A comet or tailing star is a small celestial body, usually a few kilometers in diameter, which in the parts of its orbit near the sun develops a coma produced by outgassing and usually also a glowing tail. The name comes from the ancient Greek κομήτης komḗtēs ("hair star"), derived from κόμη kómē ("head hair, mane").
Like asteroids, comets are remnants of the formation of the solar system and consist of ice, dust and loose rock . They formed in the outer, cold areas of the solar system (mostly beyond the orbit of Neptune ), where the abundant hydrogen and carbon compounds condensed into ice .
In the vicinity of the sun, the comet nucleus, which is usually only a few kilometers in size, is surrounded by a diffuse, foggy envelope called a coma , which can reach an extent of 2 to 3 million kilometers. The nucleus and coma together are also called the head of the comet . However, the most noticeable feature of the comets visible from Earth is the tail . It only forms at a distance of less than 2 AU , but can reach a length of several 100 million kilometers for large objects close to the sun. Usually it is only a few tens of millions of kilometers.
The number of newly discovered comets was around 10 per year until the 1990s and has increased significantly since then thanks to automatic search programs and space telescopes . Most of the new comets and those already observed in earlier orbits are only visible in the telescope . As they approach the sun , they begin to shine more intensely, but the development of brightness and tail cannot be precisely predicted. There are only about 10 truly impressive apparitions per century.
History of comet research
Comets already aroused great interest in the early days because they suddenly appear and behave completely differently from other celestial bodies. In ancient times and up to the Middle Ages , they were therefore often seen as heralds of fate or signs of the gods. Aristotle and Ptolemy considered them to be vapors from the earth's atmosphere, because only regular orbits should occur in the sky.
Only Regiomontanus recognized independent celestial bodies in the comets and tried in 1472 to measure an orbit. The oldest printed cometary is probably the Tractatus de Cometis, which was published in Beromünster in 1472 and in Venice in 1474 , by the Zurich city doctor Eberhard Schleusinger , who was born in Goßmannsdorf near Hofheim in Lower Franconia , and whose work forms the basis for Johannes Lichtenberger's Pronosticatio . The beginning of scientific research on comets was Tycho Brahe's knowledge that they are not phenomena of the earth's atmosphere . Because he found the comet of 1577 that it had to be at least 230 earth radii away. However, it took many decades before this assumption could prevail, and even Galileo still contradicted it. Edmond Halley succeeded in 1682 in demonstrating the tail star that appeared that year as a periodically recurring celestial body. The comet, also observed in 1607, 1531 and 1456, moves around the sun on an elongated ellipse in 76 years. Nowadays, an average of 20-30 comets are discovered each year.
The level of knowledge about comets around the middle of the 19th century can be found in Scheffel's humorous song Der Komet : “Even Humboldt, the old man of researching power,…: 'The comet, much thinner than foam, fills the largest room with the smallest mass? ? '"
Comets are divided into aperiodic comets and periodic comets based on their appearance interval . The latter are divided into long-period and short-period comets according to their orbital times .
Comets which - due to their parabolic or hyperbolic orbit - will definitely not return, or individual observations about which no statement can be made - yet - due to a lack of precise orbit determination .
- Long-period comets with an orbital period of more than 200 years presumably come from the Oort cloud , their orbital inclinations are statistically distributed and they orbit the sun both in the same direction as the planets (prograd) and in the opposite direction to the planetary orbits (retrograde). The eccentricities of their orbits are close to 1 - but the comets are usually still bound to the sun by gravity , although they need up to 100 million years for their orbit. Eccentricities greater than 1 ( hyperbolic orbits ) are rare and are mainly caused by orbital disturbances when passing the large planets . Theoretically, these comets no longer return close to the sun, but leave the solar system . In the outer area of the planetary system, however, even small forces are sufficient to make the orbit elliptical again.
- Short-period comets with orbital times of less than 200 years presumably originate from the Kuiper Belt . They usually move in the usual sense of rotation and their inclination is on average around 20 °, so they are close to the ecliptic . In more than half of the short-period comets, the greatest distance from the Sun ( aphelion ) near the orbit of Jupiter is 5 and 6 astronomical units ( Jupiter family ) . These are originally long-period comets, whose orbit was changed by the influence of Jupiter's gravity .
