wikipedia comets

7/30/2019 Wikipedia Comets

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

occasionally) exhibits a coma(or atmosphere) and/or a tail both due primarily to

the effects of solar radiation upon the comet'snucleus, which itself is a minor planet

composed of rock, dust, and ices. Due to their origins in the outer solar system and

their propensity to be highly affected (orperturbed) by relatively close approaches to

the major planets, comets' orbits are constantly changing. Some are moved intosungrazing orbits that destroy the comets when they near the Sun, while others are

thrown out of the solar system forever.

Most comets are believed to originate in a cloud (the Oort cloud) at large distances

from the Sun consisting of debris left over from the condensationof the solar nebula;

the outer edges of such nebulae arecoolenough thatwaterexists in a solid (rather

than gaseous) state. Asteroids originate via a different process, but very old comets

which have lost all theirvolatilematerials may come to resemble asteroids.

The word cometcame to the English languagethroughLatincometes. From the Greek

word kom, meaning "hair of the head," Aristotle first used the derivation komts todepict comets as "stars with hair."

Contents

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1 Physical characteristics

2 Orbital characteristics

3 Comet nomenclature

4 History of comet studyo 4.1 Early observations and thought

o 4.2 Orbital studies

o 4.3 Studies of physical characteristics

o 4.4 Debate over comet composition

5 Great comets

6 Peculiar comets

7 Comets in fiction

8 See also

9 References

10 External links

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Physical characteristics

Long-period comets are believed to originate in a distant cloud known as the Oort

cloud (after the astronomerJan Hendrik Oort who hypothesised its existence).[1] They

are sometimes perturbed from their distant orbits by gravitational interactions, falling

into extremely elliptical orbits that can bring them very close to the Sun. One theory

says that as a comet approaches the inner solar system,solar radiationcauses part of

its outer layers, composed of ice and other materials, to melt and evaporate, but thishas not been proven. The streams ofdust and gas this releases form a very large,
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extremely tenuous atmosphere around the comet called the coma, and the force

exerted on the coma by the Sun's radiation pressure and solar wind cause an enormous

tailto form, which points away from the sun. The streams of dust and gas each form

their own distinct tail, each pointed in slightly different directions. The tail made of

dust is left behind in the comet's orbit in such a manner that it often forms a curved

tail. At the same time, the ion tail, made of gases, always pointing directly away fromthe Sun, as this gas is more strongly affected by the solar wind than dust is, following

magnetic field lines rather than an orbital trajectory. While the solid body of comets

(called the nucleus) is generally less than 50km across, the coma may be larger than

the Sun, and the ion tails have been observed to extend 150 million km (1

Astronomical unit) or more.

Both the coma and tail are illuminated by the Sun, and may become visible from the

Earth when a comet passes through the inner solar system, the dust reflecting sunlight

directly and the gases glowing due toionization. Most comets are too faint to be

visible without the aid of a telescope, but a few each decade become bright enough to

be visible with the naked eye. Before the invention of the telescope, comets seemed toappear out of nowhere in the sky and gradually vanish out of sight. They were usually

considered bad omens of deaths of kings or noble men, or coming catastrophes. From

ancient sources, such as Chinese oracle bones, it is known that their appearance have

been noticed by humans for millennia. One very famous old recording of a comet is

the appearance of Halley's Comet on the Bayeux Tapestry, which records theNorman

conquest ofEngland in 1066.[2]

Surprisingly, cometary nuclei are among the darkest objects known to exist in the

solar system. The Giotto probe found that Comet Halley's nucleus reflects

approximately 4% of the light that falls on it, andDeep Space 1 discovered that

Comet Borrelly's surface reflects only 2.4% to 3% of the light that falls on it; by

comparison, asphalt reflects 7% of the light that falls on it. It is thought that complex

organic compoundsare the dark surface material. Solar heating drives off volatile

compounds leaving behind heavy long-chain organics that tend to be very dark, like

taror crude oil. The very darkness of cometary surfaces allows them to absorb the

heat necessary to drive their outgassing.

