Comet Hale-Bopp

Comet West

Comets are small Solar System bodies that orbit the Sun and, when close enough to the Sun, exhibit a visible coma (or atmosphere) and/or a tail — both primarily from the effects of solar radiation upon the comet's nucleus. The Solar System consists of the Sun and those celestial objects bound to it by Gravity. The Sun (Sol is the Star at the center of the Solar System. In Astronomy, a coma (from the Latin word for "hair" is the nebulous envelope around the nucleus of a Comet. The nucleus is the solid central part of a Comet, popularly termed a dirty snowball. Comet nuclei are themselves loose collections of ice, dust and small rocky particles, measuring a few kilometres or tens of kilometres across.

Comets have a variety of different orbital periods, ranging from a few years, to hundreds of thousands of years, while some are believed to pass through the inner Solar System only once before being thrown out into interstellar space. Short-period comets are thought to originate in the Kuiper Belt, or associated scattered disc,^[1] which lie beyond the orbit of Neptune. The Kuiper belt (ˈkaɪpɚ to rhyme with "viper" sometimes called the Edgeworth-Kuiper belt, is a region of the Solar System beyond the planets extending The scattered disc (or scattered disk) is a distant region of the Solar System that is sparsely populated by icy Minor planets known as scattered Neptune ( English|AmE] ] is the eighth and farthest Planet from the Sun in the Solar System. Long-period comets are believed to originate at a very much greater distance from the Sun, in a cloud (the Oort cloud) consisting of debris left over from the condensation of the solar nebula. The Oort cloud ( ort alternatively the Öpik-Oort Cloud) is a hypothetical spherical cloud of Comets believed to lie roughly 50 000 AU, Condensation is the change of the physical state of aggregation (or simply state of matter from gaseous phase into liquid phase The formation and evolution of the Solar System is estimated to have begun Comets are thrown from these outer reaches of the Solar System inwards towards the Sun by gravitational perturbations from the outer planets (in the case of Kuiper Belt objects) or nearby stars (in the case of Oort Cloud objects), or as a result of collisions.

Comets leave a trail of debris behind them. If the comet's path crosses Earth's path, then at that point may be meteor showers as the Earth passes through the trail of debris. The Perseid meteor shower occurs every year between August 9 and 13 when the Earth passes through the orbit of the comet Swift-Tuttle. Halley's comet is the source of the Orionid shower in October. Halley's Comet, officially designated 1P/Halley and also referred to as Comet Halley after Edmond Halley, is a Comet that can be seen every

Comets are distinguished from asteroids by the presence of a coma and/or tail, though very old comets that have lost all their volatile materials may come to resemble asteroids. Asteroids, sometimes called Minor planets or planetoids', are bodies—primarily of the inner Solar System —that are smaller than planets but Volatility in the context of Chemistry, Physics and Thermodynamics is a measure of the tendency of a substance to Vaporize. ^[2] Asteroids are also believed to have a different origin from comets, having formed in the inner Solar System rather than the outer Solar System. ^[3] Recent findings have, however, somewhat blurred the distinction between asteroids and comets;^[4] see also Asteroid: Terminology. Asteroids, sometimes called Minor planets or planetoids', are bodies—primarily of the inner Solar System —that are smaller than planets but

There are a reported 3,354 known comets as of November 2007,^[5] of which several hundred are short-period. This number is steadily increasing. However, this represents only a tiny fraction of the total potential comet population: the reservoir of comet-like bodies in the outer solar system may number one trillion. ^[6] The number of naked-eye comets averages to roughly one per year,^[7] though many of these are faint and unspectacular. When a historically bright or notable naked-eye comet is witnessed by many people, it is often considered a Great comet. A Great Comet is a Comet which becomes particularly bright and is very spectacular to a casual observer on Earth.

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. English is a West Germanic language originating in England and is the First language for most people in the United Kingdom, the United States Latin ( lingua Latīna, laˈtiːna is an Italic language, historically spoken in Latium and Ancient Rome. Greek (el ελληνική γλώσσα or simply el ελληνικά — "Hellenic" is an Indo-European language, spoken today by 15-22 million people mainly Aristotle (Greek Aristotélēs) (384 BC – 322 BC was a Greek philosopher a student of Plato and teacher of Alexander the Great. " The astronomical symbol for comets (☄) accordingly consists of a disc with a hairlike tail. Astronomical symbols are symbols used to represent various Celestial objects theoretical constructs and observational events in Astronomy.

Physical characteristics

Nucleus of comet Tempel 1 imaged by the Deep Impact impactor. 9P/Tempel, also known as Tempel 1 or 9P/Tempel 1, is a periodic Comet. Deep Impact is an ongoing NASA Space probe launched on 12 January 2005 that was designed to study the composition of the interior of the Comet The nucleus measures about 6 kilometres across.

Comet Hyakutake

Comet nuclei are in a range from 1/2 kilometer to 50 kilometers across and are composed of rock, dust, water ice, and frozen gases such as carbon monoxide, carbon dioxide, methane and ammonia. Comet Hyakutake (çʲakɯ̥take formally designated C/1996 B2) is a Comet discovered on January 30 1996 which passed very close to Earth Carbon monoxide, with the chemical formula CO is a colorless odorless tasteless yet highly toxic Gas. Carbon dioxide ( Chemical formula:) is a Chemical compound composed of two Oxygen Atoms covalently bonded to a single Methane is a Chemical compound with the molecular formula. It is the simplest Alkane, and the principal component of Natural gas. Ammonia is a compound with the formula N[[hydrogen H3]] It is normally encountered as a Gas with a characteristic pungent Odor ^[8] They are often popularly described as "dirty snowballs", though recent observations have revealed dry dusty or rocky surfaces, suggesting that the ices are hidden beneath the crust (see Debate over comet composition). Comets also contain a variety of organic compounds; in addition to the gases already mentioned, these may include methanol, hydrogen cyanide, formaldehyde, ethanol and ethane, and perhaps more complex molecules such as long-chain hydrocarbons and amino acids. An organic compound is any member of a large class of Chemical compounds whose Molecules contain Carbon. Methanol, also known as methyl alcohol, carbinol, wood alcohol, wood naphtha or wood spirits, is a Chemical compound Hydrogen cyanide is a Chemical compound with Chemical formula HCN Formaldehyde is a Chemical compound with the formula H2CO It is the simplest Aldehyde —an organic compound containing a terminal Carbonyl ETHANE is a mnemonic indicating a protocol used by Emergency services to report situations which they may be faced with especially as it relates to major incidents where In Organic chemistry, a hydrocarbon is an Organic compound consisting entirely of Hydrogen and Carbon. In Chemistry, an amino acid is a Molecule containing both Amine and Carboxyl Functional groups In Biochemistry, this ^[9][10][11] Comet nuclei are irregularly shaped: they have insufficient mass (and hence gravity) to become spherical.

