Astronomer amateur, knowing well the constellations, ORTIZ was able to observe all the big comets of the second part of the XXth century and, during his flights, he discovered himself three new comets. For reasons of delays of communications, their name was officially attributed to the only one of these wandering celestial bodies. The name of the comet 1970-VI WHITE-ORTIZ-BOLELLI, shared with an Australian and a Chilean, ORTIZ so became the first Frenchman for fifty years to register its name in the sky. He was followed, only in 1986, by Jacqueline CIFFREO, astronomer who studied the photographic patches during the observation of the comet of HALLEY.

The Conference: The granted time (50 minutes) not allowing deepen the subject, E. ORTIZ made a summary of the current knowledge concerning comets. After a brief reminder of the descriptions of comets in History and of their interpretation by the astronomers of various times, the speaker made a brief statement on the characteristics of comets, their origin, their composition, their classification, the conditions of their discovery and their observation. He evoked the comet of HALLEY and the scientific missions launched to its meeting. He also mentioned the famous comet SHOEMAKER-LEVY 9 which, in 1994, got by Jupiter, broke in several fragments which rushed on the big planet and provoked spectacular impacts.

THE COMETS

INTERPRETATION OF COMETS IN THE HISTORY

Comets, mobile, fleeting celestial bodies, in the varied forms, intrigued and frightened the (people) since the highest antiquity. We find mention on tablets of Clay of Mesopotamia, in the chronicles of the Court of China. These strange celestial bodies are generally associated to fatal events: princely death, wars, famines. In Greece, Aristote (IVth BC) was able to observe a comet and tempted a rational, regrettably erroneous explanation (he imagines a gas broadcast(emission,issue) stemming from the Earth and living in the world sublunaire). In Rome, at the beginning of our era, the philosopher SÉNÈQUE describes comets as natural phenomena which the future generations will know how to explain.

The times disturbed by the Middle Age make be reborn the dismays and the populations see comets in the form of sabres, of daggers, even of monsters. They are, again, malefic miracles, heralds of violence and explained misfortunes. In 1066, the Tapestry of Bayeux shows the companions of HAROLD terrorized by a comet. In the XVIth century, Danish astronomers Tycho BRAHÉ ( 1546-1601 ) is the first observer equipped well in material. He tries to measure the parallaxe of a comet and estimates its distance in six times that of the Moon. The measure is false but this is the first one questioned by the system of ARISTOTE. For Johannes KÉPLER ( 1571-1630 ), heir of the tables of Tycho BRAHÉ, comets are celestial bodies in the rectilinear trajectories come from the borders of the sky. Contemporary of KÉPLER, the monk physicist Galiléo GALILÉI ( 1564-1642 ) has the dea to adapt the new glasses of Approach in the Observation of the sky. He so invents the astronomical telescope in 1609 and discovers the reliefs of the Moon, the satellites of Jupiter, the phases of Venus, an oddity on Saturn. This is the end of the system of ARISTOTE and the debuts of the Modern astronomy. Edmond HALLEY ( 1656-1742 ), mathematician and astronomer, is intrigued by the phenomenon of comets. He has the occasion to observe two and he begins to define them trajectories. To confirm his works, he makes with his elder collegue Isaac NEWTON (1642-1727 ), physicist and renowned mathematician. NEWTON has already defined the trajectories of comets but does not find her calculations. In helping to reconstitute its research, HALLEY discovers the importance of NEWTON's works and he will help to publish them. It will be a major stage in the history of the science "Principiamathematica"(1686). In 1705, HALLEY suspects that the comet of 1682 describes an elliptic orbit, that she is thus periodic, and he calculates its return for 1758. He will not see it but this return will be confirmed : a German farmer rediscovers the comet in Christmas, close to the foreseen position. Comets became rare but familiar celestial bodies.

