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Comet

2007 Schools Wikipedia Selection. Related subjects: Space (Astronomy)

   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 primarily from the effects of solar radiation upon the comet's
   nucleus, which itself is a minor body composed of rock, dust, and ices.
   Comets' orbits are constantly changing: their origins are in the outer
   solar system, and they have a propensity to be highly affected (or
   perturbed) by relatively close approaches to the major planets. Some
   are moved into sungrazing 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
   condensation of the solar nebula; the outer edges of such nebulae are
   cool enough that water exists in a solid (rather than gaseous) state.
   Asteroids originate via a different process, but very old comets which
   have lost all their volatile materials may come to resemble asteroids.

   The word comet came to the English language through Latin cometes. From
   the Greek word komē, meaning "hair of the head," Aristotle first used
   the derivation komētēs to depict comets as "stars with hair."
   Comet Hale-Bopp
   Enlarge
   Comet Hale-Bopp

Physical characteristics

   Long-period comets are believed to originate in a distant cloud known
   as the Oort cloud (after the astronomer Jan Hendrik Oort who
   hypothesised its existence). 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 radiation
   causes part of its outer layers, composed of ice and other materials,
   to melt and evaporate, but this has not been proven. The streams of
   dust and gas this releases form a very large, 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 tail to 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 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 the nucleus)
   is generally less than 50 km 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
   from ionisation. 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 to appear 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 appearances 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 the Norman conquest of England in AD 1066.

   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, and
   Deep 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 compounds are the dark surface material. Solar heating drives
   off volatile compounds leaving behind heavy long-chain organics that
   tend to be very dark, like tar or 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. 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 cometary atoms and molecules.
   In these collisions, the ions will capture one or more electrons
   leading to emission of X-rays and far ultraviolet photons.

Orbital characteristics

   Orbits of Comet Kohoutek and Earth, illustrating the high eccentricity
   of the orbit and more rapid motion when closer to the Sun.
   Enlarge
   Orbits of Comet Kohoutek and Earth, illustrating the high eccentricity
   of the orbit and more 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.
   Enlarge
   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, also called periodic comets, have orbits of less than 200
   years, while long period comets have longer orbits but remain
   gravitationally bound to the Sun, and main-belt comets orbit within the
   asteroid belt. Single-apparition comets have parabolic or hyperbolic
   orbits which will cause them to permanently 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.

   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 the Oort cloud. A variety of
   mechanisms have been proposed to explain why comets get perturbed into
   highly elliptical orbits, including close approaches to other stars as
   the Sun follows its orbit through the Milky Way Galaxy; the Sun's
   hypothetical companion star Nemesis; 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
   their aphelia to coincide with a giant planet's orbital radius, with
   the Jupiter family of comets being the largest, as the histogram shows.
   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 because of perturbations by
   Jupiter. It was not found again until accidentally rediscovered by
   LINEAR in 2001.

Comet nomenclature

   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 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"). After Edmund 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 as Comet Halley.
   Similarly, the second and third known periodic comets, Comet Encke and
   Comet Biela, 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
   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-Alcock was discovered independently by
   the IRAS satellite and amateur astronomers Genichi Araki and George
   Alcock. In the past, when multiple comets were discovered by the same
   individual, group of individuals, or team, the comets' names were
   distinguished by adding a numeral to the discoverers' names; thus
   Comets Shoemaker-Levy 1– 9. Today, the large numbers of comets
   discovered by some instruments (in August 2005, SOHO discovered its
   1000th comet) 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 a provisional 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)

   Increasing numbers of comet discoveries made this procedure awkward,
   and in 1994 the International Astronomical Union approved 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 for asteroids), 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. So
   Halley's Comet, the first comet to be identified as periodic, has the
   systematic designation 1P/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).

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.

   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.
   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.

   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,
   behaviour 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. 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.

   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.

Orbital studies

   The orbit of the comet of 1680, fit to a parabola, as shown in Isaac
   Newton's Principia.
   Enlarge
   The orbit of the comet of 1680, fit to a parabola, as shown in Isaac
   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 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.

   The first suggestion that Kepler's laws of planetary motion should also
   apply to the comets was made by William Lower in 1610. 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
   and Johannes Hevelius, supported comets' linear motion.

   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 1681, the Saxon pastor
   Georg Samuel Doerfel set forth his proofs that comets are heavenly
   bodies moving in parabolas of which the sun is the focus. Then Isaac
   Newton, in his Principia Mathematica of 1687, 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.

   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. (Earlier, Robert Hooke had identified the comet of
   1664 with that of 1618, while Jean-Dominique Cassini had suspected the
   identity of the comets of 1577, 1665, and 1680. 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. 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 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. 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 ☄.

Studies of physical characteristics

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

   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 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; perhaps
          To 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. 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.

   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. 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 Whipple proposed that rather than being rocky
   objects containing some ice, comets were icy objects containing some
   dust and rock. 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.
   Comet Wild 2 exhibits jets on lit side and dark side, stark relief, and
   is dry.
   Enlarge
   Comet Wild 2 exhibits jets on lit side and dark side, stark relief, and
   is dry.

   The Stardust spacecraft, launched in February 1999, collected particles
   from the coma of Comet 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.

   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.

   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.

Debate over comet composition

   Comet Borrelly exhibits jets, yet is hot and dry.
   Enlarge
   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 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".

   The recent Deep 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.

Notable comets

Great comets

   While hundreds of tiny comets pass through the inner solar system every
   year, 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 designated Great 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
   cyanogen in the tail might poison millions, while the appearance of
   Comet Hale-Bopp in 1997 triggered the mass suicide of the Heaven's Gate
   cult. To most people, however, a great comet is simply a beautiful
   spectacle. See images of Hale-Bopp at the Comet Hale-Bopp Images
   webpage.

   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 Kohoutek in 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.

   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.

Unusual comets

   Of the thousands of known comets, some are very unusual. Comet Encke
   orbits from inside the orbit of Jupiter to inside the orbit of Mercury
   while Comet 29P/Schwassmann-Wachmann orbits in a nearly circular orbit
   entirely between Jupiter and Saturn. 2060 Chiron, whose unstable orbit
   keeps it between Saturn and Uranus, was originally classified as an
   asteroid until a faint coma was noticed. Similarly, Comet
   Shoemaker-Levy 2 was originally designated asteroid 1990 UL[3]. Some
   near-earth asteroids are thought to be extinct nuclei of comets which
   no longer experience outgassing.

   Some comets have been observed to break up. Comet Biela was 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.

   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 Kreutz 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 of Comet
   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. However, it has been suggested that the
   likely object responsible for the Tunguska event in 1908 was a fragment
   of Comet Encke. Less likely is the attribution to an Encke fragment of
   having caused the formation of the Giordano Bruno (crater) on the Moon
   in 1178.

Currently visible comets

   Comet SWAN, also designated C/2006M4, discovered by SWAN/SOHO on June
   20, 2006, as of October 6 is visible with binoculars in the predawn sky
   in the constellation Canes Venatici. Perihelion was September 28.

   The comet is moving rapidly into Aquila and was an easy binocular
   target in the early evening sky throughout the month of October. It is
   now fading, as its distance from both Earth and Sun increases.

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