In the outer solar system, comets remain frozen and are extremely difficult or impossible to detect from Earth due to their small size (though some observations of comet nuclei in the Kuiper Belt have been made^[12]). The Solar System consists of the Sun and those celestial objects bound to it by Gravity. As a comet approaches the inner solar system, solar radiation causes the water, frozen gases and other volatile materials within the comet to vaporise and stream out of the nucleus, carrying dust away with them. The Solar System consists of the Sun and those celestial objects bound to it by Gravity. The streams of dust and gas thus released form a huge, 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. In Astronomy, a coma (from the Latin word for "hair" is the nebulous envelope around the nucleus of a Comet. Radiation pressure is the Pressure exerted upon any surface exposed to Electromagnetic radiation. The solar wind is a Stream of charged particles&mdasha plasma &mdashthat are ejected from the upper atmosphere of the Sun.

The streams of dust and gas each form their own distinct tail, pointing in slightly different directions. The tail 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 points directly away from the Sun, as this gas is more strongly affected by the solar wind than is dust, following magnetic field lines rather than an orbital trajectory. While the solid nucleus of comets is generally less than 50 km across, the coma may be larger than the Sun, and ion tails have been observed to extend 1 astronomical unit (150 million km) or more. The astronomical unit ( AU or au or au or sometimes ua) is a unit of Length based on the distance from the Earth to the ^[13]

Both the coma and tail are illuminated by the Sun and may become visible from Earth when a comet passes through the inner solar system, the dust reflecting sunlight directly and the gases glowing from ionisation. EARTH was a short-lived Japanese vocal trio which released 6 singles and 1 album between 2000 and 2001 An ion is an Atom or Molecule which has lost or gained one or more Valence electrons giving it a positive or negative electrical charge 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. A telescope is an instrument designed for the observation of remote objects and the collection of Electromagnetic radiation. Occasionally a comet may experience a huge and sudden outburst of gas and dust, during which the size of the coma temporarily greatly increases in size. This happened in 2007 to Comet Holmes. 17P/Holmes is a Periodic comet in our Solar system, discovered by the British amateur astronomer Edwin Holmes on November 6, 1892.

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. Giotto was a European Robotic spacecraft mission from the European Space Agency, intended to fly by and study Halley's Comet. Halley's Comet, officially designated 1P/Halley and also referred to as Comet Halley after Edmond Halley, is a Comet that can be seen every Deep Space 1 is a Spacecraft launched on 24 October 1998 as part of NASA 's New Millennium program. Comet Borrelly (formally designated 19P/Borrelly) is a periodic Comet, which was visited by the space craft Deep Space 1. 4% to 3% of the light that falls on it; by comparison, asphalt reflects 7% of the light that falls on it. Asphalt ( is a sticky black and highly viscous liquid or semi-solid that is present in most crude Petroleums and in some natural deposits sometimes termed asphaltum 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. Tar is a viscous black Liquid derived from the Destructive distillation of organic matter Petroleum ( L petroleum, from Greek πετρέλαιον, lit 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. X-radiation (composed of X-rays) is a form of Electromagnetic radiation. ^[14] 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. An ion is an Atom or Molecule which has lost or gained one or more Valence electrons giving it a positive or negative electrical charge In these collisions, the ions will capture one or more electrons leading to emission of X-rays and far ultraviolet photons. ^[15]

The fate of comets

Eventually – typically after many orbits of the Sun – all the volatile material contained in a comet nucleus evaporates away, and the comet either disintegrates into a trail of dust or becomes a small, dark, inert lump of rock or rubble^[16] that may come to resemble an asteroid. Asteroids, sometimes called Minor planets or planetoids', are bodies—primarily of the inner Solar System —that are smaller than planets but Comets are also known to break up into large fragments, as happened with Comet Schwassmann-Wachmann 3 in 2006. 73P/Schwassmann-Wachmann, also known as Schwassmann-Wachmann 3, is a periodic Comet in our Solar system which is in the process of disintegrating This breakup may be triggered by tidal gravitational forces from the Sun or a large planet, by an "explosion" of volatile material, or for other reasons not fully explained.

Some comets meet a more spectacular end – either falling into the Sun,^[17] or smashing into a planet or other body. Collisions between comets and planets or moons were common in the early Solar System: some of the many craters on the Earth's Moon, for example, may have been caused by comets. A recent collision of a comet with a planet occurred in 1994 when Comet Shoemaker-Levy 9 broke up into pieces and collided with Jupiter. Shoemaker-Levy redirects here For other Shoemaker-Levy comets see List of periodic comets.

Many comets and asteroids collided into Earth in its early stages. Many scientists believe that comets bombarding the young Earth (about 4 billion years ago) brought the vast quantities of water that now fill the Earth's oceans, or at least a significant proportion of it. But other researchers have cast doubt on this theory. ^[18] The detection of organic molecules in comets has led some to speculate that comets and/or meteorites may have brought the precursors of life – or even life itself – to Earth. A meteorite is a natural object originating in Outer space that survives an impact with the Earth 's surface ^[10] There are still many near-Earth comets, although a collision with an asteroid is more likely than with a comet.

It is suspected that comet impacts have, over long timescales, also delivered significant quantities of water to the Earth's Moon, some of which may have survived as lunar ice. Lunar ice is water ice that is hypothesised to exist on the surface of the Moon, delivered over geological timescales by the regular bombardment of the Moon by

Orbital characteristics

Orbits of Comet Kohoutek (red) and Earth (blue), illustrating the high eccentricity of the orbit and more rapid motion when closer to the Sun. There are two other long-period comets named Kohoutek C/1969 O1 (a EARTH was a short-lived Japanese vocal trio which released 6 singles and 1 album between 2000 and 2001 In Astrodynamics, under standard assumptions, any Orbit must be of Conic section shape The Sun (Sol is the Star at the center of the Solar System.

