|of these articles is occupied with the comet as heavenly body, for further meanings, sees: Comet (term clarifying).|
a comet (derived from Greek kométes feather star, from kómē main hair) or tail star is a small heavenly body, which exhibits one at least in the sunnear parts of its course in outgassing produced coma.
In sun proximity is that usuallyonly few kilometers by a vague, foggy, coma covering mentioned surround large comet nucleus, which can reach an expansion of up to 100.000 km. Together one calls core and coma also the head of the comet. The most remarkable characteristicof the earth from visible comets is however the tail, which can achieve a visible length from 10 to 100 million kilometers.
Table of contents
history of the comet research
in the antiquity and the Middle Ages to comets frequently as fate messenger or indication of the Gods were regarded. Aristoteles and Ptolemäus regarded the comets as evaporation of the Erdatmospähre. Regiomontanus recognized in them for the first timeindependent heavenly bodies. One can set the beginning of the scientific aspect during the comet research with the realization Tycho Brahes that comets are not features of the terrestrial atmosphere. It stated 1577 that comets at least 230 Erdradien must be removed. It lastedsome time to itself this acceptance to however still intersperse could. Even Galilei contradicted this realization. Edmond Halley was to be proven it to 1682 possible in this year the emerging comets as periodically returning heavenly bodies. Also the 1607, 1531 and 1456 comet which can be provenmoves on an elongated ellipse into 76 years around the sun. Nowadays on the average 20-30 comets per year are discovered.
comet become due to their feature interval in aperiodic Komenten and periodic cometsdifferentiated. Zweitere are divided according to their scan times into long-periodic and shortperiodic comets.
- Aperiodic comets
- comets, which - due to their paraboloiden or hyperboloiden course - do not return reliably, or single observations, over because of exact lacking course regulation - still - no statementto be met can.
- Periodic comets
- comets, whose return is secured on the basis their course elements, which circle stable orbit the sun thus on one - at least a certain period -.
- Long-periodic comets with a scan time of more than 200 years probably comefrom the Oort cloud, its course inclinations are distributed statistically and them rotate the sun both in the same direction of rotation as the planets (per degrees) and in opposite direction to the planet courses (retrograd). The eccentricities of their courses lie near 1 -the comets are usually however still bound by the force of gravity to the sun, although they need for their circulation up to 100 million year. Eccentricities than 1 (hyperbolic orbits) become larger only in rare cases by orbit/trajectory disturbanceswith the passage of the large planets caused. These comets return then no more to sun proximity, but leave the solar system.
- Shortperiodic comets with scan times smaller than 200 years probably originate from the Kuipergürtel. They move usually infor usual direction of rotation and their inclination is appropriate on the average with for instance 20°, it lie thus in the proximity of the ecliptic. With more than half of the shortperiodic comets is the largest sun distance (aphelion) in the proximity of the Jupiterbahnwith 5 and 6 astronomical units (Jupiter family). It concerns thereby originally long-periodic comets, whose courses were changed by the gravitativen influence of the Jupiter.
major item: Designation of Asteroiden and comet
new discovered cometsreceived from the international astronomical union first a name, which consists of the discovery year and a large letter, that beginning with A at the 1. January and B to 16. January in the half monthly rhythm after the time of the discovery is fixed.Additionally still another number comes, so that one can differentiate between several comets in the half month. As soon as the course elements of the comet are more exactly certain, the name after following systematics a further letter is placed in front:
|P||the scan time is smaller than 200Years and/or. at least two confirmed observations of the Periheldurchgangs (periodic comet)|
|C||the scan time is more largely than 200 years|
|X||the course is not assignable|
|D||periodic comet, was lost or any longer does not exist|
|to A||one later states thatit not around a comet, but around a Asteroiden acts|
a periodic comet besides a sequential number is placed in front, as soon as a confirmed observation of a second Periheldurchgangs is present - at present (conditions: 18. November 2005) are 173 comets with at least twoconfirmed Periheldurchgängen admits, among them any longer existing do not sieve (among other things3D Biela).
The comet Hyakutake for example is led also under the designation C/1996 B2. Hyakutake was thus the second comet, which discovers 1996 in the second half of Januarybecame. Its scan time is larger than 200 years.
