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Comets usually have highly eccentric elliptical orbits, and they have a wide range of orbital periods, ranging from several years to potentially several millions of years. Short-period comets originate in the Kuiper belt or its associated scattered disc, which lie beyond the orbit of Neptune. Long-period comets are thought to originate in the Oort cloud, a spherical cloud of icy bodies extending from outside the Kuiper belt to halfway to the nearest star. Long-period comets are set in motion towards the Sun from the Oort cloud by gravitational perturbations caused by passing stars and the galactic tide. Hyperbolic comets may pass once through the inner Solar System before being flung to interstellar space. The appearance of a comet is called an apparition.
Comets are distinguished from asteroids by the presence of an extended, gravitationally unbound atmosphere surrounding their central nucleus. This atmosphere has parts termed the coma (the central part immediately surrounding the nucleus) and the tail (a typically linear section consisting of dust or gas blown out from the coma by the Sun's light pressure or outstreaming solar wind plasma). However, extinct comets that have passed close to the Sun many times have lost nearly all of their volatile ices and dust and may come to resemble small asteroids. Asteroids are thought to have a different origin from comets, having formed inside the orbit of Jupiter rather than in the outer Solar System. The discovery of main-belt comets and active centaur minor planets has blurred the distinction between asteroids and comets. In the early 21st century, the discovery of some minor bodies with long-period comet orbits, but characteristics of inner solar system asteroids, were called Manx comets. They are still classified as comets, such as C/2014 S3 (PANSTARRS). 27 Manx comets were found from 2013 to 2017.
there are 4584 known comets. However, this represents only a tiny fraction of the total potential comet population, as the reservoir of comet-like bodies in the outer Solar System (in the Oort cloud) is estimated to be one trillion. Roughly one comet per year is visible to the naked eye, though many of those are faint and unspectacular. Particularly bright examples are called "great comets". Comets have been visited by unmanned probes such as the European Space Agency's Rosetta, which became the first to land a robotic spacecraft on a comet, and NASA's Deep Impact, which blasted a crater on Comet Tempel 1 to study its interior.
Read more...: Etymology Physical characteristics Nucleus Coma Bow shock Tails Jets Orbital characteristics Short period Long period Oort cloud and Hills cloud Exocomets Effects of comets Connection to meteor showers Comets and impact on life Fear of comets Fate of comets Departure (ejection) from Solar System Volatiles exhausted Breakup and collisions Nomenclature History of study Early observations and thought Scientific approach Spacecraft missions Classification Great comets Sungrazing comets Unusual comets Largest Centaurs Observation Lost In popular culture Gallery
Etymology
The word comet derives from the Old English from the Latin or . That, in turn, is a romanization of the Greek 'wearing long hair', and the Oxford English Dictionary notes that the term already meant 'long-haired star, comet' in Greek. was derived from 'to wear the hair long', which was itself derived from 'the hair of the head' and was used to mean 'the tail of a comet'.
The astronomical symbol for comets (represented in Unicode) is , consisting of a small disc with three hairlike extensions.
Physical characteristics
Nucleus
The solid, core structure of a comet is known as the nucleus. Cometary nuclei are composed of an amalgamation of rock, dust, water ice, and frozen carbon dioxide, carbon monoxide, methane, and ammonia. As such, they are popularly described as "dirty snowballs" after Fred Whipple's model. Comets with a higher dust content have been called "icy dirtballs". The term "icy dirtballs" arose after observation of Comet 9P/Tempel 1 collision with an "impactor" probe sent by NASA Deep Impact mission in July 2005. Research conducted in 2014 suggests that comets are like "deep fried ice cream", in that their surfaces are formed of dense crystalline ice mixed with organic compounds, while the interior ice is colder and less dense.
The surface of the nucleus is generally dry, dusty or rocky, suggesting that the ices are hidden beneath a surface crust several metres thick. In addition to the gases already mentioned, the nuclei contain a variety of organic compounds, which may include methanol, hydrogen cyanide, formaldehyde, ethanol, ethane, and perhaps more complex molecules such as long-chain hydrocarbons and amino acids. In 2009, it was confirmed that the amino acid glycine had been found in the comet dust recovered by NASA's Stardust mission. In August 2011, a report, based on NASA studies of meteorites found on Earth, was published suggesting DNA and RNA components (adenine, guanine, and related organic molecules) may have been formed on asteroids and comets.
The outer surfaces of cometary nuclei have a very low albedo, making them among the least reflective objects found in the Solar System. The Giotto space probe found that the nucleus of Halley's Comet (1P/Halley) reflects about four percent of the light that falls on it, and Deep Space 1 discovered that Comet Borrelly's surface reflects less than 3.0%; by comparison, asphalt reflects seven percent. The dark surface material of the nucleus may consist of complex organic compounds. Solar heating drives off lighter volatile compounds, leaving behind larger organic compounds that tend to be very dark, like tar or crude oil. The low reflectivity of cometary surfaces causes them to absorb the heat that drives their outgassing processes.
Comet nuclei with radii of up to have been observed, but ascertaining their exact size is difficult. The nucleus of 322P/SOHO is probably only in diameter. A lack of smaller comets being detected despite the increased sensitivity of instruments has led some to suggest that there is a real lack of comets smaller than across. Known comets have been estimated to have an average density of . Because of their low mass, comet nuclei do not become spherical under their own gravity and therefore have irregular shapes.
Roughly six percent of the near-Earth asteroids are thought to be the extinct nuclei of comets that no longer experience outgassing, including 14827 Hypnos and 3552 Don Quixote.
Results from the Rosetta and Philae spacecraft show that the nucleus of 67P/Churyumov–Gerasimenko has no magnetic field, which suggests that magnetism may not have played a role in the early formation of planetesimals. Further, the ALICE spectrograph on Rosetta determined that electrons (within above the comet nucleus) produced from photoionization of water molecules by solar radiation, and not photons from the Sun as thought earlier, are responsible for the degradation of water and carbon dioxide molecules released from the comet nucleus into its coma. Instruments on the Philae lander found at least sixteen organic compounds at the comet's surface, four of which (acetamide, acetone, methyl isocyanate and propionaldehyde) have been detected for the first time on a comet.
Coma
The streams of dust and gas thus released form a huge and extremely thin atmosphere around the comet called the "coma". The force exerted on the coma by the Sun's radiation pressure and solar wind cause an enormous "tail" to form pointing away from the Sun.
The coma is generally made of water and dust, with water making up to 90% of the volatiles that outflow from the nucleus when the comet is within 3 to 4 astronomical units (450,000,000 to 600,000,000 km; 280,000,000 to 370,000,000 mi) of the Sun. The parent molecule is destroyed primarily through photodissociation and to a much smaller extent photoionization, with the solar wind playing a minor role in the destruction of water compared to photochemistry. Larger dust particles are left along the comet's orbital path whereas smaller particles are pushed away from the Sun into the comet's tail by light pressure.
Although the solid nucleus of comets is generally less than across, the coma may be thousands or millions of kilometers across, sometimes becoming larger than the Sun. For example, about a month after an outburst in October 2007, comet 17P/Holmes briefly had a tenuous dust atmosphere larger than the Sun. The Great Comet of 1811 also had a coma roughly the diameter of the Sun. Even though the coma can become quite large, its size can decrease about the time it crosses the orbit of Mars around from the Sun. At this distance the solar wind becomes strong enough to blow the gas and dust away from the coma, and in doing so enlarging the tail. Ion tails have been observed to extend one astronomical unit (150 million km) or more.
Both the coma and tail are illuminated by the Sun and may become visible when a comet passes through the inner Solar System, the dust reflects sunlight directly while the gases glow 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 to the naked eye. Occasionally a comet may experience a huge and sudden outburst of gas and dust, during which the size of the coma greatly increases for a period of time. This happened in 2007 to Comet Holmes.
In 1996, comets were found to emit X-rays. This greatly surprised astronomers because X-ray emission is usually associated with very high-temperature bodies. The X-rays are generated by the interaction between comets and the solar wind: when highly charged solar wind ions fly through a cometary atmosphere, they collide with cometary atoms and molecules, "stealing" one or more electrons from the atom in a process called "charge exchange". This exchange or transfer of an electron to the solar wind ion is followed by its de-excitation into the ground state of the ion by the emission of X-rays and far ultraviolet photons.
Bow shock
Bow shocks form as a result of the interaction between the solar wind and the cometary ionosphere, which is created by the ionization of gases in the coma. As the comet approaches the Sun, increasing outgassing rates cause the coma to expand, and the sunlight ionizes gases in the coma. When the solar wind passes through this ion coma, the bow shock appears.
The first observations were made in the 1980s and 90s as several spacecraft flew by comets 21P/Giacobini–Zinner, 1P/Halley, and 26P/Grigg–Skjellerup. It was then found that the bow shocks at comets are wider and more gradual than the sharp planetary bow shocks seen at, for example, Earth. These observations were all made near perihelion when the bow shocks already were fully developed.
The Rosetta spacecraft observed the bow shock at comet 67P/Churyumov–Gerasimenko at an early stage of bow shock development when the outgassing increased during the comet's journey toward the Sun. This young bow shock was called the "infant bow shock". The infant bow shock is asymmetric and, relative to the distance to the nucleus, wider than fully developed bow shocks.
Tails
In the outer Solar System, comets remain frozen and inactive and are extremely difficult or impossible to detect from Earth due to their small size. Statistical detections of inactive comet nuclei in the Kuiper belt have been reported from observations by the Hubble Space Telescope but these detections have been questioned. As a comet approaches the inner Solar System, solar radiation causes the volatile materials within the comet to vaporize and stream out of the nucleus, carrying dust away with them.
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 called the type II or dust tail. At the same time, the ion or type I tail, made of gases, always points directly away from the Sun because this gas is more strongly affected by the solar wind than is dust, following magnetic field lines rather than an orbital trajectory. On occasions—such as when Earth passes through a comet's orbital plane, the antitail, pointing in the opposite direction to the ion and dust tails, may be seen.
The observation of antitails contributed significantly to the discovery of solar wind. The ion tail is formed as a result of the ionization by solar ultra-violet radiation of particles in the coma. Once the particles have been ionized, they attain a net positive electrical charge, which in turn gives rise to an "induced magnetosphere" around the comet. The comet and its induced magnetic field form an obstacle to outward flowing solar wind particles. Because the relative orbital speed of the comet and the solar wind is supersonic, a bow shock is formed upstream of the comet in the flow direction of the solar wind. In this bow shock, large concentrations of cometary ions (called "pick-up ions") congregate and act to "load" the solar magnetic field with plasma, such that the field lines "drape" around the comet forming the ion tail.
