A short message about new scientific research on comets. Report: Comets

Plan

1. Comets in the history of mankind

2. Anatomy of a comet: nucleus, coma and tail

5. Shrinking shells

1. Comets in human history

Comets are the most efficient celestial bodies in the solar system.

Comets are a kind of cosmic icebergs, consisting of frozen gases of complex cosmic composition, water ice and refractory mineral matter in the form of dust and larger fragments.

Comets belong to the group of small bodies, which also includes asteroids, meteorites, meteor swarms and clouds of interplanetary dust. The solar system consists of one star, nine planets, three of which have ring systems (Saturn, Uranus, Jupiter), almost forty satellites of minor planets with sizes from hundreds of meters to hundreds of kilometers. Asteroids, comets and meteoroids are united under one name "small bodies of the solar system". Unlike other small bodies, comets have an amazing ability to develop atmospheres from relatively small nuclei that exceed all known objects in length. solar system including the sun itself. At the same time, extended atmospheres are observed in comets for a rather long period - sometimes for several years. This is the main property of the cometary nucleus - to continuously renew and maintain a huge gas-dust atmosphere. The name "comet" comes from the Greek word "cometis", which means "hairy" in Russian. Comets appeared unexpectedly in different parts of the sky, and these appearances did not have any patterns, such as the movement of the planets and the moon. Therefore, following the greatest philosopher of antiquity, Aristotle, they began to be considered atmospheric vapors rising into the zone of fire and igniting there in the form of fiery torches. However, not all scientists shared Aristotle's thought about comets. One of the most sensible in the question of the nature of comets was the Roman philosopher Seneca. Back in the 1st century AD. he expressed amazing thoughts about comets, which were fully confirmed after 15-16 centuries. As an object of scientific research, he began to carefully and regularly observe all the luminaries that appeared and were visible to the naked eye. He was the first to describe the trajectory along which the comet moved in 1472, noting every day its position relative to the stars and the direction of the tail. Unfortunately, Remamontan lived only 40 years and did not complete his research. In the 16th century, the astronomer Apian, observing the comet of 1531. came to the conclusion that its tail is always directed in the opposite direction from the Sun. However, he did not understand that the reason for this orientation of the comet's tail is the Sun itself. And finally, the most skillful observer of the Middle Ages, Tycho Brahe, decided to study the movement of comets. To determine the distance to the comet of 1577, he proposed to carry out simultaneous observations from two observatories remote from each other. He himself observed in Helsingburg, and his disciples followed the same comet in Uraniburg. Comparing these observations, Tycho Brahe determined that the comet is much further than the Moon, since the parallax, i.e. offset relative to the stars when viewed from two different points on the earth's surface, turned out to be much further than the Moon, because. its smaller than the moon. The observations of Tycho Brahe and his students proved that comets cannot be the evaporation of the Earth and other planets, but are independent bodies that need to be studied in order to understand their nature and origin.

2. Anatomy of a comet: nucleus, coma and tail

At the first acquaintance with a bright comet, which suddenly appeared in the night sky and spread a giant tail that hangs like a cornucopia over amazed observers, it may seem that this tail is the most important part of the comet, without which it would not represent anything remarkable as a cosmic object. . From a physical point of view, the tail is main reason a secondary formation that developed from a rather tiny nucleus, the most important part of a comet as a physical object. To understand this, it is enough to look at least once through a telescope at a comet that has just appeared, located at a distance of more than 3-5 AU. We will see a pale, barely luminous spherical nebula, sometimes looking like a uniform blurry disk, like an out-of-focal image of some earth, but more often in the center of this diffuse nebula, a displacement is visible, in the center of which is the comet's nucleus - the root cause of the rest of the complex of cometary phenomena. Comet nuclei are still not available for telescopic observations, because they are veiled by the luminous matter surrounding them, continuously flowing from the nuclei. The foggy atmosphere surrounding the photometric core and gradually fading away, merging with the sky background, is called a coma.

The coma, together with the nucleus, makes up the head of the comet. Away from the Sun, the head looks symmetrical, but as it approaches the Sun, it gradually becomes oval. Then the head lengthens even more and in the opposite side from the Sun, a tail develops from it. In ultraviolet light from spacecraft, a huge oval hydrogen coma was discovered, in which the visible coma, and sometimes the entire visible tail of the comet, is immersed.

Rice. 1. Anatomy of a comet: head (nucleus + coma) and tail.

So, the nucleus is the most important part of a comet. However, there is still no consensus on what it actually is. Even in ancient times, there was an idea of the comet's nucleus as a solid body, consisting of easily evaporating substances such as ice or snow. This classical model of the cometary nucleus has been significantly supplemented and developed recently. The mass of a comet's nucleus can be estimated from the motion of secondary nuclei in fissioning comets. For example, in 1957, into two secondary nuclei moving away from each other at a speed of 1.6 m/s. Assuming this speed to be parabolic, one can estimate the mass of the comet's nucleus at one hundred billion tons. F. Balde tried to determine the diameters of the nuclei of comets Pons-Winnike (1927) and Schwassmann-Wachmann (1930), which approached the Earth up to several million kilometers. His estimates, obtained from photographic measurements, give the diameters of the nuclei of these comets on the order of 0.4 km. Let us now turn to the coma of comets surrounding the icy comet nuclei in the form of a foggy atmosphere. In most comets, the coma consists of three main parts, which differ markedly in their physical characteristics:

· The area closest to the nucleus is the internal, molecular, chemical and photochemical coma.