Newly discovered comets are first given a name by the International Astronomical Union , which consists of the year of discovery and a capital letter starting with A on January 1st and B on January 16th every six months (up to Y on December 16th, the letter I will be skipped) according to the time of discovery. In addition, there is a number so that you can differentiate between several comets in half a month . As soon as the orbital elements of the comet have been determined more precisely, the name is preceded by another letter according to the following system:
|P||the orbital period is less than 200 years or at least two confirmed observations of the perihelion passage (periodic comet)|
|C.||The period of rotation is greater than 200 years.|
|X||The path cannot be determined.|
|D.||Periodic comet that has been lost or no longer exists.|
|A.||It is found afterwards that it is not a comet, but an asteroid .|
The comet Hyakutake, for example, is also known as C / 1996 B2 . So Hyakutake was the second comet to be discovered in the second half of January 1996. Its orbital period is longer than 200 years.
For example, usually a comet is also named after its discoverers, it is D / 1993 F2 also known as Shoemaker-Levy 9 out - is this is the ninth comet, the Eugene and Carolyn Shoemaker , along with David H. Levy discovered .
Since only short orbital arcs were observed in newly discovered comets, parabolic orbits are calculated first. However, since a parabola is only a mathematical borderline case and cannot occur as such in nature (even the smallest disturbance turns it into an ellipse or a hyperbola ), comets run whose orbital eccentricity is e = 1.0 (= parabola). is given, in truth either on ellipses (e <1.0) or on hyperbolas (e> 1.0). With longer observation and the acquisition of additional astrometric positions, it can then be decided whether it is ellipses or hyperbolas.
The distribution of around 660 comets is as follows: 43% parabolas, 25% long-period ellipses (orbital period over 200 years), 17% short-period ellipses (orbital period up to 200 years) and 15% hyperbolas. The high proportion of parabolas is, however, due to the short observation period of many cometary phenomena, in which elongated ellipses cannot be distinguished from a parabola. With a longer visibility of 240 to 500 days, only 3% of the comets probably describe a parabolic orbit. Thus, the ellipses should be predominant.
Discovery and observation of comets
While around 5 to 10 new comets were discovered each year up to 1900, this number has now risen to over 20. Automatic sky surveys and observations from space probes are essential to this . But there are also amateur astronomers who specialize in comet searches, especially in Japan and Australia .
The New Zealander William Bradfield was the most successful with 17 discoveries between 1972 and 1995, all of which were named after him. He systematically searched the twilight sky at a distance of up to 90 ° from the sun and spent around 100 hours a year doing this.
Bright binoculars or a special comet finder are suitable for visual observations . A weak magnification at high light intensity is important so that the relatively low surface brightness of the comet (similar to that of observing nebulae) is retained. The exit pupil should therefore correspond to that of the dark-adapted eye (approx. 7 mm).
Today cameras with highly sensitive CCD sensors are mostly used for photography . In detail photographs (about the structure of the comet's tail ), the camera is not in the stars tracked , but by means of approximated path calculation the comet itself. Most are in their discovery yet in the outer solar system and only appear as a diffuse star 15 to 20 Magnitude .
Space probes to comets
The following table shows some comets that have been visited by or are planned to visit by spacecraft :
|Borrelly||1904||Deep Space 1||2001||2200||Flyby|
|Halley||known since ancient times||Giotto||1986||596||Flyby|
Deep Impact ,
Extended Mission EPOXI
smallest comet studied
|Temple 1||1867||Deep impact||2005||500;
|Impact + flyby|
|1969||Rosetta||2014||6 or 0||Orbit of Rosetta; Landing of the Philae lander on Nov 12, 2014,
Rosetta's demise to the core on September 30, 2016
|Wild 2||1978||Stardust||2004||240||Flyby and return flight to earth ( sample return mission )|
At a great distance from the sun, comets consist only of the nucleus , which is essentially composed of water that has solidified into ice , dry ice (CO 2 ), CO ice, methane and ammonia with additions of meteorite-like small dust and mineral particles ( e.g. silicates , nickel iron ) consists. Comets are therefore often referred to as dirty snowballs (or dirty snowballs ). The observations of the Deep Impact mission have shown that (at least of the examined comets in the outer regions of the core die 1 ) outweigh the solid components with respect to the volatile elements, so that the designation snowy Dirtball (icy dirt Ball) true appears. From observations by the Giotto space probe on Comet Halley , we know that comets are surrounded by a black crust that reflects only about 4% of the light ( albedo ) - although comets are observed as spectacular luminous phenomena, their nuclei are, interestingly, the blackest objects in the solar system , much darker than, for example, asphalt , which reflects around 7% of the light.