In 1996, comets were found to emit X-rays.[3] These X-rays surprised researchers,

because their emission by comets had not previously been predicted. The X-rays are

thought to be generated by the interaction between comets and the solar wind: when

highly charged ions fly through a cometary atmosphere, they collide with cometaryatoms and molecules. In these collisions, the ions will capture one or more electrons

leading to emission of X-rays and far ultraviolet photons. [4]

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Orbital characteristics
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Orbits ofComet Kohoutekand Earth, illustrating the high eccentricity of the orbit andmore rapid motion when closer to the Sun.

Histogram of the aphelia of the 2005 comets, showing the giant planet comet families.

The abscissa is the natural logarithm of the aphelion expressed in AUs.

Comets are classified according to their orbital periods. Short period comets have

orbits of less than 200 years, whileLong period comets have longer orbits but remain

gravitationally bound to the Sun, and main-belt comets orbit within the asteroid belt.

Single-apparition comets haveparabolicorhyperbolic orbits which will cause them topermanently exit the solar system after one pass by the Sun.

Modern observations have revealed a few genuinely hyperbolic orbits, but no more

than could be accounted for by perturbations from Jupiter. If comets pervaded

interstellar space, they would be moving with velocities of the same order as the

relative velocities of stars near the Sun (a few tens of kilometres per second). If such

objects entered the solar system, they would have positive total energies, and would

be observed to have genuinely hyperbolic orbits. A rough calculation shows that there

might be 4 hyperbolic comets per century, within Jupiter's orbit, give or take one and

perhaps two orders of magnitude.[5]
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On the other extreme, the short period Comet Encke has an orbit which never places it

farther from the Sun than Jupiter. Short-period comets are thought to originate in the

Kuiper belt, whereas the source of long-period comets is thought to be theOort cloud.

A variety of mechanisms have been proposed to explain why comets get perturbed

into highly elliptical orbits, including close approaches to otherstars as the Sun

follows its orbit through the Milky WayGalaxy; the Sun's hypothetical companionstarNemesis; or an unknown Planet X.

Because of their low masses, and their elliptical orbits which frequently take them

close to the giant planets, cometary orbits are often perturbed. Short period comets

display a strong tendency for theiraphelia to coincide with agiant planet's orbital

radius, with the Jupiter family of comets being the largest, as the histogramshows. It

is clear that comets coming in from the Oort cloud often have their orbits strongly

influenced by the gravity of giant planets as a result of a close encounter. Jupiter is the

source of the greatest perturbations, being more than twice as massive as all the other

planets combined, in addition to being the swiftest of the giant planets.

A number of periodic comets discovered in earlier decades or previous centuries are

now "lost." Their orbits were never known well enough to predict future appearances.

However, occasionally a "new" comet will be discovered and upon calculation of its

orbit it turns out to be an old "lost" comet. An example is Comet 11P/Tempel-Swift-

LINEAR, discovered in 1869 but unobservable after 1908 due to perturbations by

Jupiter. It was not found again until accidentally rediscovered by LINEARin 2001.[6]

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Comet nomenclatureThe names given to comets have followed several different conventions over the past

two centuries. Before any systematic naming convention was adopted, comets were

named in a variety of ways. Prior to the early 20th century, most comets were simply

referred to by the year in which they appeared, sometimes with additional adjectives

for particularly bright comets; thus, the "Great Comet of 1680" (Kirch's Comet), the

"Great September Comet of 1882," and the "Daylight Comet of 1910" ("Great

January Comet of 1910"). AfterEdmund Halleydemonstrated that the comets of

1531, 1607, and 1682 were the same body and successfully predicted its return in

1759, that comet became known asComet Halley. Similarly, the second and third

known periodic comets, Comet Encke[7] andComet Biela,[8] were named after theastronomers who calculated their orbits rather than their original discoverers. Later,

periodic comets were usually named after their discoverers, but comets that had

appeared only once continued to be referred to by the year of their apparition.

