Komét (simbol:) nyaéta banda leutik dinasistim suryamandala nu ngorbit kanapanonpoé sarta (kadang-kadang) mintonkeunkoma (atawa atmosfir) jeung/atawa buntut— duanana utamana disababkeun ku ayana pangaruh radiasi panonpoé kanainti komét, nu mana manéna sorangan mangrupa banda leutik nu diwangun kubatu,kebul, jeungés. Orbit komét robah kalayan konstan: asalna di saluareunsistim suryamandala, sarta miboga kacenderungan kacida dipangaruhannana (atawa dikacokeun) ku planét gedé nu rélatif deukeut. Sababaraha di antarana pindah kana orbitsungrazing nu ngaruksak komét nalika manéhna deukeuteun paponpoé, sedengkeun lianna mah kapiceun ka saluareunsistim suryamandala salilalana.
lolobana komét dipercaya asalana tina halimun (Oort cloud) dina jarak nu jauh ti Srangéngé nu ngandungdebris nu tinggaleun tinakondénsasinébula srangéngé; sisi luar nébula ieu cukuptiis sahinggacai ogé aya dinakaayaan nu mangrupapadetan (laingas).Asteroid asalna ngaliwatan prosés nu béda, tapi komét nu geus kacida kolotna, nu geus leungiteun sakabéh bahanvolatile-na, bisa ogé jadi sarupaning asteroid.
Kecapkomét asalna tinabasa Inggris nu ngaliwatanbasa Laténcometes. Tina kecapbasa Yunanikomē mah, hartina téh "buuk na hulu,"Aristotélés mimiti maké kecap turunankomētēs keur ngagambarkeun komét miangka "béntang buukan."
Long-period comets are believed to originate in a distant cloud known as theOort 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 theSun. One théory says that as a comet approaches theinner solar system,solar radiation causes part of its outer layers, composed of ice and other materials, to melt and evaporate, but this has not been proven.The stréams ofdust and gas this reléases form a very large, extremely tenuous atmosphere around the comet called thecoma, and the force exerted on the coma by the Sun'sradiation pressure andsolar wind cause an enormoustail to form, which points away from the sun. The stréams of dust and gas éach form their own distinct tail, éach 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 from the 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 thenucleus) 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 (1Astronomical unit) or more.
Both the coma and tail are illuminated by the Sun and may become visible from theEarth when a comet passes through the inner solar system, the dust reflecting sunlight directly, and the gases glowing fromionisation. Most comets are too faint to be visible without the aid of atelescope, but a few éach decade become bright enough to be visible with the naked eye. Before the invention of the telescope, comets seemed to appéar out of nowhere in the sky and gradually vanish out of sight. They were usually considered bad omens of déaths of kings or noble men, or coming catastrophes. From ancient sources, such as Chinese oracle bones, it is known that their appéarances have been noticed by humans for millennia. One very famous old recording of a comet is the appéarance of Halley's Comet on theBayeux Tapestry, which records theNorman conquest ofEngland in AD 1066.[2]
Surprisingly, cometary nuclei are among thedarkest objects known to exist in the solar system. TheGiotto probe found thatComet Halley's nucleus reflects approximately 4% of the light that falls on it, andDeep Space 1 discovered thatComet 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 complexorganic compounds are the dark surface material. Solar héating drives off volatile compounds léaving behind héavy long-chain organics that tend to be very dark, liketar or crudeoil. The very darkness of cometary surfaces allows them to absorb the héat necessary to drive their outgassing.
In 1996, comets were found to emitX-rays.[3] These X-rays surprised reséarchers, 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 chargedions fly through a cometary atmosphere, they collide with cometary atoms and molecules. In these collisions, the ions will capture one or more electrons léading to emission of X-rays and far ultraviolet photons.[4]
Orbits ofComet Kohoutek andEarth, illustrating the higheccentricity of the orbit and more rapid motion when closer to theSun.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 theirorbital periods.Short period comets have orbits of less than 200 yéars, whileLong period comets have longer orbits but remain gravitationally bound to the Sun, andmain-belt comets orbit within theasteroid belt.Single-apparition comets haveparabolic orhyperbolic orbits which will cause them to permanently exit the solar system after one pass by the Sun.
modérn observations have revéaled 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 néar the Sun (a few tens of kilometres per second). If such objects entered the solar system, they would have positive total énérgies, 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.