DESCRIPTION OF COMETS

From the XVIIIth century, the descriptions become more and more objective but, already in 1590, the astronomer Johannis HÉVÉLIUS had published a catalog with drawings of tails of comets
In 1835, were published the first drawings of the comet of HALLEY and, in 1858, the first photography of a comet, that of BENNETT.
A comet appears generally in the sky as of girl diffuse spots to unpredictable positions (except as regards the periodic comets). The astronomers distinguish then a celestial body which is not single usual. This celestial body is immovable, as a nebula, but during the nights, its position moves in the sky with regard to stars. Generally, we see a tail developing. However, it happens that a comet seems already provided with its brilliant tail. This is the case of the comets which pass very near the Sun and that we discover low on the horizon in the early hours or in the twilight of the evening. When it approach of the Sun, the comet becomes more and more brilliant but it disappears then in some days under the horizon. We can to perceive later in the horizon just before the sunrise before the daylight removed it. Next nights, we can see it, more and more far from the Sun, in the highlight of its luminosity. When it is enough far from the Sun, the comet can be seen in the middle of the night, height in the sky, with a long brilliant tail, sometimes two. Certain comets are enough spent near the Earth so that their tail occupies half of the sky. Any were so brilliant that it could be seen in the daytime. There are, generally, some very beautiful comets by century. For those who were able to see them, this is an unforgettable spectacle.

TRAJECTORY OF COMETS

We saw that, according to KÉPLER, they are rectilinear trajectories. Comets come from borders of the universes and cross the solar system in considerable speeds and in big distances of the Earth. In 1610, he declares : Comets?are everywhere in the sky, uncountable.
In 1680, for NEWTON, which invented the tiny and established calculation the laws of the universal gravitation, comets move according to very lengthened(very stretched out) parabolae. They come from borders of the universes and return to it after their passage near our planet. In 1690, HALLEY thinks that certain comets describe ellipses the sun of which is the one of the foyers, what returns them periodically around the Earth. This hypothesis is confirmed during the winter of 1958-1959.
In the XVIIIth and in the XIXth century, the calculations are improved, grace in particular to the French physicist LAPLACE. Comets describe, with regard to the Sun of the trajectories which belong to the geometrical family of Conics.So, at the end of the XXth century have could classify all the comets known in three families according to their trajectories:
- Comets with elliptic trajectory, thus periodicals : about three hundred comets with short periods ( less than 10 years), average periods (between 10 and 200 years) or long periods ( 200-year-old superiors).
- Comets with parabolic trajectory (about three hundred).
- Comets with hyperbolic trajectories, (less d? Hundred).

The search for comets

NEWTON had invented the telescope, the Dutchman Christian HUIGHENS ( 1629-1695 ) perfected its optics and the astronomers had the successful instruments. The observation of the sky became the big activity of the astronomers. The new instruments allowed to see farther and the observation of celestial bodies made considerable progress. At the end of the XYIIème century, HÉVÉLIUS, one astronomers of Dantzig, drew up a directory of tails of comets. In the XVIIIth century, French astronomer MESSIER was nicknamed by LOUIS XIV "Ferret of comets". At the end of the century, Caroline HERSCHEL, sister of the big astronomer discoverer of the planet Uranus ( 1797 ), specialized in the search for comets. She discovered eight. In the XIXth century, we find the Frenchman Jean-Louis PONS, the champion absolved with 37 comets between 1801 and 1827. In the XXth century, the hunting in comets becomes an activity of the astronomers amateurs, often with instruments so simple as simple twins. In the second half of the century, young Japanese amateurs are made an eminent place (SUGANO, SAIGUSHA,?SÉKI,FUJIKAWA, IKÉYA). The scientific satellites. So discover comets (IRAS), as well as the spatial telescope HUBBLE. The big telescopes are henceforth handicaped by the cost of their application and by their sensibility which prevents them to observe near the horizon. The amateurs preserve their luck and the current champion is the australian BRADFIELD with about twenty comets to his credit.