Histogram of the aphelia of the 2005 comets, showing the giant planet comet families. In Statistics, a histogram is a Graphical display of tabulated frequencies, shown as Bars It shows what proportion of cases fall into each of In Celestial mechanics, an apsis, plural apsides (ˈæpsɨdɪːz is the point of greatest or least distance of the Elliptical orbit of an object from The abscissa is the natural logarithm of the aphelion expressed in AUs. In Mathematics, the Cartesian coordinate system (also called rectangular coordinate system) is used to determine each point uniquely in a plane The natural logarithm, formerly known as the Hyperbolic logarithm is the Logarithm to the base e, where e is an irrational The astronomical unit ( AU or au or au or sometimes ua) is a unit of Length based on the distance from the Earth to the

Most comets have elongated elliptical orbits that take them close to the Sun for a part of their orbit, and then out into the further reaches of the Solar System for the remainder. In Astrodynamics or Celestial mechanics an elliptic orbit is a Kepler orbit with the eccentricity greater than 0 and less than 1 Comets are often classified according to the length of their orbital period; the longer the period the more elongated the ellipse. The orbital period is the time taken for a given object to make one complete Orbit about another object

• Short-period comets are generally defined as having orbital periods of less than 200 years. Periodic comets are defined for these purposes as those Comets having orbital periods of less than 200 years (also known as "short-period comets" or which They usually orbit more-or-less in the ecliptic plane in the same direction as the planets. The ecliptic is the apparent path that the Sun traces out in the sky during the year Their orbits typically take them out to the region of the outer planets (Jupiter and beyond) at aphelion; for example, Comet Halley's aphelion is a little way beyond the orbit of Neptune. In Celestial mechanics, an apsis, plural apsides (ˈæpsɨdɪːz is the point of greatest or least distance of the Elliptical orbit of an object from Halley's Comet, officially designated 1P/Halley and also referred to as Comet Halley after Edmond Halley, is a Comet that can be seen every Neptune ( English|AmE] ] is the eighth and farthest Planet from the Sun in the Solar System. At the shorter extreme, Comet Encke has an orbit which never places it farther from the Sun than Jupiter. Comet Encke (officially designated 2P/Encke) is a periodic Comet, named after Johann Franz Encke, who through laborious study of its Orbit and Short-period comets are further divided into the Jupiter family (periods less than 20 years) and Halley family (periods between 20 and 200 years).

• Long-period comets have highly eccentric (elongated) orbits and periods ranging from 200 years to thousands or even millions of years. In Astrodynamics, under standard assumptions, any Orbit must be of Conic section shape (However, by definition they remain gravitationally bound to the Sun; those comets that are ejected from the solar system due to close passes by major planets are no longer properly considered as having "periods". ) Their orbits take them far beyond the outer planets at aphelia, and the plane of their orbits need not lie near the ecliptic.

• Single-apparition comets are similar to long-period comets, but have parabolic or hyperbolic trajectories which will cause them to permanently exit the solar system after passing the Sun once. In Astrodynamics or Celestial mechanics a Parabolic trajectory is a Kepler orbit with the eccentricity equal to 1 In Astrodynamics or Celestial mechanics a hyperbolic trajectory is a Kepler orbit with the eccentricity greater than 1

• Some authorities use the term periodic comet to refer to any comet with a periodic orbit (that is, all short-period comets plus all long-period comets),^[19] while others use it to mean exclusively short-period comets. ^[20] Similarly, although the literal meaning of non-periodic comet is the same as single-apparition comet, some use it to mean all comets that are not "periodic" in the second sense (that is, to also include all comets with a period greater than 200 years).

• Recently-discovered main-belt comets form a distinct class, orbiting in more circular orbits within the asteroid belt. Main-belt comets are bodies orbiting within the ( main) asteroid belt which show cometary activity during a part of their orbit The asteroid belt is the region of the Solar System located roughly between the orbits of the Planets Mars and Jupiter. ^[21][22]

Based on their orbital characteristics, short-period comets are thought to originate in the Kuiper belt or the scattered disk^[1] – a disk of objects in the transneptunian region – whereas the source of long-period comets is thought to be the far more distant spherical Oort cloud (after the Dutch astronomer Jan Hendrik Oort who hypothesised its existence). The Kuiper belt (ˈkaɪpɚ to rhyme with "viper" sometimes called the Edgeworth-Kuiper belt, is a region of the Solar System beyond the planets extending The scattered disc (or scattered disk) is a distant region of the Solar System that is sparsely populated by icy Minor planets known as scattered The Oort cloud ( ort alternatively the Öpik-Oort Cloud) is a hypothetical spherical cloud of Comets believed to lie roughly 50 000 AU, Jan Hendrik Oort ( April 28, 1900 &ndash November 5, 1992) was a Dutch Astronomer. ^[23] Vast swarms of comet-like bodies are believed to orbit the Sun in these distant regions in roughly circular orbits. Occasionally the gravitational influence of the outer planets (in the case of Kuiper Belt objects) or nearby stars (in the case of Oort cloud objects) may throw one of these bodies into an elliptical orbit that takes it inwards towards the Sun, to form a visible comet. The Sun (Sol is the Star at the center of the Solar System. Unlike the return of periodic comets whose orbits have been established by previous observations, the appearance of new comets by this mechanism is unpredictable.

Since their elliptical orbits frequently take them close to the giant planets, comets are often subject to further gravitational perturbations. Short period comets display a 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. In Celestial mechanics, an apsis, plural apsides (ˈæpsɨdɪːz is the point of greatest or least distance of the Elliptical orbit of an object from A gas giant (sometimes also known as a Jovian planet after the planet Jupiter, or giant planet) is a large Planet that is not primarily In Statistics, a histogram is a Graphical display of tabulated frequencies, shown as Bars It shows what proportion of cases fall into each of 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. These perturbations may sometimes deflect long-period comets into shorter orbital periods (Halley's Comet being a possible example). Halley's Comet, officially designated 1P/Halley and also referred to as Comet Halley after Edmond Halley, is a Comet that can be seen every

Early observations have revealed a few genuinely hyperbolic (i. e. non-periodic) trajectories, 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 trajectories. A rough calculation shows that there might be four hyperbolic comets per century, within Jupiter's orbit, give or take one and perhaps two orders of magnitude.

