Halley's Comet

{{Infobox Planet | name=1P/Halley (Halley's Comet) | image= | discoverer=prehistoric (observation);Edmond Halley (recognition of periodicity) | discovery_date=1758 (first 'predicted perihelion) | orbit_ref =) | mass= 2.2 kg |sidereal_day = 2.2 d (52.8 h) (?) | magnitude = 28.2 (in 2003) Halley is the only short-period comet that is clearly visible to the naked eye from Earth, and thus the only naked-eye comet that might appear twice in a human lifetime. Clear records of the comet's appearances were made by Chinese, Babylonian, and medieval European chroniclers, but were not recognized as reappearances of the same object at the time. The comet's periodicity was first determined in 1705 by English astronomer Edmond Halley, after whom it is now named. Halley's Comet last appeared in the inner Solar System in 1986 and will next appear in mid-2061.

During its 1986 apparition, Halley became the first comet to be observed in detail by spacecraft, providing the first observational data on the structure of a comet nucleus and the mechanism of coma and tail formation. Spellings of Edmond Halley's name during his lifetime included Hailey, Haley, Halley, Hawley, and Hawly, so its correct pronunciation is uncertain.

Computation of orbit

Halley was the first comet to be recognized as periodic. Until the Renaissance, the philosophical consensus on the nature of comets, promoted by Aristotle, was that they were disturbances in the Earth's atmosphere. This idea was disproved in 1577 by Tycho Brahe, who used parallax measurements to show that comets must lie beyond 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. His work on comets was decidedly incomplete. Although he had suspected that two comets that had appeared in succession in 1680 and 1681 were the same comet before and after passing behind the Sun (he was later found to be correct; see Newton's Comet), he was unable to completely reconcile comets into his model. Ultimately, 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. 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'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. It did not pass through its perihelion until 13 March 1759, the attraction of Jupiter and Saturn having caused a retardation of 618 days. This effect was computed prior to 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. Halley himself did not live to see the comet's return as he 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. This theory notes a passage in the Talmud which refers to "a star which appears once in seventy years that makes the captains of the ships err".

Orbit and origin

(red)|alt=The orbital paths of Halley, outlined in blue, against the orbits of Jupiter, Saturn, Uranus and Neptune, outlined in red]]

Halley's orbital period over the last three centuries has been between 75 and 76 years, though it has varied between 74 and 79 years since 240 BC. Due to Halley's highly eccentric orbit, it has one of the highest velocities, relative to the Earth, of any object in the Solar System. The 1910 passage was at a relative velocity of 70.56 km/s (157,838 mph or 254,016 km/h). Because its orbit comes close to Earth's in two places, Halley's Comet is the parent body of 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 (those with orbital periods shorter than 20 years and inclinations of 20–30 degrees or less) are called Jupiter family comets. Those like 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. To date, only 54 Halley-type comets have been observed, compared with nearly 400 identified Jupiter family comets.

The orbits of the Halley-type comets suggest that they were originally long-period comets whose orbits were perturbed by the gravity of the giant planets and directed into the inner Solar System.

meteor striking the sky below the Milky Way and to the right of Venus. Zodiacal light is also seen at the image.|alt=A meteor strikes the bottom left, while the Milky Way arcs overhead and a dawn-like light lines the lower horizon. The image was taken through a curved lens.]] Halley's Comet has probably been in its current orbit for 16,000 to 200,000 years, although it is not possible to numerically integrate its orbit for more than a few tens of apparitions, and close approaches before 837 AD can only be verified from recorded observations. The non-gravitational effects can be crucial;