SIMPLISTIC STRUTURE of A COMET

The pit(core). A comet consists d? A compact pit(core) which under l? Effect of the heat of sunbeams, frees(releases) very volatile gases. Pits have generally dimensions of l? Order of the kilometre or the dozen kilometres but we know it who(which) exceed hundred kilometres. The pit(core) coming from borders of the solar system is gradually lit(enlightened) and warmed by the beams(shelves) of the Sun. Around Saturn, warmed gases dilate? Escape from the surface and spread(diffuse) around the pit(core). The luminosity of the pit(core) depends on the nature of its surface and of his(her,its) thoughtful power albédo ) The GIOTTO probe showed that the solid surface of the comet of HALLEY possesses a particularly raised(particularly brought up) albédo, c? Is l? The darkest object of the solar system that? We met jusqu? Here.
The hair. He(it) forms then an envelope of gas and dusts, the hair or coma, which can reach(affect) gigantic dimensions (10 times the Sun). The comet, by moving on its trajectory meets the solar wind and it forms a shock wave in l? Before of coma.
The tail. In l? Back of coma, the solar wind pushes away(repels) gases and dusts to form a double tail:?a tail of plasma ( ionized gases) straight(right), bluish or iridescent, transparent, sometimes twined and a tail of dusts bent, formed by tiny solid particles which reflect with a yellow tint the sunlight.

L? ACTIVITY COMÈTAIRE

Tail of plasma:?towards 4 UA ( 600 Mkm), ices(mirrors) and ice-cold gases are sublimated and? Escape by évents or??Active beaches??On the surface of the pit(core), pulling(entailing) very fine particles of solid material(subject). So form the hair and the tail of dusts. Gases violently thrown(planned) towards l? Outside spread(diffuse) and are ionized by l? Electromagnetic action of the solar wind. They form the tail of plasma, ténue, infinitely diffuse, of variable structure and changeable shape (range, right(straight) or twined ribbon). This tail is brutally sometimes interrupted to reappear later.
Tail of dusts:?the particles of dust are pulled(entailed) by l? Ejection of gases. Their dimensions are generally insignificant, going of the thousandth of micron to the tenth of micron.
The tail of dust is nevertheless formed by solid material(subject). C? Is why she(it) sends back(dismisses) the sunlight, which has a yellow dominant. The tail of dust also has a slowness and it takes a shape determined by its position on the trajectory and l? Effect of the solar wind. The tail of the comet is always directed in the sense(direction) of the solar wind. She(it)? Spreads(widens) towards l? Before of the trajectory when the comet goes to the Sun. She(it)? Spreads(widens) towards l? Back of the trajectory when the comet? Take away from the Sun. He can then form a small tail annexes towards the Sun.
Luminosity:?L? Brightness d? A comet varies according to its distance in the sun, d? A part because of its appropriate(clean) activity which varies according to the quantity of gas and dusts that? She(it) emits(utters), and d? Somewhere else of the light that? She(it) receives from the sun and what? She(it) sends back(dismisses). The young comets are very atives and thus very brilliant, the former(old) periodic comets n? Have hardly d? Activity and are less and less brilliant.
Comets??Sun-grazers??Are particularly active and brilliant after their passage in the périhélie.
How for planets and stars, we express l? Brightness of comets??Magnitude?.
The sublimation:?the heater of the surface of the pit(core) by the beams(shelves) of the Sun provokes the direct transformation of the organic molecules frozen in free gases. These gases essentially consist of four elements which are??Bricks??Some organic matter, the CHON (carbon, hydrogen, oxygen and nitrogen). He(it) is to notice that these elements exist in l? Intersidereal space in proportions similar to those of the life on Earth. The sublimated volatile elements look d? Access of??Molecules mothers??(H2O, HCN, CO, CO2;?CH3OH, H2CO, etc.). L? Action of the solar wind separates these molecules mothers there??Molecules girls??Who are radicals, ions and free atoms (CN, a Hour, OH, O, CO +, C, CO, HP, CH3O, etc.). When the volatile material(subject) is exhausted, the comet loses its activity, its tail disappears, his(her,its) coma retracts and the pit(core) is reduced to a dark block of sluggish solid material(subject).
Origin of comets
In the XXth?century ( 1945-1950 ), hypothesis d? A vast very remote reservoir of comets (OPIK and OORT).
In 1980, hypothesis d? Another reservoir of comets from l? Orbit of Pluto:?belt of KUIPER.