In the early 20th century, the convention of naming comets after their discoverers

became common, and this remains so today. A comet is named after up to three

independent discoverers. In recent years, many comets have been discovered by

instruments operated by large teams of astronomers, and in this case, comets may be

named for the instrument. For example, Comet IRAS-Araki-Alcockwas discovered

independently by the IRAS satellite and amateur astronomers Genichi Arakiand

George Alcock. In the past, when multiple comets were discovered by the same
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individual, group of individuals, or team, the comets' names were distinguished by

adding a numeral to the discoverers' names; thus Comets Shoemaker-Levy 19.

Today, the large numbers of comets discovered by some instruments (in August 2005,

SOHO discovered its 1000th comet[9]) has rendered this system impractical, and no

attempt is made to ensure that each comet has a unique name. Instead, the comets'

systematic designations are used to avoid confusion.

Until 1994, comets were first given aprovisional designation consisting of the year of

their discovery followed by a lowercase letter indicating its order of discovery in that

year (for example, Comet Bennett 1969i was the 9th comet discovered in 1969). Once

the comet had been observed through perihelion and its orbit had been established, the

comet was given a permanent designation of the year of its perihelion, followed by a

Roman numeral indicating its order of perihelion passage in that year, so that Comet

Bennett 1969i became Comet Bennett 1970 II (it was the second comet to pass

perihelion in 1970)[10]

Increasing numbers of comet discoveries made this procedure awkward, and in 1994the International Astronomical Unionapproved a new naming system. Comets are

now designated by the year of their discovery followed by a letter indicating the half-

month of the discovery and a number indicating the order of discovery (a system

similar to that already used forasteroids), so that the fourth comet discovered in the

second half of February 2006 would be designated 2006 D4. Prefixes are also added

to indicate the nature of the comet, with P/ indicating a periodic comet, C/ indicating a

non-periodic comet, X/ indicating a comet for which no reliable orbit could be

calculated, D/ indicating a comet which has broken up or been lost, and A/ indicating

an object that was mistakenly identified as a comet, but is actually a minor planet.

After their second observed perihelion passage, periodic comets are also assigned a

number indicating the order of their discovery.[11] So Halley's Comet, the first comet

to be identified as periodic, has the systematic designation1P/1682 Q1.Comet Hale-

Bopp's designation is C/1995 O1.

There are only four objects that are cross-listed as both comets and asteroids: 2060

Chiron (95P/Chiron), 133P/Elst-Pizarro (7968 Elst-Pizarro), 60558 Echeclus

(174P/Echeclus) and 4015 Wilson-Harrington (107P/Wilson-Harrington).

[edit]

History of comet study

[edit]

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" inGilgamesh,Revelation and the Book ofEnoch as references to

comets, or possiblybolides.
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In the first book of hisMeteorology,Aristotlepropounded 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 wereplanets, 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.[12]

Instead, he described comets as a phenomenon of the upperatmosphere, where hot,dry exhalations gathered and occasionally burst into flame. Aristotle held this

mechanism responsible for not only comets, but alsometeors, the aurora borealis, and

even the Milky Way.[13]

A few later classical philosophers did dispute this view of comets.Seneca the

Younger, in hisNatural 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. [14]

However, the Aristotelean viewpoint proved more influential, and it was not until the16th century that it was demonstrated that comets must exist outside the earth's

atmosphere.

In 1577, a bright comet was visible for several months. The Danish astronomerTycho

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. [15]

[edit]

Orbital studies

The orbit of the comet of 1680, fit to aparabola, as shown inIsaac Newton's

Principia.

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 afterJohannes Keplerhad 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 planetsshould also influence the motion of other bodieshe 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.[16]
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The first suggestion that Kepler's laws of planetary motion should also apply to the

comets was made by William Lowerin 1610.[15]In the following decades, other

astronomers, including Pierre Petit, Giovanni Borelli, Adrien Auzout,Robert Hooke,

Johann Baptist Cysat, and Giovanni Domenico Cassini, all argued for comets curving

about the sun on elliptical or parabolic paths, while others, such as Christian Huygens

andJohannes Hevelius, supported comets' linear motion.[16]

The matter was resolved by thebright cometthat was discovered by Gottfried Kirch

onNovember 14, 1680. Astronomers throughout Europe tracked its position for

several months. In 1681, the Saxon pastorGeorg Samuel Doerfel set forth his proofs

that comets are heavenly bodies moving inparabolasof which the sun is the focus.