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. The LI ncoln N ear- E arth A steroid R esearch ( LINEAR) project is a cooperative project between the United States Air Force ^[24]

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"). The Great Comet of 1680, formally known as C/1680 V1 or Kirch's Comet, has the distinction of being the first Comet discovered by Telescope. The Great Comet of 1882 formally designated C/1882 R1, 1882 II, and 1882b, was a Comet which became very bright in September The Great January comet of 1910 or Great Daylight comet was a Great comet that in January 1910 upstaged the much-anticipated appearance of Halley's Comet 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. Edmond Halley FRS (ˈɛdmənd ˈhɔːlɪ ( November 8, 1656 &ndash January 14, 1742) was an English Astronomer Halley's Comet, officially designated 1P/Halley and also referred to as Comet Halley after Edmond Halley, is a Comet that can be seen every Similarly, the second and third known periodic comets, Comet Encke^[25] and Comet Biela,^[26] were named after the astronomers who calculated their orbits rather than their original discoverers. Comet Encke (officially designated 2P/Encke) is a periodic Comet, named after Johann Franz Encke, who through laborious study of its Orbit and 3D/Biela is the official designation for a lost periodic Comet discovered in 1826 by Wilhelm von Biela. 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. Comet IRAS-Araki-Alcock (C/1983 H1 was a small Comet that in 1983, made the closest approach to the earth (about 5000000 km of any comet in 200 years The Infrared Astronomical Satellite ( IRAS) was the first-ever space-based Observatory to perform a survey of the entire Sky at Infrared George Eric Deacon Alcock ( August 28 1912 Peterborough, Cambridgeshire &ndash December 15 2000) was an English 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 (but only for periodic comets); thus Comets Shoemaker-Levy 1–9. Shoemaker-Levy redirects here For other Shoemaker-Levy comets see List of periodic comets. Today, the large numbers of comets discovered by some instruments (in August 2005, SOHO discovered its 1000th comet^[27]) has rendered this system impractical, and no attempt is made to ensure that each comet has a unique name. The Solar and Heliospheric Observatory ( SOHO) is a Spacecraft that was launched on a Lockheed Martin Atlas IIAS launch vehicle on December Instead, the comets' systematic designations are used to avoid confusion. ^[28]

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 1969i (Bennett) was the 9th comet discovered in 1969). A provisional designation in astronomy is the naming convention applied to Astronomical objects immediately following their discovery Comet Bennett, formally known as C/1969 Y1 (old style 1970 II and 1969i) was one of two brilliant comets to grace the 1970s along with 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 1969i became Comet 1970 II (it was the second comet to pass perihelion in 1970)^[29]

Increasing numbers of comet discoveries made this procedure awkward, and in 1994 the International Astronomical Union approved a new naming system. In Celestial mechanics, an apsis, plural apsides (ˈæpsɨdɪːz is the point of greatest or least distance of the Elliptical orbit of an object from Roman numerals are a Numeral system originating in ancient Rome, adapted from Etruscan numerals. Comet Bennett, formally known as C/1969 Y1 (old style 1970 II and 1969i) was one of two brilliant comets to grace the 1970s along with 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. Asteroids, sometimes called Minor planets or planetoids', are bodies—primarily of the inner Solar System —that are smaller than planets but Prefixes are also added to indicate the nature of the comet:

• P/ indicates a periodic comet (defined for these purposes as any comet with an orbital period of less than 200 years or confirmed observations at more than one perihelion passage);
• C/ indicates a non-periodic comet (defined as any comet that is not periodic according to the preceding definition);
• X/ indicates a comet for which no reliable orbit could be calculated (generally, historical comets);
• D/ indicates a comet which has broken up or been lost;
• A/ indicates an object that was mistakenly identified as a comet, but is actually a minor planet. Minor planet is a term used since the 19th century to describe objects such as Asteroids that are in Orbit around the Sun but are not Planets

After their second observed perihelion passage, periodic comets are also assigned a number indicating the order of their discovery. ^[30] So Halley's Comet, the first comet to be identified as periodic, has the systematic designation 1P/1682 Q1. Halley's Comet, officially designated 1P/Halley and also referred to as Comet Halley after Edmond Halley, is a Comet that can be seen every Comet Hale-Bopp's designation is C/1995 O1. Comet Hale-Bopp ( formally designated C/1995 O1) was probably the most widely observed Comet of the twentieth century and one of the brightest Comets which first received a minor planet designation keep the latter, which leads to some odd names such as P/2004 EW₃₈ (Catalina-LINEAR).

There are only five objects that are cross-listed as both comets and asteroids: 2060 Chiron (95P/Chiron), 4015 Wilson-Harrington (107P/Wilson-Harrington), 7968 Elst-Pizarro (133P/Elst-Pizarro), 60558 Echeclus (174P/Echeclus), and 118401 LINEAR (176P/LINEAR (LINEAR 52)). TemplateInfobox Planet. --> 2060 Chiron (, or as in Χείρων) is a Planetoid in the TemplateInfobox Planet. --> 2060 Chiron (, or as in Χείρων) is a Planetoid in the 60558 Echeclus (ɪˈkɛkləs e-kek'-ləs from Έχεκλος) is a centaur in the outer Solar system. 60558 Echeclus (ɪˈkɛkləs e-kek'-ləs from Έχεκλος) is a centaur in the outer Solar system.

History of comet study

Early observations and thought

Comet Halley depicted on the Bayeux Tapestry which shows King Harold I being told of Halley's Comet before the Battle of Hastings in 1066. Halley's Comet, officially designated 1P/Halley and also referred to as Comet Halley after Edmond Halley, is a Comet that can be seen every The Bayeux Tapestry (Tapisserie de Bayeux is a 50 cm by 70 m (20 in by 230 ft long embroidered cloth which explains the events leading up to the 1066 Norman invasion of

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, or even interpreted as attacks by heavenly beings against terrestrial inhabitants. An omen (also called portent or presage) is a phenomenon that is believed to foretell the Future, often signifying the advent of change From ancient sources, such as Chinese oracle bones, it is known that their appearances have been noticed by humans for millennia. Oracle bones ( Chinese: 甲骨 Pinyin: jiǎgǔpiàn are pieces of Bone or turtle shell that were heated and cracked during divination Some authorities interpret references to "falling stars" in Gilgamesh, the Book of Revelation and the Book of Enoch as references to comets, or possibly bolides. Gilgamesh was the son of Lugalbanda and the fifth king of Uruk (Early Dynastic II first dynasty of Uruk ruling circa 2600 BC according to the Sumerian king The Book of Revelation, also called Revelation to John, Apocalypse of John ( pronounced, from the Ἀποκάλυψις Ἰωάννου The Book of Enoch is any of several works that attribute themselves to Enoch, the great-grandfather of Noah and son of Jared (

