Comet

A comet is a small body in the solar system that orbits the sun and (at least occasionally)

History of comet study

Early observations and thought

Historically, comets were thought to be unlucky, or even interpreted as attacks by heavenly beings against terrestrial inhabitants. Some authorities interpret references to "falling stars" in Gilgamesh, Revelation and the Book of Enoch as references to comets, or possibly bolides.

Related Topics:
Gilgamesh - Revelation - Enoch - Bolide

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In the first book of his Meteorology, Aristotle propounded the view of comets that would hold sway in Western thought for nearly two thousand years. He rejected the ideas of several earlier philosophers that comets were planets, or at least a phenomenon related to the planets, on the grounds that while the planets confined their motion to the circle of the Zodiac, comets could appear in any part of the sky. {{hnote|Aristotle, l. 1. c. 6.}} Instead, he described comets as a phenomenon of the upper atmosphere, where hot, dry exhalations gathered and occasionally burst into flame. Aristotle held this mechanism responsible for not only comets, but also meteors, the aurora borealis, and even the Milky Way.{{hnote|Aristotle, l. 1. c. 7.}}

Related Topics:
Meteorology - Aristotle - Planet - Zodiac - Atmosphere - Meteor - Aurora borealis - Milky Way

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A few later classical philosophers did dispute this view of comets. Seneca the Younger, in his Natural Questions, observed that comets moved regularly through the sky and were undisturbed by the wind, behavior more typical of celestial than atmospheric phenomena. While he conceded that the other planets do not appear outside the Zodiac, he saw no reason that a planet-like object could not move through any part of the sky, humanity's knowledge of celestial things being very limited.{{hnote|Sagan, pp. 23–24}} However, the Aristotelean viewpoint proved more influential, and it was not until the 16th century that it was demonstrated that comets must exist outside the earth's atmosphere.

Related Topics:
Seneca the Younger - Natural Questions - Wind - 16th century

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In 1577, a bright comet was visible for several months. The Danish astronomer Tycho Brahe used measurements of the comet's position taken by himself and other, geographically separated observers to determine that the comet had no measureable parallax. Within the precision of the measurements, this implied the comet must be at least four times more distant from the earth than the moon.{{hnote|ESO, Part I}}

Related Topics:
1577 - Danish - Tycho Brahe - Parallax

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Orbital studies

Although comets had now been demonstrated to be in the heavens, the question of how they moved through the heavens would be debated for most of the next century. Even after Johannes Kepler had determined in 1609 that the planets moved about the sun in elliptical orbits, he was reluctant to believe that the laws that governed the motions of the planets should also influence the motion of other bodies—he believed that comets travel among the planets along straight lines. Galileo Galilei, although a staunch Copernicanist, rejected Tycho's parallax measurements and held to the Aristotelean notion of comets moving on straight lines through the upper atmosphere.{{hnote|Prasar, Part II}}

Related Topics:
Johannes Kepler - 1609 - Elliptical - Laws that governed the motions of the planets - Galileo Galilei - Copernicanist

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The first suggestion that Kepler's laws of planetary notion should also apply to the comets was made by William Lower in 1610.{{hnote|ESO, Part I}} In the following decades, other astronomers, including Pierre Petit, Giovanni Borelli, Adrien Auzout, Robert Hooke, and Jean-Dominique Cassini, all argued for comets curving about the sun on elliptical or parabolic paths, while others, such as Christian Huygens and Johannes Hevelius, supported comets' linear motion.{{hnote|Prasar, Part II}}

Related Topics:
William Lower - 1610 - Pierre Petit - Giovanni Borelli - Adrien Auzout - Robert Hooke - Jean-Dominique Cassini - Christian Huygens - Johannes Hevelius

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The matter was resolved by the bright comet that was discovered by Gottfried Kirch on November 14, 1680. Astronomers throughout Europe tracked its position for several months. In his Principia Mathematica of 1687, Isaac Newton proved that an object moving under the influence of his inverse square law of universal gravitation must trace out an orbit shaped like one of the conic sections, and he demonstrated how to fit a comet's path through the sky to a parabolic orbit, using the comet of 1680 as an example.{{hnote|Newton, Lib. 3, Prop. 41.}}