Then Isaac Newton, in hisPrincipia Mathematicaof 1687, proved that an object

moving under the influence of his inverse square law ofuniversal 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.[17]

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 similarorbital elements, and he was

further able to account for the slight differences in their orbits in terms of gravitational

perturbation by Jupiterand Saturn. Confident that these three apparitions had been

three appearances of the same comet, he predicted that it would appear again in 1758-

9.[18] (Earlier, Robert Hooke had identified the comet of 1664 with that of 1618,[19]

while Jean-Dominique Cassini had suspected the identity of the comets of 1577, 1665,

and 1680.[20] Both were incorrect.) Halley's predicted return date was later refined by a

team of three French mathematicians: Alexis Clairaut,Joseph Lalande, andNicole-

Reine Lepaute, who predicted the date of the comet's 1759 perihelion to within one

month's accuracy.[21] 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.

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 wasComet Encke (official designation

2P/Encke). Over the period 1819-1821 the Germanmathematician and physicistJohann Franz Enckecomputed 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.[22] By 1900, seventeen comets had been observed at more

than one perihelion passage and recognized as periodic comets. As of April 2006, 175

comets have achieved this distinction, though several have since been destroyed or

lost. In ephemerides, comets are often denoted by the symbol.

To meet Wikipedia's quality standards, this article or section may require

cleanup.Please discuss this issue on the talk page, or replace this tag with amore specific message.

Editing help is available.This article has been tagged since June 2006.
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[edit]

Studies of physical characteristics

Comets have highly elliptical orbits. Note the two distinct tails.

Isaac Newton described comets as compact, solid, fixed, and durable bodies: in otherwords, a kind of planet, 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.

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

"From his huge vapouring train perhaps to shake

Reviving moisture on the numerous orbs,

Thro' which his long ellipsis winds; perhapsTo lend new fuel to declining suns,

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

James Thomson, "The Seasons" (1730; 1748).

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.[23]In 1836, the German mathematician Friedrich Wilhelm

Bessel, after observing streams of vapor in the 1835 apparition of Comet Halley,

proposed that thejet forces of evaporating material could be great enough to

significantly alter a comet's orbit and argued that the non-gravitational movements ofComet Encke resulted from this mechanism.[24]
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However, another comet-related discovery overshadowed these ideas for nearly a

century. Over the period 18641866 the Italian astronomerGiovanni Schiaparelli

computed the orbit of the Perseidmeteors, and based on orbital similarities, correctly

hypothesized that the Perseids were fragments ofComet Swift-Tuttle. The link

between comets and meteor showers was dramatically underscored when in 1872, a

major meteor shower occurred from the orbit ofComet Biela, which had beenobserved to split into two pieces during its 1846 apparition, and never seen again after

1852.[25] 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.

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 Whippleproposed that rather than being

rocky objects containing some ice, comets were icy objects containing some dust and

rock.[26] This "dirty snowball" model soon became accepted. It was confirmed when

an armada ofspacecraft(including the European Space Agency's Giotto probe and theSoviet Union'sVega 1 andVega 2) flew through the coma of Halley's comet in 1986

to photograph the nucleus and observed the jets of evaporating material. The

American probeDeep Space 1 flew past the nucleus ofComet Borrelly onSeptember

21, 2001 and confirmed that the characteristics of Comet Halley are common on other

comets as well.

Comet Wild 2 exhibits jets on lit side and dark side, stark relief, and is dry.

The Stardustspacecraft, launched in February 1999, collected particles from the coma

ofComet Wild 2 in January 2004, and returned the samples to Earth in a capsule in

January 2006. Claudia Alexander, a program scientist for Rosetta from NASA's Jet

Propulsion Laboratory who 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 on 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.[27]

Forthcoming space missions will add greater detail to our understanding of what

comets are made of. In July 2005, theDeep Impactprobe blasted a crater on Comet

Tempel 1 to study its interior. And in 2014, the EuropeanRosettaprobe will orbit

comet Comet Churyumov-Gerasimenko and place a small lander on its surface.