Visible coma, or coma of radicals.

Ultraviolet, or atomic coma.

The dimensions of these three coma are markedly affected by the heliocentric distance of the comet. The most impressive part of a comet is its tail. Comet tails almost always point away from the Sun. Tails consist of dust, gas and ionized particles. Therefore, depending on the composition, the particles of the tails are repelled in the opposite direction from the Sun by forces emanating from the Sun, but having different physical strength, nature: dust and neutral gas are mainly affected by the forces of radiation pressure, which counteract the force of gravity and, in the case of normal tails, exceed them. Comet tails fall into three main types. These types were discovered by the scientist Bredikhin.

1st type: the values of the 1st reach several tens, and sometimes several thousand F.A. Bredikhin believed that the values of the 1st should be a multiple of 22.3. By appearance- these are rectilinear tails, creeping along an extended radius vector; their outlines are irregular, often screw-shaped; in addition, tails of the first type may consist of a set of individual filaments or rays.

2nd type: 1st values are between 0.6 and 2.5. These include tails, in appearance they resemble a strongly curved horse or an ox horn. At the end of such tails, stripes of a doublet structure are often observed, directed towards the comet's nucleus. These strips are called synchronous.

3rd type: 1st values take different values between 0 and 2.5. In appearance, these are short straight tails, representing one complete synchrony, starting directly from the nucleus.

3. Rays

Quite often, thin rectilinear rays are observed in the tails of the 1st type. Most general properties rays are: the rays are located symmetrically about the axis of the tail and approximately in the direction of the extended radius - vector; the first (short) rays appear at large angles to the tail axis and lengthen as they approach the axis; the motion of the rays perpendicular to the axis of the tail has the character of slamming; often the rays acquire a spiral shape; sometimes the rays are strongly curved.

Rays are plasma formations. Therefore, it is most likely that the rays are cometary plasma compressed into fibers under the action of external, magnetic and electric fields. Sometimes ray systems are observed associated with cloud formations moving with high accelerations in the comet's tail. Along with the cloud formations, their ray systems also moved. For example, at the comet Morehouse on October 15-17, 1908, ray systems were observed simultaneously emerging from the head of the comet. Also, the rays can still come out of the tail of comets.

4. Galos

Halo formation in comets consists in the appearance of a system of expanding concentric luminous rings against the background of the diffuse glow of the coma. Expanding at a speed of 1-2 km / s, the halos gradually merge with the sky background and become invisible. The most prominent halos were observed in the heads of bright comets. Halos were first discovered by Schmidt in the head of the bright comet Donati (1858). After that, halos were found in comets Pons-Brooks, Halley, Alcock and Honda.

5. Shrinking shells

The phenomenon of shrinking shells was discovered in Comet Morehouse (1908). Observations showed that the shells arose at approximately the same distance from the nucleus, with the tops of the shells appearing first.

To a non-astronomer, a comet is a blurry object in the sky. Most people don't show much interest in them. For an astronomer, a de comet is a frozen body made up of a variety of ice and dust, or as astronomer Fred Whipple said, it is a “dirty snowball”. What makes comets so interesting to astronomers? Here are a few reasons: They are not predictable. Comets can suddenly flare up or go out of sight for hours. A comet may lose its tail or develop multiple tails. Sometimes they can even split into two or more parts, so that through a telescope, several comets can be seen moving across the sky at the same time.

Comets represent some of the most ancient, virtually untouched, objects in the solar system. It seems that their certain compositions represent the initial appearance of a vast nebula, which subsequently, condensing, forms a star and planets. AT last years it was found that comets "undermined" the progress of life on Earth. Many astronomers believe that comet collisions with the Earth brought a large amount of water, which now makes up the Earth's oceans. These oceans made it possible for life to rise to its feet. On the other hand, dinosaurs are a vivid example of how comet impacts with the earth can bring extinction to some life forms. Arguing logically, one can come to the conclusion that other periods of mass extinction could be the result of such collisions. What would life be like on Earth if these collisions never happened?

Comets are like machines of lies. It is very exciting to watch a comet, for example, Galileo's comet with a period of 75 years, and think what life was like when this comet last time watched. The same thoughts fill your mind when you watch the comets that traveled through the skies of the Earth hundreds, thousands or even millions of years ago.

Not so far back in the past, comets were considered a bad omen. The ancient writings of China and Europe three thousand years ago speak of random great comets flying across the sky and terrible events that, according to the people of that time, happened due to the fault of these comets. In not so distant times, the oral traditions of the indigenous inhabitants of the North and South America, like the Pacific Islanders, say that comets were a terrible sight. In general, various societies branded comets for wars, earthquakes, epidemics, and even the death of leaders.

What is a Comet?

As mentioned above, a comet is basically a ball of ice and dust. Usually a comet is less than ten kilometers in diameter. They spend most of their time frozen outside of our solar system. The figure below shows all the components of the comet. At this stage of the discussion, a comet is nothing more than a nucleus. With the exception of a few supposedly dead comets and a couple of suspicious asteroids that randomly show gas emissions as a comet, the nucleus is usually not visible from Earth. By the time a comet is visible from the ground, it has usually become a dot.

