The appearance of a flashy comet in the night sky has attracted people's attention since ancient times. These phenomena were recorded in chronicles and bred different emotions under the influence of the ideas and superstitions of that time. More often than not the procession of a new comet in the sky used to be associated with forthcoming disasters, epidemics, king's deaths, uprisings, decline of empires and all kinds of other sorrows! Nowdays, the passage of a bright comet is also a significant event, engaging many astronomers professionals and addicts as well as a lot of inquisitive people. With their unpredictable appearance these strange objects give variety and dynamics to the still and impressive picture of the starry sky. Each comet is unique with some typical characteristics - structure, development, sky path, brightness and so on. That's why they make us follow, take photos and describe them for so many nights, extracting valuable information about them as facts and personal impressions!
The name of comets derives from Greek and means “a long - haired star”. Actually, the ancient Greeks, influenced by Aristotel, regarded comets as terrestrial fumes in the upper layers of atmosphere. Not until the 1577 the Danish astronomer Tiho Brake organizes observations of a bright comet from two remote points and proves that comets are far outside the limits of the earth atmosphere.
 
 

      The comets visible with the naked eye are cosmic bodies consisted of two not sharply delineated shining parts - a head and a tail. Comets are bodies consisted of two frozen gases, orbiting in the Solar system. When a comet is away from the sun it is invisible. As it approaches to the it the frozen gases began to evaporate and a beautiful tail is formed. It radiates light as a result from the influence of the solar wind. The comets' tails are always oriented towards a direction opposed to the Sun.

      The folk name of comets is “tailed stars”. Big comets have heads with diameter hundreds of thousands kilometers and tails dozen of millions kilometers long. A huge part of comets, which can be seen only with telescopes, however, have far smaller size and do have not tails. Through comets' tails and heads stars whose brightness almost doesn't abate can be seen. This shows that the shining substance in comets has extremely small density.

      Therefore these are huge objects from strongly rarefied matter with too small masses and that's why comets are also called “visible nothing”. The nucleus is a conglomeration of solid fragments and easily evaporable frozen substances. Its shape can considerably differ from a sphere. The nucleus of the Haley's comet resembles a dumb - bell with size 15x8x8 km. The comet Hale - Bop has unusually big nucleus - about 40km. The gas - dusty atmosphere is called Coma. It is a result from the sublimation of the substance from the surface of the comet's nucleus under the influence of the sun's rays. As the coma takes away from the sun it lessens and disappears.
      The whole comet is immersed in a rarefied cloud of hydrogen, which cannot be seen from the Earth.
The iodine tail or the tail from the first type, is a result from the effect of the magnetic field, caused by the solar wind. Its length is very big and can reach up to 100 kilometers. The reason why it shines is the fluorescence.

     Long ago Newton thought that comets move around the sun in ellipses like planets. His contemporary Edmond Haley calculated the orbits of more than 20 comets. Three of them (the last one was observed by him in 1682), had very close orbits. He supposed that they weren't three comets, but only one. It moved round the sun in ellipse with a period of 76 years. Haley predicted the next appearance of the comet in 1758, when it was really seen. Later on this comet was given the name Haley's comet. This brilliantly confirmed Haley's assumptions and Newton's theory of gravitation and its effects.
      Some comets have such large orbits that they need hundreds or thousands of years for only one tour. Such a comet is the one from 1680, which could be seen 7 000 years B. C. and will be visible again near the 10 500 year. Of course, there are comets that appear quite more often, like the famous comet of Eke, which passes a minimal distance from the sun every 3 years and 4 months. All comets from this type are very weak and can be usually seen only with telescope.
      Therefore , the comets moving around the sun can be conditionally divided into three groups: short periodical (with a period under 20 years), middle periodical (with a period between 20 and 200 years) and long periodical (with a period over 200 years). Of course, their orbits are constantly being disturbed, conditionally from the major planets - Jupiter, Saturn, Uranus and Neptune. Such disturbances have led to the decomposition of the comet Showmakersaw - Lavy and the supervened catastrophic collision between the fragments of the nucleus and Jupiter in the summer of 1994.

      According to the adopted UPI's model, comets are “dirty snow balls” when they are in a long distance away from the sun. As they approach the Sun, they heat and molecules and atoms are liberated from the solid nucleus. They form coma and later - ionic and dusty tail.