Rosetta observed the Deep Impact event, and with its set of very sensitive instrumentsfor cometary investigations, it used its capabilities to observe Tempel 1 before, during
http://en.wikipedia.org/wiki/Italyhttp://en.wikipedia.org/wiki/Giovanni_Schiaparellihttp://en.wikipedia.org/wiki/Giovanni_Schiaparellihttp://en.wikipedia.org/wiki/Perseidshttp://en.wikipedia.org/wiki/Meteorhttp://en.wikipedia.org/wiki/Comet_Swift-Tuttlehttp://en.wikipedia.org/wiki/Comet_Bielahttp://en.wikipedia.org/wiki/Comet_Bielahttp://en.wikipedia.org/wiki/Comets#_note-19http://en.wikipedia.org/wiki/Fred_Lawrence_Whipplehttp://en.wikipedia.org/wiki/Fred_Lawrence_Whipplehttp://en.wikipedia.org/wiki/Comets#_note-Whipplehttp://en.wikipedia.org/wiki/Spacecrafthttp://en.wikipedia.org/wiki/Spacecrafthttp://en.wikipedia.org/wiki/European_Space_Agencyhttp://en.wikipedia.org/wiki/European_Space_Agencyhttp://en.wikipedia.org/wiki/Giotto_missionhttp://en.wikipedia.org/wiki/Soviet_Unionhttp://en.wikipedia.org/wiki/Soviet_Unionhttp://en.wikipedia.org/wiki/Vega_1http://en.wikipedia.org/wiki/Vega_1http://en.wikipedia.org/wiki/Vega_2http://en.wikipedia.org/wiki/Vega_2http://en.wikipedia.org/wiki/Deep_Space_1http://en.wikipedia.org/wiki/Comet_Borrellyhttp://en.wikipedia.org/wiki/Comet_Borrellyhttp://en.wikipedia.org/wiki/September_21http://en.wikipedia.org/wiki/September_21http://en.wikipedia.org/wiki/September_21http://en.wikipedia.org/wiki/2001http://en.wikipedia.org/wiki/Comet_Wild_2http://en.wikipedia.org/wiki/Stardust_(spacecraft)http://en.wikipedia.org/wiki/81P/Wildhttp://en.wikipedia.org/wiki/Comets#_note-20http://en.wikipedia.org/wiki/Deep_Impact_(space_mission)http://en.wikipedia.org/wiki/9P/Tempelhttp://en.wikipedia.org/wiki/9P/Tempelhttp://en.wikipedia.org/wiki/Rosetta_space_probehttp://en.wikipedia.org/wiki/Rosetta_space_probehttp://en.wikipedia.org/wiki/67P/Churyumov-Gerasimenkohttp://en.wikipedia.org/wiki/Image:Comet_wild_2.jpghttp://en.wikipedia.org/wiki/Image:Comet_wild_2.jpghttp://en.wikipedia.org/wiki/Italyhttp://en.wikipedia.org/wiki/Giovanni_Schiaparellihttp://en.wikipedia.org/wiki/Perseidshttp://en.wikipedia.org/wiki/Meteorhttp://en.wikipedia.org/wiki/Comet_Swift-Tuttlehttp://en.wikipedia.org/wiki/Comet_Bielahttp://en.wikipedia.org/wiki/Comets#_note-19http://en.wikipedia.org/wiki/Fred_Lawrence_Whipplehttp://en.wikipedia.org/wiki/Comets#_note-Whipplehttp://en.wikipedia.org/wiki/Spacecrafthttp://en.wikipedia.org/wiki/European_Space_Agencyhttp://en.wikipedia.org/wiki/Giotto_missionhttp://en.wikipedia.org/wiki/Soviet_Unionhttp://en.wikipedia.org/wiki/Vega_1http://en.wikipedia.org/wiki/Vega_2http://en.wikipedia.org/wiki/Deep_Space_1http://en.wikipedia.org/wiki/Comet_Borrellyhttp://en.wikipedia.org/wiki/September_21http://en.wikipedia.org/wiki/September_21http://en.wikipedia.org/wiki/2001http://en.wikipedia.org/wiki/Comet_Wild_2http://en.wikipedia.org/wiki/Stardust_(spacecraft)http://en.wikipedia.org/wiki/81P/Wildhttp://en.wikipedia.org/wiki/Comets#_note-20http://en.wikipedia.org/wiki/Deep_Impact_(space_mission)http://en.wikipedia.org/wiki/9P/Tempelhttp://en.wikipedia.org/wiki/9P/Tempelhttp://en.wikipedia.org/wiki/Rosetta_space_probehttp://en.wikipedia.org/wiki/67P/Churyumov-Gerasimenko