Since only small regions of the core are outgassed, as explained in more detail in the section on coma, according to more recent ideas it is assumed that the surface is formed by a type of rock debris, which consists of rocks that are too heavy for the gravitational attraction of the core to overcome. Giotto also discovered tiny particles that are rich in the elements carbon (C), hydrogen (H), oxygen (O) and nitrogen (N) and are therefore also called CHON particles. These could come from a thin layer of soot covering the surface of the core, which would explain the low albedo. The current Rosetta mission should provide more information .
|H 2 O||100|
|H 2 CO||1|
|CH 3 OH||2|
|C 2 H 2||0.1|
|C 2 H 6||0.3|
|CH 2 CO||<0.03|
|CH 3 CHO||0.02|
|CH 3 CH 2 OH||<0.05|
|CH 3 OCH 3||<0.45|
|NH 2 CHO||0.01|
|CH 3 CN||0.02|
|HC 3 N||0.02|
|H 2 S||1.5|
|H 2 CS||0.02|
As soon as a comet approximates the orbit of Jupiter at a distance of about 5 AU when approaching the sun , the interaction between the solar wind and the comet forms a shell-shaped coma that also shows ray-like structures near the core. It is created by the sublimation of volatile substances on the side facing the sun, which carry away dust particles embedded in the ice . According to the observations of the Giotto probe, this sublimation only takes place on about 10 to 15% of the comet's surface, and the volatile substances evidently only escape from fragile areas of the black crust. The mother molecules escaping at these points form the internal coma. Through further heating, ionization and dissociation , the coma increases in size and forms the finally visible coma of ions and radicals. It is still surrounded by an atomic hydrogen halo that emits ultraviolet rays , which is also known as the UV coma and reached a diameter of 150 million kilometers at comet Hale-Bopp in 1997. Since the ozone layer is impermeable to UV radiation, the UV coma can only be examined from outside the earth's atmosphere .
The components of the coma are "blown away" by radiation pressure and solar wind, so that a tail forms within the orbit of Mars , or more precisely two tails:
- A narrow, elongated tail (type I tail) , which consists essentially of molecular ions and is also called a plasma tail . The radiation pressure is not sufficient to explain these particles, so that in 1951 Ludwig Biermann postulated particle radiation emanating from the sun, which is now known as the solar wind , as an explanation for this. Today it is assumed that the cometary ions are driven by an interaction with the solar magnetic field, which is carried along by the charged particles of the solar wind.
- A diffuse, curved tail (Type II tail) , also called a dust tail . The small dust particles that form this tail are influenced by the radiation pressure of the sun, the effect of which can be explained by a split into two components:
- A radial component that opposes the force of gravity and how it decreases quadratically with the distance from the sun. This works like an effective decrease of the solar gravitational force, the dust particles therefore move on "pseudo-Kepler orbits", which differ for dust particles of different sizes, since the radiation pressure depends on the particle size. This leads to a relatively strong fanning of the dust tail compared to the plasma tail.
- The other effective component of the radiation pressure is opposite to the direction of movement of the dust particles and leads to a deceleration of the particles which are larger than the wavelength of the light, that is, larger than about 0.5 µm. In the long term, these particles move in the same way as other interplanetary dust on spiral orbits towards the sun ( Poynting-Robertson effect ).
- Very rarely, with special orbit constellations, is a counter -tail ( type III tail , anti -tail ) visible. However, this is not a separate tail, but only a geometric projection effect: When the earth moves between the sun and the comet, part of the dust tail, due to its curvature, appears to protrude beyond the comet's head.