If the ion tail loading is sufficient, the magnetic field lines are squeezed together to the point where, at some distance along the ion tail, magnetic reconnection occurs. This leads to a "tail disconnection event". This has been observed on a number of occasions, one notable event being recorded on 20 April 2007, when the ion tail of Encke's Comet was completely severed while the comet passed through a coronal mass ejection. This event was observed by the STEREO space probe.
In 2013, ESA scientists reported that the ionosphere of the planet Venus streams outwards in a manner similar to the ion tail seen streaming from a comet under similar conditions."
Jets
Uneven heating can cause newly generated gases to break out of a weak spot on the surface of comet's nucleus, like a geyser. These streams of gas and dust can cause the nucleus to spin, and even split apart. In 2010 it was revealed dry ice (frozen carbon dioxide) can power jets of material flowing out of a comet nucleus. Infrared imaging of Hartley 2 shows such jets exiting and carrying with it dust grains into the coma.
Orbital characteristics
Most comets are small Solar System bodies with 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. Comets are often classified according to the length of their orbital periods: The longer the period the more elongated the ellipse.
Short period
Periodic comets or short-period comets are generally defined as those having orbital periods of less than 200 years. They usually orbit more-or-less in the ecliptic plane in the same direction as the planets. Their orbits typically take them out to the region of the outer planets (Jupiter and beyond) at aphelion; for example, the aphelion of Halley's Comet is a little beyond the orbit of Neptune. Comets whose aphelia are near a major planet's orbit are called its "family". Such families are thought to arise from the planet capturing formerly long-period comets into shorter orbits.
At the shorter orbital period extreme, Encke's Comet has an orbit that does not reach the orbit of Jupiter, and is known as an Encke-type comet. Short-period comets with orbital periods less than 20 years and low inclinations (up to 30 degrees) to the ecliptic are called traditional Jupiter-family comets (JFCs). Those like Halley, with orbital periods of between 20 and 200 years and inclinations extending from zero to more than 90 degrees, are called Halley-type comets (HTCs). , 94 HTCs have been observed, compared with 725 identified JFCs.
Recently discovered main-belt comets form a distinct class, orbiting in more circular orbits within the asteroid belt.
Because their elliptical orbits frequently take them close to the giant planets, comets are subject to further gravitational perturbations. Short-period comets have a tendency for their aphelia to coincide with a giant planet's semi-major axis, with the JFCs being the largest group. 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. These perturbations can deflect long-period comets into shorter orbital periods.
Based on their orbital characteristics, short-period comets are thought to originate from the centaurs and the Kuiper belt/scattered disc —a disk of objects in the trans-Neptunian 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 hypothesized its existence). Vast swarms of comet-like bodies are thought 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 toward the Sun to form a visible comet. Unlike the return of periodic comets, whose orbits have been established by previous observations, the appearance of new comets by this mechanism is unpredictable. When flung into the orbit of the sun, and being continuously dragged towards it, tons of matter are stripped from the comets which greatly influence their lifetime; the more stripped, the shorter they live and vice versa.
Long period
Long-period comets have highly eccentric orbits and periods ranging from 200 years to thousands or even millions of years. An eccentricity greater than 1 when near perihelion does not necessarily mean that a comet will leave the Solar System. For example, Comet McNaught had a heliocentric osculating eccentricity of 1.000019 near its perihelion passage epoch in January 2007 but is bound to the Sun with roughly a 92,600-year orbit because the eccentricity drops below 1 as it moves farther from the Sun. The future orbit of a long-period comet is properly obtained when the osculating orbit is computed at an epoch after leaving the planetary region and is calculated with respect to the center of mass of the Solar System. By definition long-period comets 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". The orbits of long-period comets take them far beyond the outer planets at aphelia, and the plane of their orbits need not lie near the ecliptic. Long-period comets such as C/1999 F1 and C/2017 T2 (PANSTARRS) can have aphelion distances of nearly with orbital periods estimated around 6 million years.
Single-apparition or non-periodic comets are similar to long-period comets because they also have parabolic or slightly hyperbolic trajectories when near perihelion in the inner Solar System. However, gravitational perturbations from giant planets cause their orbits to change. Single-apparition comets have a hyperbolic or parabolic osculating orbit which allows them to permanently exit the Solar System after a single pass of the Sun. The Sun's Hill sphere has an unstable maximum boundary of . Only a few hundred comets have been seen to reach a hyperbolic orbit (e > 1) when near perihelion that using a heliocentric unperturbed two-body best-fit suggests they may escape the Solar System.
, only two objects have been discovered with an eccentricity significantly greater than one: 1I/ʻOumuamua and 2I/Borisov, indicating an origin outside the Solar System. While ʻOumuamua, with an eccentricity of about 1.2, showed no optical signs of cometary activity during its passage through the inner Solar System in October 2017, changes to its trajectory—which suggests outgassing—indicate that it is probably a comet. On the other hand, 2I/Borisov, with an estimated eccentricity of about 3.36, has been observed to have the coma feature of comets, and is considered the first detected interstellar comet. Comet C/1980 E1 had an orbital period of roughly 7.1 million years before the 1982 perihelion passage, but a 1980 encounter with Jupiter accelerated the comet giving it the largest eccentricity (1.057) of any known solar comet with a reasonable observation arc. Comets not expected to return to the inner Solar System include C/1980 E1, C/2000 U5, C/2001 Q4 (NEAT), C/2009 R1, C/1956 R1, and C/2007 F1 (LONEOS).
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), whereas others use it to mean exclusively short-period comets. 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).
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. Comets from interstellar space are moving with velocities of the same order as the relative velocities of stars near the Sun (a few tens of km per second). When such objects enter the Solar System, they have a positive specific orbital energy resulting in a positive velocity at infinity (v_{\infty}\!) and have notably 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.
Oort cloud and Hills cloud
The Oort cloud is thought to occupy a vast space starting from between to as far as from the Sun. This cloud encases the celestial bodies that start at the middle of our solar system—the sun, all the way to outer limits of the Kuiper Belt. The Oort cloud consists of viable materials necessary for the creation of celestial bodies. The planets we have today, exist only because of the planetesimals (chunks of leftover space that assisted in the creation of planets) that were condensed and formed by the gravity of the sun. The eccentric made from these trapped planetesimals is why the Oort Cloud even exists. Some estimates place the outer edge at between . The region can be subdivided into a spherical outer Oort cloud of , and a doughnut-shaped inner cloud, the Hills cloud, of . The outer cloud is only weakly bound to the Sun and supplies the long-period (and possibly Halley-type) comets that fall to inside the orbit of Neptune. The inner Oort cloud is also known as the Hills cloud, named after J. G. Hills, who proposed its existence in 1981. Models predict that the inner cloud should have tens or hundreds of times as many cometary nuclei as the outer halo; it is seen as a possible source of new comets that resupply the relatively tenuous outer cloud as the latter's numbers are gradually depleted. The Hills cloud explains the continued existence of the Oort cloud after billions of years.
Exocomets
Exocomets beyond the Solar System have also been detected and may be common in the Milky Way. The first exocomet system detected was around Beta Pictoris, a very young A-type main-sequence star, in 1987. A total of 11 such exocomet systems have been identified , using the absorption spectrum caused by the large clouds of gas emitted by comets when passing close to their star. For ten years the Kepler space telescope was responsible for searching for planets and other forms outside of the solar system. The first transiting exocomets were found in February 2018 by a group consisting of professional astronomers and citizen scientists in light curves recorded by the Kepler Space Telescope. After Kepler Space Telescope retired in October 2018, a new telescope called TESS Telescope has taken over Kepler's mission. Since the launch of TESS, astronomers have discovered the transits of comets around the star Beta Pictoris using a light curve from TESS. Since TESS has taken over, astronomers have since been able to better distinguish exocomets with the spectroscopic method. New planets are detected by the white light curve method which is viewed as a symmetrical dip in the charts readings when a planet overshadows its parent star. However, after further evaluation of these light curves, it has been discovered that the asymmetrical patterns of the dips presented are caused by the tail of a comet or of hundreds of comets.
Effects of comets
Connection to meteor showers
As a comet is heated during close passes to the Sun, outgassing of its icy components also releases solid debris too large to be swept away by radiation pressure and the solar wind. If Earth's orbit sends it through that trail of debris, which is composed mostly of fine grains of rocky material, there is likely to be a meteor shower as Earth passes through. Denser trails of debris produce quick but intense meteor showers and less dense trails create longer but less intense showers. Typically, the density of the debris trail is related to how long ago the parent comet released the material. The Perseid meteor shower, for example, occurs every year between 9 and 13 August, when Earth passes through the orbit of Comet Swift–Tuttle. Halley's Comet is the source of the Orionid shower in October.
Comets and impact on life
Many comets and asteroids collided with Earth in its early stages. Many scientists think that comets bombarding the young Earth about 4 billion years ago brought the vast quantities of water that now fill Earth's oceans, or at least a significant portion of it. Others have cast doubt on this idea. The detection of organic molecules, including polycyclic aromatic hydrocarbons, in significant quantities in comets has led to speculation that comets or meteorites may have brought the precursors of life—or even life itself—to Earth. In 2013 it was suggested that impacts between rocky and icy surfaces, such as comets, had the potential to create the amino acids that make up proteins through shock synthesis. The speed at which the comets entered the atmosphere, combined with the magnitude of energy created after initial contact, allowed smaller molecules to condense into the larger macro-molecules that served as the foundation for life. In 2015, scientists found significant amounts of molecular oxygen in the outgassings of comet 67P, suggesting that the molecule may occur more often than had been thought, and thus less an indicator of life as has been supposed.
It is suspected that comet impacts have, over long timescales, also delivered significant quantities of water to Earth's Moon, some of which may have survived as lunar ice. Comet and meteoroid impacts are also thought to be responsible for the existence of tektites and australites.
Fear of comets
Fear of comets as acts of God and signs of impending doom was highest in Europe from AD 1200 to 1650. The year after the Great Comet of 1618, for example, Gotthard Arthusius published a pamphlet stating that it was a sign that the Day of Judgment was near. He listed ten pages of comet-related disasters, including "earthquakes, floods, changes in river courses, hail storms, hot and dry weather, poor harvests, epidemics, war and treason and high prices".
By 1700 most scholars concluded that such events occurred whether a comet was seen or not. Using Edmond Halley's records of comet sightings, however, William Whiston in 1711 wrote that the Great Comet of 1680 had a periodicity of 574 years and was responsible for the worldwide flood in the Book of Genesis, by pouring water on Earth. His announcement revived for another century fear of comets, now as direct threats to the world instead of signs of disasters. Spectroscopic analysis in 1910 found the toxic gas cyanogen in the tail of Halley's Comet, causing panicked buying of gas masks and quack "anti-comet pills" and "anti-comet umbrellas" by the public.