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 its dynamics were chaotic and unpredictable on long timescales. Halley's projected lifetime could be as long as 10 million years. More recent work suggests that Halley will evaporate, or split in two, within the next few tens of thousands of years, or will be ejected from the Solar System within a few hundred thousand years. Evaporation of this dirty ice releases dust particles, which travel with the gas away from the nucleus. Gas molecules in the coma absorb solar light and then re-radiate it at different wavelengths, a phenomenon known as fluorescence, whereas dust particles scatter the solar light. Both processes are responsible for making the coma visible. As a fraction of the gas molecules in the coma are ionized by the solar ultraviolet radiation, Changes in the flow of the solar wind can cause disconnection events, in which the tail completely breaks off from the nucleus. and its average density is about 0.6 g/cm³, indicating that it is made of large number of small pieces, held together very loosely, forming a structure known as a rubble pile. Ground-based observations of coma brightness suggested that Halley's rotation period was about 7.4 days. Images taken by the various spacecraft, along with observations of the jets and shell, suggested a period of 52 hours. Given the irregular shape of the nucleus, Halley's rotation is likely to be complex. Although only 25% of Halley's surface was imaged in detail during the flyby missions, they revealed an extremely varied topography, with hills, mountains, ridges, depressions, and at least one crater. with traces of hydrocarbons although more recent sources give a value of 10% for carbon monoxide and also include traces of methane and ammonia. The dust particles were found to be 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. Thus, despite appearing brilliant white to observers on Earth, Halley's Comet is in fact pitch black. The surface temperature of evaporating "dirty ice" ranges from 170 K (−103 °C) at higher albedo to 220 K (−53 °C) at low albedo; Vega 1 found Halley's surface temperature to be in the range 300–400 K (30–130 °C). This suggested only 10% of Halley's surface was active, and that large portions of it were coated in a layer of dark dust, which retained heat.

Apparitions

Halley's calculations enabled the comet's earlier appearances to be found in the historical record. The following table sets out the astronomical designations for every apparition of Halley's Comet from 240 BC, the earliest widespread sighting. For example, "(1P/1982 U1, 1986 III, 1982i" indicates that for the perihelion in 1986, Halley's Comet was the first period comet known (designated 1P) and 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. Perihelion dates 1607 and later are in the Gregorian calendar, while perihelion dates of 1531 and earlier are in the Julian calendar.

1P/−239 K1, −239 (25 May 240 BC)

1P/−163 U1, −163 (12 November 164 BC)

1P/−86 Q1, −86 (6 August 87 BC)

1P/−11 Q1, −11 (10 October 12 BC)

1P/66 B1, 66 (25 January 66 AD)

1P/141 F1, 141 (22 March 141)

1P/218 H1, 218 (17 May 218)

1P/295 J1, 295 (20 April 295)

1P/374 E1, 374 (16 February 374)

1P/451 L1, 451 (28 June 451)

1P/530 Q1, 530 (27 September 530)

1P/607 H1, 607 (15 March 607)

1P/684 R1, 684 (2 October 684)

1P/760 K1, 760 (20 May 760)

1P/837 F1, 837 (28 February 837)

1P/912 J1, 912 (18 July 912)

1P/989 N1, 989 (5 September 989)

1P/1066 G1, 1066 (20 March 1066)

1P/1145 G1, 1145 (18 April 1145)

1P/1222 R1, 1222 (28 September 1222)

1P/1301 R1, 1301 (25 October 1301)

1P/1378 S1, 1378 (10 November 1378)

1P/1456 K1, 1456 (9 June 1456)

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 (13 March 1759)

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

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

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

Next perihelion predicted 28 July 2061

Prior to 1066

on a clay tablet between 22–28 September 164 BC, Babylon, Iraq. British Museum.]] Halley may have been recorded as early as 467 BC, but this is uncertain. A comet was recorded in ancient Greece between 466 and 468 BC; its timing, location, duration, and associated meteor shower all suggest it was Halley. A large meteor the size of a "wagonload" that landed in northern Greece during the comet's apparition was a local attraction for 500 years. Chinese chroniclers also mention a comet in that year.

The first certain appearance of Halley's Comet in the historical record is a description from 240 BC, in the Chinese chronicle Records of the Grand Historian or Shiji, which describes a comet that appeared in the east and moved north. The only surviving record of the 164 BC apparition is found on two fragmentary Babylonian tablets, now owned by the British Museum. 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 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 August 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.

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.

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. This apparition was the only one to occur during ben Hananiah's lifetime.

The 141 AD apparition was recorded in Chinese chronicles. The 374 AD and 607 approaches each came within 0.09 AU of the Earth. Its tail may have stretched 60 degrees across the sky. It was recorded by astronomers in China, Japan, Germany and the greater Middle East.

1066

.|alt=White embroidery showing several people on the left pointing to Halley, top right, over a tower]] In 1066, the comet was seen in England and thought to be an omen: later that year Harold II of England died at the Battle of Hastings; 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 surviving accounts describe it as appearing to be four times the size of Venus and shining with a light equal to a quarter of that of the Moon. Halley came within 0.10 AU of the Earth at that time.

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."

