In Astronomy, a comet is a small star shining of the Solar system, whose Orbite generally has the form of a very lengthened ellipse, and often accompanied by a long luminous trail due to the interaction at speed high between comet in the vicinity of the Sun and various forces emanating of the Sun: solar wind, pressure of radiation and gravitation.
The word comet comes from the Greek “komêtês” (Κομήτης) which means hairy ; it is due to the immense made tail of gas and dust from approximately 30 to 80 million kilometers.
The name of a cometPut aside the comet of Halley, or that of Encke, the name of a comet is allotted officially by a commission of the International Astronomical Union (UAI, English IAU), whose seat is with Washington. Certain historical comets do not comprise a name: thus one speaks about large comet of 1843.
First processThe names of the first two observers of a new comet are allotted to him officially; thus one speaks about comet Hauls-Bopp, discovered in July 1995 by Allan Hale and Thomas Bopp.
This process comprises disadvantages, in particular since the discovery of many comets by space probes: the probe LINEAR discovered several tens of them, and probes it SoHO several hundreds.
Intermediate processBefore 1995, a new comet often had a double denomination. The first, provisional, on standby of the final denomination. Two years after its discovery, the final name was allotted to him according to the following criteria: the year of the passage to the perihelion, followed by a sequence number noted in Roman numerals indicating its place inside the year in question (example: 1994 IV). This process comprised also many disadvantages (in particular for comets at short period). For comets at short period, one put a sequence number in front of p: comet 19/P Borrelly.
New processSince January 1st, 1995, the official name of a new comet comprises to other indications in addition.
Thus the official name of comet Hauls-Bopp is C/1995 O1 :
- C indicates that it is about a comet at long period (possibly not periodical); one will put p for a comet at short period, or D for a disappeared comet;
- 1995 indicates that the comet was discovered in 1995;
- O indicates that she was discovered during the first fortnight of July (each letter of the alphabet, except I, corresponds to one half-month);
- 1 states finally that it is about first comet discovered during this period.
EnumerationOne knows at present more than 2.000 comets which were indexed.
One of the most famous comets is the Halley's Comet, who reappears all the 75 or 76 years.
Other famous comets:
- comet McNaught (C/2006 P1) the any last very brilliant
- comet Hauls-Bopp (C/1995 O1)
- comet Hyakutake (C/1995 Y1)
- comet Shoemaker-Levy 9 (D/1993 F2), destroyed by collision with Jupiter in July 1994
- comet 109P/Swift-Tuttle
- comet 55P/Tempel-Tuttle
- comet 19P/Borrelly, visited by the probe Deep Space 1
- comet 9P/Tempel 1, struck and photographed by the probe Deep Impact
- comet Wild 2, flown over by the probe Stardust
Comets selected to be flown over by space probes:
- comet 67P/Churyumov-Gerasimenko, will be visited by the Sonde Rosetta
DescriptionA comet is composed primarily of three parts: the core, the hair and tails. The core and the hair constitute the head of comet.
At the time of the last passage of the Halley's Comet in 1986,6 space probes (ICE, Vega-1, Vega-2, Sakigake, Suisei and Giotto) passed very close to comet and recorded invaluable data and images for our knowledge of comets.
The coreThe assumption of constitution of the core most commonly allowed and confirmed by the recent space experiments, is that it would be a solid body made up of ices (water, carbon monoxide, carbon dioxide) and agglomerated matters meteoritic (model known as of the " snowball sale" proposed by Fred Whipple). These ices sublimate under the action of the solar radiation and give rise to hair, then with the tails.
The diameter of the core (nonspherical) is estimated between a few hundred meters and a few tens of kilometers.
The greatest dimension of the core of Halley's Comet, of oblong form, is approximately 15 km; the volume of its core was estimated at 500 km ³, for a mass of 10 14 kg, which corresponds to an average density of 200 kg/m ³.
The hairThe hair, or coma , consists of atoms, gases and dust from the core of comet and released in the form of gas jets. Very quickly the ultraviolet radiation emanating from the Sun breaks the atoms and the molecules (phenomenon of Ionization). The brightness of hair is stronger near the core.
Its diameter generally lies between 50 000 and 250 000 km, with extreme limits of 15 000 and 1 800 000 km. The hair is frequently identified with the head of comet, being given the low relative diameter of the core, but sometimes the hair can make to 250 gigameters (million km) of diameter.
The analyzes of gas of hair of Halley's Comet indicate that this one contains 80% of Eau, 10% of Carbon monoxide, 3% of Carbon dioxide, 2% of Méthane, less than 1,5% of Ammoniac and 0,1% of Hydrocyanic acid.
TailsAn important comet has two visible tails in general:
- a tail made up of a plasma, rectilinear and being maintained contrary to the Sun (like a shade), pushed at high-speed (about 500 km/s) by the solar wind; the changes of polarity of the solar wind produce ruptures in the tail of plasma which is reconstituted in the hours which follow;
a broader tail made up of dust pushed by the pressure of solar radiation, and curved in the plan of the orbit. Thanks to work of Michael Finson and Ronald Probstein (1968), which implemented the assumptions of Theodor Bredichin (1885) which did themselves following those of Bessel, one can model the tail of dust. The trajectories (Keplerian) of the grains can thus be analyzed according to the duration of emission (Synchrones) or according to their size (syndynes).
a third envelope, invisible with optical instruments, but detected thanks to the Radioastronomy, is the hydrogen tail which extends on considerable dimensions.