Since the Gitto Space Telescope photographed the nucleus of Comet Halley in 1986, we know that the nucleus of the comet likely has a surface that would more accurately be described as a black crust. Halley's comet is also long, about 12 km, and it is also considered that its nucleus is from 1 to 50 km in diameter. Comet Hail-Bopp in 1997 had a nucleus with a radius of about 40 km in diameter.

The black crust of the nucleus helps the comet to retain heat and, due to it, turn some glaciers under the crust into gas. Under pressure from within, the cloudless but frozen landscape begins to churn in places. And as a result, the weakest areas of the crust collapse under pressure and the gas shoots out like a geyser. Astronomers call it a jet. Any dust that has mixed with the gas is also thrown out. The more jets appear, the shell of rarefied gas and dust forms a point.

Comet Halley photographed by the Gitto telescope in 1986. Pay attention to the active areas that throw dust and gas into outer space. This then forms a dot.

A comet can typically have a dot several thousand kilometers in diameter, depending on the comet's distance from the Sun and the size of the nucleus. The latter plays a priority role. One of the greatest comets in history was the Great Comet of 1811, which was also mentioned in Leo Tolstoy's War and Peace. It was one of the few comets discovered in history that was discovered with a relatively small telescope and in an unusually long distance from the Sun, more than half of Jupiter's orbit. The core was about 30 - 40 km. At one point in September-October 1811, the point reached a diameter, according to rough estimates, equivalent to the diameter of the Sun.

Even if the dot becomes quite large in size, it can shrink sharply around the intersection of the orbit of Mars. At such a distance, the radiation of the Sun will be sufficient to literally blow off gas and dust from the core and point. This destructive process is responsible for the creation of the comet's tail, its best-known part.

Tail.

When a comet flies into Earth's orbit, it has a potentially large tail. The current record for a comet tail is that of the Great Comet of 1843. Its length was more than 250 million kilometers. This means that if you mentally place the comet itself in the center of the Sun, then the tail would cross the orbit of Mars!

Where do comets come from?

Our solar system began as a vast cloud of gas and dust. This cloud slowly revolved around the very young Sun and the particles inside this cloud collided with each other. During these collisions, some particles disappeared, some grew in size, and later were to become a planet.

During this early period, comets bolzhnoby filled the solar system. Their collisions with young planets played a major role in their development and growth. Ice-covered comets became the main material of the nascent atmosphere. planets, and scientists are now deeply convinced that comets brought Water to Earth, which gave birth to life.

A year later, comets no longer fill our solar system, as they did 4 billion years ago. Telescopes today can see 10-20 comets at a time. Most comets are now located outside the solar system. Mainly in Oort cloud and Kuiper belt. The Oort cloud is just a hypothesis, because no one has seen him yet.

Bibliography

For the preparation of this work, materials from the site http://referat2000.bizforum.ru/ were used.

Comets represent some of the most ancient, virtually untouched, objects in the solar system. It seems that their certain compositions represent the initial appearance of a vast nebula, which subsequently, condensing, forms a star and planets. In recent years it has been found that comets have undermined the progress of life on Earth. Many astronomers believe that comet collisions with the Earth brought a large amount of water, which now makes up the Earth's oceans. These oceans made it possible for life to rise to its feet. On the other hand, dinosaurs are a vivid example of how comet impacts with the earth can bring extinction to some life forms. Arguing logically, one can come to the conclusion that other periods of mass extinction could be the result of such collisions. What would life be like on Earth if these collisions never happened?

Comets are like machines of lies. It's very exciting to watch a comet, like Gallileo's Comet, with a period of 75 years, and think about what life was like when that comet was last seen. The same thoughts fill your mind when you watch the comets that traveled through the skies of the Earth hundreds, thousands or even millions of years ago.

Not so far back in the past, comets were considered a bad omen. The ancient writings of China and Europe three thousand years ago speak of random great comets flying across the sky and terrible events that, according to the people of that time, happened due to the fault of these comets. In a time not so distant, oral tradition from the natives of the Americas, as well as from the Pacific Islanders, speaks of comets as a terrible sight. In general, various societies branded comets for wars, earthquakes, epidemics, and even the death of leaders.

What is a Comet?

Comet Halley photographed by the Gitto telescope in 1986. Pay attention to the active areas that throw dust and gas into outer space. This then forms a dot.

A comet can typically have a dot several thousand kilometers in diameter, depending on the comet's distance from the Sun and the size of the nucleus. The latter plays a priority role. One of the greatest comets in history was the Great Comet of 1811, which was also mentioned in Leo Tolstoy's War and Peace. It was one of the few comets discovered in history that was discovered with a relatively small telescope and at an unusually large distance from the Sun, more than half the orbit of Jupiter. The core was about 30 - 40 km. At one point in September-October 1811, the point reached a diameter, according to rough estimates, equivalent to the diameter of the Sun.

Tail.

Where do comets come from?

