail [that] may represent the passage of Halley's Comet in 87 BC." Gurzadyan and Vardanyan argue that "Tigranes could have seen Halley's Comet when it passed closest to the Sun on Aug. 6 in 87 BC" as the comet would have been a "most recordable event"; for ancient Armenians it could have heralded the New Era of the brilliant King of Kings.

10 October 12 BC (1P/−11 Q1, −11)

The apparition of 12 BC was recorded in the Book of Han by Chinese astronomers of the Han Dynasty who tracked it from August through October. It passed within 0.16AU of the Earth.

Halley's return in 12 BC, only a few years distant from the 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. However, there are other explanations for the phenomenon, such as planetary conjunctions, and there are also records of other comets that appeared closer to the date of Jesus' birth.

25 January 66 AD (1P/66 B1, 66)

If, as has been suggested, the reference in the Talmud to "a star which appears once in seventy years that makes the captains of the ships err" (see above) refers to Halley's Comet, it may be a reference to the 66 AD appearance, because this passage is attributed to the Rabbi Yehoshua ben Hananiah. The 66 AD apparition was the only one to occur during ben Hananiah's lifetime.

141 - 760



Computations of Halley's orbit show that it must have appeared nine times between the years 141 and 760.

  • 1P/141 F1, 141 (22 March 141). This apparition was recorded in Chinese chronicles.
  • 1P/218 H1, 218 (17 May 218)
  • 1P/295 J1, 295 (20 April 295)
  • 1P/374 E1, 374 (16 February 374) Came within 0.09AU of the Earth.
  • 1P/451 L1, 451 (28 June 451)
  • 1P/530 Q1, 530 (27 September 530)
  • 1P/607 H1, 607 (15 March 607) Came within 0.09AU of the Earth.
  • 1P/684 R1, 684 (2 October 684) The apparition was recorded in Europe in one of the sources used by the compiler of the 1493 Nuremberg Chronicles. Chinese records also report it as the "broom star".
  • 1P/760 K1, 760 (20 May 760)


28 February 837 (1P/837 F1, 837)

In 837, Halley's Comet may have passed as close as 0.03 AU (3.2 million miles; 5.1 million kilometres) from Earth, by far its closest approach. Its tail may have stretched 60 degrees across the sky. It was recorded by astronomers in Chinamarker, Japanmarker, Germanymarker and the Islamic world.

18 July 912 (1P/912 J1, 912)

In 912, Halley's Comet is recorded in the Annals of Ulster, which state "A dark and rainy year. A comet appeared."

5 September 989 (1P/989 N1, 989)

The 989 apparition may have been seen by Eilmer of Malmesbury when he was a young boy, as he may have referred to it when writing about the 1066 apparition (see below).

20 March 1066 (1P/1066 G1, 1066)



In 1066, the comet was seen in Englandmarker and thought to be an omen: later that year Harold II of England died at the Battle of Hastingsmarker; it was a bad omen for Harold, but a good omen for the man who defeated him, William the Conqueror. The comet is represented on the Bayeux Tapestry as a fiery star, and the accounts that have been preserved represent it as having appeared to be four times the size of Venus, and to have shone with a light equal to a quarter of that of the Moon. Halley came within 0.10AU of the Earth.

This appearance of the comet is also noted in the Anglo-Saxon Chronicle. Eilmer of Malmesbury may have seen it in 989, as he wrote of the comet in 1066: "You've come, have you?…You've come, you source of tears to many mothers, you evil. I hate you! It is long since I saw you; but as I see you now you are much more terrible, for I see you brandishing the downfall of my country. I hate you!"

The Irish Annals of the Four Masters recorded the comet as "A star [that] appeared on the seventh of the Calends of May, on Tuesday after Little Easter, than whose light the brilliance or light of the moon was not greater; and it was visible to all in this manner till the end of four nights afterwards."

Chaco Native Americans in New Mexicomarker may have recorded the 1066 apparition in their petroglyphs.

1145-1222

The next two apparitions were as follows:

  • 1P/1145 G1, 1145 (18 April 1145) Recorded by the monk Eadwine. The 1986 apparition exhibited a fan tail similar to Eadwine's drawing.
  • 1P/1222 R1, 1222 (28 September 1222) Some claim that Genghis Khan was inspired to turn his conquests toward Europe by this apparition.


25 October 1301 (1P/1301 R1, 1301)



The 1301 apparition may have been seen by the artist Giotto di Bondone, who represented the Star of Bethlehem as a fire-coloured comet in the Nativity section of his the Arena Chapelmarker cycle, completed in 1305.

10 November 1378 (1P/1378 S1, 1378)

No record has survived of the 1378 apparition.

9 June 1456 (1P/1456 K1, 1456)

In 1456, the Ottoman Empire invaded the Balkans, culminating in the Siege of Belgrade in July of that year. The then Pope, Calixtus III, ordered special prayers be taken in a Papal Bull. In 1470, the humanist scholar Platinamarker wrote in his Lives of the Popes that,

A hairy and fiery star having then made its appearance for several days, the mathematicians declared that there would follow grievous pestillence, 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. In the eighteenth century, a Frenchman, in anger at the Church, further embellished the story by claiming that the Pope had "excommunicated" Halley's Comet, though this story was most likely his own invention.