Certain comets (Arend-Roland, in April 1957) presented a " anti-queue" that one can explain. It is about part of the tail of dust (near to the core) made up of coarse grains which, by effect of prospect when the Earth crosses the plan of the cometary orbit, seems to point towards the Sun.
Their dimensions are considerable: lengths from 30 to 80 gigameters (billion kilometers) are relatively frequent.
OrbitsThe orbits of comets are defined using six parameters (orbital elements): T (the period), ω (argument of latitude of the perihelion), Ω (longitude of the ascending node), I (slope), Q (perihelic distance), E (eccentricity). When a new comet is discovered, after at least three distinct observations, one models a first orbit by taking E = 1 (one supposes the parabolic orbit). Then, when the observations are specified, one seeks the best osculatory orbit by refining the value of the eccentricity E.
The comets are known as conventionally at short period when their period is lower than 200 years. They would be originating in the Ceinture of Kuiper.
The comets of which the period is higher than 200 years are supposed to come from the Nuage of Oort.
The comets attached to the solar system have an orbit whose eccentricity is lower than 1 (elliptic orbits, therefore periodic comets). There exist some rare cases of comets whose eccentricity is higher than 1 (hyperbolic orbits, therefore nonperiodic comets): either they are comets coming from the outside of the solar system, or it acts of comets whose orbit underwent disturbances such as they will leave the solar system.
Modification of the orbital elements
When a comet passes near the large planets (primarily Jupiter), it undergoes gravitational disturbances which can modify some of its orbital elements. Thus the Shoemaker-Levy comet struck Jupiter in 1994 because at the time of its preceding passage, this comet had passed sufficiently close to this planet so that at the same time its orbit is modified and its core broken up into a multitude of elements distributed along the orbit.
The orbital elements of a comet can also be modified in a nonforeseeable way by the activity of the core (nongravitational disturbances).
For these reasons the orbital elements of a comet are never final and must be recomputed at the time of each passage (in the case of comets at short period).
Parameters of some comets
Here some of the parameters of some known comets.
Comets and shooting starsShooting star swarms (for example: Perseids, Orionides, Géminides) are associated with comets. The dust lost by a comet at the time of a passage is distributed along the orbit of this one by forming a kind of vast cloud. If it occurs that the Earth, in its annual orbital movement, crosses such a cloud, one then witnesses a shooting star rain more or less dense according to the activity and the nature of comet. These " stars filantes" , which is thus not, seem to come from the same direction of the sky (the radian ), a little as when one is in a rectilinear tunnel and that one with the impression that the edges of this one converge towards the same point. The radian bears the name of the constellation of which the shooting stars seem to come.
Cometary dust, when they penetrate in the upper atmosphere of the Earth warm up and ionize, producing the luminous trail which one knows.
One can associate the swarms of shooting stars with comets (which they can resow in dust at the time of a passage) or with old comets. Thus, swarm of the Perseids celebrates it (maximum of intensity on August 12th) is associated with the comet Swift-Tuttle.
One found traces of comet observations in Chinese annals, but also on Babylonian shelves. Their true nature, nor their periodicity had not been found before the work carried out by Edmond Halley in 1682.
A comet is a Astéroïde of irregular form which can reach ten kilometers in diameter, it consists of ice and dust. The comets were in the beginning a luminous halation which appeared episodically in the sky, and which was interpreted, according to its aspect and the historical period, like a sign of good or bad forecasts. In fact, they become visible only when they approach the Sun, the action of this last causes gas emissions and dust which reflects solar light.
The first results obtained by the mission Stardust considerably modified the assumptions concerning the formation of comets. Indeed the grains taken in the coma of the comet Wild 2 by this mission and brought back on Earth contain Olivine, material which can be synthesized only at very high temperatures (1 300 K). One is thus brought to think that the comet cores were formed near the Sun and were ejected thereafter towards the Cloud of Oort.
In particular because of the space experiments, the scientific study of comets at the 20th century revealed their true nature.
Space experimentsKnowledge on comets, in particular those their cores, progressed considerably since the advent of the space experiments. On the whole, it is currently nearly ten space missions which contributed to better knowing the cometary cores.
the European probe Giotto, launched on July 2nd, 1985 by a rocket ARIANE 1 to approach the core of the Halley's Comet to less than 500 km;
- the Japanese probe Sakigate, in direction of Halley;
- the Japanese probe Suisei, in direction of Halley;
- two Soviet probes Vega 1 and Vega 2, which, after having detached a module towards the planet Venus took a certain number of stereotypes of the coma of Halley;
- the American probe Deep Space 1, launched in 2001, which studied the core of the comet 19P/Borrelly;
- the American probe Deep Impact, launched in 2005, aimed to dig an artificial crater by collision between the core of the comet Tempel 1 and an impactor;
- the American probe Stardust, launched in 2004, was intended to collect and bring back on Earth of cometary dust coming from the comet Wild 2;
- the European probe Rosetta launched in 2005 in direction of the comet 67P/Churyumov-Gerasimenko which it will reach in November 2014.
- the European probe SoHO intended to study the Sun uninterrupted and which of this fact made it possible to discover comets which finished their life in " tombant" in the Sun;
- Differences between comets, asteroids, meteors, meteorites
- a site amateur dedicated to the comets
- comets for null the
Be-X-old: Камэта Simple: Makes Zh-min-nan: Tn̂g-boé-chheⁿ
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