During this early period, comets filled the solar system. Their collisions with young planets played a major role in their development and growth. Ice-covered comets became the main material of the nascent atmosphere. planets, and scientists are now deeply convinced that comets brought Water to Earth, which gave birth to life.

abstract

in astronomy

"Comets"

student 11 "A" class

Korneeva Maxim

Plan:

1. Introduction.

2. Historical facts, the beginning of the study of comets.

3. The nature of comets, their birth, life and death.

4. The structure, composition of the comet.

6. Conclusion.

7. List of literary sources.

1. Introduction.

Comets are among the most spectacular bodies in the solar system. These are a kind of space icebergs, consisting of frozen gases of a complex chemical composition, water ice and refractory mineral matter in the form of dust and larger fragments. Every year, 5-7 new comets are discovered, and quite often, once every 2-3 years, a bright comet with a large tail passes near the Earth and the Sun. Comets are of interest not only to astronomers, but also to many other scientists: physicists, chemists, biologists, historians ... Quite complex and expensive research is constantly being carried out. What caused such a keen interest in this phenomenon? It can be explained by the fact that comets are a capacious and far from fully explored source of information useful to science. For example, comets “told” scientists about the existence of the solar wind, there is a hypothesis that comets are the cause of life on earth, they can provide valuable information about the origin of galaxies ... But it should be noted that the student does not receive a very large amount of knowledge in this area due to limited time. Therefore, I would like to replenish my knowledge, as well as learn more interesting facts on this topic.

2. Historical facts, the beginning of the study of comets.

When did people first think about the bright tailed “stars” in the night sky? The first written mention of the appearance of a comet dates back to 2296 BC. The movement of the comet through the constellations was carefully observed by Chinese astronomers. To the ancient Chinese, the sky was a huge country, where the bright planets were the rulers, and the stars were the authorities. Therefore, the ancient astronomers considered the constantly moving comet to be a messenger, a courier delivering dispatches. It was believed that any event on starry sky was preceded by a decree of the heavenly emperor, delivered by a comet-messenger.

Ancient people were terrified of comets, prescribing to them many earthly cataclysms and misfortunes: pestilence, famine, natural disasters... They were afraid of comets because they could not find a sufficiently understandable and logical explanation for this phenomenon. This is where the myths about comets come from. The ancient Greeks imagined any comet bright enough and visible to the naked eye with a head with flowing hair. Hence the name was formed: the word “comet” comes from the ancient Greek “cometis”, which means “hairy” in translation.

Aristotle was the first to try to scientifically substantiate the phenomenon. Not noticing any regularity in the appearance and movement of comets, he proposed to consider them as flammable atmospheric vapors. Aristotle's opinion became generally accepted. However, the Roman scholar Seneca tried to refute the teachings of Aristotle. He wrote that "the comet has its own place between celestial bodies..., it describes its path and does not go out, but only moves away." But his astute assumptions were considered reckless, since Aristotle's authority was too high.

But due to uncertainty, lack of consensus and explanation for the phenomenon of “tailed stars”, people continued to consider them as something supernatural for a long time. They saw fiery swords, bloody crosses, burning daggers, dragons, severed heads in comets ... The impressions from the appearance of bright comets were so strong that even enlightened people, scientists succumbed to prejudice: for example, famous mathematician Bernoulli said that the tail of a comet is a sign of God's wrath.

In the Middle Ages, scientific interest in the phenomenon reappeared. One of the outstanding astronomers of that era, Regiomontanus, treated comets as objects of scientific research. Regularly observing all the luminaries that appeared, he was the first to describe the trajectory of movement and the direction of the tail. In the 16th century, the astronomer Apian, making similar observations, came to the conclusion that the tail of a comet is always directed in the direction opposite to the Sun. A little later, the Danish astronomer Tycho Brahe began to observe the movement of comets with the highest accuracy for that time. As a result of his research, he proved that comets - celestial bodies, more distant than the Moon, and thus refuted Aristotle's theory of atmospheric evaporation.

But, despite the research, getting rid of prejudice was very slow: for example, Louis XIV was very afraid of the comet of 1680, because he considered it a harbinger of his death.

The greatest contribution to the study of the true nature of comets was made by Edmond Halley. His main discovery was the establishment of the periodicity of the appearance of the same comet: in 1531, in 1607, in 1682. Fascinated by astronomical research, Halley became interested in the motion of the comet of 1682 and started calculating its orbit. He was interested in the path of its movement, and since Newton had already carried out similar calculations, Halley turned to him. The scientist immediately gave the answer: the comet will move in an elliptical orbit. At the request of Halley, Newton set out his calculations and theorems in the treatise "De Motu", that is, "On Motion". Having received the help of Newton, he began to calculate cometary orbits from astronomical observations. He managed to collect information about 24 comets. Thus appeared the first catalog of cometary orbits. In his catalog, Halley found that the three comets were very similar in their characteristics, from which he concluded that these were not three different comets, but periodic appearances of the same comet. The period of its appearance was equal to 75.5 years. Subsequently, it was named Halley's comet.

After Halley's catalog, several more catalogs appeared, where all comets that appeared both in the distant past and at the present time are entered. Of these, the most famous are: the catalog of Balde and Obaldia, as well as, first published in 1972, the catalog of B. Marsden, which is considered the most accurate and reliable.

3. The nature of comets, their birth, life and death.

Where do the “tailed stars” come from? Until now, lively discussions have been going on about the sources of comets, but a single solution has not yet been developed.