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. Meteorology (from Greek grc μετέωρος metéōros, "high in the sky" and grc -λογία -logia) is the Interdisciplinary Aristotle (Greek Aristotélēs) (384 BC – 322 BC was a Greek philosopher a student of Plato and teacher of Alexander the Great. 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. A planet, as defined by the International Astronomical Union (IAU is a celestial body Orbiting a Star or stellar remnant that is Zodiac denotes an annual cycle of twelve stations along the Ecliptic, the apparent path of the sun across the heavens through the Constellations that divide the ecliptic ^[31] Instead, he described comets as a phenomenon of the upper atmosphere, where hot, dry exhalations gathered and occasionally burst into flame. Temperature and layers The temperature of the Earth's atmosphere varies with altitude the mathematical relationship between temperature and altitude varies among five Aristotle held this mechanism responsible for not only comets, but also meteors, the aurora borealis, and even the Milky Way. The Milky Way (a translation of the Latin Via Lactea, in turn derived from the Greek Γαλαξίας (Galaxias sometimes referred to simply ^[32]

A few later classical philosophers did dispute this view of comets. Seneca the Younger, in his Natural Questions, observed that comets moved regularly through the sky and were undisturbed by the wind, behavior more typical of celestial than atmospheric phenomena. Lucius Annaeus Seneca (often known simply as Seneca, or Seneca the Younger; Σένεκας in Ancient Greek literature (c 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. ^[33] However, the Aristotelian 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.

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. Halley's Comet, officially designated 1P/Halley and also referred to as Comet Halley after Edmond Halley, is a Comet that can be seen every The Bayeux Tapestry (Tapisserie de Bayeux is a 50 cm by 70 m (20 in by 230 ft long embroidered cloth which explains the events leading up to the 1066 Norman invasion of England is a Country which is part of the United Kingdom. Its inhabitants account for more than 83% of the total UK population whilst its mainland ^[34]

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 measurable parallax. Tycho Brahe, born Tyge Ottesen Brahe ( December 14 1546 &ndash October 24 1601) was a Danish nobleman Parallax is an apparent displacement or difference of orientation of an object viewed along two different lines of sight and is measured by the angle or semi-angle of inclination between Within the precision of the measurements, this implied the comet must be at least four times more distant from the earth than the moon. ^[35]

Orbital studies

The orbit of the comet of 1680, fit to a parabola, as shown in Isaac Newton's Principia. In Mathematics, the parabola (pəˈræbələ from the Greek παραβολή) is a Conic section, the intersection of a right circular Sir Isaac Newton, FRS (ˈnjuːtən 4 January 1643 31 March 1727) Biography Early years See also Isaac Newton's early life and achievements The Philosophiæ Naturalis Principia Mathematica ( Latin: "mathematical principles of natural philosophy" often 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. Johannes Kepler (ˈkɛplɚ ( December 27 1571 &ndash November 15 1630) was a German Mathematician, Astronomer In Mathematics, an ellipse (from the Greek ἔλλειψις literally absence) is a Conic section, the locus of points in a In Astronomy, Kepler's Laws of Planetary Motion are three mathematical laws that describe the motion of Planets in the Solar System. Galileo Galilei, although a staunch Copernicanist, rejected Tycho's parallax measurements and held to the Aristotelian notion of comets moving on straight lines through the upper atmosphere. Galileo Galilei (15 February 1564 &ndash 8 January 1642 was a Tuscan ( Italian) Physicist, Mathematician, Astronomer, and Philosopher ^[36]

The first suggestion that Kepler's laws of planetary motion should also apply to the comets was made by William Lower in 1610. ^[35] 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. Pierre Petit (1832-1909 was a French photographer born in Aups, in Provence. Giovanni Alfonso Borelli ( Pisa January 28, 1608 - December 31, 1679) was a Renaissance Italian Physiologist Adrien Auzout ( January 28, 1622 &ndash May 23, 1691) was a French Astronomer. Robert Hooke, FRS (18 July 1635 – 3 March 1703 was an English Natural philosopher and Polymath who played an important role in the Johann Baptist Cysat ( Latinized as Cysatus; in French, Jean-Baptiste Cysat) (c Giovanni Domenico Cassini ( June 8, 1625 &ndash September 14, 1712) was an Italian Mathematician, Astronomer Christiaan Huygens (ˈhaɪgənz in English ˈhœyɣəns in Dutch) ( April 14, 1629 &ndash July 8, 1695) was a Dutch Johannes Hevelius ( Latin) also called Johannes Hewel Johann Hewelke Johannes Höwelcke in German, or Jan Heweliusz (in Polish) ^[36]

The matter was resolved by the bright comet that was discovered by Gottfried Kirch on November 14, 1680. The Great Comet of 1680, formally known as C/1680 V1 or Kirch's Comet, has the distinction of being the first Comet discovered by Telescope. Gottfried Kirch ( Kirche, Kirkius) (December 18 1639—July 25 1710 was a German astronomer. Events 1533 - Conquistadors from Spain under the leadership of Francisco Pizarro arrive in Cajamarca, Inca 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. The Free State of Saxony (Freistaat Sachsen ˈzaksən Swobodny Stat Sakska is the easternmost federal state of Germany. In Mathematics, the parabola (pəˈræbələ from the Greek παραβολή) is a Conic section, the intersection of a right circular 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. Sir Isaac Newton, FRS (ˈnjuːtən 4 January 1643 31 March 1727) Biography Early years See also Isaac Newton's early life and achievements The Philosophiæ Naturalis Principia Mathematica ( Latin: "mathematical principles of natural philosophy" often Principia In Physics, an inverse-square law is any Physical law stating that some physical Quantity or strength is inversely proportional Gravitation is a natural Phenomenon by which objects with Mass attract one another In Mathematics, a conic section (or just conic) is a Curve obtained by intersecting a cone (more precisely a circular Conical surface ^[37]