Usually a comet is designated additionally after its discoverers, then for example D/1993 F2 is led also under the designation Shoemaker Levy 9 - it concerns here the ninth comet, that Eugene and Carolyn Shoemaker as well as David Levy discovered.
with again discovered comet only short course elements are visible there, first parabolic courses are computed. With rising observation period can then be decided whether it itself overEllipses or hyperbolas acts. From zirka 660 examined comets the following distribution shows up: 43% parabolas, 25% long-periodic ellipses (rotating time over 200 years), 17% shortperiodic ellipses (rotating time up to 200 years) and 15% hyperbolas. The high portion of parabolas is howeverto the too short observation period of many comet features to be attributed, with those by a parabola not be differentiated days only more can describe elongated ellipses 3% of the comets a parabola course with a longer visibility of 240-500. Thus the ellipses might be prevailing(see also).
in large distance of the sun comets exist only of the core, inExists substantial ones from frozen water , dry ice, CO ice , methane and ammonia with admixtures from meteorite-similar small dust and mineral particles ( for example silicates, nickel irons), calls one comets therefore frequently dirty Schneebälle or dirty snowballs. From observations thatSpace probe Giotto to comet Halley knows one that comets by black crust surrounded are, which only zirka 4% of the light reflected (albedo) - although comets as spectacular Leuchterscheinungen are observed, are their cores thus ironically thoseblackest objects of the solar system, substantially more darkly than for example bitumen, that approx. 7% of the light reflect.
There only small regions of the core outgassing, as in the section coma is more near described, one assumes after newer conceptionsthe surface is formed by a kind rock debris, which consists of Gesteinsbrocken, which are too heavy, in order to overcome the gravitative attraction of the core. Giotto discovered also tiny particles, those richly at the elements carbon (C), hydrogen (H), oxygen (O) and nitrogen (N) are and therefore also CHON particles to be called. These could originate from a thin soot layer, which covers the surface of the core and so that would explain the low albedo. The current Rosettamission is to supply closer information.
A specialPortion of the discovery of the structure of the comets had Fred Whipple.
|O 100 CO||20|
|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 3OCH 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 asa comet during the approximation to the sun in a distance from approximately 5 AE the Jupiterbahn crosses, forms the dish-shaped coma, which shows also radiate-like structures in core proximity. It results from sublimation of easily volatile substances on thatto sun-turn side, which drag embedded dust particle along in the ice. After the observations of the probe Giotto this sublimation takes place only at approximately 10 to 15% of the comet surface, the volatile substances escapes obviously only in fragile places of the black crust. Thosein these places escaping nut/mother molecules form the internal coma. By further heating, ionization and dissociation the coma continues to become larger and forms the finally visible coma from ions and radicals. This still becomes from a hydrogen atomic hydrogen radiating in the ultravioletis surrounded, also the UV coma called and with the comet resound Bopp 1997 a diameter of 150 million kilometers reached. Since the atmospheric ozone layer for the UV-RADIATION is impermeable, the UV coma can be examined only with satellites.
- A narrow, long stretched tail (type i tail), that essentially from molecular ions exists and also Plasma tail is called. For these particles the radiation pressure is not sufficient as explanation, so that Ludwig Biermann 1951 a particle radiation outgoing from the sun, which is called today solar wind, when explanation postulated for this. Today one assumes the comet-likeIons by a reciprocal effect with the solar magnetic field to be propelled, which is carried by the charged particles of the solar wind.
- A vague, curved tail (type II tail), which also dust tail is called. The small dust particles, which form this tail, becomeby the radiation pressure of the sun affects, whose effect can be explained by a fragmentation in two components:
- A radial component, which is against-arranged the gravitation strength and decreases like these squarely with the distance to the sun. This works like an effectiveAcceptance of the solar gravitation strength, which dust particles move therefore on „pseudo Keplerbahnen “, which differ for dust particles of different size, since the radiation pressure is dependent on the particle size. This leads to a relatively strong fanning out of the dust tail in the comparison to the plasma tail.
- Thoseother effective component of the radiation pressure is against-arranged the direction of motion of the dust particles and leads to a deceleration of the particles, which are larger than the wavelength of the light, i.e., more largely than about 0.5 µm. These particles move on a long-term basis just as howthe other interplanetary dust on spiral courses direction sun (Poynting Robertson effect).
- Very rarely, at special course constellations, a Gegenschweif (type iii tail) is visible. Here it concerns however not independent tail, but only around a geometrical projection effect:If the earth between sun and comet through-moves, a part of the dust tail, under its curvature, exceeds apparently over the comet head.
The waste of a comet became with „new “comets, which come the first time into sun proximity, on approximately10 to 50 tons per second estimated, after repeated sun approximation the mass loss sinks on less than 0.1 t/s. These small subject quantities from maximally 0.03 to 0.2 per cent of the comet mass per sun passage mean that the tails only a very small densityexhibit. The enormous brightness of the tails explains itself in case of the dust tail by the large surface of the microscopically small dust particles, in the plasma tail carries even each atom and/or. Molecule to luminosity. This supplies in the comparison to the size of the comet nucleusan increase of luminosity around many orders of magnitude.
the high portion of easily volatile substances like for example water and Carbon monoxide means that the comet nuclei must have developed in the outside range of the solar system. The Planetesimale outthe range of the outside planets, which were not gathered by the gas giants, thereby to a large part it was so strongly strewn that they left the solar system. Only about 10 per cent of these strewn bodies form the Oort cloud. ThoseObjects beyond the Neptunbahn were subject to this scattering process not and form the Kuipergürtel.