Back in the 18th century, Herschel, observing nebulae, suggested that comets are small nebulae moving in interstellar space. In 1796, Laplace, in his book Exposition of the System of the World, expressed the first scientific hypothesis about the origin of comets. Laplace considered them fragments of interstellar nebulae, which is incorrect due to differences in the chemical composition of both. However, his assumption that these objects are of interstellar origin was supported by the presence of comets with near-parabolic orbits. Laplace also considered short-period comets to have come from interstellar space, but were once captured by Jupiter's gravity and transferred to short-period orbits. Laplace's theory has supporters at the present time.

In the 1950s, the Dutch astronomer J. Oort proposed a hypothesis about the existence of a cometary cloud at a distance of 150,000 AU. e. from the Sun, formed as a result of the explosion of the 10th planet of the solar system - Phaeton, which once existed between the orbits of Mars and Jupiter. According to Academician V.G. Fesenkov, the explosion occurred as a result of too close approach of Phaethon and Jupiter, since during such an approach, due to the action of colossal tidal forces, a strong internal overheating of Phaethon arose. The force of the explosion was enormous. To prove the theory, one can cite the calculations of Van Flandern, who studied the distribution of elements of 60 long-period comets and came to the conclusion that 5 million years ago, between the orbits of Jupiter and Mars, a planet with a mass of 90 Earth masses (comparable in mass to Saturn) exploded. As a result of such an explosion, most of the matter in the form of nuclei of comets (fragments of the ice crust), asteroids and meteorites left the solar system, some of it lingered on its periphery in the form of the Oort cloud, some of the matter remained in the former orbit of Phaethon, where it still circulates in asteroids, comet nuclei and meteorites.

Fig.: Paths of long-period comets to the outskirts of the solar system (Phaethon explosion?)

Some cometary nuclei have retained relict ice under a loose heat-insulating layer of a refractory component, and short-period comets moving in almost circular orbits are still sometimes discovered in the asteroid belt. An example of such a comet is the Smirnova-Chernykh comet, discovered in 1975.

At present, the hypothesis of gravitational condensation of all bodies of the solar system from the primary gas-dust cloud, which had a chemical composition similar to that of the sun, is considered generally accepted. In the cold zone, the clouds condensed giant planets: Jupiter, Saturn, Uranus, Neptune. They absorbed the most abundant elements of the protoplanetary cloud, as a result of which their masses increased so much that they began to capture not only solid particles, but also gases. In the same cold zone, ice nuclei of comets were also formed, which partly went to the formation of giant planets, and partly, as the masses of these planets grew, they began to be thrown by them to the periphery of the solar system, where they formed a “reservoir” of comets - the Oort cloud.

As a result of studying the elements of almost parabolic cometary orbits, as well as applying the methods of celestial mechanics, it was proved that the Oort cloud really exists and is quite stable: its half-life is about one billion years. At the same time, the cloud is constantly replenished from various sources, so it does not cease to exist.

F. Whipple believes that in the solar system, in addition to the Oort cloud, there is also a closer region densely populated by comets. It is located beyond the orbit of Neptune, contains about 10 comets, and it is it that causes those noticeable perturbations in the motion of Neptune that were previously attributed to Pluto, since it has a mass two orders of magnitude greater than the mass of Pluto. This belt could have been formed as a result of the so-called "diffusion of cometary orbits", the theory of which was most fully developed by the Riga astronomer K. Steins. It consists in a very slow accumulation of small planetary perturbations, which results in a gradual contraction of the semi-major axis of the comet's elliptical orbit.

Diagram of diffusion of cometary orbits:

Thus, over millions of years, many comets that previously belonged to the Oort cloud change their orbits so that their perihelions (the closest distance from the Sun) begin to concentrate near the giant planet Neptune, the most distant from the Sun, which has a large mass and an extended sphere of action. Therefore, the existence of the comet belt predicted by Whipple beyond Neptune is quite possible.

In the future, the evolution of the cometary orbit from the Whipple belt proceeds much more rapidly, depending on the approach to Neptune. When approaching, a strong transformation of the orbit occurs: Neptune with its magnetic field acts in such a way that after leaving its sphere of action, the comet begins to move in a sharply hyperbolic orbit, which leads either to its ejection from the solar system, or it continues to move inside the planetary system, where it can again be exposed to the impact of the giant planets, or it will move towards The sun in a stable elliptical orbit, with its aphelion (the point of greatest distance from the Sun) showing belonging to the Neptune family.

According to E.I. Kazimirchak-Polonskaya, diffusion leads to the accumulation of circular cometary orbits also between Uranus and Neptune, Saturn and Uranus, Jupiter and Saturn, which are also sources of cometary nuclei.

A number of difficulties in the capture hypothesis, especially during the time of Laplace, in explaining the origin of comets, prompted scientists to look for other sources of comets. So, for example, the French scientist Lagrange, based on the absence of sharp initial hyperbolas, the presence of only direct motions in the system of short-period comets in the Jupiter family, put forward a hypothesis about the eruptive, that is, volcanic, origin of comets from various planets. Lagrange was supported by Proctor, who explained the existence of comets in the solar system by the strongest volcanic activity on Jupiter. But in order for a fragment of Jupiter's surface to overcome the planet's gravitational field, it would need to be given an initial velocity of the order of 60 km/s. The appearance of such velocities during volcanic eruptions is unrealistic, therefore the hypothesis of the eruptive origin of comets is considered physically untenable. But in our time it is supported by a number of scientists, developing additions and clarifications to it.

