A small Solar System body made of ice and dust. Comets are asteroidal in appearance at distances of many astronomical units from the Sun (when they consist of a bare, inactive nucleus) and are often spectacularly active when nearer to the Sun. The characteristic bright head (coma) and streaming tails (both dust and ions) are created by solar heating, which causes sublimation of the ices (and entrainment of the dust) with subsequent solar-induced emission of light from gas molecules and scattered light from the dust. The source of comets of long orbital periods is believed to be a spherical halo cloud about the Sun called the Oort Cloud - representing a region to which cometary planetesimals scattered gravitationally after their formation 6 thousand million years ago. Oort Cloud comets are loosely bound in a solar orbit at distances of c.50 000 astronomical units. Comets with orbital periods shorter than about 200 years are thought to originate in an inner extension of the Oort Cloud, termed the Kuiper Belt.
Observable comets are occasionally scattered into the inner Solar System by the gravitational fields of nearby stars and giant molecular clouds. There are about 50 known periodic comets in the Jupiter family whose aphelion distance is near Jupiter's, their orbits having been affected by gravitational interactions with that planet. These are potential targets for spacecraft missions, and represent the most primitive material, dating back to the origin of the Solar System. International Cometary Explorer encountered Comet GiacobiniZinner in 1985, and a small armada of spacecraft (Sakigake and Suisei, VEGA 1 and 2, Giotto) encountered Halley's comet in 1986. In 2004, the Stardust probe captured particles from Comet Wild 2 and returned to Earth in January 2006. In 2005, NASA's Deep Impact Spacecraft carried a 372 kg (820 lbs) impactor into space which it released directly into the path of Comet Tempel 1. In a mission designed to punch a crater in the surface of the comet, the projectile successfully shattered on impact throwing up a huge plume of icy debris. The data collected by Deep Impact will be studied by scientists who hope to gain new information on the Solar System's original composition.
From spacecraft and telescopic measurements it is now known that the nucleus of a typical comet is a few km in size, irregularly shaped and very dark; dust in the coma contains carbon and silicate; there is evidence of polymerized organic molecules; the gases of the coma include water, carbon monoxide, carbon dioxide, ammonia, methane, and hydrocarbons; the dirty snowball hypothesis of astronomer Fred Whipple is basically confirmed. Cometary impacts on Earth over geologic time may have been an important source of volatile material and organics contributing to the pre-biotic environment. Cometary and asteroidal impacts may also have contributed to periodic extinctions of species in the past.
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. 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 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."
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). 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 tail made of dust is left behind in the comet's orbit in such a manner that it often forms a curved tail.
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 ionization. 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 1066. 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;
In 1996, comets were found to emit X-rays.
Orbital characteristics
Comets are classified according to their orbital periods. Short period 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. 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; 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.
A number of periodic comets discovered in earlier decades or previous centuries are now "lost." However, occasionally a "new" comet will be discovered and upon calculation of its orbit it turns out to be an old "lost" comet.
Comet nomenclature
The names given to comets have followed several different conventions over the past two centuries. 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. 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. 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.
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. So Halley's Comet, the first comet to be identified as periodic, has the systematic designation 1P/1682 Q1.
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.
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.
A few later classical philosophers did dispute this view of comets.
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.
Orbital studies
Although comets had now been demonstrated to be in the heavens, the question of how they moved through the heavens would be debated for most of the next century. Even after Johannes Kepler had determined in 1609 that the planets moved about the sun in elliptical orbits, he was reluctant to believe that the laws that governed the motions of the planets should also influence the motion of other bodies—he believed that comets travel among the planets along straight lines.
The first suggestion that Kepler's laws of planetary motion should also apply to the comets was made by William Lower in 1610.
The matter was resolved by the bright comet that was discovered by Gottfried Kirch on November 14, 1680. 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. 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. (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. When the comet returned as predicted, it became known as Comet Halley or Halley's Comet (its official designation is 1P/Halley).
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.
Studies of physical characteristics
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;
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; 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. 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. 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.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. And in 2014, the European Rosetta probe will orbit comet Comet Churyumov-Gerasimenko and place a small lander on its surface. 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
As late as 2002, there is conflict on how much ice is in a comet.
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 make any impact on the general public. About every decade or so, a comet will become bright enough to be noticed by a casual observer — such comets are often designated Great Comets. 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.
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. 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.
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.
Some comets have been observed to break up.
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.
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.
The comet is moving rapidly into Bootes and will become an easy binocular target in the early evening sky throughout the month of October.
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