In 1705, Edmond Halley applied Newton's method to twenty-four cometary apparitions that had occurred between 1337 and 1698. Edmond Halley FRS (ˈɛdmənd ˈhɔːlɪ ( November 8, 1656 &ndash January 14, 1742) was an English Astronomer 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. The elements of an orbit are the parameters needed to specify that Orbit uniquely given a model of two point-masses obeying the Newtonian laws of motion and the Confident that these three apparitions had been three appearances of the same comet, he predicted that it would appear again in 1758–9. ^[38] (Earlier, Robert Hooke had identified the comet of 1664 with that of 1618,^[39] while Jean-Dominique Cassini had suspected the identity of the comets of 1577, 1665, and 1680. ^[40] 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. This article is about the country For a topic outline on this subject see List of basic France topics. Alexis Claude de Clairault (or Clairaut) ( May 3, 1713 – May 17, 1765) was a French Mathematician and Joseph Jérôme Lefrançais de Lalande ( July 11, 1732 &ndash April 4, 1807) was a French Astronomer and writer Nicole-Reine Étable de la Brière Lepaute (1723-1788 was a French Astronomer. ^[41] When the comet returned as predicted, it became known as Comet Halley or Halley's Comet (its official designation is 1P/Halley). Halley's Comet, officially designated 1P/Halley and also referred to as Comet Halley after Edmond Halley, is a Comet that can be seen every Its next appearance will be 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. A telescope is an instrument designed for the observation of remote objects and the collection of Electromagnetic radiation. The second comet to be discovered to have a periodic orbit was Comet Encke (official designation 2P/Encke). Comet Encke (officially designated 2P/Encke) is a periodic Comet, named after Johann Franz Encke, who through laborious study of its Orbit and 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 that they were same comet, and successfully predicted its return in 1822. Germany, officially the Federal Republic of Germany ( ˈbʊndəsʁepuˌbliːk ˈdɔʏtʃlant is a Country in Central Europe. Johann Franz Encke ( 23 September 1791 – 26 August 1865) was a German Astronomer, born in Hamburg. ^[25] 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 ☄. An ephemeris (plural ephemerides; from the Greek word ἐφήμερος ephemeros "daily" is a table of values that gives the positions of

Studies of physical characteristics

Comets have highly elliptical orbits. In Astrodynamics or Celestial mechanics an elliptic orbit is a Kepler orbit with the eccentricity greater than 0 and less than 1 Note the two distinct tails:Cyan for gas tail, grey for dust tail.

Isaac Newton described comets as compact and durable solid bodies moving in oblique orbits, and their tails as thin streams of vapor emitted by their nuclei, ignited or heated by the sun. Sir Isaac Newton, FRS (ˈnjuːtən 4 January 1643 31 March 1727) Biography Early years See also Isaac Newton's early life and achievements The nucleus is the solid central part of a Comet, popularly termed a dirty snowball. Newton suspected that comets were the origin of the life-supporting component of air. Newton also believed that the vapors given off by comets might replenish the planets' supplies of water (which was gradually being converted into soil by the growth and decay of plants), and the sun's supply of fuel —

As early as the 18th century, some scientists had made correct hypotheses as to comets' physical composition. James Thomson ( 11 September, 1700 &ndash 27 August, 1748) was a Scottish Poet and playwright known for his masterpiece In 1755, Immanuel Kant hypothesized that comets are composed of some volatile substance, whose vaporization gives rise to their brilliant displays near perihelion. Immanuel Kant (ɪmanuəl kant 22 April 1724 12 February 1804 was an 18th-century German Philosopher from the Prussian city of Königsberg ^[42] 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. Friedrich Wilhelm Bessel (22 July 1784 &ndash 17 March 1846 was a German Mathematician, Astronomer, and systematizer of the Bessel functions The jet force is a rocket-like force due to Newton's third law which states that for every action there is an equal and opposite Reaction. Comet Encke (officially designated 2P/Encke) is a periodic Comet, named after Johann Franz Encke, who through laborious study of its Orbit and ^[43]

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. Italy (Italia officially the Italian Republic, (Repubblica Italiana is located on the Italian Peninsula in Southern Europe, and on the two largest Honors and Awards Awards Gold Medal of the Royal Astronomical Society (1872 Bruce Medal (1902 Named The Perseids (ˈpɝsiːɨdz pûr'sē-ĭdz is a prolific Meteor shower associated with the 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 was never seen again after 1852. 3D/Biela is the official designation for a lost periodic Comet discovered in 1826 by Wilhelm von Biela. ^[26] 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. Fred Lawrence Whipple ( November 5, 1906 – August 30, 2004) was an American Astronomer. ^[44] 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. A spacecraft is a Vehicle or machine designed for Spaceflight. The European Space Agency ( ESA) established in 1975 is an intergovernmental organisation dedicated to the exploration of space, currently with 17 member Giotto was a European Robotic spacecraft mission from the European Space Agency, intended to fly by and study Halley's Comet. The Union of Soviet Socialist Republics (USSR was a constitutionally Socialist state that existed in Eurasia from 1922 to 1991 Vega 1 (along with its twin Vega 2) is a Soviet Space probe part of the Vega program. Vega 2 (along with Vega 1) is a Soviet Space probe part of the Vega program. 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. Deep Space 1 is a Spacecraft launched on 24 October 1998 as part of NASA 's New Millennium program. Comet Borrelly (formally designated 19P/Borrelly) is a periodic Comet, which was visited by the space craft Deep Space 1. Events 1217 - The Estonian tribal leader Lembitu of Lehola was killed in a battle against Teutonic Knights. Year 2001 ( MMI) was a Common year starting on Monday according to the Gregorian calendar.

Although comets formed in the outer Solar System, radial mixing of material during the early formation of the Solar System is thought to have redistributed material throughout the proto-planetary disk,^[45] so comets also contain crystalline grains which were formed in the hot inner Solar System. This is seen in comet spectra as well as in sample return missions.