Fate of comets
Departure (ejection) from Solar System
If a comet is traveling fast enough, it may leave the Solar System. Such comets follow the open path of a hyperbola, and as such, they are called hyperbolic comets. Solar comets are only known to be ejected by interacting with another object in the Solar System, such as Jupiter. An example of this is Comet C/1980 E1, which was shifted from an orbit of 7.1 million years around the Sun, to a hyperbolic trajectory, after a 1980 close pass by the planet Jupiter. Interstellar comets such as 1I/ʻOumuamua and 2I/Borisov never orbited the Sun and therefore do not require a 3rd-body interaction to be ejected from the Solar System.
Volatiles exhausted
Jupiter-family comets and long-period comets appear to follow very different fading laws. The JFCs are active over a lifetime of about 10,000 years or ~1,000 orbits whereas long-period comets fade much faster. Only 10% of the long-period comets survive more than 50 passages to small perihelion and only 1% of them survive more than 2,000 passages. Eventually most of the volatile material contained in a comet nucleus evaporates, and the comet becomes a small, dark, inert lump of rock or rubble that can resemble an asteroid. Some asteroids in elliptical orbits are now identified as extinct comets. Roughly six percent of the near-Earth asteroids are thought to be extinct comet nuclei.
Breakup and collisions
The nucleus of some comets may be fragile, a conclusion supported by the observation of comets splitting apart. A significant cometary disruption was that of Comet Shoemaker–Levy 9, which was discovered in 1993. A close encounter in July 1992 had broken it into pieces, and over a period of six days in July 1994, these pieces fell into Jupiter's atmosphere—the first time astronomers had observed a collision between two objects in the Solar System. Other splitting comets include 3D/Biela in 1846 and 73P/Schwassmann–Wachmann from 1995 to 2006. Greek historian Ephorus reported that a comet split apart as far back as the winter of 372–373 BC. Comets are suspected of splitting due to thermal stress, internal gas pressure, or impact.
Comets 42P/Neujmin and 53P/Van Biesbroeck appear to be fragments of a parent comet. Numerical integrations have shown that both comets had a rather close approach to Jupiter in January 1850, and that, before 1850, the two orbits were nearly identical.
Some comets have been observed to break up during their perihelion passage, including great comets West and Ikeya–Seki. Biela's Comet was one significant example when it broke into two pieces during its passage through the perihelion in 1846. These 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 minor meteor shower, the Andromedids, occurs annually in November, and it is caused when Earth crosses the orbit of Biela's Comet.
Some comets meet a more spectacular end – either falling into the Sun 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 Moon, for example, may have been caused by comets. A recent collision of a comet with a planet occurred in July 1994 when Comet Shoemaker–Levy 9 broke up into pieces and collided with Jupiter.
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 when they appeared, sometimes with additional adjectives for particularly bright comets; thus, the "Great Comet of 1680", the "Great Comet of 1882", and the "Great January Comet of 1910".
After Edmond Halley demonstrated that the comets of 1531, 1607, and 1682 were the same body and successfully predicted its return in 1759 by calculating its orbit, that comet became known as Halley's Comet. Similarly, the second and third known periodic comets, Encke's Comet and Biela's Comet, 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 appearance.
In the early 20th century, the convention of naming comets after their discoverers became common, and this remains so today. A comet can be named after its discoverers or an instrument or program that helped to find it. For example, in 2019, astronomer Gennady Borisov observed a comet that appeared to have originated outside of the solar system; the comet was named C/2019 Q4 (Borisov) after him.
History of study
Early observations and thought
From ancient sources, such as Chinese oracle bones, it is known that comets have been noticed by humans for millennia. Until the sixteenth century, comets 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.
Aristotle (384–322 BC) was the first known scientist to utilize various theories and observational facts to employ a consistent, structured cosmological theory of comets. He believed that comets were atmospheric phenomena, due to the fact that they could appear outside of the zodiac and vary in brightness over the course of a few days. Aristotle's cometary theory arose from his observations and cosmological theory that everything in the cosmos is arranged in a distinct configuration. Part of this configuration was a clear separation between the celestial and terrestrial, believing comets to be strictly associated with the latter. According to Aristotle, comets must be within the sphere of the moon and clearly separated from the heavens. Also in the 4th century BC, Apollonius of Myndus supported the idea that comets moved like the planets. Aristotelian theory on comets continued to be widely accepted throughout the Middle Ages, despite several discoveries from various individuals challenging aspects of it.
In the 1st century AD, Seneca the Younger questioned Aristotle's logic concerning comets. Because of their regular movement and imperviousness to wind, they cannot be atmospheric, and are more permanent than suggested by their brief flashes across the sky. He pointed out that only the tails are transparent and thus cloudlike, and argued that there is no reason to confine their orbits to the zodiac. In criticizing Apollonius of Myndus, Seneca argues, "A comet cuts through the upper regions of the universe and then finally becomes visible when it reaches the lowest point of its orbit." While Seneca did not author a substantial theory of his own, his arguments would spark much debate among Aristotle's critics in the 16th and 17th centuries.
Also in the 1st century, Pliny the Elder believed that comets were connected with political unrest and death. Pliny observed comets as "human like", often describing their tails with "long hair" or "long beard". His system for classifying comets according to their color and shape was used for centuries.
In India, by the 6th century astronomers believed that comets were celestial bodies that re-appeared periodically. This was the view expressed in the 6th century by the astronomers Varāhamihira and Bhadrabahu, and the 10th-century astronomer Bhaṭṭotpala listed the names and estimated periods of certain comets, but it is not known how these figures were calculated or how accurate they were.
In the 11th century Bayeux Tapestry, Halley's Comet is depicted portending the death of Harold and the triumph of the Normans at the Battle of Hastings.
According to Norse mythology, comets were actually a part of the Giant Ymir's skull. According to the tale, Odin and his brothers slew Ymir and set about constructing the world (Earth) from his corpse. They fashioned the oceans from his blood, the soil from his skin and muscles, vegetation from his hair, clouds from his brains, and the sky from his skull. Four dwarves, corresponding to the four cardinal points, held Ymir's skull aloft above the earth. Following this tale, comets in the sky, as believed by the Norse, were flakes of Ymir's skull falling from the sky and then disintegrating.
In 1301, the Italian painter Giotto was the first person to accurately and anatomically portray a comet. In his work Adoration of the Magi, Giotto's depiction of Halley's Comet in the place of the Star of Bethlehem would go unmatched in accuracy until the 19th century and be bested only with the invention of photography.
Astrological interpretations of comets proceeded to take precedence clear into the 15th century, despite the presence of modern scientific astronomy beginning to take root. Comets continued to forewarn of disaster, as seen in the Luzerner Schilling chronicles and in the warnings of Pope Callixtus III. In 1578, German Lutheran bishop Andreas Celichius defined comets as "the thick smoke of human sins ... kindled by the hot and fiery anger of the Supreme Heavenly Judge". The next year, Andreas Dudith stated that "If comets were caused by the sins of mortals, they would never be absent from the sky."
Scientific approach
Crude attempts at a parallax measurement of Halley's Comet were made in 1456, but were erroneous. Regiomontanus was the first to attempt to calculate diurnal parallax by observing the great comet of 1472. His predictions were not very accurate, but they were conducted in the hopes of estimating the distance of a comet from the Earth.
In the 16th century, Tycho Brahe and Michael Maestlin demonstrated that comets must exist outside of Earth's atmosphere by measuring the parallax of the Great Comet of 1577. Within the precision of the measurements, this implied the comet must be at least four times more distant than from Earth to the Moon. Based on observations in 1664, Giovanni Borelli recorded the longitudes and latitudes of comets that he observed, and suggested that cometary orbits may be parabolic. Galileo Galilei, one of the most renowned astronomers to date, even attempted writings on comets in The Assayer. He rejected Brahe's theories on the parallax of comets and claimed that they may be a mere optical illusion. Intrigued as early scientists were about the nature of comets, Galileo could not help but throw about his own theories despite little personal observation. Maestlin's student Johannes Kepler responded to these unjust criticisms in his work Hyperaspistes. Jakob Bernoulli published another attempt to explain comets (Conamen Novi Systematis Cometarum) in 1682.
Also occurring in the early modern period was the study of comets and their astrological significance in medical disciplines. Many healers of this time considered medicine and astronomy to be inter-disciplinary and employed their knowledge of comets and other astrological signs for diagnosing and treating patients.
Isaac Newton, in his Principia Mathematica of 1687, proved that an object moving under the influence of gravity by an inverse square law 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 describes comets as compact and durable solid bodies moving in oblique orbit and their tails as thin streams of vapor emitted by their nuclei, ignited or heated by the Sun. He suspected that comets were the origin of the life-supporting component of air. He also pointed out that comets usually appear near the Sun, and therefore most likely orbit it. On their luminosity, he stated, "The comets shine by the Sun's light, which they reflect," with their tails illuminated by "the Sun's light reflected by a smoke arising from coma".
In 1705, Edmond Halley (1656–1742) applied Newton's method to 23 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 caused 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. 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 Halley's Comet.
As early as the 18th century, some scientists had made correct hypotheses as to comets' physical composition. In 1755, Immanuel Kant hypothesized in his Universal Natural History that comets were condensed from "primitive matter" beyond the known planets, which is "feebly moved" by gravity, then orbit at arbitrary inclinations, and are partially vaporized by the Sun's heat as they near perihelion. In 1836, the German mathematician Friedrich Wilhelm Bessel, after observing streams of vapor during the appearance of Halley's Comet in 1835, proposed that the jet forces of evaporating material could be great enough to significantly alter a comet's orbit, and he argued that the non-gravitational movements of Encke's Comet resulted from this phenomenon.
In the 19th century, the Astronomical Observatory of Padova was an epicenter in the observational study of comets. Led by Giovanni Santini (1787–1877) and followed by Giuseppe Lorenzoni (1843–1914), this observatory was devoted to classical astronomy, mainly to the new comets and planets orbit calculation, with the goal of compiling a catalog of almost ten thousand stars. Situated in the Northern portion of Italy, observations from this observatory were key in establishing important geodetic, geographic, and astronomical calculations, such as the difference of longitude between Milan and Padua as well as Padua to Fiume. In addition to these geographic observations, correspondence within the observatory, particularly between Santini and another astronomer Giuseppe Toaldo, about the importance of comet and planetary orbital observations.
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 and appeared to be supported by the observations of an armada of spacecraft (including the European Space Agency's Giotto probe and the Soviet Union's Vega 1 and Vega 2) that flew through the coma of Halley's Comet in 1986, photographed the nucleus, and observed jets of evaporating material.