On the other extreme, the short periodComet Encke has an orbit which never places it farther from the Sun thanJupiter. Short-period comets are thought to originate in theKuiper belt, wheréas 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 theMilky WayGalaxy; the Sun's hypothetical companion starNemesis; or an unknownPlanet X[rujukan?].
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 thehistogram shows. It is cléar 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 gréatest 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 éarlier decades or previous centuries are now "lost." Their orbits were never known well enough to predict future appéarances. 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 Comet11P/Tempel-Swift-LINEAR, discovered in 1869 but unobservable after 1908 because of perturbations by Jupiter. It was not found again until accidentally rediscovered byLINEAR in 2001.[5]
The 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 éarly 20th century, most comets were simply referred to by the yéar in which they appéared, 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 Halley demonstrated 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[6] andComet Biela,[7] were named after the astronomers who calculated their orbits rather than their original discoverers. Later, periodic comets were usually named after their discoverers, but comets that had appéared only once continued to be referred to by the yéar of their apparition.
In the éarly 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 yéars, many comets have been discovered by instruments operated by large téams of astronomers, and in this case, comets may be named for the instrument. For example,Comet IRAS-Araki-Alcock was discovered independently by theIRAS satellite and amateur astronomersGenichi Araki andGeorge Alcock. In the past, when multiple comets were discovered by the same individual, group of individuals, or téam, the comets' names were distinguished by adding a numeral to the discoverers' names; thus CometsShoemaker-Levy 1–9. Today, the large numbers of comets discovered by some instruments (in August 2005,SOHO discovered its 1000th comet[8]) has rendered this system impractical, and no attempt is made to ensure that éach comet has a unique name. Instéad, the comets' systematic designations are used to avoid confusion.
Until 1994, comets were first given aprovisional designation consisting of the yéar of their discovery followed by a lowercase letter indicating its order of discovery in that yéar (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 yéar of itsperihelion, followed by aRoman numeral indicating its order of perihelion passage in that yéar, so that Comet Bennett 1969i becameComet Bennett 1970 II (it was the second comet to pass perihelion in 1970)[9]
Incréasing numbers of comet discoveries made this procedure awkward, and in 1994 theInternational Astronomical Union approved a new naming system. Comets are now designated by the yéar 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 alréady used forasteroids), so that the fourth comet discovered in the second half of February 2006 would be designated 2006D4. 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 aminor planet. After their second observed perihelion passage, periodic comets are also assigned a number indicating the order of their discovery.[10] 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.
Historically, comets were thought to be unlucky, or even interpreted as attacks by héavenly beings against terrestrial inhabitants. Some authorities interpret references to "falling stars" inGilgamesh,Revelation and the Book ofEnoch as references to comets, or possiblybolides.
In the first book of hisMeteorology,Aristotle propounded the view of comets that would hold sway in Western thought for néarly two thousand yéars. He rejected the idéas of several éarlier philosophers that comets wereplanets, or at léast a phenomenon related to the planets, on the grounds that while the planets confined their motion to the circle of theZodiac, comets could appéar in any part of the sky.[11] Instéad, 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, theaurora borealis, and even theMilky Way.[12]
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 thewind, behavior more typical of celestial than atmospheric phenomena. While he conceded that the other planets do not appéar outside the Zodiac, he saw no réason that a planet-like object could not move through any part of the sky, humanity's knowledge of celestial things being very limited.[13] However, the Aristoteléan viewpoint proved more influential, and it was not until the 16th century that it was demonstrated that comets must exist outside the éarth's atmosphere.