NAMING

Comets were registered(recorded) in The Hague in the office of l? Union astronommique international ( I.A.U) since the 1950s, messages must be sent to Smithsonian Institute, Service of the fast Satellites, Director Brian MARSDEN, Cambrige, Massachussetts, U.S.A. Simplified address:?I.A.U. Office exchange for Astronomical Telegrams, Cambridge, Mas. U.S.A..
Every comet carries(wears) its year.
She(It) receives besides a letter d? Temporary alphabet (in small letters) in l? Order of the discoveries of l? Year:?1990a, 1990b, 1990c, etc.
If a discoverer is alone before all the others, the comet receives its name:
Comet DONATI on 1965 Comet AUSTIN 1989
- If there are two or three independent discoverers, their names are attached:
IKEYA-SEKI on 1965 WHITE-ORTIZ-BOLELLI on 1970
HONDA-MRKOS-PADJUSAKOVA on 1945
- If a big number of persons saw together the comet, she(it) does not receive official name:?comet of January, 1910 Comet of Johanesbourg
For the naming, after confirmation, observation and calculation d? Orbit, the comet enters the Catalog with its name, date of its passage in the périhélie and its row(rank) in l? Year in Roman?numerals:
In 1973, Lubos KOHOUTEK discovers two comets in week d? Interval:
The first one is baptized on 1973 e. Very weak, she(it) quickly disappears The second is baptized 1973 F. She(it) quickly grows and is announced as the comet of the century. In 1973, it will have been the 12th to pass in the périhélie. She(it) enters the Catalog as KOHOUTEK on 1973 XII.
Particular cases
- If the same observer indicates several comets, these receive a number d? Order in Roman?numerals:
SCHWASSMAN and A.A. WACHMANN discovered together several comets, they entered the Catalog with their number d? Personal?nature:
SCHWASSMAN-WACHMANN I (on 1925 II)
SCHWASSMAN-WACHMANN II
- If a comet is discovered by a satellite, she(it) carries(wears) the name of this satellite:
In 1990, IRAS had discovered five comets, in?particular:
IRAS-ARAKI-ALCOCK on 1983 (closed to encounter)
- Comets discovered by the Chinese receive the name of the look-out post, not that of the discoverer:
Comet TSUCHINSAN ( Mauve Mountain)
- The periodic comets?carry(wear) l? Initial P undated:
P / PONS-WINNECKE P / GIACOBINI ZINNER
- The periodic already indicated but recognized comets carry(wear) generally the name of l? Astronomer who recognized their orbit or their periodicity:
P / HALLEY P / LEXEL P / CROMMELIN
- Exceptionally, the name of the discoverer maybe supplanted by that of l? Astronomer having calculated l? Orbit and the periodicity:
A comet MECHAIN became P / ENCKE
The naming:?she(it) is assured(insured) by the Office(Desk) of the fast Satellites Cambridge ( Massachusets), in association with l? International Astronomic Union ( I.A.U)..
Brian MARSDEN was very for a long time the responsible.
Catalogs
In the end of the XVIIth century:?HEVELIUS ( Dantzig):?directory of the tails of comets.
In the end of the XVIIIth century:?MESSIER:?catalog of??Nebula?.
In the end of the XXth century:?more than 1200 comets were recognized.
Catalogs of Brian MARSDEN, Don YEOMANS, KRONCK.
New Catalog:?since ?the 90s, the multiplicity of the discoveries of comets and d? Asteroids by artificial satellites led(drove) l? I.A.U. has to establish a more precise identity in comets:
A letter defines the nature of l? Heavenly object (C for comet, P for periodical).
Follows the date of discovery and, possibly, a letter of characterization:
Eg:?K for a comet belonging to the group of KREUTZER.
A number can follow to clarify the row(rank) of passage.
The date of passage in the périhélie and the row(rank) in l? Year in Roman numerals.
Eg:?C / 1970 K1=1970 VI (Comet WHITE-ORTIZ-BOLELLI)

THE FRENCH DISCOVERIES OF the XXÈME CENTURY?

920?:?Comet QUENISSET
1970:?Comet WHITE (Australian) - ORTIZ ( Frenchman) - BOLELLI ( Argentine)
1986:?Comet CIFFREO by Jacqueline CIFFREO by observing an image of the comet of HALLEY
At the end of the 80s:?comets MAURY by Alain MAURY during second??Survey??Of Palomar (replaced by Jean MUELLER who examines since Schmidt de la Survey's patches).
1997:?Comet MEUNIER-DUPOUY C / 1997 J2 by Michel MEUNIER and Philippe DUPOUY. First comet to have been discovered by amateurs in CCD, she(it) was discovered during? A follow-up of the comet MUELLER J1.
Amateurs supported by professional astronomers discover more and more d? Asteroids and of comets on patches resulting from big telescopes. Cf. List of distribution(broadcasting) the AUDE.