On 22 January 2014, ESA scientists reported the detection, for the first definitive time, of water vapor on the dwarf planet Ceres, the largest object in the asteroid belt. The detection was made by using the far-infrared abilities of the Herschel Space Observatory. The finding is unexpected because comets, not asteroids, are typically considered to "sprout jets and plumes". According to one of the scientists, "The lines are becoming more and more blurred between comets and asteroids." On 11 August 2014, astronomers released studies, using the Atacama Large Millimeter/Submillimeter Array (ALMA) for the first time, that detailed the distribution of HCN, HNC, , and dust inside the comae of comets C/2012 F6 (Lemmon) and C/2012 S1 (ISON).
Spacecraft missions
• The Halley Armada describes the collection of spacecraft missions that visited and/or made observations of Halley's Comet 1980s perihelion. The space shuttle Challenger was intended to do a study of Halley's Comet in 1986, but exploded shortly after being launched.
• Deep Impact. Debate continues about how much ice is in a comet. In 2001, the Deep Space 1 spacecraft obtained high-resolution images of the surface of Comet Borrelly. It was found that the surface of comet Borrelly is hot and dry, with a temperature of between , and extremely dark, suggesting that the ice has been removed by solar heating and maturation, or is hidden by the soot-like material that covers Borrelly. In July 2005, the Deep Impact probe blasted a crater on Comet Tempel 1 to study its interior. The mission yielded results suggesting that the majority of a comet's water ice is below the surface and that these reservoirs feed the jets of vaporized water that form the coma of Tempel 1. Renamed EPOXI, it made a flyby of Comet Hartley 2 on 4 November 2010.
• Ulysses. In 2007, the Ulysses probe unexpectedly passed through the tail of the comet C/2006 P1 (McNaught) which was discovered in 2006. Ulysses was launched in 1990 and the intended mission was for Ulysses to orbit around the sun for further study at all latitudes.
• Stardust. Data from the Stardust mission show that materials retrieved from the tail of Wild 2 were crystalline and could only have been "born in fire", at extremely high temperatures of over . 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. As a result, comets also contain crystalline grains that formed in the early, hot inner Solar System. This is seen in comet spectra as well as in sample return missions. More recent still, the materials retrieved demonstrate that the "comet dust resembles asteroid materials". These new results have forced scientists to rethink the nature of comets and their distinction from asteroids.
• Rosetta. The Rosetta probe orbited Comet Churyumov–Gerasimenko. On 12 November 2014, its lander Philae successfully landed on the comet's surface, the first time a spacecraft has ever landed on such an object in history.
Classification
Great comets
Approximately once a decade, a comet becomes bright enough to be noticed by a casual observer, leading such comets to be designated as great comets. 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 Earth when at its brightest, it has 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 but became an extremely impressive comet.
The Great Comet of 1577 is a well-known example of a great comet. It passed near Earth as a non-periodic comet and was seen by many, including well-known astronomers Tycho Brahe and Taqi ad-Din. Observations of this comet led to several significant findings regarding cometary science, especially for Brahe.
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. The first great comet of the 21st century was C/2006 P1 (McNaught), which became visible to naked eye observers in January 2007. It was the brightest in over 40 years.
Sungrazing comets
A sungrazing comet is a comet that passes extremely close to the Sun at perihelion, generally within a few million kilometers. Although small sungrazers can be completely evaporated during such a close approach to the Sun, larger sungrazers can survive many perihelion passages. However, the strong tidal forces they experience often lead to their fragmentation.
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 remainder 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.
Unusual comets
Of the thousands of known comets, some exhibit unusual properties. Comet Encke (2P/Encke) orbits from outside the asteroid belt to just inside the orbit of the planet Mercury whereas the Comet 29P/Schwassmann–Wachmann currently travels in a nearly circular orbit entirely between the orbits of Jupiter and Saturn. 2060 Chiron, whose unstable orbit is 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 .
Largest
The largest known periodic comet is 95P/Chiron at 200 km in diameter that comes to perihelion every 50 years just inside of Saturn's orbit at 8 AU. The largest known Oort cloud comet is suspected of being Comet Bernardinelli-Bernstein at ≈150 km that will not come to perihelion until January 2031 just outside of Saturn's orbit at 11 AU. The Comet of 1729 is estimated to have been ≈100 km in diameter and came to perihelion inside of Jupiter's orbit at 4 AU.
Centaurs