      In the middle of XX century, it was UPI's model that was given publicity. According to him the comets 'nucleus not only are small bodies with a size about some kilometers consisting of frozen gases in which the dusty particles are mixed but also whenever the Sun ‘'dresses'' the comet with a crown and a tail it helps itself with a substance taken from the comets itself which it blows and wastes among the panets space. With each tour the comet abates and in the end when it has lost a considerable part of its initial mass the forces of repulsion overbear the attraction of gravity and the substance it consists of intersperses along the orbit in the form of debris, rocks, stones, gas and very sheer dust, which keep on moving in space like a huge swarm. When our planet passes near one of these swarms, a lot of fragments fall to it accelerated by gravity. In our atmosphere they fire and disappear after they have described a bright trace in the sky - the fragments from the comet live their last minutes, appearing like falling stars to us.

      A cloud of hundred milliards cometary nucleuses is about 50 000 AU - 200 000 AU away from the Sun, according to the theory of the Dutch astronomer Oort. When under the gravity influence of a star passing near the Sun a nucleus directs towards it, enters a new orbit, close to the Sun and turns into a “normal” comet. Oort's cloud is a residuum from that substance the solar system was once formed from.

      The first information about the chemical composition of comets was received in 1864 from Donate, when emissive lines of hydrogen were identified in the spectrum of the comet Vineke.
      Later spectral observations add a lot of new lines of radicals, ions, atoms and molecules including the organic ones. On a spectrogram of the Haley's comet in 1985 were identified 8 lines of diatomic and 12 of triatomic carbon, 12 lines of cyanogen, 9 lines of the carbonic group CH, one line of ionized calcium. Lines of neutral and ionized water molecules, carbon dioxide, carbon sulfite, ionized diatomic nitrogen and many others.

      The question about the chemical composition of comets is closely related to the problem about the origin and development of life on Earth. About 10 10 th power tones substance from space fall on Earth every year. The cosmic dust has mainly cometary origin. The famous meteorite Tungun, which is probably a part of the Enke's nucleus, has enriched the region of the Podkamennaq Tungunska river with mercury, iodine, selenium, zinc, silver, sodium, potassium, bromine, rubidium, strontium, lead and so on. According to different evaluations the earth's crust consist of about 0.5 to 5% cometary substance. It is obvious, however, that we own at least a part of the microelements we couldn't live without to comets.

    Despite being beautiful, comets can give different types of information about the origin of planets, because they are the wandering “children” of our system. They existed when it was created 4.6 milliard years before and hasn't changed a lot for the reason that they lived in the outward spheres of the Solar system. The magic of comets probably is due to their ability to make us look back on the past. They appear on the horizon and after a while they are already gone. For example, the comet Hale - Bopp will come back after 2400 years - in 4397. If its discoverers are lucky, it will still bear their names.

 

     Let's remember some of the last brightest comets. We begin with the bright comet Bennett from 1970, which was observed in spring.

      In February and march 1976 before sunrise the comet West, which was remarkable with its fan - shaped tail, could be seen.
The appearance of 1P/Halley in 1986 was preceded by too noisy comments in the media, many of which were with incorrect and even contradictory information. It passes through its perihelly in 9 th February of 1986 .

      Far more attractive, however, is the comet Hale - Bopp /C/1995 01 - Hale-Bopp/, at the end of the winter and during the spring of 1997, which remains in the memories of many people with its double tail (gas and dusty) in different shades, which could be clearly seen even among the bright city lights.It is on its perihelly on1 st April.

     On May 5 th after sunset was the first time when the second expected comet C/2001 Q4 (Neat) was possible to be seen.The comet is on its peruhelly on 15 th of May . In the last nights of May Q4 has already dimmer shine - about 4, m 5 and keeps falling.

      As long ago as February 2003 there was a powerful eruption on the Sun and at the same time the brightness of the comet Neat sharply increased. In this moment it just crosses the orbit of Mercury and a long tail of gas and dust appears in it. Despite the huge thrown out cloud of hot gas and the bright eruption of radiation, the nucleus of the comet remains hole.

      This event happens so close to the Sun that people cannot watch it, but the satellite SOHO, which constantly makes researches of the Sun, records all the details of this sky performance. At the beginning of March the comet Neat takes away from the Sun, but despite this it remains bright enough, with 2 m , although it weakens as it gets further. Observers from the South hemisphere were able to watch the comet with binoculars at least a month more.

