Comet Halley

Comet Halley, officially designated 1P/Halley, more generally known as Halley's Comet after Edmond Halley, is the best-known and the brightest of the "short-period" comets from the Kuiper belt that visit the inner solar system in years or decades-long orbits rather than the millennial periods of comets from the Oort Cloud.

The most standard pronunciation of "Halley" is [] (IPA), to rhyme with "valley". The once-standard alternate pronunciation [] (to rhyme with "Bailey") led to rock and roll singer Bill Haley naming his band Bill Haley & His Comets.

Composition

The Giotto space probe found the comet's surface to be rich in carbon. Of the volume of material ejected by Halley: 80% was water, 10% carbon monoxide, and 2.5% a mix of methane and ammonia. Other hydrocarbons, iron, and sodium were detected in trace amounts.

Cyanogen gas is present in trace amounts.

Material from the comet is responsible for two meteor showers each year, the Eta Aquarids in May and the Orionids in October.

Early history

Having perceived that the elements of the comet of 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 in Prague), Halley concluded that all three comets were in fact the same object returning every 76 years. After a rough estimate of the perturbations the comet would sustain from the attraction of the planets, he predicted its return for 1757. 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 1759; the attraction of Jupiter and Saturn having caused a retardation of 618 days, as was computed by a team of three French mathematicians, Alexis Clairault, Joseph Lalande, and Nicole-Reine Lepaute, previously to its return. Halley did not live to see the comet's return, having died in 1742.

Halley's calculations enabled the comet's earlier appearances to be found in the historical record:

• When the comet was observed in 1456, it passed very near to the Earth; its tail extended over 60° of the heavens and took the form of a sabre.
• In 1066, the comet was thought to be an omen: later that year Harold II of England died at the Battle of Hastings. It is shown on the Bayeux Tapestry, and the accounts which have been preserved represent it as having then appeared to be four times the size of Venus, and to have shone with a light equal to a fourth of that of the Moon.
• It is calculated that Comet Halley may have passed as close as 0.03 AU (3.2 million miles) from Earth in the year 837.
• Some have suggested that the comet's appearance in 12 BC might explain the Biblical story of the Star of Bethlehem. The artist Giotto would have observed the comet in 1301 and his depiction of the Star of Bethlehem in the Nativity in the Arena Chapel cycle completed in 1305 is a candidate for an early depiction.
• Historical records show that Chinese astronomers observed the comet's appearance in 240 BC and possibly as early as 2467 BC. Observations of appearances after 240 BC are recorded by Chinese, Japanese, Babylonian and Islamic astronomers.

Recent history

The comet returned in 1835, 1910 and 1986.

The 1910 approach was notable for several reasons: as well as being the first approach of which photographs exist, it was relatively close, making the comet a spectacular sight. Indeed, on 19 May, the comet transited the Sun's disk, and the Earth actually passed through its tail. This proved worrisome in some quarters, as the comet's tail was known to contain poisonous cyanogen gas. However, the gas is so dilute that there were no ill-effects of the passage through the tail.

The 1986 approach was less favorable for Earth observers: the comet did not achieve the spectacular brightness of some previous approaches, and with increased light pollution from urbanization, many people never saw the comet at all. However, the development of space travel allowed scientists the opportunity to study a comet at close quarters, and several probes were launched to do so. Most spectacularly, the Giotto space probe, launched by the European Space Agency, made a close pass of the comet's nucleus. Other probes included the Soviet Union's Vega 1 and Vega 2, and two Japanese probes, Suisei and Sakigake.

Halley will next return in 2061.

Dates of perihelia

Comet Halley returned to perihelion on the following dates. It is usually visible to the unaided eye for a few months around perihelion.

Trivia

• Two of the comet's visits - 1835 and 1910 - are the same years as the birth and death of the American novelist Mark Twain. He wrote in 1909, "I came in with Halley's Comet in 1835. It is coming again next year, and I expect to go out with it."
• According to one story, first appearing in a posthumous biography in 1475 and later embellished and popularized by Pierre-Simon Laplace, Pope Callixtus III excommunicated the 1456 apparition of Halley's Comet, believing it to be an ill omen for the Christian defenders of Belgrade, who were at that time being besieged by the armies of the Ottoman Empire. However, no known primary source supports the authenticity of this account.
• Having first seen it as a young boy in 989, Eilmer of Malmesbury declared prophetically in 1066: "You've come, have you?...You've come, you source of tears to many mothers. 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" (William of Malmesbury, Deeds of the English Kings, Ch. 225, ISBN 0-19-820678-X).
• In the Talmud, it is mentioned that "There is a star which appears once in seventy years that makes the captains of the ships err" (Horioth, chap. III (http://www.sacred-texts.com/jud/t09/hor03.htm)). It probably refers to the 66 CE perihelion.

Halley's Comet in fiction

• Arthur C. Clarke's novel 2061: Odyssey Three includes a detailed description of a manned mission to Halley's Comet.
• In Heart of the Comet, a novel by Gregory Benford and David Brin (1987), a multinational team colonizes Halley's Comet, building a habitat within the ice.
• In an episode of Futurama, entitled "A Taste Of Freedom", it is mentioned that Earth once fought a war "to take back Halley's Comet". Comet Halley has also been mined for water ice in the series.
• In the computer game Shadow of the Comet, the passing of the Comet, combined with a special vantage point, is the only time(presumably) certain entities can be summoned.

External links

• cometography.com (http://cometography.com/pcomets/001p.html)
• seds.org (http://www.seds.org/~spider/spider/Comets/halley.html)
• Bayeux Tapestry (http://www.getty.edu/artsednet/resources/Space/Stories/halleys.html)
• Vega 2 Images of Comet Halley (http://pages.preferred.com/%7Etedstryk/vega2.html)ca:Cometa Halley

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