Centaurs typically behave with characteristics of both asteroids and comets. Centaurs can be classified as comets such as 60558 Echeclus, and 166P/NEAT. 166P/NEAT was discovered while it exhibited a coma, and so is classified as a comet despite its orbit, and 60558 Echeclus was discovered without a coma but later became active, and was then classified as both a comet and an asteroid (174P/Echeclus). One plan for Cassini involved sending it to a centaur, but NASA decided to destroy it instead.
Observation
A comet may be discovered photographically using a wide-field telescope or visually with binoculars. However, even without access to optical equipment, it is still possible for the amateur astronomer to discover a sungrazing comet online by downloading images accumulated by some satellite observatories such as SOHO. SOHO's 2000th comet was discovered by Polish amateur astronomer Michał Kusiak on 26 December 2010 and both discoverers of Hale–Bopp used amateur equipment (although Hale was not an amateur).
Lost
A number of periodic comets discovered in earlier decades or previous centuries are now lost comets. Their orbits were never known well enough to predict future appearances or the comets have disintegrated. However, occasionally a "new" comet is discovered, and calculation of its orbit shows it 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. There are at least 18 comets that fit this category.
In popular culture
The depiction of comets in popular culture is firmly rooted in the long Western tradition of seeing comets as harbingers of doom and as omens of world-altering change. Halley's Comet alone has caused a slew of sensationalist publications of all sorts at each of its reappearances. It was especially noted that the birth and death of some notable persons coincided with separate appearances of the comet, such as with writers Mark Twain (who correctly speculated that he'd "go out with the comet" in 1910) and Eudora Welty, to whose life Mary Chapin Carpenter dedicated the song "Halley Came to Jackson".
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, Earth passed through the comet's tail, and erroneous newspaper reports inspired a fear that cyanogen in the tail might poison millions, whereas the appearance of Comet Hale–Bopp in 1997 triggered the mass suicide of the Heaven's Gate cult.
In science fiction, the impact of comets has been depicted as a threat overcome by technology and heroism (as in the 1998 films Deep Impact and Armageddon), or as a trigger of global apocalypse (Lucifer's Hammer, 1979) or zombies (Night of the Comet, 1984). In Jules Verne's Off on a Comet a group of people are stranded on a comet orbiting the Sun, while a large manned space expedition visits Halley's Comet in Sir Arthur C. Clarke's novel 2061: Odyssey Three.
Gallery
File:Comet_C2020F3_NEOWISE_over_California_desert_landscape.png|Comet C/2020 F3 NEOWISE
File:Comet P1 McNaught02 - 23-01-07-edited.jpg|Comet C/2006 P1 (McNaught) taken from Victoria, Australia 2007
File:Great Comet of 1882.jpg|The Great Comet of 1882 is a member of the Kreutz group
File:Great Comet 1861.jpg|Great Comet 1861
File:X-rays from Hyakutake.jpg|Comet Hyakutake (X-ray, ROSAT satellite)
File:Asteroid P2013 P5 v2.jpg|"Active asteroid" 311P/PANSTARRS with several tails
File:NASA-14090-Comet-C2013A1-SidingSpring-Hubble-20140311.jpg|Comet Siding Spring (Hubble; 11 March 2014)
File:Comets WISE.jpg|Mosaic of 20 comets discovered by the WISE space telescope
File:PIA22419-Neowise-1stFourYearsDataFromDec2013-20180420.gif|NEOWISE – first four years of data starting in December 2013
File:Lovejoy-hi1a srem dec12 14.gif|C/2011 W3 (Lovejoy) heads towards the Sun
File:ITS Impact.gif|View from the impactor in its last moments before hitting Comet Tempel 1 during the Deep Impact mission
;Videos
File:NASA Developing Comet Harpoon for Sample Return.ogv|NASA is developing a comet harpoon for returning samples to Earth
File:Encke tail rip off.ogg|Comet Encke loses its tail
| 彗星的彗核、彗发和彗尾:
• 上排:深度撞击号拍摄的坦普尔1号彗星彗核和罗塞塔号拍摄的彗星67P
• 中排:霍姆斯彗星和星尘号拍摄的维尔特二号彗星彗核
• 下排:1997年于克罗埃西亚拍摄的海尔博普彗星和ISS上拍摄到的C/2011 W3彗星
|}
彗星(Κομήτης,Komet,Comet),俗称扫把星,是由冰构成的太阳系小天体(SSSB)。当其朝向太阳接近时,会被加热并且开始释气,展示出可见的大气层,也就是彗发,有时也会有彗尾。这些现象是由太阳辐射和太阳风共同对彗核作用造成的。彗核是由松散的冰、尘埃、和小岩石构成的,大小从P/2007 R5的数百米至海尔博普彗星的数十公里不等,彗尾可能延伸长达一天文单位。
彗星的轨道周期范围也很大,可以从几年到几百万年。短周期彗星来自超越至海王星轨道之外的柯伊伯带,或是与离散盘有所关联。长周期彗星被认为起源于欧特云,这是在古柏带外面,伸展至最近恒星一半距离上,由冰冻天体构成的球壳。长周期彗星受到路过恒星和银河潮汐的引力摄动而直接朝向太阳前进。双曲线轨道的彗星可能在进入内太阳系之前曾经被沿著双曲线轨迹被抛射至星际空间,则只会穿越太阳系一次。来自太阳系外,在银河系内可能是常见的系外彗星也曾经被检测到。
彗星与小行星的区别通常只在于存在著包围彗核的大气层,未受到引力的拘束而扩散著。这些大气层有一部分被称为彗发(在中央包围著彗核的大气层),其它的则是彗尾(受到来自太阳的太阳风电浆和光压作用,从彗发被剥离的气体、尘埃、和带电粒子,通常呈线性延展的部分)。然而,熄火彗星因为已经接近太阳许多次,几乎已经失去了所有可挥发的气体和尘埃,所以就显得类似于小的小行星。小行星被认为与彗星有著不同的起源,是在木星轨道内侧形成的,而不是在太阳系的外侧。主带彗星和活跃的半人马小行星的发现,已经使得小行星和彗星之间的差异变得模糊不清。
,已知的彗星有6,619颗,而且这个数量还在稳定的增加中。然而,这只是潜在彗星族群中微不足道的数量:估计在外太阳系的储藏所内类似的彗星体数量可能达到一兆颗。尽管大多数的彗星都是暗淡和不够引人注目的,但平均大概每年会有一颗裸眼可见的彗星,其中特别明亮的就会被称为「大彗星」。
在2014年1月22日,ESA科学家的报告首次明确的指出在矮行星谷神星,也是小行星带中最大的天体,有水气存在。这项检测是通过赫歇尔太空望远镜使用远红外线技术完成的。此一发现是出人意料之外的,因为彗星,不是小行星,才会有这种典型的「喷流萌芽和羽流」。根据其中一位科学家的说法:「彗星和小行星之间的区隔是越来越模糊了」。
古代也有彗星出现的记录,古人一般认为彗星是凶兆搜索。
Read more...: 命名和语源 研究的历史 早期的观测和推论 轨道的研究 物理性质的研究 近代的发现 物理性质 彗核 彗发 彗尾 喷流 与流星雨的关系 轨道特性 短周期 长周期 欧特云和希尔云 彗星的死亡 从太阳系排出 耗尽挥发物质 瓦解(分裂) 失踪 碰撞 命名规则 著名的彗星 大彗星 掠日彗星 不寻常的彗星 观测 对人类文化的影响 大众文化 相关条目 注释 进阶读物