The loss of material from a comet for "new" comets that come close to the Sun for the first time was estimated to be around 10 to 50 tons per second. After several times approaching the Sun, the loss of mass drops to less than 0.1 t / s. These small amounts of matter of a maximum of 0.03 to 0.2 percent of the comet's mass per sun pass mean that the tails have only a very low density. In the case of the dust tail, the enormous brightness of the tails is explained by the large surface of the microscopic dust particles; in the plasma tail, even every atom or molecule contributes to the luminosity. Compared to the size of the comet's nucleus, this leads to an increase in luminosity by many orders of magnitude .
Origin and dissolution
Comets are the remains from the formation of the solar system ( primordial objects) - and not younger fragments that have arisen from later collisions of other, larger celestial bodies.
The high proportion of volatile substances in the comet's nuclei, such as water and carbon monoxide , and the discovery of clathrates means that they must have formed in extremely cold environments (<100 K) and thus in the outer region of the solar system. Most of the planetesimals in the area of the outer planets were probably collected by the four gas giants in the early days of the solar system . Due to the orbital disturbances affecting the other particles , many of them were so strongly scattered that they left the solar system. It is believed that about 10 percent of these scattered bodies formed the distant Oort cloud . The objects closer to, but circling beyond the orbit of Neptune, were less subject to this scattering process and formed the Kuiper Belt .
The Oort cloud and partly the Kuiper belt are the reservoir for most comets, the number of which could be in the billions. Since long-period comets are heavily scattered by the large planets, especially by Jupiter, when they cross the inner region of the solar system, they can only be identified as former members of the Oort cloud for a few passes. A mechanism is therefore necessary to bring the comets that are still visible today from their orbits far from the sun into close proximity to the sun. For the short-period comets from the Kuiper belt, one suspects collisions of original Kuiper belt objects , whereby fragments get into the interior of the solar system. The scattering process of long-period comets is not yet known. Weak tidal effects from nearby stars or the gravitation of larger trans-Neptunian objects can cause gradual changes in orbit and deflect the distant, cold cometary nuclei in a long orbit towards the sun, which leads to the discovery of new comets every year. Some later disappear, never to be seen again, others remain in periodic orbits . However, the influence of passing stars or as yet undiscovered planets ( Planet X ) or the now disproved idea of a companion star to the sun ( Nemesis ) is discussed as a cause.
If the comets entering the inner solar system contain a lot of ice and they come close to the sun, some can also become freely visible - as was very clearly the case with Ikeya-Seki (1965) or Hale-Bopp (1997).
But comets lose a small part of their mass with each orbit around the sun , especially volatile components of the outer layer of the core. The closer the perihelion of the orbit is to the sun, the more violent this process is, because the ice sublimates more quickly and larger particles are also carried away by the outgassing of the rock. Therefore, after a few thousand orbits of the sun , the comet's core can hardly be recognized as such. This period of time is significantly shorter than the age of the solar system.
As the ice evaporates, the rock in the core loses its cohesion and the comet gradually dissolves. This can take place through division (as with Comet Biela 1833), through Jupiter's influence ( Shoemaker-Levy 9 1994) or through gradual distribution of the particles along their original orbit. The latter is the cause of most shooting star swarms .
Differentiation from other celestial bodies
The distinction between asteroids and comets is not always clear. It is believed that some of the objects classified as asteroids with highly elliptical orbits, for example the centaurs , are “burned out” cometary nuclei covered by a thick layer of non-volatile substances. On the other hand, the object originally classified as an asteroid (2060) Chiron has been classified as a comet since the discovery of a coma and named 95P / Chiron according to the comet nomenclature .
Today, contrary to its original definition , the term comet is often used in both popular science and scientific language for all presumably icy minor planets. Examples of this are the objects of the Kuiper Belt and the Oort Cloud, which contain highly volatile substances but are never heated enough to form a coma due to their distance from the sun. It is assumed that such objects are more similar to comet nuclei than asteroids from the asteroid belt, but solar radiation is only strong enough to form a coma through a sublimation process at perihelion distances within Jupiter's orbit.