Related Topics:
Bright comet - Gottfried Kirch - November 14 - 1680 - Principia Mathematica - 1687 - Isaac Newton - Inverse square law - Universal gravitation - Conic section - Parabolic

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In 1705, Edmond Halley applied Newton's method to twenty-four cometary apparitions that had occurred between 1337 and 1698. He noted that three of these, the comets of 1531, 1607, and 1682, had very similar orbital elements, and he was further able to account for the slight differences in their orbits in terms of gravitational perturbation by Jupiter and Saturn. Confident that these three apparitions had been three appearances of the same comet, he predicted that it would appear again in 1758-9. {{hnote|Halley (1705)}} (Earlier, Robert Hooke had identified the comet of 1664 with that of 1618, {{hnote|Pepys, 1 March 1664/5}} while Jean-Dominique Cassini had suspected the identity of the comets of 1577, 1665, and 1680. {{hnote|Sagan, pp. 42–43}} Both were incorrect.) Halley's predicted return date was later refined by a team of three French mathematicians: Alexis Clairaut, Joseph Lalande, and Nicole-Reine Lepaute, who predicted the date of the comet's 1759 perihelion to within one month's accuracy. {{hnote|Sagan, p. 83}} When the comet returned as predicted, it became known as Comet Halley or Halley's Comet (its official designation is 1P/Halley). Its next appearance is due in 2061.

Related Topics:
1705 - Edmond Halley - Orbital element - Jupiter - Saturn - French - Alexis Clairaut - Joseph Lalande - Nicole-Reine Lepaute - Comet Halley - 2061

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Among the comets with short enough periods to have been observed several times in the historical record, Comet Halley is unique in consistently being bright enough to be visible to the naked eye. Since the confirmation of Comet Halley's periodicity, many other periodic comets have been discovered through the telescope. The second comet to be discovered to have a periodic orbit was Comet Encke (official designation 2P/Encke). Over the period 1819-1821 the German mathematician and physicist Johann Franz Encke computed orbits for a series of cometary apparitions observed in 1786, 1795, 1805, and 1818, concluded they were same comet, and successfully predicted its return in 1822.{{hnote|Kronk, '2P/Encke'}} By 1900, seventeen comets had been observed at more than one perihelion passage and recognized as periodic comets. As of January 2005, 164 comets have achieved this distinction, though several have since been destroyed or lost.

Related Topics:
Telescope - Comet Encke - 1819 - 1821 - German - Johann Franz Encke - 1822

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Studies of physical characteristics

:Hast thou ne'er seen the Comet's flaming flight?

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Isaac Newton described comets as compact, solid, fixed, and durable bodies: in one word, a kind of planets, which move in very oblique orbits, every way, with the greatest freedom, persevering in their motions even against the course and direction of the planets; and their tail as a very thin, slender vapour, emitted by the head, or nucleus of the comet, ignited or heated by the sun. Comets also seemed to Newton absolutely requisite for the conservation of the water and moisture of the planets; from their condensed vapours and exhalations all that moisture which is spent on vegetations and putrefactions, and turned into dry earth, might be resupplied and recruited; for all vegetables were thought to increase wholly from fluids, and turn by putrefaction into earth. Hence the quantity of dry earth must continually increase, and the moisture of the globe decrease, and at last be quite evaporated, if it have not a continual supply. Newton suspected that the spirit which makes the finest, subtilest, and best part of our air, and which is absolutely requisite for the life and being of all things, came principally from the comets.

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Another use which he conjectured comets might be designed to serve, is that of recruiting the sun with fresh fuel, and repairing the consumption of his light by the streams continually sent forth in every direction from that luminary —

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:"From his huge vapouring train perhaps to shake

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:Reviving moisture on the numerous orbs,

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:Thro' which his long ellipsis winds; perhaps

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:To lend new fuel to declining suns,

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:To light up worlds, and feed th' ethereal fire."