命名和语源
彗星以其拖著的长尾巴而得名,「彗」的本意就是帚。《说文》纪载:「彗,埽竹也。」。中国古人把彗星叫做「星孛」,《春秋》记载,鲁文公14年(前613年)「秋七月,有星孛入于北斗」。这是世界上关于哈雷彗星的最早记录。根据董仲舒的说法,「孛者,恶之所生也。谓之孛者,言其孛之有所妨蔽,暗乱不明之貌也」。实际上在中国古代,彗星常与灾厄联系在一起,刘向也说「孛者,乱臣矣,篡杀之表也」,但也有刘知几等人早就表示这种说法是无稽之谈。
《晋书·天文志》载有:「彗星所谓扫星,本类星,末类彗,小者数寸,长或经天。彗星本无光,傅日而为光,故夕见则东指,晨见则西指。在日南北皆随日光而指,顿挫其芒,或长或短。」准确的描述了彗星的形态。
西方语言中的「彗星」一词(如法语:;德语:Komet;英语:comet,古英文:cometa),
源自拉丁文的comēta或comētēs,这是拉丁化的希腊文κομήτης。在牛津英语字典,这个词是'κομήτης'(' ἀστὴρ '),意思是希腊文的"长发明星,彗星"。Κομήτης是从κομᾶν("留著长发")转变过来的,其本身又是从κόμη(意思是"头上的头发")转变过来的,而其意思是"彗星的尾巴"。希腊哲学家兼科学家亚里斯多德是第一位使用这个延伸出来的字κόμη, κομήτης,来形容他看见的"长著头发的星星"。彗星的天文学符号是 (Unicode ☄ U+2604),由一个小圆盘和三根如头发突起的短线段组成。
研究的历史
由于彗星无论是形貌或运行规律,都和人类平常观测的行星和恒星大相迳庭,因此早期彗星始终为不少人所著迷,且由于彗星的出现时间常和不少历史上大事件的发生时间相近,因此部分民族甚至视彗星为神或灾祸的象徵。
早期的观测和推论
在望远镜发明之前,彗星好像无论在何处出现,都会慢慢的消失不见。它们通常都被认为是不好的预兆,会为国王或男性的贵族带来灾难、死亡,甚至被解释为上天对地球上居民的攻击。来自古代的资料,例如中国的甲骨文,知道数千年来人类就曾经发现过彗星。乌鲁克的国王吉尔伽美什将之解释为"流星",而启示录、以诺书等则称之为彗星,或可能是火流星。一个很有名的古老记录,是出现在贝叶挂毯上的哈雷彗星,这幅挂毯描述的是1066年诺曼征服英格兰的事迹。
亚里士多德在他的第一本书,天象论中对彗星看法的论调,主导了西方对彗星的思潮将近两千年。他否决了几个早期哲学家认为彗星是行星,或至少是一种与行星有关天象的想法,理由是行星局限于黄道上,并且是种圆周运动,但彗星可能出现在天空中的任何部分。取而代之的是,他描述彗星是地球大气层上层的现象,是在炎热、乾燥的环境下聚集和偶然喷出的火焰。亚里斯多德认为这种机制不仅形成彗星,还包括流星、极光,甚至是银河。
有几位后来的哲学家对彗星的看法提出异议。塞内卡延续阿波罗尼奥斯彗星是独立星体的观点,在他的天问指出,彗星在天空中有规律的移动,并且不受风影响的性质,这种不受干扰的行为比较像天体而不是大气中典型的现象。尽管他认为其它的行星不会出现在黄道之外,但是类似地球的天体没有理由不能在天空的任何地方出现,人类对天体的认识是非常有限的。然而,亚里斯多德的观点被证明更有影响力,直到16世纪,彗星还被认为是大气层内,而不是大气层之外的现象。
在1577年,一颗明亮的彗星出现了好几个月。丹麦的天文学家第谷·布拉赫使用他自己和别人在不同地点测量的彗星位置,试图测量出彗星的视差。但在测量的精确度范围内,测不出任何视差,这暗示了彗星的距离比月球到地球距离至少还要远4倍以上。
在《科学的奇迹》(Marvels of Science)中,作者提到:笛卡尔、欧拉等人相信,整个宇宙充满在一种难以察觉的介质中,他们称其为以太。无数的行星和恒星漂浮在以太海洋中。在这种介质中彗星则起到清道夫发作用,以防止以太集结成块,使其保持稳定适当的稀薄状态。
轨道的研究
虽然彗星现在已经被证明是天体,但是它们在天空上是如何移动的,却在下个世纪成为辩论的主题。即使稍后约翰·克卜勒在1609年确定行星是以椭圆轨道环绕著太阳,他认为定律管辖的是行星运动,应该不会影响到其它天体的运动-他相信彗星是在行星之间以直线运动。伽利略虽然坚信哥白尼学说,拒绝第谷的视差测量并且包容亚里斯多德认为彗星是通过大气层上层直线运动的观念。
在1610年,威廉·罗耳是第一位建议行星运动的克卜勒定律也适用于彗星的人。在接下来的数十年,其他的天文学家,包括Pierre Petit、Giovanni Borelli、Adrien Auzout、罗伯特·虎克、Johann Baptist Cysat、和乔瓦尼·多梅尼科·卡西尼也都主张彗星是以椭圆或抛物线的曲线路径绕著太阳;但是,如克里斯蒂安·惠更斯和约翰·赫维留的部分学者依然认为彗星是以直线运动。
这件事经由Gottfried Kirch在1680年11月14日发现的亮彗星得到解决,整个欧洲的天文学家追踪这颗彗星的位置达数个月。在1681年,萨克逊的牧师进一步的证明这颗彗星是以抛物线运行的天体,并且太阳在其中的一个焦点上。然后艾萨克·牛顿在他1687年发表的数学原理中证明了一个在与距离平方成反比的万有引力影响下运动的物体,它的轨道所形成的轨迹形状是圆锥曲线,并且使用1680年的彗星做例子,说明彗星在天球上经过的路径与抛物线是如何吻合的。
在1705年,爱德蒙·哈雷应用牛顿的方法分析了在1337年至1698年间出现的23颗彗星。他注意到1531年、1607年和1682年的彗星有著非常相似的轨道要素,他进一步考虑到木星和土星的引力摄动对轨道造成的微小差异,更有信心确认这三颗彗星是同一颗彗星的一再出现,他并预测这颗彗星在1758至1759年间会再出现。(稍早些,罗伯特·虎克认定1664年和1618年的彗星是同一颗,同时乔凡尼·卡西尼曾怀疑1577年、1665年、和1680年的,但两者都不正确。)哈雷预测的回归日期后来被三位法国数学家的小组:亚历克西斯·克劳德·克莱罗、约瑟夫·拉朗德和妮可-雷讷·勒波特,再精算过,他们预测这颗彗星的近日点落在1759年,准确在一个月内。当这颗彗星如预测的回来时,它被命名为哈雷彗星(稍后的正式名称为1P/Halley),下次将于2061年回归。
在历史上,彗星的周期不仅要够短,还要每次都够明亮,才能够被记录好几次。哈雷彗星是唯一每次都够亮,在经过太阳系的内侧时能以肉眼看见的彗星。自哈雷彗星的周期被确认之后,通过望远镜的使用,发现了许多其它的周期彗星。第二颗被发现周期的彗星是恩克彗星(官方正式的名称是2P/Encke)。德国数学家兼物理学家约翰·弗朗茨·恩克在1819-21年间计算一系列彗星的轨道,他观察到1786年、1795年、1805年、和1818年的彗星,得出的结论是它们是同一颗彗星,并且成功的预测它在1822年的回归。到1900年,已经有17颗彗星被观察到多次通过近日点,并被认定是周期彗星。,已有271颗周期彗星被辨识出来,不过其中有几颗已经瓦解或是失踪了。
物理性质的研究
艾萨克·牛顿描述彗星是在倾斜轨道上运动的紧密和持久的固体,它们的尾巴是由核心排放出,被太阳加热或点燃的稀薄气体。牛顿怀疑彗星是支援空气中生命起源的元件,他也相信彗星排放的蒸气和太阳供应的燃料,可以补充行星的水(经由植物的增长和腐烂还逐渐转变成行星上的土壤)。
在18世纪初期,一些科学家对彗星的组成已经做了正确的假设。在1755年,伊曼努尔·康德假设彗星是由一些挥发性物质组成,当它们接近近日点时因为汽化而呈现辉煌的亮度。在1836年,德国数学家弗里德里希·威廉·贝塞尔在观察1835年的哈雷彗星喷发出来的气流之后,认为喷射力大到足以改变一颗彗星的轨道,。
然而,另一个有关彗星的发现掩盖了这个想法将近一世纪之久。在1864至1866年间,义大利天文学家乔凡尼·斯基亚帕雷利计算英仙座流星雨的轨道,基于轨道的相似性,它正确的指出该流星雨是斯威夫特-塔特尔彗星的片段。彗星和流星雨之间的联系,在1872年被戏剧性的强调,在比拉彗星的轨道上发生了重大的流星雨,而这颗彗星在1846年出现时被观测到分裂成两半,并且在1852年后就未曾再见到。"碎石银行"结构的彗星模型出现了,在模型中,彗星是由松散的小岩石堆积而成,并涂上了冰冷的外层。
在20世纪中叶,这种模型呈现出了一些缺点:尤其是,它不能解释只有少量冰冻物质的物体,可以在经过近日点数次之后,依然可以继续的蒸发出气体而持续完美的展现。在1950年,弗雷德·惠普尔提出这一点,认为彗星不是岩石包覆著一些冰,而是冰冻的物质包含了一些尘埃和岩石。这"脏雪球"模型很快的就被接受,并且来自庞大的太空船观测资料,似乎也支持这样的见解。这些太空船包括ESO的乔托号探测器和苏联的Vega 1和Vega 2,它们在1986年穿越过哈雷彗星的彗发,拍摄了彗核的影像,和观察了挥发性物质的彗尾。
近代的发现
关于彗星含有多少冰的辩论仍然持续著。在2001年,NASA的深空一号小组,在NASA的喷射推进实验室工作,获得19P/包瑞利彗星表面的高解析影像。他们宣布包瑞利彗星展现出性质不同的喷流,是热且乾燥的。假设彗星包含水和其他的冰,领导人,美国地质调查局的Laurence Soderblom博士说:光谱显示表面是热和乾燥的。令人惊讶的是我们没有看见水冰的痕迹。然而,他又提出冰可能隐藏在下方,而表面因为太阳的加热已经乾涸,也或许包表面覆盖著非常黑的,像煤灰的材料掩盖了地壳表面任何冰的踪迹。
在2005年7月,深度撞击探测器在坦普尔1号彗星上撞出一个坑穴以研究它的内部。这个任务的结果显示彗星的冰水大部份都是在表面下,这些储藏的水升华形成了彗发,提供了坦普尔1号彗星喷流所需要的蒸发水。之后,它改名为EPOXI,在2010年11月4日飞掠过哈特雷二号彗星。
在1999年2月发射的星尘号太空船,在2004年1月蒐集了维尔特二号彗星来自彗发的颗粒,并且在2006年1月用荚舱将样品送回地球。克劳迪雅亚历山大,在NASA的喷射推进实验是从事彗星模型建构多年,向space.com报告她对喷流数量的惊讶,它们的外观在黑暗侧和明亮侧是一样的,它们能从彗星的表面举起大块的岩石,此一事实表明维尔特二号彗星不是松散黏合的瓦砾堆。
更多来自星尘任务的资料显示来自维尔特二号彗星尾巴物质的结晶可能仅能在火中生成。虽然彗星是在太阳系的外侧形成的,但在太阳系早期的形成时间,径向的物质混合有可能重新分配了原始行星盘的所有物质,所以彗星也包含了在炙热的太阳系内侧形成的结晶颗粒。这在彗星的光谱,以及样本返回任务都能见到。近来还有更多,取回的物质表明"彗星尘埃类似于小行星的物质"。这些新的结果迫使科学家重新思考彗星和小行星在本质上的区别和差异。
在2011年4月,来自亚历桑纳大学的科学家发现维尔特二号彗星中有液态水存在的证据。他们找到了铁和必须有水存在下才能形成的硫化铜矿物。此一发现粉碎了彗星从来没有得到足够使大量冰块融化的温暖环境的现有范例。
即将进行的太空任务将增加能让我们更清楚认识彗星的组成。欧洲的罗塞塔探测器将前往67P/楚留莫夫-格拉希门克彗星;在2014年,它将进入环绕这颗彗星的轨道和安放一个小登陆艇到它的表面。
物理性质
彗星由彗核、彗发和彗尾组成。彗核和彗发构成彗头。
彗核
彗星的核心固体结构称为彗核,是由水冰、岩石、和冻结的气体(如二氧化碳、一氧化碳、甲烷和氨等)融合在一起组成的。1950年代,美国天文学家弗雷德·惠普尔提出「彗星的内核是由含冰的凝聚物组成」的假说,这个「彗星模型」后来令彗星普遍的被大众(包含惠普尔本人)昵称为「冰污球」(Icy dirtballs)或「脏雪球」(Dirty Snowballs)。
彗核的表面一般是乾燥、尘土或岩石飞扬的,这暗示冰是隐藏在表面数公尺厚的的地壳之下。除了已经提到的气体,彗核还包含各种各样的有机化合物,它们可能包括甲醇、氰化氢、甲醛、乙醇、和乙烷,或许还有更复杂的分子,如长链的烃类和氨基酸。在 2009年,从NASA星尘任务带回的彗星尘埃中发现了氨基酸中的甘氨酸。在2011年8月,NASA一份根据在地球上发现的陨石所做的报告指出,已经发现DNA和RNA的元件(腺嘌呤、鸟嘌呤、及相关的有机分子),可能已经在小行星和彗星上形成。