Comet Wild 2 exhibits jets on lit side and dark side, stark relief, and is dry. Comet 81P/Wild, also known as Wild 2, is a Comet named after Swiss astronomer Paul Wild (pronounced Vilt) who discovered it in 1978

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. Stardust is an American interplanetary mission of the NASA Jet Propulsion Laboratory, whose primary purpose was to investigate the makeup of the Comet 81P/Wild, also known as Wild 2, is a Comet named after Swiss astronomer Paul Wild (pronounced Vilt) who discovered it in 1978 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. ^[46]

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. Deep Impact is an ongoing NASA Space probe launched on 12 January 2005 that was designed to study the composition of the interior of the Comet 9P/Tempel, also known as Tempel 1 or 9P/Tempel 1, is a periodic Comet. And in 2014, the European Rosetta probe will orbit comet Comet Churyumov-Gerasimenko and place a small lander on its surface. Rosetta is a European Space Agency -led Robotic spacecraft mission launched in 2004 intended to study the comet 67P/Churyumov-Gerasimenko. 67P/Churyumov-Gerasimenko is the designation of a Comet with a current Orbital period of 6

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 than Deep Impact itself to view the comet.

Debate over comet composition

Comet Borrelly exhibits jets, yet is hot and dry. Comet Borrelly (formally designated 19P/Borrelly) is a periodic Comet, which was visited by the space craft Deep Space 1.

Debate continues about how much ice is in a comet. In 2001, NASA's Deep Space 1 team, working at NASA's Jet Propulsion Lab, obtained high-resolution images of the surface of Comet Borrelly. Deep Space 1 is a Spacecraft launched on 24 October 1998 as part of NASA 's New Millennium program. Comet Borrelly (formally designated 19P/Borrelly) is a periodic Comet, which was visited by the space craft Deep Space 1. 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". ^[47]

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 reservoirs feed the jets of vaporised water that form the coma of Tempel 1. Deep Impact is an ongoing NASA Space probe launched on 12 January 2005 that was designed to study the composition of the interior of the Comet ^[48]

However, more recent data from the Stardust mission show that materials retrieved from the tail of comet Wild 2 were crystalline and could only have been "born in fire. Stardust is an American interplanetary mission of the NASA Jet Propulsion Laboratory, whose primary purpose was to investigate the makeup of the Comet 81P/Wild, also known as Wild 2, is a Comet named after Swiss astronomer Paul Wild (pronounced Vilt) who discovered it in 1978 "^[49][50] More recent still, the materials retrieved demonstrate that the "comet dust resembles asteroid materials. "^[51][52][53] These new results have forced a rethink about the very nature of comets and their distinction from asteroids. ^[54]

Notable comets

Great comets

Main article: Great Comet

While hundreds of tiny comets pass through the inner solar system every year, very few are noticed by the general public. A Great Comet is a Comet which becomes particularly bright and is very spectacular to a casual observer on Earth. 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. A Great Comet is a Comet which becomes particularly bright and is very spectacular to a casual observer on Earth. 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. Halley's Comet, officially designated 1P/Halley and also referred to as Comet Halley after Edmond Halley, is a Comet that can be seen every Cyanogen is the Chemical compound with the formula ( C[[nitrogen N]]2 Comet Hale-Bopp ( formally designated C/1995 O1) was probably the most widely observed Comet of the twentieth century and one of the brightest Heaven's Gate was an American UFO religion based in San Diego California and led by Marshall Applewhite (1932-1997 and Bonnie Nettles (1927-1985 To most people, however, a great comet is simply a beautiful spectacle.

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. There are two other long-period comets named Kohoutek C/1969 O1 (a 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. There is another long-period comet West C/1978 A1 (aka 1977 IX 1978a ^[55]

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. Comet Hyakutake (çʲakɯ̥take formally designated C/1996 B2) is a Comet discovered on January 30 1996 which passed very close to Earth The first great comet of the 21st century was Comet McNaught, which became visible to naked eye observers in January 2007. Comet McNaught, also known as the Great Comet of 2007 and given the designation C/2006 P1, is a non-periodic Comet discovered on It was the brightest in over 40 years.

Sungrazing comets

Main article: Sungrazing comet

The Great Comet of 1882, is a member of the Kreutz group

A Sungrazing comet is a comet that passes extremely close to the Sun at perihelion, sometimes within a few thousand kilometres of the Sun's surface. A sungrazing comet is a Comet that passes extremely close to the Sun at Perihelion - sometimes within a few thousand kilometres of the Sun's surface The Great Comet of 1882 formally designated C/1882 R1, 1882 II, and 1882b, was a Comet which became very bright in September The Kreutz Sungrazers (ˈkrɔɪts are Sungrazing comets of a particular family While small sungrazers can be completely evaporated during such a close approach to the Sun, larger sungrazers can survive many perihelion passages. The Sun (Sol is the Star at the center of the Solar System. In Celestial mechanics, an apsis, plural apsides (ˈæpsɨdɪːz is the point of greatest or least distance of the Elliptical orbit of an object from However, the strong tidal forces they experience often lead to their fragmentation. The tidal force is a secondary effect of the Force of Gravity and is responsible for the Tides It arises because the gravitational acceleration experienced

About 90% of the sungrazers observed with SOHO are members of the Kreutz group, which all originate from one giant comet that broke up into many smaller comets during its first passage through the inner solar system. The Solar and Heliospheric Observatory ( SOHO) is a Spacecraft that was launched on a Lockheed Martin Atlas IIAS launch vehicle on December The Kreutz Sungrazers (ˈkrɔɪts are Sungrazing comets of a particular family The Solar System consists of the Sun and those celestial objects bound to it by Gravity. ^[56] The other 10% contains some sporadic sungrazers, but four other related groups of comets have been identified among them: the Kracht, Kracht 2a, Marsden and Meyer groups. The Marsden and Kracht groups both appear to be related to Comet 96P/Machholz, which is also the parent of two meteor streams, the Quadrantids and the Arietids. Comet 96P/Machholz or 96P/Machholz 1 is a periodic comet discovered on May 12, 1986 by Donald Machholz in Loma Prieta, California A meteor shower, some of which are known as a "meteor storm" or "meteor outburst" is a celestial event where a group of meteors are observed to radiate from one point The Quadrantids are a strong January Meteor shower. The radiant of this shower is an area inside the Constellation Boötes. The Arietids are a strong Meteor shower that lasts from May 22 to July 2 each year and peaks on June 7. ^[57]