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As early as the 18th century, some scientists had made correct hypotheses as to comets' physical composition. In 1755, Immanuel Kant hypothesized that comets are composed of some volatile substance, whose vaporization gives rise to their brilliant displays near perihelion.{{hnote|Sagan, p. 77}} In 1836, the German mathematician Friedrich Wilhelm Bessel, after observing streams of vapor in the 1835 apparition of Comet Halley, proposed that the jet forces of evaporating material could be great enough to significantly alter a comet's orbit and argued that the non-gravitational movements of Comet Encke resulted from this mechanism.{{hnote|Sagan, p. 117}}

Related Topics:
1755 - Immanuel Kant - Friedrich Wilhelm Bessel - Jet force - Comet Encke

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However, another comet-related discovery overshadowed these ideas for nearly a century. Over the period 1864–1866 the Italian astronomer Giovanni Schiaparelli computed the orbit of the Perseid meteors, and based on orbital similarities, correctly hypothesized that the Perseids were fragments of Comet Swift-Tuttle. The link between comets and meteor showers was dramatically underscored when in 1872, a major meteor shower occurred from the orbit of Comet Biela, which had been observed to split into two pieces during its 1846 apparition, and never seen again after 1852.{{hnote|Kronk, '3D/Biela'}} A "gravel bank" model of comet structure arose, according to which comets consist of loose piles of small rocky objects, coated with an icy layer.

Related Topics:
1864 - 1866 - Italian - Giovanni Schiaparelli - Perseid - Meteor - Comet Swift-Tuttle - 1872 - Comet Biela - 1846 - 1852

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By the middle of the twentieth century, this model suffered from a number of shortcomings: in particular, it failed to explain how a body that contained only a little ice could continue to put on a brilliant display of evaporating vapor after several perihelion passages. In 1950, Fred Lawrence Whipple proposed that rather than being rocky objects containing some ice, comets were icy objects containing some dust and rock.{{hnote|Whipple (1950)}} This "dirty snowball" model soon became accepted. It was confirmed when an armada of spacecraft (including the European Space Agency's Giotto probe and the Soviet Union's Vega 1 and Vega 2) flew through the coma of Halley's comet in 1986 to photograph the nucleus and observed the jets of evaporating material. The American probe Deep Space 1 flew past the nucleus of Comet Borrelly on September 21 2001 and confirmed that the characteristics of Comet Halley are common on other comets as well.

Related Topics:
1950 - Fred Lawrence Whipple - Spacecraft - European Space Agency - Giotto - Soviet Union - Vega 1 - Vega 2 - 1986 - Deep Space 1 - Comet Borrelly - September 21 - 2001

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The Stardust spacecraft, launched in February 1999, has already collected particles from the coma of Comet Wild 2 in January 2004, and will return the samples to Earth in a capsule in 2006. Claudia Alexander, a program scientist for Rosetta from NASA's Jet Propulsion Laboratory who has has modeled comets for years, reported to space.com about her astonishment at the number of jets, their appearance on the dark side of the comet as well as the light side, their ability to lift large chunks of rock from the surface of the comet and the fact that comet Wild 2 is not a loosely-cemented rubble pile.http://www.space.com/scienceastronomy/stardust_results_040617.html

Related Topics:
''Stardust'' - 1999 - Comet Wild 2 - 2004 - 2006

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Forthcoming space missions will add greater detail to our understanding of what comets are made of. In July 2005, the Deep Impact probe blasted a crater on Comet Tempel 1 to study its interior. And in 2014, the European Rosetta probe will orbit comet Comet Churyumov-Gerasimenko and place a small lander on its surface.

Related Topics:
2005 - ''Deep Impact'' - Comet Tempel 1 - 2014 - ''Rosetta'' - Comet Churyumov-Gerasimenko

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Debate over comet composition

As late as 2002, there is conflict on how much ice is in a comet. NASA's Deep Space 1 team, working at NASA's Jet Propulsion Lab, obtained high-resolution images of the surface of comet Borrelly. They announced that comet Borrelly exhibits distinct jets, yet has a hot, dry surface. The assumption that comets contain water and other ices led Dr. Laurence Soderblom of the U.S. Geological Survey to say, "The spectrum suggests that the surface is hot and dry. It is surprising that we saw no traces of water ice." However, he goes on to suggest that the ice is proabably hidden below the crust as "either the surface has been dried out by solar heating and maturation or perhaps the very dark soot-like material that covers Borrelly's surface masks any trace of surface ice".http://www.jpl.nasa.gov/releases/2002/release_2002_80.html

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The recent Deep Impact probe has also yielded preliminary results suggesting there is less ice in comets then originally predicted.

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