彗核表面的反照率非常的低,使它们成为太阳系内反照率最低的物体。乔托号太空探测器发现哈雷彗星的彗核只反射了大约4%照射在它上面的光线 ,深空一号发现包瑞利彗星表面反射落在它上面的光线少于3%;相较之下,落在沥青表面的光都还有7%能被反射。彗核表面黑暗的物质材料可能包括复杂的有机化合物。太阳的热驱动了较轻的挥发物,留下了较重的有机化合物,往往都是黑色的,像是焦油或是原油。彗星表面相对较低的反照率使它们可以吸收更多需要的热量,驱动释气的程序。
曾经观察过的彗核直径有超过的,但是要确定其确实的大小是很困难的。P/2007 R5的彗核直径大约只有100–200公尺。尽管仪器非常灵敏,但是缺乏较小的彗星可供检测彗核的大小,使得一些人认为彗核的直径不会小于。从已知的彗星估计,彗核的平均密度大约是0.6g/cm3,彗核的低质量使彗核不会因为自己的重力造成球形,因此它们的外型是不规则的。
大约6%的近地小行星被认为是熄火彗星,它们的彗核已不再释放出气体,包括(14827) Hypnos(睡神星)和(3552) Don Quixote(唐吉诃德)。
彗发
在彗星的周围围绕著的尘埃和气体形成一个巨大且稀薄的大气层,称为彗发,彗发受到太阳风和太阳的辐射压导致背向太阳的巨大尾巴,称为彗尾。
彗发通常都由和尘埃构成,其中90%都是当彗星距离太阳就从彗核挥发出来的水。的母分子主要是通过光解和很多规模较小的光电离,还有太阳风扮演光化学的小角色而被摧毁(分解) 。较大的尘埃粉尘粒子沿著彗星轨道的路径留下,而更小的粒子被光压推入彗星的尾巴。
虽然固体的彗核一般都小于的直径,但彗发可能有数千或数百万公里的直径,有时会变得比太阳还要大。。例如,17P/霍姆斯彗星在2007年10月爆发之后大约一个月的短时间,巨大的大气层就比太阳还要大;1811年大彗星的彗发也大致与太阳的直径相当。但即使彗发再大,在它跨越火星,大约距离太阳,它的大小就会衰减。在这个距离上,太阳风已经足够强大,可以将气体和尘埃吹离彗发,使尾巴增大。
当一颗彗星穿越内太阳系时,彗发和尾巴都会被太阳照亮而能够看得见,尘埃会直接反射阳光,而气体会因为离子化而发光。大多数的彗星因为太暗淡,没有望远镜的协助依然看不见,但每几十年总会有亮到肉眼足以直接看见的彗星。偶尔,会遇到彗星突然爆发出大量的气体和尘埃,这时彗发的大小会增加一段时期。在2007年,17P/霍姆斯彗星就发生这样的现象。
在1996年,发现彗星辐射出X射线。这使天文学家大为吃惊,因为X射线通常与高温天体相关联。X射线是彗星与太阳风的交互作用生成的:当高度电离的太阳风离子飞过彗星的大气层时,它们与彗星大气层中的原子和分子撞击,会从它们获得一个或多个电子,这个过程称为」电荷交换」。这种交换或转让一个电子给太阳风中的离子让离子去激发回到基态,导致辐射出X射线和远紫外线光子 。
彗尾
在太阳系的外缘,彗星依然在冰冻和不活跃的状态时,由于体积很小,因此很难甚至无法从地球上观测到。来自哈伯太空望远镜的观测报告,提出在古柏带内不活跃彗核的统计报告,但是这些检测不仅受到质疑,并且无法独立验证。当彗星接近太阳系的内侧时,太阳辐射造成彗核内部挥发性物质蒸发,并且从核心向外喷出,同时会带走一些尘埃粒子。
气体和尘埃流会形成指向不同方向,自己独特的彗尾。尘埃形成弯曲的尾巴会被抛在轨道的后方,通常称为第二型彗尾。同时,离子尾,或是第一型彗尾总是指向背向太阳的地方,因为它们受到太阳风的作用远比尘埃更强烈,因此是沿著磁场线而不是轨道的轨迹。在某些场合,如当地球穿越过彗星的轨道平面和我们从侧面看见彗星,可能会看见与尘埃尾指向相反的尘埃尾,称为彗翎(反尾)(在环绕太阳彗星前方的彗尾,与尾端的尘埃尾共线)。
对彗翎的观察在太阳风的发现上有意义深远的贡献。离子尾是彗发的微粒被太阳紫外线辐射电离后形成的。一旦粒子被电离,它们获得净正电核,并反过来在彗星附近引发」诱导磁层」。彗星和它的诱导磁层形成太阳风粒子向外流动的障碍。因为彗星的轨道速度和太阳风的速度都是超音速,弓形震波会在彗星运动和太阳风流动方向的前缘形成。在这些弓形震波,大量的彗星离子(称为」拾取离子」)被凝聚和集中,并且载入太阳风的磁场和电浆,这样的场线"披盖"在彗星的周围形成了离子尾。
如果离子尾的负载已经足够了,则磁场线会在那个点上挤在一起,在沿著离子尾的某个距离上会发生磁重联,这会导致"尾断离事件"。这种现象已经被观测到好几次,在2007年4月20日就有一次值得注意的事件。当恩克彗星通过日冕抛射的物质的时候,它的离子尾就完全的被截断了。日地关系天文台观测到了这次的事件。
在2013年,欧洲太空总署的科学家报告金星的电离层向外扩张的方式类似于一颗彗星在类似条件下形成的离子尾。
喷流
不均匀的加热会导致气体从彗核表面较薄弱的点,像间歇泉一样爆发出来。这些气体和尘埃流会导致彗核旋转,甚至分裂。在2010年,科学家发现乾冰(冻结的二氧化碳)可以驱动物质从彗核流出成为喷流。红外线的影像显示,哈特雷二号彗星的喷流携带灰尘颗粒进入彗发。
与流星雨的关系
由于释气的缘故,彗星会留下一些固体的碎片。如果彗星的路径跨越地球的路径,当地球经过彗尾碎片的踪迹,就有可能形成流星雨。例如,每年8月9日至12日,当地球穿越斯威夫特-塔特尔彗星的路径时,形成的英仙座流星雨;哈雷彗星是10月份的猎户座流星雨的来源。
轨道特性
大多数彗星都是细长椭圆轨道的太阳系小天体,它们的轨道只有一小部分接近太阳,剩馀的大部分都在深远的太阳系外缘。彗星通常都以轨道周期的长短来分类:轨道周期越长的椭圆也越细长。
短周期
短周期彗星的定义一般是指周期短于200年的彗星。它们的轨道通常在黄道的上下,并且运行方向与行星相同。它们轨道的远日点通常在外行星的区域(木星和超越其外);例如,哈雷彗星的远日点就在海王星之外不远处。彗星轨道的远日点靠近哪一颗行星,它就是该行星的彗星"家庭"这些家庭成员被认为是起因于被行星捕获到周期较短轨道上的长周期彗星。
周期最短的极端,恩克彗星的轨道不会抵达木星的轨道,并且称为恩克型彗星。短周期彗星中,周期短于20年和低倾角(不超过30度)的被称为木星族彗星 。像哈雷彗星的,轨道周期在20至200年之间,轨道倾角从0至超过90度的,称为哈雷族彗星。,只有72颗哈雷族彗星被观测过,相较之下木星族彗星则几乎有470颗。
最近发现的主带彗星形成一个独立的类别,不仅轨道在小行星带内,而且还接近圆形。
因为其椭圆轨道经常会带它们接近巨大的行星,彗星会受到进一步的重力扰动。短周期彗星的远日点有趋近于气体巨星轨道半径的趋势。很显然的,来自欧特云的彗星在接近巨大行星的时候,经常会受到这些行星强烈的影响。木星是最大的扰动源,因为它的质量是其他行星质量总和的两倍。这些扰动可以将长周期彗星的轨道转变成短周期的轨道。
基于其轨道特徵,有些短周期彗星被认为起源于半人马和古柏带/离散盘 —一个在海王星外侧的盘状区域—而长周期彗星的来源被认为是更遥远的一个球形的欧特云(以提出存在这个假想球壳的何兰天文学家杨·亨德里克·欧特的名字命名)。一般认为在这个以太阳为中心,大致成球形的遥远地区内,在大致是圆形的轨道上,存在著许多类似彗星的天体。偶尔,外侧行星的影响力(这种情形通常是对古柏带的天体),或是邻近的恒星(这种情形通常是对欧特云的天体)可能会将这些天体中的一颗抛入椭圆形的轨道,将它带向太阳成为可以看见的彗星。不同于回归的短周期彗星,没有之前的观测资料可以建立它们的轨道,通过这个机制产生的新彗星,其外观是不可预知的。
长周期
长周期彗星有较高的离心率轨道和范围从200年至数千乃至百万年的周期,在近日点附近时,离心率大于1并不完全意味著这颗彗星会逃离太阳系。例如,麦克诺得彗星在2007年1月(历元)接近近日点时的日心吻切轨道离心率是1.000019,但是它受到太阳的引力约束,周期约为92,600年,因为在它远离太阳之后离心率已降至1以下。长周期彗星将来的轨道需要再它远离行星所在的区域以后,再以太阳系的中心计算吻切轨道的历元,才能确定。依据定义,长周期彗星依然受到太阳引力的约束;这些彗星在接近主要的行星时可能会被弹出太阳系,因此就无须考虑它原本的「周期」是否正确。长周期彗星的轨道会带它们进入远离外行星的远日点,而且它们的轨道平面也不需要躺在黄道面附近。像威斯特彗星和C/1999 F1这些长周期彗星在重心座标系的拱点距离接近70,000天文单位,估计轨道周期大约长达600万年。
单次出现或非周期彗星都类似长周期彗星,这是因为它们在进入内太阳系接近近日点时,都有抛物线或略呈双曲线的轨迹。但是,这可能是巨大行星的摄动导致它们的轨道发生改变。单次出现或是有著抛物或双曲吻切的彗星,会使它们在接近太阳一次之后,就永远的离开太阳系。太阳的希尔球是一个不稳定的球体,最大的范围可以达到230,000 AU。只有少数的数百颗彗星在接近近日点的附近时曾被观测到双曲线轨道(e > 1),在使用无摄动的日心二体最加拟合才认为它们可能会逃出太阳系。
已经观测过的彗星,没有离心率明显大于1的所以没有明确的证据可以指出有起源于太阳系外的彗星。C/1980 E1彗星的在1982年通过近日点之前的周期大约是710万年,但是它在1980年与木星遭遇而被加速,使它成为已知彗星中离心率最大的(1.057)。预测不会再返回内太阳系的彗星包括C/1980 E1、C/2000 U5、C/2001 Q4 (NEAT)、C/2009 R1、C/1956 R1、和C/2007 F1 (LONEOS)。
有些机构使用周期彗星这个术语泛指轨道有周期性的彗星(也就是包括所有的短周期彗星和长周期彗星),而其他人使用它时则完全仅意味著短周期彗星。同样的,虽然无周期彗星字面的意义是与「仅出现一次的彗星」是相同的,但有些人的意思是所有在有生之年不能看见第二次的彗星(也就是包括周期在200年以上的长周期彗星)。
早期的观测显示有几颗彗星的轨迹真的是双曲线轨道彗星(也就是无周期彗星),但都未超过被木星摄动而被加速的可能范围。如果彗星充斥在星际空间内,它们的移动速度应该与临近太阳的恒星有著相同数量级的相对速度(每秒数十公里的速度)。如果这样的天体进入太阳系,它们应该有正值的特殊轨道能量,并将真正的观测到有著双曲线轨道。粗略的计算显示,每世纪应该有4颗双曲线轨道的彗星进入木星轨道的内侧,并有著1或2等级的星等。
欧特云和希尔云
彗星的死亡
从太阳系排出
如果一颗彗星有足够快的速度运行,那么它可以离开太阳系;这就是双曲线情况的彗星。到目前为止,已知会弹出太阳系的彗星都是曾和太阳系的其它天体,像是木星,发生过交互作用(参见摄动)。所有已知的彗星都起源于太阳系内,而不是以高速度的双曲线轨道进入太阳系。
耗尽挥发物质
木星族彗星(JFC)和长周期彗星(LPC,参见前述的"轨道特性")似乎遵循非常不同的衰退法则。木星族彗星的活动大约是10,000年,或是1,000次的公转,而长周期彗星消失得更快。只有10%的长周期彗星能够通过短距离的近日点50次依然存活著,而只有1%能超过2,000次。最终,大部分彗星的挥发性材料都会蒸发掉,使得彗星成为小而黑的惰性岩石,或是类似于小行星的废墟。
瓦解(分裂)
彗星也会碎裂成为碎片,例如:比拉彗星(3D/Biela)于1846年发生分裂,1872年彗核完全分开,结果在1872、1885、1892年都引起非常壮观的流星暴,每小时流星数达3000∼15000颗左右。73P/Schwassmann–Wachmann从1995年也开始发生这样的现象。
这些分裂可能是太阳或大行星引力导致的潮汐力造成的,或是由于挥发性物质的"爆炸",还是其他尚未完全明了的原因。
失踪
许多在数十年前或前个世纪发现的彗星现在已经成为失踪者了。它们或因为轨道不明确而难以预测未来的出现,或是已经瓦解了。然而,偶尔会发现一颗"新"彗星,但它们的轨道计算显示,这是旧有的"失踪"彗星。一个例子是11P/Tempel–Swift–LINEAR,在1869年发现,但在1908年受到木星的摄动就失踪了,直到2001年才意外的被LINEAR再度发现。
碰撞
有些彗星有著更壮观的结束- 要么落入太阳,或是粉碎后进入另一颗行星或天体。在太阳系的早期,彗星和行星或卫星之间的碰撞是很常见的:例如,地球的卫星表面有许多的撞击坑,有些可能就是彗星造成的。最近一次彗星与行星的撞击发生在1994年7月,粉碎了的舒梅克·利维九号彗星与木星相撞。
在早期的阶段,有许多彗星和小行星因相撞而进入地球。许多科学家认为彗星的轰击为年轻的地球(40亿年前)带来了大量的水,形成了目前铺满地球的海洋,即使不是全部也是很大的一部分。但也有其它的研究人员对这个理论产生质疑。在彗星上检测到一些有机分子,使得有人推论彗星或陨石可能为地球带来了生命的前身- 甚至就是生物本身。依然有许多彗星是近地彗星,但是地球与小行星撞击的机率还是高于彗星。