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and after the impact. At a distance of about 80 million kilometres from the comet,

Rosetta was the only spacecraft other then Deep Impact itself to view the comet.

[edit]

Debate over comet composition

Comet Borrelly exhibits jets, yet is hot and dry.

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".[28]

The recent Deep Impact probe has also yielded results suggesting that the majority of

a comets water ice is below the surface, and that these resevoirs feed the jets of

vaporised water that form the coma of Tempel 1.[1]

[edit]

Great comets

While hundreds of tiny comets pass through the inner solar system every year, only a

very few comets make any impact on the general public. About every decade or so, a

comet will become bright enough to be noticed by a casual observer such comets

are often designatedGreat Comets. In times past, bright comets often inspired panic

and hysteria in the general population, being thought of as bad omens. More recently,

during the passage of Halley's Comet in 1910, the Earth passed through the comet's

tail, and erroneous newspaper reports inspired a fear that cyanogenin the tail might

poison millions, while the appearance ofComet Hale-Boppin 1997 triggered the

mass suicide of the Heaven's Gate cult. To most people, however, a great comet is

simply a beautiful spectacle.
http://en.wikipedia.org/w/index.php?title=Comet&action=edit&section=8http://en.wikipedia.org/wiki/Comet_Borrellyhttp://en.wikipedia.org/wiki/Comets#_note-21http://en.wikipedia.org/wiki/Comets#_note-21http://en.wikipedia.org/wiki/Deep_Impact_(space_mission)http://www.brown.edu/Administration/News_Bureau/2005-06/05-072.htmlhttp://en.wikipedia.org/w/index.php?title=Comet&action=edit&section=9http://en.wikipedia.org/wiki/Great_Comethttp://en.wikipedia.org/wiki/Great_Comethttp://en.wikipedia.org/wiki/Cyanogenhttp://en.wikipedia.org/wiki/Cyanogenhttp://en.wikipedia.org/wiki/Comet_Hale-Bopphttp://en.wikipedia.org/wiki/Comet_Hale-Bopphttp://en.wikipedia.org/wiki/Comet_Hale-Bopphttp://en.wikipedia.org/wiki/Heaven's_Gate_(cult)http://en.wikipedia.org/wiki/Image:Comet_borrelly.jpghttp://en.wikipedia.org/wiki/Image:Comet_borrelly.jpghttp://en.wikipedia.org/w/index.php?title=Comet&action=edit&section=8http://en.wikipedia.org/wiki/Comet_Borrellyhttp://en.wikipedia.org/wiki/Comets#_note-21http://en.wikipedia.org/wiki/Deep_Impact_(space_mission)http://www.brown.edu/Administration/News_Bureau/2005-06/05-072.htmlhttp://en.wikipedia.org/w/index.php?title=Comet&action=edit&section=9http://en.wikipedia.org/wiki/Great_Comethttp://en.wikipedia.org/wiki/Cyanogenhttp://en.wikipedia.org/wiki/Comet_Hale-Bopphttp://en.wikipedia.org/wiki/Heaven's_Gate_(cult)


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Predicting whether a comet will become a great comet is notoriously difficult, as

many factors may cause a comet's brightness to depart drastically from predictions.