The comets lose a small part of their mass with each circulation around the sun. With these mass losses it acts above all the volatile components thatoutside layer, so that they are to be recognized after some hundred sun circulation hardly still as comets. This time interval is clearly shorter than the age of the solar system. The long-periodic comets become besides during their crossing of the internal range of the solar system of thatlarge planets, particularly by Jupiter, so strongly strewn that they can be identified only for few passages as former members of the Oort cloud. Is thus a mechanism necessary, that the today still visible comets from their sun-far coursesin sun proximity brings. For the shortperiodic comets from the Kuipergürtel one assumes for this collisions of original Kuipergürtelobjekte, whereby fragments arrive the inside the solar system. The scattering process of long-periodic comets is not well-known yet, it becomes however frequently the influence of by-pulling starsor yet planet (planet X) did not discover and/or still an unknown quantity Begleitstern of the sun (Nemesis) as a cause called.
demarcation to other heavenly bodies
the distinction between Asteroiden and comets is not always completely clearly. One assumes that some the objects with strongly elliptical courses, for example the Centauren is, classified as Asteroiden, „burned out “comet nuclei, which are covered by a thick layer of non volatile substances. On the other hand that becomes originally than Asteroid 2060 Chiron classified object since the discovery of a coma as comet classifies and in accordance with the comet nomenclature 95P/Chiron mentioned.
Today the term becomes often comet both in the popular-scientific and in the scientific linguistic usage against its original definition for all probably icySmall planets uses. Examples for this are the objects of the Kuipergürtels and the Oort cloud, which contain volatile substances, but due to their distance of the sun strongly enough to be never warmed up, in order to form a coma. From such objects becomes howeverassumed the fact that their structure resembles rather the comet nuclei as the Asteroiden from the Asteroidengürtel but only with Periheldistanzen within the Jupiterbahn is the solar radiation strongly enough that by a sublimation process a coma can be formed.
meteor stream andMeteorites
the particles of the dust tail distribute themselves along the comet course around the sun. How Giovanni Schiaparelli showed, meteor stream arise , if the earth crosses this course. The most well-known meteor stream are the Leoniden and the Perseiden. These rivers are as meteors easily observably. Usually the comet material glows with the transit flight by the terrestrial atmosphere, and so so far still no meteorites were discovered, which come free of doubts from comets. For some very rare meteorite types, like for example the CI-Chondriten, becamea connection to comets suggested, a proof could not be furnished however so far yet. Also micrometeorites originate predominantly from the Asteroidengürtel, although a comet-like component is discussed also here.
The direct investigation of comet material is however for the understanding thatEmergence of our solar system of great importance, so that complex space missions with space probes are accomplished such as Deep Impact or Rosetta, which examines the comet material locally. By the Stardust - mission succeeded it first times, to samples in the form of smallest particles outto bring and for investigations in terrestrial laboratories to the order place the coma of a comet to the earth back.
comets particularly worth mentioning
- the Halley's cometwas the first comet, which (1705 of Edmond Halley) was recognized as periodic and whose core could be photographed by space probes (1986).
- The Encke comet has the shortest scan time of all well-known comets with 3.31 years, can but not to be any longer with the naked eye observed.
- The comet Ikeya Seki is considered as one of the brightest comets of the last millenium. It reached 60fache brightness of the full moon approximately in October 1965 those and was during the day clearly visible beside the sun
- The comet Shoemaker Levy 9 broke within the gravitation range of the Jupiter. Its fragments struck between 16 July and that 22. July 1994 on the planet up.
- The comet resound Bopp was from 1996 to 1997 more than 18 months with naked eyevisibly and holds thereby the record under all well-known comets.
- The comet game 2 is the first comet, from whose coma by a probe particles were in-collected. The samples were returned in the year 2006 to the earth.
- The comet temple 1became the goal of the Deep Impact mission of NASA, with to 4. July 2005 a 372 kg heavy, mainly projectile with a relative speed of 10 km/s on the comet, consisting of copper, hit. With the probe andwith numerous earthsupported telescopes, in addition, with the space telescope Hubble and the ESA space probe Rosetta the developed particle deaf cloud was observed.
- On the comet Tschurjumow Gerasimenko is to land 2014 for the first time during the Rosetta - mission a probe.