In 1577, a bright comet was visible for several months. TheDanish astronomerTycho Brahe used méasurements of the comet's position taken by himself and other, géographically separated, observers to determine that the comet had no méasuréableparallax. Within the precision of the méasurements, this implied the comet must be at léast four times more distant from the éarth than the moon.[14]
Although comets had now been demonstrated to be in the héavens, the question of how they moved through the héavens would be debated for most of the next century. Even afterJohannes Kepler had determined in 1609 that the planets moved about the sun inelliptical orbits, he was reluctant to believe that thelaws 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 staunchCopernicanist, rejected Tycho's parallax méasurements and held to the Aristoteléan notion of comets moving on straight lines through the upper atmosphere.[15]
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 byJupiter andSaturn. Confident that these three apparitions had been three appéarances of the same comet, he predicted that it would appéar again in 1758-9.[17] (éarlier, Robert Hooke had identified the comet of 1664 with that of 1618,[18] while Jéan-Dominique Cassini had suspected the identity of the comets of 1577, 1665, and 1680.[19] Both were incorrect.) Halley's predicted return date was later refined by a téam of threeFrench mathematicians:Alexis Clairaut,Joseph Lalande, andNicole-Reine Lepaute, who predicted the date of the comet's 1759 perihelion to within one month's accuracy.[20] When the comet returned as predicted, it became known asComet Halley or Halley's Comet (its official designation is1P/Halley). Its next appéarance 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 thetelescope. The second comet to be discovered to have a periodic orbit wasComet Encke (official designation2P/Encke). Over the period 1819-1821 theGerman mathematician and physicistJohann 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.[6] 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. Inephemerides, comets are often denoted by the symbol☄.
Isaac Newton described comets as compact, solid, fixed, and durable bodies: in other words, a kind of planet, which move in very oblique orbits, every way, with the gréatest 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 héad, ornucleus of the comet, ignited or héated 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 éarth, might be resupplied and recruited; for all vegetables were thought to incréase wholly from fluids, and turn by putrefaction into éarth. Hence the quantity of dry éarth must continually incréase, and the moisture of the globe decréase, 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 stréams continually sent forth in every direction from that luminary —
As éarly 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 néar perihelion.[21] In 1836, the German mathematicianFriedrich Wilhelm Bessel, after observing stréams of vapor in the 1835 apparition of Comet Halley, proposed that thejet forces of evaporating material could be gréat enough to significantly alter a comet's orbit and argued that the non-gravitational movements ofComet Encke resulted from this mechanism.[22]
However, another comet-related discovery overshadowed these idéas for néarly a century. Over the period 1864–1866 theItalian astronomerGiovanni Schiaparelli computed the orbit of thePerseidmeteors, and based on orbital similarities, correctly hypothesized that the Perseids were fragments ofComet Swift-Tuttle. The link between comets and metéor showers was dramatically underscored when in 1872, a major metéor shower occurred from the orbit ofComet Biela, which had been observed to split into two pieces during its 1846 apparition, and never seen again after 1852.[7] A "gravel bank" modél 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 modél 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.[23] This "dirty snowball" modél soon became accepted. It was confirmed when an armada ofspacecraft (including theEuropean Space Agency'sGiotto 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 212001 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.
TheStardust spacecraft, launched in February 1999, collected particles from the coma ofComet Wild 2 in January 2004, and returned the samples to éarth in a capsule in January 2006. Claudia Alexander, a program scientist for Rosetta from NASA's Jet Propulsion Laboratory who has modéled comets for yéars, reported to space.com about her astonishment at the number of jets, their appéarance 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.[24]
Forthcoming space missions will add gréater detail to our understanding of what comets are made of. In July 2005, theDeep Impact probe blasted a crater onComet Tempel 1 to study its interior. And in 2014, the EuropéanRosetta probe will orbit cometComet Churyumov-Gerasimenko and place a small lander on its surface.
Rosetta observed the Deep Impact event, and with its set of very sensitive instruments for cometary investigations, it used its capabilities to observe Tempel 1 before, during 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.