Broadly speaking, if a comet has a large and active nucleus, will pass close to the Sun,

and is not obscured by the Sun as seen from the Earth when at its brightest, it will

have a chance of becoming a great comet. However, Comet Kohoutekin 1973

fulfilled all the criteria and was expected to become spectacular, but failed to do so.Comet West, which appeared three years later, had much lower expectations (perhaps

because scientists were much warier of glowing predictions after the Kohoutek

fiasco), but became an extremely impressive comet.[29]

The late 20th century saw a lengthy gap without the appearance of any great comets,

followed by the arrival of two in quick succession Comet Hyakutake in 1996,

followed by Hale-Bopp, which reached maximum brightness in 1997 having been

discovered two years earlier. As yet, the 21st century has not seen the arrival of any

great comets.

[edit]

Peculiar comets

Of the thousands of known comets, some are very unusual. Comet Encke orbits from

inside the orbit of Jupiter to inside the orbit ofMercurywhile Comet

29P/Schwassmann-Wachmann orbits in a nearly circular orbit entirely between

Jupiter and Saturn.[30]2060 Chiron, whose unstable orbit keeps it between Saturn and

Uranus, was originally classified as an asteroid until a faint coma was noticed.[31]

Similarly, Comet Shoemaker-Levy 2was originally designated asteroid 1990 UL3.[32]

Some near-earth asteroids are thought to be extinct nuclei of comets which no longerexperience outgassing.

Some comets have been observed to break up. Comet Bielawas one significant

example, breaking into two during its 1846 perihelion passage. The two comets were

seen separately in 1852, but never again after that. Instead, spectacular meteor

showers were seen in 1872 and 1885 when the comet should have been visible. A

lesser meteor shower, the Andromedids, occurs annually in November, and is caused

by the Earth crossing Biela's orbit.[33]

Several other comets have been seen to break up during their perihelion passage,

including great comets West and Comet Ikeya-Seki. Some comets, such as the KreutzSungrazers, orbit in groups and are thought to be pieces of a single object that has

previously broken apart.

Another very significant cometary disruption was that ofComet Shoemaker-Levy 9,

which was discovered in 1993. At the time of its discovery, the comet was in orbit

around Jupiter, having been captured by the planet during a very close approach in

1992. This close approach had already broken the comet into hundreds of pieces, and

over a period of 6 days in July 1994, these pieces slammed into Jupiter's atmosphere

the first time astronomers had observed a collision between two objects in the solar