Unusual comets

Of the thousands of known comets, some are very unusual. Comet Encke orbits from outside the main asteroid belt to inside the orbit of Mercury while Comet 29P/Schwassmann-Wachmann travels in a nearly circular orbit entirely between Jupiter and Saturn. Comet Encke (officially designated 2P/Encke) is a periodic Comet, named after Johann Franz Encke, who through laborious study of its Orbit and Comet 29P/Schwassmann-Wachmann, also known as Schwassmann-Wachmann 1, was discovered on 15 November 1925 by Arnold Schwassmann and ^[58] 2060 Chiron, whose unstable orbit keeps it between Saturn and Uranus, was originally classified as an asteroid until a faint coma was noticed. TemplateInfobox Planet. --> 2060 Chiron (, or as in Χείρων) is a Planetoid in the ^[59] Similarly, Comet Shoemaker-Levy 2 was originally designated asteroid 1990 UL₃. ^[60] Roughly six percent of the near-earth asteroids are thought to be extinct nuclei of comets which no longer experience outgassing. Near-Earth asteroids (NEAs are Asteroids whose Orbits are close to Earth 's orbit ^[61]

Some comets have been observed to break up during their perihelion passage, including great comets West and Ikeya-Seki. There is another long-period comet West C/1978 A1 (aka 1977 IX 1978a There are two comets named Ikeya-Seki C/1965 S1 (this one and C/1967 Y1, a Comet Biela was one significant example, breaking into two during its 1846 perihelion passage. 3D/Biela is the official designation for a lost periodic Comet discovered in 1826 by Wilhelm von Biela. The two comets were seen separately in 1852, but never again afterward. Instead, spectacular meteor showers were seen in 1872 and 1885 when the comet should have been visible. A meteor shower, some of which are known as a "meteor storm" or "meteor outburst" is a celestial event where a group of meteors are observed to radiate from one point A lesser meteor shower, the Andromedids, occurs annually in November, and is caused by the Earth crossing Biela's orbit. The Andromedids Meteor shower is associated with the Comet 3D/Biela, which broke up after its 1852 Perihelion. ^[62]

Another significant cometary disruption was that of Comet Shoemaker-Levy 9, which was discovered in 1993. Shoemaker-Levy redirects here For other Shoemaker-Levy comets see List of periodic comets. 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. ^[63] 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. ^[64] It has also been suggested that the object likely to have been responsible for the Tunguska event in 1908 was a fragment of Comet Encke. The Tunguska Event, or Tunguska explosion, was a powerful Explosion that occurred near the Podkamennaya (Lower Stony Tunguska River in what is now ^[65]

Observation

Example of a comet's path plotted by sky software (Sky Map Pro)

A new comet may be discovered photographically using a wide-field telescope or visually with binoculars. A telescope is an instrument designed for the observation of remote objects and the collection of Electromagnetic radiation. Binocular telescopes, or binoculars (also known as field glasses are two identical or Mirror - symmetrical telescopes mounted side-by-side and However, even without access to optical equipment, it is still possible for the amateur astronomer to discover a Sun-grazing comet online by downloading images accumulated by some satellite observatories such as SOHO. The Solar and Heliospheric Observatory ( SOHO) is a Spacecraft that was launched on a Lockheed Martin Atlas IIAS launch vehicle on December ^[28][66]

Comets visible to the naked eye are fairly infrequent, but comets that put on fine displays in amateur class telescopes (50 mm to 100 cm) occur fairly often — as often as several times a year, occasionally with more than one in the sky at the same time. Commonly available astronomical software will plot the orbits of these known comets. They are fast compared to other objects in the sky, but their movement is usually subtle in the eyepiece of a telescope. However, from night to night, they can move several degrees, which is why observers find it useful to have a sky chart such as the one in the adjoining illustration.

The type of display presented by the comet depends on its composition and how close it comes to the sun. Because the volatility of a comet's material decreases as it gets further from the sun, the comet becomes increasingly difficult to observe as a function of not only distance, but the progressive shrinking and eventual disappearance of its tail and the reflective elements it carries. Comets are most interesting when their nucleus is bright and they display a long tail, which to be seen sometimes requires a large field of view best provided by smaller telescopes. Therefore, large amateur instruments (apertures of 25 cm or larger) that have fainter light grasp do not necessarily confer an advantage in terms of viewing comets. The opportunity to view spectacular comets with relatively small aperture instruments in the 8 cm to 15 cm range is more frequent than might be guessed from the relatively rare attention they get in the mainstream press.

Currently visible comets

Comet C/2007 W1 (Boattini), discovered November 28, 2007 by Andrea Boattini (Mt Lemmon Survey), is as of mid May 2008 at about magnitude +6, potentially visible to the naked eye in the early evening sky in the constellation Pyxis. Comet C/2007 W1 (Boattini discovered November 28 2007 by Andrea Boattini (Mt Lemmon Survey is as of mid May 2008 at about magnitude +6 potentially visible to the naked See also Pyxis (ˈpɪksɪs Box) is a minor southern Constellation introduced by Nicolas Louis de Lacaille under the name Pyxis Nautica. It will be lost in sunglow through June but reappears as a morning object in July at a predicted peak of +4. Perigee will be 24 June 2008.

References

See also

• List of comets

Further Reading

Schechner, Sara J. Non-periodic Comets are seen only once They are usually on near- Parabolic orbits that will not return to the vicinity of the Sun for thousands of years if ever Comets, Popular Culture, and the Birth of Modern Cosmology. Princeton, New Jersey: Princeton University Press. 1997.

External links

• Comet C/2001 RX14 (LINEAR) near to galaxy NGC 3726 @ SKY-MAP.ORG, SDSS. Captured on Dec. 14th 2002
• Cometography.com
• David Jewitt overview of the comets
• Comets Page at NASA's Solar System Exploration
• ESSAY ON COMETS, which gained the first of Dr. Fellowes's prizes, proposed to those who had attended the University of Edinburgh within the last twelve years. By David Milne. Publisher: Edinburgh, Printed for A. Black; 1828. (a searchable facsimile at the University of Georgia Libraries; DjVu & layered PDF format)
• Everything you wanted to know about comets and asteroids — Provided by New Scientist. DjVu (pronounced Déjà vu) is a Computer File format designed primarily to store scanned images especially those containing text and line New Scientist is a weekly International science magazine and website covering recent developments in science and technology for a general English -speaking
• Listing of newly discovered comets
• Comets at the Open Directory Project
• Seiichi Yoshida's Comet Information website
• Source of useful comet-related material on the Web
• Animation and static graphics of current and past comets
• The Starry Mirror - Comet News
• Comets Astronomy Cast episode #19, includes full transcript. The Open Directory Project ( ODP) also known as dmoz (from directory Astronomy Cast is an educational Nonprofit Podcast discussing various topics in the field of Astronomy.