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Halley's Comet

Halley's Comet or Comet Halley  is the best-known of the short-period comets and is visible from Earth every 75–76 years.[2][10] Halley's Comet 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.[11] Other naked-eye comets may be brighter and more spectacular, but will appear only once in thousands of years. Halley's Comet returns to the inner Solar System have been observed and recorded by astronomers since at least 240 BCE. 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.[12]
During its 1986 apparition, Halley's Comet became the first 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.[13][14] These observations supported a number of longstanding hypotheses about comet construction, particularly Fred Whipple's "dirty snowball" model, which correctly predicted that Halley's Comet would be composed of a mixture of volatile ices – such as water, carbon dioxide and ammonia – and dust. The missions also provided data which substantially reformed and reconfigured these ideas; for instance it is now understood that the surface of Halley's Comet is largely composed of dusty, non-volatile materials, and that only a small portion of it is icy.


Comet Halley is commonly pronounced /ˈhæli/, rhyming with valley, or /ˈhli/, rhyming with daily.[15] Spellings of Edmond Halley's name during his lifetime included Hailey, Haley, Hayley, Halley, Hawley, and Hawly, so its contemporary pronunciation is uncertain.[16]

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 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.[17]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),[18] 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.[19]
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).[19] 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 gravitational attraction of the planets, he predicted its return for 1758.[20]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.[21]
This effect was computed prior to its return (with a one-month error to 13 April)[22] by a team of three French mathematicians, Alexis Clairaut, Joseph Lalande, and Nicole-Reine Lepaute.[23] Halley himself did not live to see the comet return, as he died in 1742.[24] 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.[25] The comet was first named in Halley's honour by French astronomer Nicolas Louis de Lacaille in 1759.[25]The possibility has been raised that 1st-century Jewish astronomers had already recognized Halley's Comet as periodic.[26] This theory notes a passage in the Talmud[27] which refers to "a star which appears once in seventy years that makes the captains of the ships err."[28]

Orbit and origin

Halley's orbital period over the last 3 centuries has been between 75–76 years, though it has varied between 74–79 years since 240 BCE.[25][29] Its orbit around the Sun is highly elliptical, with an orbital eccentricity of 0.967 (with 0 being a perfect circle and 1 being a parabolic trajectory). The perihelion, the point in the comet's orbit when it is nearest the Sun, is just 0.6 Astronomical units (AU, approximately the distance of Earth from the Sun).[a] This is between the orbits of Mercury and Venus. Its aphelion, or farthest distance from the Sun, is 35 AU (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.
The orbit is inclined by 18° to the ecliptic, with much of it lying south of the ecliptic, but is retrograde (true inclination is 162°).[30] Due to the retrograde 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).[31] 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.[32] However, observations conducted around the time of Halley's Comet's appearance in 1986 suggest that the Eta Aquarid meteor shower might not originate from Halley's Comet, though it might be perturbed by the comet.[33]
Halley is classified as a periodic or short-period comet; one with an orbit lasting 200 years or less.[34] This contrasts it with long-period comets, whose orbits last for thousands of years. Periodic comets have an average inclination to the ecliptic of only ten degrees, and an orbital period of just 6.5 years, so Halley's orbit is atypical.[25] 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.[34][35] To date, only 54 Halley-type comets have been observed, compared with nearly 400 identified Jupiter-family comets.[36]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.[34]
If Halley was once a long-period comet, it is likely to have originated in the Oort Cloud,[35] a sphere of cometary bodies that has an inner edge of 20,000–50,000 AU. Conversely the Jupiter-family comets are believed to originate in the Kuiper belt,[35] a flat disc of icy debris between 30 AU (Neptune's orbit) and 50 AU from the Sun (in the scattered disc). Another point of origin for the Halley-type comets has been proposed. In 2008, a trans-Neptunian object with a retrograde orbit similar to Halley's was discovered, 2008 KV42, whose orbit takes it from just outside that of Uranus to twice the distance of Pluto. It may be a member of a new population of small Solar System bodies that serves as the source of Halley-type comets.[37]Halley's Comet has probably been in its current orbit for 16,000–200,000 years, although it is not possible to numerically integrate its orbit for more than a few tens of apparitions, and close approaches before 837 CE can only be verified from recorded observations.[38] The non-gravitational effects can be crucial;[38] as Halley 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 four days.[39]
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.[40] 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.[35] Observations by D.W. Hughes suggest that Halley's nucleus has been reduced in mass by 80–90% over the last 2000–3000 revolutions.[14]

Structure and composition

The Giotto and Vega missions gave planetary scientists their first view of Halley's surface and structure. Like all comets, as Halley nears the Sun, its volatile compounds (those with low boiling points, such as water, carbon monoxide, carbon dioxide and other ices) begin to sublime from the surface of its nucleus.[41] This causes the comet to develop a coma, or atmosphere, up to 100,000 km across.[3] 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.[11] As a fraction of the gas molecules in the coma are ionized by the solar ultraviolet radiation,[11] pressure from the solar wind, a stream of charged particles emitted by the Sun, pulls the coma's ions out into a long tail, which may extend more than 100 million kilometers into space.[41][42] Changes in the flow of the solar wind can cause disconnection events, in which the tail completely breaks off from the nucleus.[13]
Despite the vast size of its coma, Halley's nucleus is relatively small: barely 15 kilometers long, 8 kilometers wide and perhaps 8 kilometers thick.[b] Its shape vaguely resembles that of a peanut.[3] Its mass is relatively low (roughly 2.2 × 1014 kg)[4] and its average density is about 0.6 g/cm3, indicating that it is made of a large number of small pieces, held together very loosely, forming a structure known as a rubble pile.[5] 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.[14] Given the irregular shape of the nucleus, Halley's rotation is likely to be complex.[41] Although only 25% of Halley's surface was imaged in detail during the flyby missions, the images revealed an extremely varied topography, with hills, mountains, ridges, depressions, and at least one crater.[14]
Halley is the most active of all the periodic comets, with others, such as Comet Encke and Comet Holmes, displaying activity one or two orders of magnitude weaker.[14] Its day side (the side facing the Sun) is far more active than the night side. Spacecraft observations showed that the gases ejected from the nucleus were 80% water vapor, 17% carbon monoxide and 3–4% carbon dioxide,[43] with traces of hydrocarbons[44] although more recent sources give a value of 10% for carbon monoxide and also include traces of methane and ammonia.[45] 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.[41] The dust particles decreased in size down to the limits of detection (~0.001 µm).[13] 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. Subsequent observations showed Halley's deuterium ratio to be far higher than that in found in Earth's oceans, making such comets unlikely sources for Earth's water.[41]
Giotto provided the first evidence in support of 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 ices on their surfaces to sublimate (change directly from a solid to a gas), and jets of volatile material to burst outward, creating the coma. Giotto showed that this model was broadly correct,[41] though with modifications. Halley's albedo, for instance, is about 4%, meaning that it reflects only 4% of the sunlight hitting it; about what one would expect for coal.[46] 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.[13] Together, these observations suggested that Halley was in fact predominantly composed of non-volatile materials, and thus more closely resembled a "snowy dirtball" than a "dirty snowball".[14][47]
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