EVOLUTION OF COMETS

In every passage near the Sun, a comet loses approximately 1 % of its mass. We consider that the comet of HALLEY which, in the périhélie, loses approximately 50 tons of material(subject) and gas per second, should last approximately 150 000 years. The comet ENCKE, in short period, is in fast decrease.
Dislocations:?since l? Antiquity, it is said that comets broke in several fragments. ARISTOTE would have been a witness in his youth of l? Explosion d? A comet. The most famous testimony is the case of the comet BIELLA which, in 1846, in l? Approach the Sun? Is broken in several fragments and n? Is more reappeared.
Trainées of dusts:?during its trajectory, a comet abandons grains(beads) of material(subject).
If the Earth cuts l? Orbit of the comet, it crosses these clouds of dust and grains(beads) (generally tiny) penetrate in l? Atmosphere where they waste away without reaching(affecting) the ground. Their speed is enormous. If bigger objects arrive jusqu? On the ground, they create an impact and a crater but they are pulverized.
A crossing heavenly object l? Ground atmosphere shows itself by a meteor collectively called??Falling?star?. The passage of the Earth in dusts of the trail d? A comet provokes in the sky of streams (swarms) of meteors (rains d? Stars).
Remarkable swarms:
? HALLEÏDES:?trail of the comet of HALLEY,
Aquarides:?in the constellation of the Aquarius, in April-May (max on May 8th)
Orionides:?in the constellation d? Orion at the end of October (max 21 oct.)
BIELIDES:?trail of the comet of BIELA, broken in 1846
In the constellation d? Andromède (in the South of Cassiopée) in 1877.
Collisions:?the collisions of comets with celestial bodies of solar systems are inevitable but fortunately relatively rare. The modern means d? Observation of the sky allowed recently d? Register(record) collisions which, formerly, would be crossed(spent) unnoticed.
Collision with the Sun:
In?August 20th, 1979: photos grip by the American satellite P78-1 (US DOD)
Percussion of the comet HOWARD-KOOMAN-MICHAELS on 1979 XI
Speed of percussion estimated(esteemed) at km/s 560 (2 016 000 kph)
Collision with Jupiter:?sights of the impacts of SHOEMAKER-LEVY 9 on Jupiter in 1994


In?March 23rd, 1993: comet?SHOEMAKER-LEVY 9: a train of comets. Observation confirmed by the big telescopes:?21 fragments. D? After the calculations of the JPL, (Pasadena, U.S.A.), the comet had been arrested in 1929 by Jupiter and? Was broken during the 25th passage on July 7th, 1992 (bigger fragments:?G, a Hour, K, L). In the end of May, 1993, Brian MARSDEN published a report announcing the collision of the comet with Jupiter by July 16th, 1994. (Speed:?215 600 kph)
From 16 till 22 July 1994, collision of 21 fragments under the supervision of the biggest telescopes of the world (visible also by the amateurs). The biggest fragment ( G ) struck Jupiter July 18th, 1992 provoking a ball of light bigger than the planet. The panache d? Impact persisted more d? One hour and left with the surface a cloud of the size of the Earth. Scars remained visible during almost? One year.
First world observation of the collision d? A comet with a planet and a warning on the damages d? Such a collision.


Collision with the Earth:
Fortunately, no major collision since l? Appearance of l? Man except an event badly clarified (accident of Toungonska? Tunguska - on June 30th, 1908).
Probably a small pit(core) cométaire which n? Reached(affected) the ground but which provoked an audible explosion in 1000 km and slept trees on a 50 km beam(shelf).
The searches(researches) continue to identify l? Object which struck the Earth there is 65 My and provoked an extinction of mass of the human beings among whom dinosaurs (crater of Chicxulub off Yucatan). Cf. Bill BRYSON??In Pair?of shorts History of Everything?.