人们怀疑彗星的撞击,在长时间的尺度上,也能运送大量的水给地球的卫星,所以可能有一些月球冰会留存下来。
彗星和陨石的撞击被认为是玻璃陨石和澳洲玻璃陨体的成因。
命名规则
在过去的两个世纪,彗星的命名有几个不同的规则。在通过有系统的命名约定之前,有许多不同的命名方法。在20世纪的初期之前,大多数的彗星只简单的依据它们出现的时间命名,特别是明亮的大彗星都只提及年份:像是"1680年大彗星"(C/1680 V1,Kirch's Comet)、"1882年9月大彗星(C/1882 R1)、和"1910年白昼大彗星(1910年1月大彗星)。
在爱德蒙·哈雷表明1531年、1607年、和1682年的彗星是同一颗,并且很成功的预测它在1759年回归,这颗彗星就被称为哈雷彗星。相同的,第二颗和第三颗周期彗星恩克彗星和比拉彗星也都是以计算它们轨道的天文学家,而不是最初(原始)发现者的名字命名。之后,周期彗星通常就以发现者的名字命名,但也只有第一次,之后的出现就以通过近日点的年份表示。
在20世纪初期,以发现者的名字为彗星命名变得非常普遍,并且迄今依然是如此,一颗彗星可以使用三位独立发现者的名字。在最近这些年,许多彗星是由许多天文学家组织的大型团队机构发现的,就以这个机构的名称做为彗星的名字。例如,IRAS—荒贵—阿尔科克彗星(Comet IRAS–Araki–Alcock)是红外线天文卫星(IRAS)、和业馀天文学家玄一荒木与乔治·阿尔科克独立发现的。在过去,当多颗彗星是由同一个人、独力的团队或团队发现时,会在彗星的名称之后附加上数字(但限定是周期彗星),用来区别这些彗星;像是舒梅克-李维1至9号。现在,因为一些组织发现的彗星数量众多,使得这样的命名变得不切实际,也未能试图确保每颗彗星有一个唯一的名称。取而代之的是,使用系统化的彗星型号,藉以避免混淆。
直到1994年,彗星都会先给与一个临时名称,这是以发现的年份配合发现的先后顺序加上一个小写的英文字母(例如,1969 i(班尼特彗星)是1969年发现的第9颗彗星)。一旦观测到这颗彗星通过近日点,并且确定了它的轨道之后,就根据它通过近日点的年份和顺序的罗马数字给与永久性的名称(这编号通常是该年结束后二年才能编好)。所以彗星1969 i就成为彗星1970 II(它是1970年通过近日点的第二颗彗星),又如舒梅克·利维九号彗星的名称分别为1993e和1994Ⅹ。
但越来越多的彗星被发现,而且有些是在通过近日点之后才被发现,使这套系统显得不切实际。于是国际天文学联合会在1994年推出新的彗星命名系统。从1995年开始,彗星在一年中以每半个月为单位使用一个字母和数字来指示发现的顺序(这个系统和用于小行星的类似),所以,例如在2006年2月下半月发现的第4颗彗星,将被命名为2006 D4。此外,还添加前缀字母来显示彗星的性质:
• P/:确认为周期彗星(目地在定义任何周期短于200年的彗星,或是确认已经观测通过近日点超过一次以上的彗星);P前面再加上周期彗星总表编号。所以,哈雷彗星,第一颗被确认周期的彗星,在系统内的名称是1P/1682 Q1。
• C/ 标示无周期的彗星或周期超过200年的彗星。例如,海尔博普彗星的名称为C/1995 O1。
• X/ 标示没有可靠的轨道元素可以计算的彗星(一般来说都是历史上的彗星)。
• D/ 标示不再回归或已经消失、分裂或失踪的彗星。
• A/ 标示被错误归类为彗星,但其实是小行星的天体。
• I/ 标示来自太阳系外部的小天体,如A/2017 U1由于轨道表明来自太阳系外,后被命名为1I/'Omuamua,I表示Interstellar,即星际来客。
最初被当成小行星命名的彗星,在确认后仍然维持原有的名称,但会加上前缀字母,例如(Catalina–LINEAR)。
在太阳系内,暨是彗星又是小行星的天体已经有五颗,它们分别是:
• 95P/开朗=2060开朗
• 107P/威尔逊-哈灵顿=4015威尔逊-哈灵顿
• 133P/Elst-Pizarro=7968 Elst-Pizarro
• 174P/Echeclus=60558厄开克洛斯
• 176P/LINEAR=118401LINEAR
如果彗星破碎,分裂成数个以上的彗核,则在编号后加上-A、-B..以区分每个彗核。回归彗星方面,如彗星再次被观测到回归时,则在P/(或可能是D/)前加上一个由IAU小行星中心给定的序号,以避免该彗星回归时重新标记。例如哈雷彗星有以下标记:1P/1682 Q1=1P/1910 A2=1P/1982 U1=1P/Halley=哈雷彗星。
著名的彗星
大彗星
虽然每年都有数以百计的小彗星进入内太阳系,但很少受到一般民众的注意。大约每十年但不尽如此,会有一颗彗星亮到无须刻意观察就能看见- 这种彗星通常被称为大彗星。在过去的时代,明亮的彗星往往引发一般民众的恐慌和歇斯底里的反应,被认为是不好的徵兆。最近,在1910年重返的哈雷彗星,因为地球会通过它的彗尾,报纸上错误的报导激起民众对氰化物的恐惧,认为可能会毒害数以万计的生命,1997年海尔-波普彗星的出现,引起天堂之门教徒大规模的自杀潮。
预测一颗彗星是否能成为大彗星是很困难的,因为有许多因素都会影响到彗星偏离预测的亮度,而不知能否成为大彗星。概括的说,如果彗星有一颗庞大和活跃的核,并且足够接近太阳,在最亮时没有被太阳遮掩而能从地球看到,它就有机会成为大彗星。然而,1973年的柯侯德彗星符合前述所有的标准,被预测会成为壮观的大彗星,但结果并非如此。三年后出现的威斯特彗星,大家对它的期望并不高(或许因为对柯侯德彗星预测的惨败,使科学家们在预测上趋于保守),但却成为令人印象深刻的彗星。
在20世纪末期,有很长的一段时间没有出现大彗星,然后有两颗大彗星接踵出现。在1996年继海尔-波普彗星之后,百武彗星随即现身,并在1997年达到最大亮度。21世纪的第一颗大彗星是C/2006 P1(麦克诺特),它在2007年1月成为肉眼可见的彗星,并且是40年来最亮的彗星。
掠日彗星
掠日彗星是指近日点极为接近太阳的彗星,有时其距离可接近至太阳表面仅数千公里。较小的掠日彗星会在接近太阳时被完全蒸发掉,而较大的彗星则可通过近日点多次。然而,太阳强大的潮汐力通常仍会使它们分裂。
SOHO观测到的掠日彗星大约90%都是克鲁兹族的成员,它们源自一颗在第一次进入内太阳系时就被碎裂成许多小彗星的巨大彗星。其它10%则包含一些零星的彗星,以及4个已经确定有所关联的群体:分别为科里切特族(Kracht)、科里切特2a族、马斯登族(Marsden)及迈耶族(Meyer)。马斯登族和科里切特族或许与96P周期彗星——梅克贺兹一号彗星有所关联,这颗彗星也可能是象限仪座流星雨和白羊座流星雨的母彗星。
不寻常的彗星
已知的数千颗彗星中,有些是很不寻常的。恩克彗星的轨道从小行星带的外侧进入到行星的水星轨道内侧,而29P/施瓦斯曼·瓦茨曼彗星的轨道接近圆形,并且允型在木星和土星轨道之间。在土星和天王星之间的凯龙轨道并不稳定,起出被归类为小行星,直到注意到它有著暗淡的彗发,才被认为是彗星。同样的,137P/舒梅克·利维2号彗星起初也被当成小行星。大约百分之六的近地小行星被认为是不再能排出气体的熄火彗星。
有些彗星,包括威斯特彗星和池谷关彗星,在通过近日点时被观察到分裂的现象。3D/比拉彗星是一个值得注意的例子,它在1846年通过近日点时分裂成两块,在1852年还观测到这两颗分离的彗星,但之后就没有再看见。取而代之的是在彗星该回归的1872年和1885年出现了壮观的流星雨。在每年的11月,当地球跨越过比拉彗星的轨道时,都会出现一个较小的流星雨:仙女座流星雨。
另一颗值得注意的彗星是撞毁的舒梅克-李维九号彗星,它是在1993年被发现的。在发现的时候,这颗彗星的轨道环绕著木星,它是在1992年非常接近木星而被捕获的。如此靠近的距离使这颗彗星碎裂成数百片,并在1994年7月花费了六天的时间陆续撞击到木星上。1908年的通古斯事件也被认为可能是类似的事件,有可能是恩克彗星的碎片造成的。
观测
使用广视野望远镜摄影或双筒望远镜都可能发现新彗星。然而,即使没有光学设备,业馀天文学家依然可以从线上下载一些卫星的影像,像是SOHO卫星,发现掠日彗星。在2010年12月26日,业馀天文学家Michał Kusiak发现了第2,000颗SOHO的彗星,在可预见的未来,这个数字还会稳定的持续增加。
肉眼可见的彗星是非常罕见的,但业馀的天文望远镜(口径50mm至100mm)就能精细显示的彗星倒是相当的多-每年都有好几颗,有时在一个夜晚,甚至同一个时间就能在夜空中看见好多颗。通常可以用天文软体绘制这些已知彗星的轨道。相较于其它天体,它们会快速的移动,而在望远镜的目镜中,它们的移动通常是很容易察觉的。但是,夜复一夜,它们的移动量也只有几度,这就是为甚么观察者使用星图就很容易发现它们,就像是在毗邻的图示。
彗星显示的类型取决于其组成和与太阳接近的程度。因为一颗彗星的物质挥发会随著它与太阳距离的增加而减少,彗星变得越来越难观测,不只是因为它的距离,还有它的尾巴和用于反射的元素量逐渐的萎缩。
最引人注目的彗星是有著明亮的核心和展示出长长的尾巴,有时需要广视野的小望远镜或双筒望远镜才能获得最好的景象。因此,大型的业馀仪器(口径或更大)虽然有更好的集光力,但在观赏彗星时不一定会有优势。使用至等级的小口径仪器能观赏到的壮观彗星很频繁的,但较少受到注意,而其机会远高于受到媒体关注而非常罕见的大彗星。
彗星被认为也会绕著其它的恒星运转,但是对目前的系外行星侦测法而言,它们因太远和太小而难以被检测到。
对人类文化的影响
彗星奇特的形态,加上偶尔才能看到,古代许多地区的人们都把它视作上天的一种徵兆。在中国古代,人们把它看作灾祸降临的不祥之兆,称之为「灾星」。欧洲曾经把它当作上帝给予的预示。钱锺书说:「古人每借天变以谏诫帝王」,「以彗星为『天教』、荧惑为『天罚』」,「然君主复即以此道还治臣工,有灾异则谴咎公卿」。
大众文化
在流行文化中,彗星常常被叙述为是预示世界末日和改变世界的预兆,而这个观点也牢固地根植于西方的传统看法中。哈雷彗星每次的回归都会在各种类型的出版物上创造一系列耸动的新闻。其中特别受到注意的是,一些名人的出生和死亡与这颗彗星的回归,像是马克·吐温(他曾预测自己会在彗星于1910年回归时辞世,而他的预言后来也确实成真),和尤多拉·韦尔蒂(1909年出生),玛丽·翠萍·卡本特以专曲哈雷来到杰克逊于1987年成名。
在科幻,彗星撞击被用来描述克服技术困难与威胁的英雄主义(彗星撞地球,1998年的影片),或是用来触发全球的危机(路西法的锤子,1979年影片),或成批的僵尸(彗星夜,1984年影片)。近期描述撞击的有儒勒·凡尔纳的远离彗星和zh-cn:图苇·杨松;zh-hk:图苇·杨松;zh-tw:朵贝·杨笙;的姆米谷的彗星,而大型的载人太空探测有亚瑟·查理斯·克拉克的小说:2061太空漫游。
日本动画也常有使用彗星的元素。早期如宇宙战舰大和号中,来自河外星系的外星人建造的一个机动要塞,以一层类似白矮星的简并态物质掩饰下高速飞向银河系侵略时,给远处的人类以为只是一颗白色的慧星。电视动画夏洛特中,有一颗和影片标题相同的彗星,接近地球时使一部分青少年拥有了超能力。电影动画你的名字中,以架空历史的2013年一颗接近地球的彗星,一个女孩发梦中和男孩交换了灵魂,真相是天赋的预知的超能力。
相关条目
• 彗星酒
• 彗星列表
• 大崩溃 (书)
• 凯撒神庙
• 地球撞击坑列表
注释
进阶读物
• .
• Brandt, J.C. and Chapman, R.D.: Introduction to comets, Cambridge University Press 2004
• 《彗星─性质及观测方法》,香港太空馆编制,香港市政局出版,1986年初版,1997年3月再版;ISBN 962-7054-08-8/UC 10641
Text | Count |
---|---|
清史稿 | 2 |
新唐书 | 34 |
五代会要 | 3 |
清史纪事本末 | 10 |
三国志 | 2 |
明史 | 41 |
大越史记全书 | 16 |
南史 | 5 |
通志 | 9 |
辽史 | 3 |
元史 | 19 |
契丹国志 | 1 |
宋史 | 50 |
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