Heavenly chameleons. Misconceptions about comets Does a planet's tail always follow it?

If you look at a comet through a telescope, you can see that it has a "head" and a "tail". The "head" is a large cloud of glowing gas called the comet's epicenter. The epicenter can reach over 1,609,300 kilometers in diameter. These gases are so light that the solar winds blow them back. Thus, a "tail" is formed.

As a comet approaches the Sun, its "tail" gets bigger and bigger because the pressure of the solar winds increases. As the comet moves away from the Sun into the cold Universe, the pressure of the solar winds decreases, but still they continue to blow out the gases of the comet. For this reason, the "tail" of a comet is always directed away from the Sun.

At the epicenter of a comet, you can sometimes see a small, shining point of light. This point of light is called the nucleus of a comet. Astronomers believe that the core is a mixture of ice and dust particles, forming a ball up to 50 kilometers in diameter.

When revolving around the Sun, most comets move in elongated orbits. They are shaped like a long, thick cigar. It takes a comet thousands of years to complete one circle in its orbit.

Three or four times a century, a comet passes so close to the Sun that its bright, shining "tail" is easily visible from Earth. We can only observe a comet when it passes close to the Sun. The Sun then turns the comet's ice into gas. Radiation from the Sun passes through the gases and ionizes them, which causes the gases to glow.

If you stand facing the radiant, you can see a few meteors, although bright, but with very short trajectories. The trajectories seem short because the meteors are flying almost straight at you. But, fortunately, the elements of meteor showers are very small and do not reach the ground.

Detailed information about meteors and comets can be found on the North American Meteor Network website (Web.InfoAve.Net/~meteorobs), on the Gary Kronk website (comets.amsmeteors.org) and on the website of the International Meteor Organization (www.imo.net).

All about comets

Comets, giant clumps of ice and mud, move slowly across the sky and look like blurry spots, followed by a trail of gas; they come from the depths of the solar system. These space wanderers have always aroused interest. Every 75–77 years, Halley's famous Comet approaches the Sun and the Earth. If you failed to see her in 1986, then try again in 2061! Don't want to wait that long? Well, there are other comets. For example, the less famous comet Hale-Bopp (which recently approached Earth) is much brighter than Halley's comet.

Many people confuse meteors and comets, but it's easy to tell the difference. A flash generated by a meteor lasts seconds, and a comet is visible for several days, weeks and even months. Meteors move quickly through the sky and flash for a brief moment because they enter the Earth's atmosphere at a distance of about 150 km from the observer. And when observing comets, it seems that they are moving slowly, because they are separated from us by many millions of kilometers. Meteors are quite common, and comets that are easy to see with the naked eye appear on average only once a year or even less frequently.

Astronomers used to describe comets as consisting of a head and a tail (or tails). Subsequently, a bright point of light in the head of a comet began to be called core. Today we know that the nucleus is the comet, the so-called "dirty snowball", a mixture of ice, frozen gases (such as carbon monoxide and carbon dioxide) and solid particles (dust or dirt) (Fig. 4.3). All other visible parts of the comet are simply the result of the evaporation of ice from the nucleus.

Rice. 4.3. A comet is, in essence, a dirty snowball

If the comet is far from the Sun, it is only a nucleus; She doesn't have a head or a tail yet. The diameter of this ice ball can be tens of kilometers or just a couple of kilometers. By astronomical standards, this is very small, and since the nucleus glows only by reflected light from the Sun, a distant comet is almost invisible and therefore difficult to detect.

Photographs of the nucleus of Halley's comet taken by the European Space Agency (ESA) probe showed that this irregularly shaped ice lump has a dark-colored crust (very similar to a scoop of vanilla ice cream sprinkled with chocolate). Alas, comets are not so tasty, but for the eyes it is a real pleasure! But as soon as the Sun heats up the surface of the core a little, jets of gas and dust begin to escape from it, like geysers, into the surrounding space. (Well, bark! No sense!)

As the comet approaches the Sun, the ice in its core begins to evaporate and streams of gas and dust are ejected into space. Gas and dust form around the nucleus a kind of misty luminous cloud called coma(coma); this term comes from the Latin word for "hair" and has nothing to do with the patient's coma (just kidding). Almost everyone confuses a coma with the head of a comet, but the head, strictly speaking, consists of a coma and a nucleus.

The glow of the comet's coma is partly the light of the Sun, reflected by millions of tiny dust particles, and partly a weak radiation emanating from the atoms and molecules of the coma.

The dust and gas contained in the comet's coma are subjected to the action of perturbing forces, so the comet's tails form.