      It was discovered in November 2002 with telescope that works on the project for searching of near Earth asteroids NEAT (Near Earth Asteroid Tracking), that's why its official name is Neat C/2002 V1. When the astronomers noticed it, the comet was 349 million kilometers away from the Sun and it looked like an object with star magnitude 17, m 3. But as it approached to our star, it got much brighter.
The researches of its trajectory show that this comet moves around the Sun among a very stretched elliptical orbit and makes a tour for about 37 000 years. The comet is on the perihelly of its orbit on 18 th of February .

      The last brighter comet is the comet Ikeya - Zhang / 153P/Ikeya - Zhang/ which suddenly arrived in the spring of 2002. Here is the place to mention the spring comets in 2004. In the beginning of spring the chance to add two more notable comets to the chronology is given. The first - C/2002 T7 /Linear/, is watched by lovers from the middle of February to the first nights of March. On April 23 rd it is in its perihelly with shine about 2 m ,5 (star magnitude). In the last nights of May its shine is about 4 m and keeps falling. Suddenly in the spring sky a third bright comet appears. On March 23 rd the Australian astronomer - lover William Bradfield discovers a new comet which bears his name - C/2004 F4 (Bradfield) and gets much brighter soon after that. It is in its perihelly on April 17 th .

      After summarizing all data from the chronology, it turns out that from the last nights of May 2004 until June 10 th two comets could be seen: C/2002 T7 Linear - low above the eastern horizon and the comet C/2001 Q4 Neat - high in the north - west. We really had the unique chance to watch two bright comets at the same time, but of course by using binoculars.
The discovery that millions of new comets are going to direct towards us from the outward parts of the Solar system will make some people happy and others-be afraid of this. About 2.4 million comets are going to direct toward the Earth, as some of them can even come into collision with it. However, DO NOT PANIC-comets are not going to come before millions of years have elapsed.

       A group of astronomers from JPL, Pasadena have mapped the traffic of those comets that are close to the Sun and have determined the destabilizations they would give to the Oort's cloud. Most of the long - periodical comets come from that spherical reservoir of comets that is far beyond the planets' orbits. The availability of some milliards in the region between 1 and 2 light years away from the Sun is estimated. When a star passes near the cloud, the gravitational effect could “shoot” many comets to the Sun. Sirius, the brightest star in the sky at present, is going to approach the Earth at a distance of 7.5 light years after 66 000 years. The Barnard's star is going to get closer to our planet in a distance of 3.7 light years after 10 000 years. Proksima from Kentavyr, the closest star to us, is going to reach a distance of 3.2 light years after 28 000 years. Although the three of them are going to get too closer to us, they won't have impact on the Oort's cloud. But the influence of the star GL 710 is much more dangerous. After 1.4 million years (a really short period in astronomy) it is going to enter the Oort's cloud and pass the Sun at a distance of 1.1 light year. A group of astronomers has estimated the effect that the proximity of GL 710, whose mass is the half of the Sun's, would have. The star would be able to direct about 2.4 million stars to orbits that would eventually cross ours. The “storm” will continue 2 million years, so only one long - periodical comet will appear on the Earth's path yearly. In comparison with the present intensity of comets - two yearly, GL 710 is going to double the cometary traffic with about 50 %.

      This comet won 't be a danger for Earth in the near future. NASA made this experiment to learn more about comets and their structures.

      Of course, it is not surprising that curious things happen to comets sometimes. One of them is the case about the comets that disappeared in the beginning of February 2002. Comets are two types: Those, which come from the Cuiper's zone, about 30-60 au away and the so - called NICs, which come from the remote Oort's cloud at a distance of more than 10 000 au. They pass through the Solar system's interior about dozen of times each year. What happens to those frozen bodies when they stop leaving their clearly visible tails remains a mystery. According to Harold Levysson and his colleagues, 99% of the comets of Oort, which enter our Solar system destroy and turn into dust. Levison makes this conclusion after using computer models of thousand of comets and following their development. Almost all comets just disappear. Yet, their destruction remains a mystery up to a point. Why? One of the Levison's hypothesis is that comets undergo a thermal shock and internal gas pressure is formed when they enter our system which is comparatively warmer than the cold places of the open space they come from. There is another possibility. Perhaps Oort's comets have weaker structure than the comets of Cuiper. The case with the invisible comets is similar to this. These comets, which outnumber the usual ones about 400 times, can be a serious danger to Earth. The British astronomer Chandra Vicramasingh says, “ it is possible to reconsider our stand on the security of the Earth”.