As late as 2002, there is conflict on how much ice is in a comet. NASA's Deep Space 1 téam, 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. Géological 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 probably 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".[25]
The recentDeep Impact probe has also yielded results suggesting that the majority of a comet's water ice is below the surface, and that these resevoirs feed the jets of vaporised water that form the coma of Tempel 1.
While hundreds of tiny comets pass through the inner solar system every yéar, only a very few comets are noticed by 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 éarth passed through the comet's tail, and erronéous newspaper reports inspired a féar thatcyanogen in the tail might poison millions, while the appéarance ofComet Hale-Bopp in 1997 triggered the mass suicide of theHeaven's Gate cult. To most péople, however, a gréat comet is simply a béautiful spectacle. See images of Hale-Bopp at theComet Hale-Bopp Images webpage.
Predicting whether a comet will become a gréat comet is notoriously difficult, as many factors may cause a comet's brightness to depart drastically from predictions. Broadly spéaking, 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 éarth when at its brightest, it will have a chance of becoming a gréat comet. However,Comet Kohoutek in 1973 fulfilled all the criteria and was expected to become spectacular, but failed to do so.Comet West, which appéared three yéars later, had much lower expectations (perhaps because scientists were much warier of glowing predictions after the Kohoutek fiasco), but became an extremely impressive comet.[26]
The late 20th century saw a lengthy gap without the appéarance of any gréat comets, followed by the arrival of two in quick succession —Comet Hyakutake in 1996, followed by Hale-Bopp, which réached maximum brightness in 1997 having been discovered two yéars éarlier. As yet, the 21st century has not seen the arrival of any gréat comets.
Of the thousands of known comets, some are very unusual. Comet Encke orbits from inside the orbit of Jupiter to inside the orbit ofMercury while Comet29P/Schwassmann-Wachmann orbits in a néarly circular orbit entirely between Jupiter andSaturn.[27]2060 Chiron, whose unstable orbit keeps it between Saturn andUranus, was originally classified as an asteroid until a faint coma was noticed.[28] Similarly,Comet Shoemaker-Levy 2 was originally designated asteroid1990 UL3.[29] Somenear-earth asteroids are thought to be extinct nuclei of comets which no longer experience outgassing.
Some comets have been observed to bréak up.Comet Biela was one significant example, bréaking into two during its 1846 perihelion passage. The two comets were seen separately in 1852, but never again after that. Instéad, spectacular metéor showers were seen in 1872 and 1885 when the comet should have been visible. A lesser metéor shower, the Andromedids, occurs annually in November, and is caused by the éarth crossing Biela's orbit.[30]
Several other comets have been seen to bréak up during their perihelion passage, including gréat comets West andComet Ikeya-Seki. Some comets, such as theKreutz Sungrazers, 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 alréady 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.[31] However, it has been suggested that the likely object responsible for theTunguska event in 1908 was a fragment of Comet Encke. Less likely is the attribution to an Encke fragment of having caused the formation of theGiordano Bruno (crater) on the Moon in 1178.
Komét SWAN, nu ditandaan ogé minangka C/2006M4, kapanggih kuSWAN/SOHO tanggal 20 Juni 2006,as of October 6 katempo maké toropong di langit wanci janari leutik dina konstélasiCanes Venatici. Périhéliumna mah 28 Séptémber.
Komét ieu meta kacida gancangna kaAquila tur jadi targé nu gampang pikeun ditoropong di langit wanci munggaran peuting salila bulan Oktober. Kiwari mah geuspudar alatan nambahna jarak ti Bumi ka Srangéngé.
ESSAY ON COMETSArchived 2005-04-02 diWayback Machine, which gained the first of Dr. Fellowes's prizes, proposed to those who had attended the University of Edinburgh within the last twelve yéars. By David Milne. Publisher: Edinburgh, Printed for A. Black; 1828.(a searchable facsimile at the University of Georgia Libraries;DjVu &layered PDFArchived 2008-04-09 diWayback Machine format)
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