Under the influence of the solar wind, dust particles are thrown away in the direction opposite to the Sun (Fig. 4.4), forming a dust cloud. tail comets.

Rice. 4.4. The tail of a comet points away from the sun

The dust tail glows with reflected light from the sun. It is even, sometimes with a slight curvature, and pale yellow.

Coma again?

The first rule of comet watching is: get out of the city! Although the nucleus of a comet may be only 8-16 km in diameter, the coma that forms around it sometimes reaches tens of thousands or even hundreds of thousands of kilometers in diameter. Gases are emitted from the core in the same way as smoke from a cigarette. Dispersing, they gradually disappear from sight. Therefore, the size of a comet's coma depends not only on how much material the comet emits, but also on the sensitivity of the human eye or film (or electronic sensor). The apparent size of the coma also depends on how dark the sky is. A bright comet in the center of the city seems much smaller than outside the city, where the sky is much darker.

Some of the gas in the coma ionized, i.e. acquires an electric charge, under the influence of ultraviolet radiation from the Sun. In this state, the gases are exposed to solar wind, an invisible stream of electrons and protons emitted by the Sun into outer space (for more details, see Chapter 10). The solar wind throws the electrified cometary gas in a direction also opposite to the Sun, resulting in the formation of the comet's ion or plasma tail. The plasma tail is like an airport windsock: it tells astronomers watching a comet which way the solar wind is "blowing" at the point in space where the comet is.

Unlike a dust tail, a comet's plasma tail is blue in color and "fibrous" in appearance, and sometimes even twisted or torn.

Sometimes some part of the plasma tail breaks away from the comet and flies away in the direction that the tail "points". Then the comet (like a lizard) forms a new plasma tail. Comet tails can range from millions to hundreds of millions of kilometers in length.

When a comet's head is facing the Sun, its tail (or tails) flutter behind it. When a comet circles the Sun and heads out of the solar system, its tail is still pointing away from the Sun, so now the comet is following its tail! Thus, the comet behaves in relation to the Sun, as a courtier - in relation to the emperor: he never turns his back on his master. As shown in fig. 4.4, a comet can move clockwise or counterclockwise, but in any case, its tail will always be directed away from the Sun.

The coma and tails of a comet are components of the process of its disappearance. The nucleus gives off gas and dust, forming a coma, and the tails are already lost by the comet forever - they simply dissipate. By the time the comet has gone far beyond the orbit of Jupiter (which is where most comets come from), there will again be only one nucleus left of it. But the dust it lost could one day "fall" to Earth in a meteor shower if it crossed its orbit.

Every few years, a comet appears bright enough and well positioned in the sky so that it can be easily seen with the naked eye or with small binoculars. I cannot say when such a comet will arrive, because the comets that astronomers accurately predict in the near future will not be particularly bright. But the fact is that almost all bright and amazingly beautiful comets were discovered, not predicted.

From modern photographs, one can easily get acquainted with the variety of forms of comets and trace the changes in these forms, which make it possible to call comets celestial chameleons - they are so changeable.

Large and bright comets that were observed with the naked eye, as a rule, were all with tails. Comets are small and dim, often have barely visible short tails, visible only in photographs, and sometimes they don’t even have tails at all. Many comets are visible only through a telescope, like faint specks of mist, fuzzy at the edges; they are called telescopic. But any bright comet is telescopic, small and weak when it is far from the Sun. Her tail appears and grows as she approaches the Sun, and with distance from it again decreases and disappears. Comets, like lizards, are able to lose their tails and regenerate them.

The apparent size and brightness of a comet depend, of course, also on its distance from the Earth. A huge comet that has slipped far away from us may appear small, and vice versa. Knowing three determinations of the comet's position in the sky, made at different times, it is already possible to calculate its orbit and then take into account the influence of the distance from the Earth on the appearance of the comet. Of course, in order for its orbit to be calculated more reliably, it is necessary to have not three, but a large number of observations over its position.

The brightness of a comet (corrected for the influence of distance from the Earth) varies with its distance from the Sun in different ways, but usually much faster than inversely proportional to the square of the distance, as was first established by Prof. S.V. Orlov in Moscow. For example, when approaching the Sun twice, the brightness of a comet increases ten to twenty times. This shows that comets shine with more than just reflected light. Otherwise, the brightness of the comets would change like the brightness of the planets, that is, simply inversely proportional to the square of the distance, and when approaching the Sun twice, it would increase only four times. The laws of change in the brightness of comets were studied in more detail by S.K. Vsekhsvyatsky and B.Yu. Levin.

The tail of a comet, as you know, is always directed in the direction opposite to the Sun, and when the comet moves away from the Sun, the tail moves ahead of the comet - almost the only case in nature among creatures with a tail ...