Meteor streams and meteorites
The particles of the dust tail are distributed along the comet's orbit around the sun. As Giovanni Schiaparelli has shown, meteor currents occur when the earth crosses this orbit. The most famous meteor streams are the Leonids and the Perseids . These currents are easily observable as shooting stars. Most of the cometary material burns up when it flies through the earth's atmosphere , and so no meteorites have been discovered that are undoubtedly from comets. A connection to comets has been suggested for some very rare meteorite types, such as the CI chondrites , but no evidence has yet been provided. Also micro-meteorites originate predominantly from the asteroid belt, although here a cometary component is discussed.
However, the direct study of cometary material is of great importance for understanding the formation of our solar system, so complex space missions are carried out with space probes such as Deep Impact or Rosetta , which examine the cometary material on site. The Stardust mission made it possible for the first time to bring samples back to Earth in the form of tiny particles from the coma of a comet and to make them available for investigations in terrestrial laboratories.
Especially noteworthy comets
- The Halley's Comet was the first comet (1705 of Edmond Halley was detected) as being periodic and the core of probes could be photographed (1986).
- The Great Comet of 1744 was the first to have its own monograph . Gottfried Heinsius used it to calculate his or her path that was visible for months, the changes in shape of the coma and the exact length of the tail (52 million km).
- The Encke comet (discovered in 1818) has 3.31 years, however, the shortest orbital period of all known comets can not be observed with the naked eye more.
- Comet Biela (1845/46) was the first tail star whose decay was observed.
- The outgassing into the coma was observed for the first time at Comet Donati (1858) . According to the artist, it was the most beautiful object of the century (see picture).
- The Comet 1882 II ( "Great September Comet") drew on his perihelion before and behind the solar disk over, with its tail was also seen in the daytime.
- The Johannesburg Comet - almost simultaneously with Halley - made 1910 a unique year for two Great Comets .
- The comet Ikeya-Seki is considered to be one of the brightest comets of the last millennium. In October 1965 it reached around 60 times the brightness of the full moon and was clearly visible next to the sun during the day .
- The comet Kohoutek (1973/74) has a particularly long orbit and is likely to originate from the Oort cloud . It won't come this close to Earth for 75,000 years.
- The comet Shoemaker-Levy 9 broke in Jupiter's gravity range. Its 21 fragments hit the planet between July 16 and 22, 1994, and their traces could be seen for several weeks.
- Comet Hale-Bopp was visible to the naked eye for more than 18 months from 1996 to 1997 and thus holds the record among all known comets.
- Comet Tempel 1 was the target of NASA's Deep Impact mission, during which on July 4, 2005 a 372 kg projectile, consisting mainly of copper, hit the comet at a relative speed of 10 km / s. The resulting particle dust cloud was observed with the probe itself and with numerous earth-based telescopes, but also with the Hubble space telescope and the ESA space probe Rosetta.
- Comet Wild 2 is the first comet from whose coma particles were collected by a probe. The samples were returned to Earth in 2006.
- At the end of October 2007, comet 17P / Holmes increased its apparent magnitude from 17 to 2.5 mag within about 36 hours. The comet, which suddenly appeared 500,000 times brighter than usual, was visible as a conspicuous object in the sky with the naked eye.
- Churyumov-Gerasimenko is the comet on which a probe landed gently for the first time in 2014 as part of the Rosetta mission.
Sungrazer (sun streaker)
Sun streakers are a group of comets that come extremely close to the sun or even move through the solar corona . The majority of the Sungrazers belong to the Kreutz group . The SOHO solar probe was able to photograph over 1000 such comets. Estimates of their total number are over 200,000 objects. The Sungrazers are often torn apart by the strong tidal forces of the sun. Most sun streaks are therefore small fragments with a diameter of 10 m and less. The conspicuous Comet Ikeya-Seki could be seen in daylight, so its diameter was estimated to be several kilometers.
Of the 10,713 near-earth objects cataloged as of February 2014 , 94 are comets and 10,619 are asteroids. This means that a little under one percent of all earth orbit cruisers that harbor a potential risk of collision with the earth are comets. Out of a total of 5253 known comets, almost 2% are earth orbit cruisers (as of November 2014). However, these figures do not allow an estimate of the probability of an impact with the earth. The risk of comet impacts is generally more difficult to assess than that of asteroids, whose orbits are comparatively more stable and better known. There are or were detection, monitoring and risk assessment systems that record both asteroids and comets (such as Catalina Sky Survey or LONEOS ) and systems that only record asteroids and no comets, such as ATLAS , LINEAR , NEAT or Sentry .