There are also other hypotheses about the origin of comets that have not received such widespread, as hypotheses about the interstellar origin of comets, about the Oort cloud and the eruptive formation of comets.

4. Structure, composition of the comet.

The small nucleus of a comet is its only solid part, almost all of its mass is concentrated in it. Therefore, the nucleus is the root cause of the rest of the complex of cometary phenomena. Comet nuclei are still inaccessible to telescopic observations, since they are veiled by the luminous matter surrounding them, continuously flowing from the nuclei. Using high magnifications, one can look into the deeper layers of the luminous gas-dust shell, but what remains will still significantly exceed the true dimensions of the core in size. The central cluster visible in the atmosphere of a comet visually and in photographs is called the photometric nucleus. It is believed that in its center is the actual nucleus of the comet, that is, the center of mass is located. However, as the Soviet astronomer D. O. Mokhnach showed, the center of mass may not coincide with the brightest region of the photometric nucleus. This phenomenon is called the Mokhnach effect.

The foggy atmosphere surrounding the photometric core is called the coma. The coma together with the nucleus make up the head of the comet - a gaseous shell, which is formed as a result of the heating of the nucleus when approaching the Sun. Away from the Sun, the head looks symmetrical, but as it approaches it, it gradually becomes oval, then elongates even more and in the opposite side from the Sun, a tail develops from it, consisting of gas and dust that make up the head.

The nucleus is the most important part of a comet. However, there is still no consensus on what it actually is. Even in the days of Laplace, there was an opinion that the nucleus of a comet - solid, consisting of easily evaporating substances such as ice or snow, quickly turning into a gas under the influence of solar heat. This classic icy model of the cometary nucleus has been significantly expanded in recent years. Whipple's model of the nucleus, a conglomerate of refractory stony particles and a frozen volatile component (methane, carbon dioxide, water, etc.), enjoys the greatest recognition. In such a core, ice layers of frozen gases alternate with dust layers. As the gases warm up, evaporating, they carry clouds of dust with them. This makes it possible to explain the formation of gas and dust tails in comets, as well as the ability of small nuclei to outgas.

According to Whipple, the mechanism for the outflow of matter from the nucleus is explained as follows. In comets that have made a small number of passages through perihelion - the so-called "young" comets - the surface protective crust has not yet had time to form, and the surface of the nucleus is covered with ice, so gas release proceeds intensively by direct evaporation. The spectrum of such a comet is dominated by reflected sunlight, which makes it possible to spectrally distinguish “old” comets from “young” ones. Usually "young" are called comets with semi-major axes of orbits, since it is assumed that they first penetrate into the inner regions of the solar system. “Old” comets are comets with a short period of revolution around the Sun, which repeatedly passed their perihelion. In “old” comets, a refractory screen is formed on the surface, since during repeated returns to the Sun, the surface ice, thawing, “contaminates”. This screen well protects the ice under it from exposure to sunlight.

The Whipple model explains many cometary phenomena: abundant outgassing from small nuclei, the cause of non-gravitational forces that deviate the comet from the calculated path. The streams flowing from the nucleus create reactive forces, which lead to secular accelerations or decelerations in the movement of short-period comets.

There are also other models that deny the existence of a monolithic core: one represents the core as a swarm of snowflakes, the other as an accumulation of stone and ice blocks, the third says that the core periodically condenses from the particles of a meteor swarm under the influence of planetary gravity. Whipple's model is considered the most plausible.

The masses of comet nuclei are currently determined extremely uncertainly, so we can talk about the probable mass range: from several tons (microcomets) to several hundreds, and possibly thousands of billions of tons (from 10 to 10 - 10 tons).

A cometary coma surrounds the nucleus in the form of a hazy atmosphere. For most comets, the coma consists of three main parts, which differ markedly in their physical parameters:

1) the closest area adjacent to the nucleus - internal, molecular, chemical and photochemical coma,

2) visible coma, or coma of radicals,

3) ultraviolet, or atomic coma.

At a distance of 1 a. e. from the Sun, the average diameter of the inner coma is D= 10 km, visible D= 10 - 10 km and ultraviolet D= 10 km.

The most intense physical and chemical processes take place in the internal coma: chemical reactions, dissociation and ionization of neutral molecules. In a visible coma, consisting mainly of radicals (chemically active molecules) (CN, OH, NH, etc.), the process of dissociation and excitation of these molecules under the action of solar radiation continues, but less intensively than in the internal coma.

Figure: Ultraviolet photo of Comet Hyakutake.

L. M. Shulman, based on the dynamic properties of matter, proposed dividing the cometary atmosphere into the following zones:

1) near-wall layer (area of evaporation and condensation of particles on the ice surface),

2) circumnuclear region (area of gas-dynamic motion of matter),

3) transition area,

4) the area of free-molecular expansion of cometary particles into interplanetary space.

But not every comet should necessarily have all the listed atmospheric regions.

As the comet approaches the Sun, the diameter of the visible head grows day by day, after passing the perihelion of its orbit, the head increases again and reaches its maximum size between the orbits of the Earth and Mars. In general, for the entire set of comets, the diameters of the heads are within wide limits: from 6000 km to 1 million km.

Comet heads take on a variety of shapes as a comet orbits. Away from the Sun, they are round, but as they approach the Sun, under the influence of solar pressure, the head takes the form of a parabola or catenary.