The comet consists of several parts, very different in nature. Therefore, misunderstandings often arise if one speaks of one or another property of a comet, without indicating which part of it, in fact, is being discussed.

In a comet, one should distinguish nucleus(more precisely, visible core), head(also called coma, if the comet has no tail) and tail. The head, or coma, is the brightest part of the comet, being brighter in the center, where the likeness of an asterisk, often hazy, is usually seen. This is the visible nucleus of the comet. Only it, perhaps, is a continuous solid body, but rather, that it also consists of separate solid parts.

The size of the nuclei is very small; they are difficult to even measure. For example, in 1910 Halley's Comet passed exactly between the Earth and the Sun. If its solid and opaque core were more than 50 km in diameter, it would be visible as a black dot against the background of the radiant solar disk. Meanwhile, nothing of the kind - not even the slightest shadow on the Sun was noticed. In 1927, the comet Pons-Winnecke came very close to the Earth. At its core, strong telescopes did not notice the slightest disk. It follows that it was less than 2 km in diameter. From an estimate of its brightness, assuming that it is a solid body and reflects the light of the Sun to the same extent as the surface of the Moon, it could be concluded that its diameter is only 400 m. It is more likely, however, that the core consists of not one, but from many boulders, but even smaller and moved away from each other. Many other facts speak in favor of this conclusion, which we will get acquainted with in the following chapters.

Sometimes the stellar nucleus of a comet is surrounded by a rather sharply defined bright haze, which some observers also include in the concept of the nucleus. This also sometimes leads to misunderstandings.

The nucleus of a telescopic and generally faint comet is always surrounded by a large nebulous mass, rather blurred at the edges. It is more or less round in shape and brighter towards the core, but often becomes oblong as it approaches the Sun. Then its elongation is directed along the line connecting the comet's nucleus with the Sun. Sometimes, from such a nebulous mass or coma, a thin beam of light, often several beams, extends in the direction opposite to the Sun, giving the comet the appearance of an onion. In brighter comets, as they approach the Sun, such a thin "bulbous" tail develops into a wide and long tail, and then the coma gets the name of the head.

The front of the head, or the shell of the comet's nucleus, as it is also called, has the shape of a paraboloid. If we rotate a parabola around its axis, then the surface described by it will be a paraboloid. There were cases when a comet formed several shells, as if nested into each other like children's split wooden balls.

1957 gave us two bright comets with remarkable tails. One of them was opened by Arend and Roland in Belgium, and the other by Mrkos in Czechoslovakia. Perhaps you, the reader, have seen them too?

When the comet moves away from the Sun, the phenomena occur in the reverse order, i.e., the tail becomes shorter and less bright, then only an oblong coma remains, and finally, the comet turns into just a hazy speck with or even without a nucleus.

The appearance, development and change in the appearance of the tail in different comets occur very differently, and even in one comet they do not proceed symmetrically with respect to the moment of its passage through perihelion. It happens that on some days the tail suddenly weakens in brightness, then increases again. The overall brilliance of the comet also exhibits occasional irregular fluctuations. Some comets were observed, usually temporarily, with two or even three tails at once, although an inexperienced observer can always mistake rectilinear or slightly curved rays forming one tail for separate tails. Something of this kind was discovered in 1944 by the Soviet scientist S.V. Orlov, studying the drawings of the 1744 Shezo comet, which, according to contemporaries, supposedly had six tails.

It was often observed how from the nucleus of large comets ejected from time to time, sometimes at intervals of only a few hours, bright clouds, gradually receding into the tail and, as it were, melting in it over time.

The totality of such observations, especially those compared with changes in the spectra of comets (which we will discuss below), depicts comets as very capricious and changeable creatures.

The variability of these celestial chameleons makes it difficult to study them, but at the same time allows you to penetrate deeper into the mystery of their structure and development. But before talking in more detail about the physical nature of the shaggy celestial wanderers, we will pay attention to their movement.

After passing through the tail of Halley's comet, the Earth played the role of a kind of probe. Unfortunately, scientists at that time did not have space rockets (more than 47 years remained before the launch of the first artificial Earth satellite). Meanwhile, then it was enough to rise above the earth's atmosphere to be directly in the comet's tail and collect a certain amount of cometary dust and gas for analyses.

It should be noted that the Earth has repeatedly passed through the tails of comets and the effect has always been the same - the substance of the tails of various comets did not have any effect on the processes in the Earth's atmosphere.