So far, no cometary impact has been confirmed with certainty in the history of the earth. In 1978 the Slovak astronomer Ľubor Kresák put forward the thesis that the Tunguska event of 1908 could have been triggered by a fragment of the periodic comet Encke . It is assumed that smaller comets, or comet fragments, leave small traces on the earth, since their ice evaporates when they enter the atmosphere and their rock components could still be scattered in the atmosphere. In 2013, researchers suggested that an unusual stone made of Libyan desert glass found in the Libyan desert could have been created by the impact of a comet.
In 1984, paleontologists David M. Raup and J. John Sepkoski found a periodicity of about 26 million years for the extinction events in the fossil record . Two teams of astronomers, Daniel P. Whitmire and Albert A. Jackson IV, as well as Marc Davis , Piet Hut and Richard A. Muller , independently of one another suggested a still undiscovered dwarf star companion of the sun as a possible cause . This, christened Nemesis , is said to cause a cyclical increase in the number of comets that get into the interior of the solar system through its disturbance effect on the Oort cloud, which would result in statistically more frequent comet impacts on earth with this periodicity. Subsequent studies of the extinction and impact events based on more recent data turned out differently.
Much progress has been made in the study of comets and the Kuiper Belt over the past 20 years, but there are still many unanswered questions:
- The composition of the coma is now very well understood through spectral analysis, but very little is known about the molecular composition of the nucleus and the parent molecules escaping from the nucleus. It is possible that comets contain organic molecules that are similar or even more complex than those found in meteorites. In preparation for the Rosetta mission , 16 different amino acids have already been identified in simulated comets . Many exobiologists therefore have high hopes for further research on the comets. Some theories on the origin of life assume that organic molecules from meteorites or comets favored or even made the origin of life on earth possible. Supporters of panspermia suspect even more complex biological molecules or possibly even simple forms of life among the CHON particles.
- According to current theories, the comets originated from the Oort cloud at a shorter distance from the sun than those from the Kuiper belt. To confirm this, differences in chemical composition should be demonstrated.
- The mechanism by which the objects in the Oort cloud are scattered into the interior of the solar system is not yet known.
- There are signs of a slight clustering of long-period comets in the direction of the solar apex . Should this be confirmed in closer investigations, it would have an impact on our understanding not only of the Oort cloud, but also of the interstellar medium in the vicinity of the solar system.
- At least one, but presumably several geological events were caused by the impact of large extraterrestrial bodies for which comets as well as asteroids come into consideration, such as the geological transition from the Cretaceous to the Tertiary as a result of the KT impact .
- The earth has a significantly larger proportion of water than other bodies in the inner solar system, for which some scientists blame large cometary impacts (see origin of terrestrial water ). However, previous measurements of the hydrogen isotope ratios in some comets do not agree well with the hydrogen isotope ratio of terrestrial oceanic water, but this could also be due to the fact that the comets measured were not representative.
For millennia, mankind has interpreted the sudden appearance of comets as a bad omen of coming misfortune, of wars and catastrophes, but also occasionally as signs of miracles . Even the scientifically open-minded 17th century was still involved in this magization , and astronomers of Johannes Kepler's rank also interpreted comets as "ominous" (in the sense of the origin of the word). It was not until Edmund Halley's discovery of periodicity in 1682 that the fear of comets subsided. Magical attributions are still made today, as can be seen from the mass suicide of Heaven's Gate members when the comet Hale-Bopp appeared in 1997.
According to ancient reports, it appeared in 44 BC. During celebrations in honor of Venus Genetrix shortly after the assassination of Julius Caesar, a very bright hair star in the Roman sky for several days. The appearance was interpreted by the Romans as a sign of the deification of Caesar and the ascent of his soul into heaven. The comet Caesar (also called 'Sidus Iulium' in antiquity ) was promoted by Emperor Augustus and became part of the cult of the state god Divus Iulius and thus an integral part of Roman mythology.
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- Astronomical objects
- Small body (astronomy)
- List of visited bodies in the solar system
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