system.[34] However, it has been suggested that the object responsible for the

Tunguska eventin 1908 was a fragment of Comet Encke.
http://en.wikipedia.org/wiki/Comet_Kohoutekhttp://en.wikipedia.org/wiki/Comet_Westhttp://en.wikipedia.org/wiki/Comets#_note-22http://en.wikipedia.org/wiki/Comet_Hyakutakehttp://en.wikipedia.org/w/index.php?title=Comet&action=edit&section=10http://en.wikipedia.org/wiki/Mercury_(planet)http://en.wikipedia.org/wiki/Mercury_(planet)http://en.wikipedia.org/wiki/29P/Schwassmann-Wachmannhttp://en.wikipedia.org/wiki/Saturn_(planet)http://en.wikipedia.org/wiki/Comets#_note-23http://en.wikipedia.org/wiki/Comets#_note-23http://en.wikipedia.org/wiki/2060_Chironhttp://en.wikipedia.org/wiki/2060_Chironhttp://en.wikipedia.org/wiki/Uranus_(planet)http://en.wikipedia.org/wiki/Comets#_note-24http://en.wikipedia.org/w/index.php?title=137P/Shoemaker-Levy&action=edithttp://en.wikipedia.org/w/index.php?title=137P/Shoemaker-Levy&action=edithttp://en.wikipedia.org/w/index.php?title=1990_UL3&action=edithttp://en.wikipedia.org/w/index.php?title=1990_UL3&action=edithttp://en.wikipedia.org/wiki/Comets#_note-25http://en.wikipedia.org/wiki/Comets#_note-25http://en.wikipedia.org/wiki/Near-earth_asteroidhttp://en.wikipedia.org/wiki/3D/Bielahttp://en.wikipedia.org/wiki/3D/Bielahttp://en.wikipedia.org/wiki/Comets#_note-26http://en.wikipedia.org/wiki/Comet_Ikeya-Sekihttp://en.wikipedia.org/wiki/Comet_Ikeya-Sekihttp://en.wikipedia.org/wiki/Kreutz_Sungrazershttp://en.wikipedia.org/wiki/Kreutz_Sungrazershttp://en.wikipedia.org/wiki/Kreutz_Sungrazershttp://en.wikipedia.org/wiki/Comet_Shoemaker-Levy_9http://en.wikipedia.org/wiki/Comets#_note-27http://en.wikipedia.org/wiki/Tunguska_eventhttp://en.wikipedia.org/wiki/Tunguska_eventhttp://en.wikipedia.org/wiki/Comet_Kohoutekhttp://en.wikipedia.org/wiki/Comet_Westhttp://en.wikipedia.org/wiki/Comets#_note-22http://en.wikipedia.org/wiki/Comet_Hyakutakehttp://en.wikipedia.org/w/index.php?title=Comet&action=edit&section=10http://en.wikipedia.org/wiki/Mercury_(planet)http://en.wikipedia.org/wiki/29P/Schwassmann-Wachmannhttp://en.wikipedia.org/wiki/Saturn_(planet)http://en.wikipedia.org/wiki/Comets#_note-23http://en.wikipedia.org/wiki/2060_Chironhttp://en.wikipedia.org/wiki/Uranus_(planet)http://en.wikipedia.org/wiki/Comets#_note-24http://en.wikipedia.org/w/index.php?title=137P/Shoemaker-Levy&action=edithttp://en.wikipedia.org/w/index.php?title=1990_UL3&action=edithttp://en.wikipedia.org/wiki/Comets#_note-25http://en.wikipedia.org/wiki/Near-earth_asteroidhttp://en.wikipedia.org/wiki/3D/Bielahttp://en.wikipedia.org/wiki/Comets#_note-26http://en.wikipedia.org/wiki/Comet_Ikeya-Sekihttp://en.wikipedia.org/wiki/Kreutz_Sungrazershttp://en.wikipedia.org/wiki/Kreutz_Sungrazershttp://en.wikipedia.org/wiki/Comet_Shoemaker-Levy_9http://en.wikipedia.org/wiki/Comets#_note-27http://en.wikipedia.org/wiki/Tunguska_event


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References

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2. ^ Britain's Bayeux Tapestry, scene 1. Reading Museum Service (2000-2004).

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3. ^ First X-Rays from a Comet Discovered. Retrieved on2006-03-05.

4. ^ Probing space weather with comets. Retrieved on2006-03-05.

5. ^ Cowley lecture 34. Retrieved on2005-03-05. - 404 error as of last access

6. ^ Kronk, Gary W. (20012005). Cometography. Retrieved on 2006-03-05.,

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7. ^ Kronk, '2P/Encke'8. ^ Kronk, '3D/Biela'

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13. ^ Aristotle, l. 1. c. 7.

14. ^ Carl Sagan & Ann Druyan (1985). Comet. New York: Random House.

ISBN 0-394-54908-2, 2324.

15. ^a

b

(2003)A Brief History of Comets, part I. European Southern Observatory.16. ^ ab Prasar, Vigyan (2001).Development of Cometary Thought, Part II.

17. ^Newton, I.S. (1687).Philosophi Naturalis Principia Mathematica.

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25. ^ Kronk, '3D/Biela'

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28. ^NASA Spacecraft Finds Comet Has Hot, Dry Surface. JPL (2002).


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30. ^ Kronk, '29P/Schwassmann-Wachmann 1'

31. ^ Kronk, '95P/Chiron'

32. ^ Kronk, '137P/Shoemaker-Levy 2'
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33. ^ The Andromedids ("Bielids"). Gary W. Kronk's Comets & Meteor Showers.


34. ^ Kronk, 'D/1993 F2 Shoemaker-Levy 9'

1. European Southern Observatory. (2003). "A Brief History of Comets."

Available online: Part I,Part II.2. Vigyan Prasar (2001). "Development of Cometary Thought." Available

online:Part I, Part II.
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