S. V. Orlov proposed the following classification of comet heads, taking into account their shape and internal structure:

1. Type E; - observed in comets with bright coma, framed from the side of the Sun by luminous parabolic shells, the focus of which lies in the nucleus of the comet.

2. Type C; - observed in comets whose heads are four times weaker than type E heads and resemble an onion in appearance.

3. Type N; - observed in comets that lack both coma and shells.

4. Type Q; - observed in comets that have a weak protrusion towards the Sun, that is, an anomalous tail.

5. Type h; - observed in comets, in the head of which uniformly expanding rings are generated - halos with a center in the nucleus.

The most impressive part of a comet is its tail. The tails are almost always directed away from the Sun. The tails are made up of dust, gas, and ionized particles. Therefore, depending on the composition, the particles of the tails are repelled in the opposite direction from the Sun by forces emanating from the Sun.

F. Bessel, studying the shape of the tail of Halley's comet, first explained it by the action of repulsive forces emanating from the Sun. Subsequently, F. A. Bredikhin developed a more advanced mechanical theory of comet tails and proposed to divide them into three separate groups, depending on the magnitude of the repulsive acceleration.

An analysis of the spectrum of the head and tail showed the presence of the following atoms, molecules and dust particles:

1. Organic C, C, CCH, CN, CO, CS, HCN, CHCN.

2. Inorganic H, NH, NH, O, OH, HO.

3. Metals - Na, Ca, Cr, Co, Mn, Fe, Ni, Cu, V, Si.

4. Ions - CO, CO, CH, CN, N, OH, HO.

5. Dust - silicates (in the infrared).

The mechanism of the glow of cometary molecules was deciphered in 1911 by K. Schwarzschild and E. Kron, who came to the conclusion that this is the mechanism of fluorescence, that is, the re-emission of sunlight.

Sometimes rather unusual structures are observed in comets: rays emerging from the nucleus at different angles and forming a radiant tail in the aggregate; galos - systems of expanding concentric rings; contracting shells - the appearance of several shells constantly moving towards the nucleus; cloud formations; omega-shaped bends of the tails that appear when the solar wind is inhomogeneous.

Fig.: A comet with a radiant tail.

There are also non-stationary processes in the heads of comets: flashes of brightness associated with increased short-wave radiation and corpuscular streams; division of nuclei into secondary fragments.

5. Modern research comets.

Project "Vega".

Project "Vega" ("Venus - Halley's Comet") was one of the most difficult in history space research. It consisted of three parts: the study of the atmosphere and surface of Venus with the help of landers, the study of the dynamics of the atmosphere of Venus with the help of balloon probes, the flight through the coma and the plasma shell of Halley's comet.

The automatic station "Vega-1" was launched from the Baikonur Cosmodrome on December 15, 1984, 6 days later it was followed by "Vega-2". In June 1985, they passed one after another near Venus, having successfully completed research related to this part of the project.

But the most interesting was the third part of the project - the study of Halley's comet. Spacecraft for the first time had to "see" the nucleus of a comet, elusive for ground-based telescopes. The meeting of Vega-1 with the comet took place on March 6, and Vega-2 on March 9, 1986. They passed at a distance of 8900 and 8000 kilometers from its core.

The most important task in the project was to study the physical characteristics of the comet's nucleus. For the first time, the core was considered as a spatially resolved object, its structure, dimensions, infrared temperature were determined, and estimates of its composition and characteristics of the surface layer were obtained.

At that time, it was not yet technically possible to land on the comet's nucleus, since the meeting speed was too high - in the case of Halley's comet, this is 78 km / s. It was dangerous even to fly too close, as cometary dust could destroy the spacecraft. The flight distance was chosen taking into account quantitative characteristics comets. Two approaches were used: remote measurements using optical instruments and direct measurements of matter (gas and dust) leaving the core and crossing the spacecraft's trajectory.

Optical instruments were placed on a special platform, developed and manufactured jointly with Czechoslovak specialists, which turned during the flight and tracked the comet's trajectory. With its help, three scientific experiments were carried out: television shooting of the nucleus, measurement of the infrared radiation flux from the nucleus (thus, the temperature of its surface was determined) and the infrared radiation spectrum of the internal “near-nuclear” parts of the coma at wavelengths from 2.5 to 12 micrometers in order to determine its composition. Investigations of IR radiation were carried out using an infrared spectrometer IKS.

The results of optical studies can be formulated as follows: the core is an elongated monolithic body of irregular shape, the dimensions of the major axis are 14 kilometers, and about 7 kilometers in diameter. Every day, several million tons of water vapor leave it. Calculations show that such evaporation can come from an icy body. But at the same time, the instruments found that the surface of the nucleus is black (reflectivity less than 5%) and hot (about 100 thousand degrees Celsius).

Measurements of the chemical composition of dust, gas and plasma along the flight path showed the presence of water vapor, atomic (hydrogen, oxygen, carbon) and molecular (carbon monoxide, carbon dioxide, hydroxyl, cyan, etc.) components, as well as metals with an admixture of silicates.

The project was implemented with extensive international cooperation and with the participation of scientific organizations many countries. As a result of the Vega expedition, scientists first saw a cometary nucleus and obtained a large amount of data on its composition and physical characteristics. The rough diagram was replaced by a picture of a real natural object that had never been observed before.