Sonnenstreifer are a group of comet those the sun extremely close come and/or by the solar corona move. By the solar probe SOHO could be photographed over 1000 such comets. Estimationstheir total number amount to over 200.000 objects. The Sungrazer is torn by the strong tidal forces of the sun often apart. Most Sonnenstreifer is less therefore small fragments with a diameter of 10 m and. The remarkable comet Ikeya Seki was to see however even with daylight, so that its diameter became estimated on several kilometers. The majority of the Sungrazer belong to the Kreutz group .
comet and the star of Betlehem
as consequence of some impressive comet features became in 16. and 17. Century a comet as explanation for the star mentioned in the Bible suggested of Betlehem. None was from that today well-known comets at the questionable time (7-4 v. Chr.) in the sky visibly, although Chinese astronomers of a comet inYear 5 v. Chr. report. This was however a rather unspektakuläres small object and therefore probably not identical to the star of Betlehem. In earlier times a comet applied besides usually than mischief messenger and would be hardly as announcement for thoseBirth of the Messiah interpreted.
open ones questions
in the last 20 years were obtained in the study of the comets as well as the Kuipergürtels large progress, it give however still many open questions:
- Is by spectrographic analysis thoseComposition of the coma meanwhile very well understood, over the molecular composition of the core and the nut/mother molecules escaping from the core is however still very few well-known. Possibly organic molecules those come than those are similarly or even still more complex into cometsin meteorites were found. Many Exobiologen set therefore large hopes for the further study of the comets. Some theories for the emergence of the life assume organic molecules from meteorites or comet the emergence of the life on earthfavoured or only made possible. The trailers of the Panspermie possibly assume even still more complex biological molecules or even simple ways of life under the CHON particles.
- According to the present theories the comets from the Oort cloud are in smaller distance to the sundeveloped as those from the Kuipergürtel. In order to confirm this, differences in the chemical composition should be proven.
- The mechanism, by which the objects of the Oort cloud are strewn the inside the solar system, is not well-known yet.
- There are signsfor an easy amassment of long-periodic comets toward the Sonnenapex. If this should be confirmed with more exact investigations, this would have not only effects on our understanding of the Oort cloud, but also the interstellar medium in the environment of the solar system.
- At least one, probably however several earth-history events was caused by the Impakt large extraterrestial body, for which beside Asteroiden also comets are possible. So for instance the earth-history transition from the chalk to the Tertiary period by the KT-Impakt.
- The earthit has compared to other bodies internal solar system a clearly larger Wasseranteil for which by some scientists large comet impacts are made responsible (see origin of the terrestrial water). However past measurements of the hydrogen abundance ratios in some comets are not correct well with thatHydrogen abundance ratio of terrestrial oceanic water which in addition, because of it be could that the measured comets were not representative.
- treasurer/Kretlow: Comets observe, practical guidance for amateur observers, stars and space publishing house, 1999, ISBN 3-87973-924-2
- John C. Brandt, Robert D, Chapman: Introduction ton of Comets, Cambridge University press, 2nd OD. 2004, ISBN 0-52100-466-7
- Gary W. Kronk: Cometography: Vol.1 Ancient - 1799: A Catalogue OF Comets, Cambridge University press, 1999, ISBN 0-52158-504-X
- Gary W. Kronk: Cometography: Vol.2 1800 - 1899, Cambridge University press, 2003, ISBN 0-52158-505-8
- S.V.M. Clube, W.M. Napier,M.E. Bailey:The Origin OF Comets, Pergamon press, 1990, ISBN 0-08034-858-0
- Gerhard thin head: “New comets - bad prophets”, in:
Philobiblon 18, Stuttgart 1974
- Charnley S.B., Rodgers S.D., Kuan Y. - J., Huang H. - C.: Biomolecules in The interstellarly medium and inComets. Advances in space Research (pdf) - (discussion over the origin of the proven organic molecules)
- horns J., Evans N.W., Bailey M.E., ashtray D.J.: The population OF Comet Like Bodies into the solar system. Mon. Emergency. Roy. Astron. Soc. 343(2003) 1057 (pdf) - (suggestion of of a new taxonomy for comet-similar bodies)
Web on the left of
|Commons: Comet - pictures, videos and/or audio files|
|Wiktionary: Comet - word origin, synonymsand translations|
- material video: When did the last comet hit? (From the television broadcast alpha Centauri)
- comet introduction
- the nine planets: Comet
- Paul Wiegert's PhD thesis: The evolution OF Long Period Comets
- ESA Pressemitteeilung to the Rosetta mission
- IAU - „official one “astronomical names
- Observable comets, list of the IAU (English)
- homepage of the VdS specialized group comet
- fascinating amateur photos of comet
|This article was taken up to the list of exzellenter articles.|