Astronomers, as well as many amateur astronomers, closely followed all the changes that took place in the tail and head of Halley's comet from the moment it was discovered by M. Wolf on September 11, 1909, until the last observation on June 15, 1911.

For the entire period of observations of Halley's comet during its appearance 1909 - 1911. more than a thousand of its astronegatives, more than a hundred spectrograms, many hundreds of drawings of the comet and a large number of determinations of its equatorial coordinates at various points in time were obtained. All this rich material made it possible to study in detail the nature of the comet's motion in orbit, to study the change in brightness and geometric dimensions of the head and tail with a change in the heliocentric distance, to study the types of tails, the structural features and chemical composition of the head and tail, as well as a number of other physical parameters of the comet's nucleus and the environment. its atmosphere.

The main results of the study of the vast and varied material, consisting of 26 points, were published by Bobrovnikov in 1931. AAAAAAAAAAAAAAAAAAAAAAAAAAA

The nature and origin of Halley's comet

The orbital elements of comets undergo significant changes as the comet approaches the planets. A particularly strong transformation of the cometary orbit occurs during close encounters of comets with one of the giant planets. This circumstance must be taken into account when studying secular changes in the elements of the orbits of comets, both in the past and in the future. Such calculations make it possible to establish from where cometary nuclei come to the inner regions of the solar system, as well as to solve the problem of the origin of short-period comets. Through the joint efforts of such prominent astronomers as Epic, Oort, Marsden, Sekanina, Everhart, K.A. Steins, E.I. Kazimirchak-Polonskaya proved the existence of an inexhaustible reservoir of cometary nuclei on the periphery of the solar system, which was called the "Epik-Oort cloud".

How did the Epik-Oort comet cloud form on the outskirts of the solar system? At present, the hypothesis of gravitational condensation of all bodies of the solar system from the primary gas-dust cloud, which had the same chemical composition as the Sun, is generally accepted. The giant planets Jupiter, Saturn, Uranus and Neptune with their numerous satellites condensed in the cold zone of the protoplanetary cloud. The remnants of protoplanetary matter may still be observed near these planets in the form of rings. The giant planets absorbed the most abundant elements of the protoplanetary cloud, and their masses increased so much that they easily began to capture not only dust 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 giant planets grew, they began to be thrown last to the periphery of the solar system, where they formed a grandiose source of comets - the Epik-Oort cloud .

The nucleus of Halley's comet in the distant past was probably one of the countless icy cometary nuclei of the Epik-Oort cloud. Revolving around the Sun in an almost parabolic orbit with a period of 106 - 107 years, this core could not be observed from the Earth even at perihelion, which should have been far beyond the planetary system. But one day, perhaps as a result of a significant transformation of the primary orbit by some star of our Galaxy, which passed not far from the Epik-Oort cloud, the nucleus of Halley's comet turned out to be in close proximity to Neptune and was captured by it into its cometary family. We now know approx. There are 10 comets of this family, and, of course, there are much more of them, but due to observational selection, we see only those of them whose perihelia are located near the Earth.

Among the 10 comets of the Neptune family, three of them, including Halley's Comet, are characterized by reverse motion in orbit. The same period as that of Halley's comet, i.e. 76 years, has another comet from this family - comet de Vico, but it was observed only in one appearance (in 1846) and has not been seen since. Only Halley's comet has already been observed during 30 perihelion returns.

Conclusion

Halley's comet was the first short-period comet discovered "at the tip of a pen". The honor of the greatest discovery belongs to the English scientist E. Halley. Careful calculations of the motion of this comet, subsequently made by the astronomers Clairaut, Lalande and Lepoute, gave results that were fully confirmed when the comet, having completed a complete revolution around the Sun, reappeared before astonished observers in March 1759. It was a real triumph of the law of universal gravitation, discovered Newton, and after that the name of Halley's comet, who predicted its appearance, firmly stuck to the comet.

Comprehensive studies of Halley's comet, both from Earth and from space, will help shed light on the possible function of cometary nuclei - to influence the origin and development of life on Earth. This could happen because cometary nuclei collided with the Earth quite often, especially in the early stages of the development of the planetary system.

Scientists believe that comets will make it possible to study the primary matter of the solar system in a relatively unchanged state, since, unlike the planets, they have not undergone profound structural changes as a result of gravity, heat and volcanic activity. It is assumed that comet nuclei consist of relic material and were formed by accretion (sticking together) even before the time when the planets were formed, i.e., about 4.6 billion years ago. Therefore, comets keep the "golden key" from the door, behind which lies the secret of the origin of the larger bodies of the solar system.

ON THE. Belyaev, K.I. Churyumov. Halley's comet and its observation. Moscow, 1985, p. 56.