NASA is currently preparing three large expeditions. The first one is called “Stardust” (“Stardust”). It involves the launch in 1999 of a spacecraft that will pass 150 kilometers from the nucleus of comet Wild 2 in January 2004. Its main task is to collect cometary dust for further research using a unique substance called “aerogel”. The second project is called “Contour” (“COmet Nucleus TOUR”). The device will be launched in July 2002. It will encounter Comet Encke in November 2003, Comet Schwassmann-Wachmann 3 in January 2006, and finally Comet d'Arrest in August 2008. It will be equipped with sophisticated technical equipment, which will make it possible to obtain high-quality photographs of the nucleus in various spectra, as well as to collect cometary gas and dust. The project is also interesting because the spacecraft with the help of the Earth's gravitational field can be reoriented in 2004-2008 to a new comet. The third project is the most interesting and difficult. It is called "Deep Space 4" and is part of a research program called "NASA New Millennium Program". It is supposed to land on the nucleus of comet Tempel 1 in December 2005 and return to Earth in 2010. spacecraft explores the nucleus of a comet, collects and delivers soil samples to Earth.

Figure: Project Deep Space 4.

The most interesting events in the last few years become: the appearance of comet Hale-Bopp and the fall of comet Schumacher-Levy 9 on Jupiter.

Comet Hale-Bopp appeared in the sky in the spring of 1997. Its period is 5900 years. This comet is associated with some Interesting Facts. In the fall of 1996, American amateur astronomer Chuck Shramek transmitted to the Internet a photograph of a comet, which clearly showed a bright white object of unknown origin, slightly flattened horizontally. Shramek called it a "Saturn-like object" (Saturn-like object, abbreviated as "SLO"). The size of the object was several times larger than the size of the Earth.

Rice.: SLO is a mysterious satellite of a comet.

The reaction of official scientific representatives was strange. The picture of Shramek was declared a fake, and the astronomer himself was a hoaxer, but no intelligible explanation for the nature of SLO was offered. The picture posted on the Internet caused an explosion of occultism, spreading a huge number of stories about the coming end of the world, "dead planet ancient civilization”, evil aliens preparing to take over the Earth with a comet, even the expression: “What the hell is going on?” (“What the hell is going on?”) was paraphrased into “What the Hale is going on?”… It is still not clear what kind of object it was, what its nature is.

Fig.: Mystical “eyes” of a comet.

Preliminary analysis showed that the second "core" is a star in the background, but subsequent images disproved this assumption. Over time, the “eyes” connected again, and the comet took on its original form. This phenomenon has also not been explained by any scientist.

Thus, the Hale-Bopp comet was not a standard phenomenon, it gave scientists new occasion for reflection.

Figure: Comet Hale-Bopp in the night sky.

Another sensational event was the fall in July 1994 of the short-period comet Schumacher-Levy 9 on Jupiter. The nucleus of the comet in July 1992, as a result of its approach to Jupiter, was divided into fragments, which subsequently collided with the giant planet. Due to the fact that the collisions took place on the night side of Jupiter, earthly researchers could only observe flashes reflected by the planet's satellites. The analysis showed that the diameter of the fragments is from one to several kilometers. 20 comet fragments fell on Jupiter.

Fig.: The fall of comet Schumacher-Levy 9 on Jupiter.

Fig.: Photograph of Jupiter in the IR range after the impact of the comet.

Scientists say that the breakup of a comet into pieces is a rare event, the capture of a comet by Jupiter is an even rarer event, and a collision big comet with the planet - an extraordinary cosmic event.

Recently, in an American laboratory, on one of the most powerful computers Intel Teraflop with a capacity of 1 trillion operations per second, a model of a comet falling with a radius of 1 kilometer to Earth was calculated. The calculations took 48 hours. They showed that such a cataclysm would be fatal for mankind: hundreds of tons of dust would rise into the air, blocking access to sunlight and heat, a giant tsunami would form when it fell into the ocean, devastating earthquakes would occur ... According to one hypothesis, dinosaurs died out as a result of the fall of a large comet or asteroid. In the state of Arizona, there is a crater with a diameter of 1219 meters, formed after the fall of a meteorite 60 meters in diameter. The explosion was equivalent to the explosion of 15 million tons of TNT. It is assumed that the famous Tunguska meteorite of 1908 had a diameter of about 100 meters. Therefore, scientists are now working on the creation of a system for the early detection, destruction or deflection of large space bodies flying near our planet.

6. Conclusion.

Thus, it turned out that, despite their careful study, comets are still fraught with many mysteries. Some of these beautiful “tailed stars” that shine from time to time in the evening sky can pose a real danger to our planet. But progress in this area does not stand still, and, most likely, our generation will already witness a landing on a cometary nucleus. Comets are not yet of practical interest, but their study will help to understand the basics, the causes of other events. The comet is a cosmic wanderer, it passes through very remote regions inaccessible for research, and it may "know" what is happening in interstellar space.

7. Sources of information:

K. I. Churyumov “Comets and their observation” (1980)

· Internet: NASA server (www.nasa.gov), Chuck Shramek's page and other resources.

B. A. Vorontsov-Velyaminov “Laplace” (1985)

· “Soviet Encyclopedic Dictionary” (1985)

B. A. Vorontsov-Velyaminov “Astronomy: a textbook for grade 10” (1987)