1531-1759

The comet returned four times between 1531 and 1759, the last of which was predicted by Edmond Halley with the help of Sir Isaac Newton, applying Newton's Theory of Revolving Orbits.

  • 1P/1531 P1, 1531 (26 August 1531)
  • 1P/1607 S1, 1607 (27 October 1607)
  • 1P/1682 Q1, 1682 (15 September 1682)
  • 1P/1758 Y1, 1759 I, 1758 (13 March 1759)


16 November 1835 (1P/1835 P1, 1835 III, 1835c)

American satirist and writer Mark Twain was born on November 30, 1835, exactly two weeks after the comet's perihelion. In his biography, he said, "I came in with Halley's Comet in 1835. It's coming again next year [1910], and I expect to go out with it. The Almighty has said no doubt, 'Now here are these two unaccountable freaks; they came in together, they must go out together.' " Twain died on April 21, 1910, the day following the comet's subsequent perihelion. The 1985 fantasy film The Adventures of Mark Twain is inspired by this.

20 April 1910 (1P/1909 R1, 1910 II, 1909c)

The April 1910 approach, which came into view around April 20, was notable for several reasons: it was the first approach of which photographs exist, and the comet made a relatively close approach of 0.15AU, making it a spectacular sight. Indeed, on May 18, the Earth actually passed through the tail of the comet. The media, despite the pleas of astronomers, wove sensational tales of mass cyanide poisoning engulfing the planet. In reality, the gas is so diffuse that the world suffered no ill effects from the passage through the tail.

Halley's 1910 apparition should not be confused with the Great Daylight Comet of 1910, which surpassed Halley in brilliance and was actually visible in broad daylight for a short time about four months before Halley made its appearance.

9 February 1986 (1P/1982 U1, 1986 III, 1982i)

Halley's Comet in 1986


Halley's 1986 apparition was the least favorable on record. The comet and the Earth were on opposite sides of the Sun in February, 1986, creating the worst viewing circumstances for Earth observers for the last 2000 years. Additionally, with increased light pollution from urbanization, many people never saw the comet at all. Further, the comet appeared brightest when it was almost invisible from the northern hemisphere in March and April, prompting many amateur astronomers to travel to the southern hemisphere for a glimpse of the interloper. However, the development of space travel allowed scientists the opportunity to study the comet at close quarters, and several probes were launched to do so. The Sovietmarker Vega 1 started returning images of Halley on 1986 March, and the first ever of its nucleus, and made its flyby on March 6, followed by Vega 2 making its flyby on March 9. On March 14, the Giotto space probe, launched by the European Space Agencymarker, made the closest pass of the comet's nucleus. There were also two Japanesemarker probes, Suisei and Sakigake. The probes were unofficially known as the Halley Armada.

The first person to visually observe the comet on its 1986 return was amateur astronomer Stephen James O'Meara on January 24, 1985. O'Meara used a home-built 24" telescope on top of Mauna Keamarker to detect the magnitude 19.6 comet. As for the naked eye observing, it was Stephen Edberg (then serving as the Coordinator for Amateur Observations at NASAmarker's Jet Propulsion Laboratorymarker) and Charles Morris who were the first to observe Halley's Comet with the naked eye in its 1986 apparition.

Based on data retrieved by Astron, the largest ultraviolet space telescope of the time, during its Halley's Comet observations in December 1985, a group of Sovietmarker scientists developed a model of the comet's coma. The comet was also observed from space by the International Cometary Explorer. Originally International Sun-Earth Explorer 3, the probe was renamed and freed from its Lagrangian point location in Earth's orbit to intercept comets 21P/Giacobini-Zinner and Halley.

Two Space Shuttle missions — the ill-fated STS-51-L (the Challenger disaster) and STS-61-E — were scheduled to observe Comet Halley from low Earth orbit. STS 61-E would have been flown by Challenger in March 1986, carrying the ASTRO-1 platform to study the comet. The mission was canceled, and ASTRO-1 would not fly until late 1990 on STS-35.

Post-1986 observations

On February 12, 1991, at a distance of 14.4 AU from the Sun, Halley underwent an outburst that lasted for several months, releasing a cloud of dust 300,000 km across. Halley was most recently observed in 2003 by three of the Very Large Telescopesmarker at Paranal, Chile, when Halley's magnitude was 28.2. The telescopes observed Halley, at the faintest and furthest any comet has ever been imaged, in order to verify a method for finding very faint Trans-Neptunian objects. Astronomers are now able to observe the comet at any point in its orbit.

Future

The next predicted perihelion of Halley's Comet is July 28, 2061, when it is expected to be better positioned for observation than during the 1985-1986 apparition, as it will be on the same side of the Sun as Earth. It is expected to have an apparent magnitude of -0.3, compared with only +2.1 for the 1986 apparition. On September 9, 2060, Halley will pass within 0.98 AU of Jupiter, and then on August 20, 2061 pass within 0.0543 AU (8.1 million kilometres) of Venus.

In 2134, Halley is expected to pass within 0.09 AU (13.6 million kilometres) of the Earth. Its apparent magnitude is expected to be -2.0.

Notes

External links




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