1145–1835

by Giotto, purportedly depicting Halley.|alt=The wise men and several animals cluster around the baby Jesus, while a comet-like object streaks overhead]] The 1145 apparition was recorded by the monk Eadwine. The 1986 apparition exhibited a fan tail similar to Eadwine's drawing. The 1301 apparition may have been seen by the artist Giotto di Bondone, who represented the Star of Bethlehem as a fire-colored comet in the Nativity section of his Arena Chapel cycle, completed in 1305. No record survives of the 1376 apparition.

In 1456, the Ottoman Empire invaded the Balkans, culminating in the Siege of Belgrade in July of that year. In a Papal Bull, Pope Calixtus III ordered special prayers be said for the city's protection. In 1470 the humanist scholar Bartolomeo Platina 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 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. In the 18th century, a Frenchman further embellished the story, in anger at the Church, by claiming that the Pope had "excommunicated" Halley's Comet, though this story was most likely his own invention.

After witnessing a bright light in the sky (which most historians have identified as Halley's Comet, visible in Ethiopia in 1456), Emperor Zara Yaqob, ruler from 1434–1468, founded the city of Debre Berhan (tr. City of Light) and made it his capital for the remainder of his reign.

Halley's periodic returns have been subject to scientific investigation since the 16th century. The three apparitions from 1531 to 1682 were noted by Edmond Halley, enabling him to predict its 1759 return. Streams of vapour observed during the comet's 1835 apparition prompted astronomer Friedrich Wilhelm Bessel to propose that the jet forces of evaporating material could be great enough to significantly alter a comet's orbit.

1910

The 1910 approach, which came into view around 20 April, was notable for several reasons: it was the first approach of which photographs exist, and the first for which spectroscopic data were obtained. Furthermore, the comet made a relatively close approach of 0.15AU, One of the substances discovered in the tail by spectroscopic analysis was the toxic gas cyanogen, which led astronomer Camille Flammarion to claim that, when Earth passed through the tail, the gas "would impregnate the atmosphere and possibly snuff out all life on the planet." In reality, as other astronomers were quick to point out, the gas is so diffuse that the world suffered no ill effects from the passage through the tail.

American satirist and writer Mark Twain was born on 30 November 1835, exactly two weeks after the comet's perihelion. In his autobiography, published in 1909, he said,

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. 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 21 April 1910, the day following the comet's subsequent perihelion. The 1985 fantasy film The Adventures of Mark Twain was inspired by the quotation.

Halley's 1910 apparition is distinct from the Great Daylight Comet of 1910, which surpassed Halley in brilliance and was actually visible in broad daylight for a short period, approximately four months before Halley made its appearance.

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 2,000 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. Halley's approach was first detected by astronomers David Jewitt and G. Edward Danielson on 16 October 1982 using the 5.1 m Hale telescope at Mount Palomar and a CCD camera. The first person to visually observe the comet on its 1986 return was amateur astronomer Stephen James O'Meara on 24 January 1985. O'Meara used a home-built 24-inch telescope on top of Mauna Kea to detect the magnitude 19.6 comet. On 8 November 1985, Stephen Edberg (then serving as the Coordinator for Amateur Observations at NASA's Jet Propulsion Laboratory) and Charles Morris were the first to observe Halley's Comet with the naked eye in its 1986 apparition.

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 Soviet Vega 1 started returning images of Halley on 4 March 1986, and the first ever of its nucleus,

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 Soviet 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 (ended by the Challenger disaster) and STS-61-E — were scheduled to observe Halley's Comet from low Earth orbit. STS 61-E was a Challenger mission scheduled for March 1986, carrying the ASTRO-1 platform to study the comet. Due to the suspension of America's manned space program after the Challenger explosion, the mission was canceled, and ASTRO-1 would not fly until late 1990 on STS-35.

After 1986

On 12 February 1991, at a distance of from the Sun, Halley underwent an outburst that lasted for several months, releasing a cloud of dust 300,000 km across. Astronomers are now able to observe the comet at any point in its orbit. 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 9 September 2060, Halley has been calculated to pass within of Jupiter, and then on 20 August 2061, pass within of Venus. In 2134, Halley is expected to pass within of the Earth. Its apparent magnitude is expected to be −2.0.

References

Bibliography

External links

Halley's nucleus by Giotto spacecraft (ESA link)

Image of Halley in 1986 by Giotto spacecraft (NASA link)

cometography.com

seds.org

Orbital simulation from JPL (Java) / Ephemeris

Donald Yeomans, "Great Comets in History"

A brief history of Halley's Comet (Ian Ridpath)

Halley's Comet,12 BC

Category:Comets Category:Comets visited by spacecraft Category:Halley's Comet Category:Mark Twain