Small celestial bodies of the solar system. Small bodies of the solar system. solar system

The Universe consists of a huge number of cosmic bodies. Every night we can contemplate stars in the sky that seem very small, although this is not the case. In fact, some of them are many times larger than the Sun. It is assumed that a planetary system is formed near each lonely star. For example, near the Sun a solar system was formed, consisting of eight large, as well as small, comets, black holes, cosmic dust, etc.

The Earth is a cosmic body because it is a planet, a spherical object that reflects sunlight. Seven other planets are also visible to us only because they reflect the light of a star. In addition to Mercury, Venus, Mars, Uranus, Neptune and Pluto, which was also considered a planet until 2006, the Solar System also contains a huge number of asteroids, which are also called minor planets. Their number reaches 400 thousand, but many scientists agree that there are more than a billion of them.

Comets are also cosmic bodies moving along elongated trajectories and approaching the Sun at a certain time. They consist of gas, plasma and dust; Overgrown with ice, they reach a size of tens of kilometers. Comets gradually melt as they approach the star. Due to the high temperature, the ice evaporates, forming a head and tail, reaching amazing sizes.

Asteroids are cosmic bodies of the Solar System, also called minor planets. Their main part is concentrated between Mars and Jupiter. They consist of iron and stone and are divided into two types: light and dark. The first of them are lighter, the second are heavier. Asteroids have irregular shapes. It is assumed that they were formed from the remains of cosmic matter after the formation of the main planets, or they are fragments of a planet located between Mars and Jupiter.

Some cosmic bodies reach the Earth, but when passing through thick layers of the atmosphere, they become heated during friction and are torn into small pieces. Therefore, relatively small meteorites fell on our planet. This phenomenon is by no means uncommon; asteroid fragments are stored in many museums around the world; they were found in 3,500 places.

There are not only large objects in space, but also tiny ones. For example, meteoroids are bodies up to 10 m in size. Cosmic dust is even smaller, up to 100 microns in size. It appears in the atmospheres of stars as a result of gas emissions or explosions. Not all cosmic bodies have been studied by scientists. These include black holes, which exist in almost every galaxy. They cannot be seen, only their location can be determined. Black holes have a very strong attraction, so they don't even let light escape from them. They annually absorb huge volumes of hot gas.

Cosmic bodies have different shapes, sizes, locations in relation to the Sun. Some of them are combined into separate groups to make them easier to classify. For example, asteroids located between the Kuiper belt and Jupiter are called Centaurs. Vulcanoids are believed to be located between the Sun and Mercury, although no objects have yet been discovered.

PLAN

Introduction

1. Asteroids

2. Meteorites

3. Small fragments

5. Search for planets in the solar system

Literature

Introduction

In addition to the large planets and their satellites, many so-called small bodies move in the Solar System: asteroids, comets and meteorites. Small bodies of the Solar System range in size from hundreds of microns to hundreds of kilometers.

Asteroids. From the point of view of physics, asteroids or, as they are also called, small planets, are dense and durable bodies. Based on their composition and properties, they can be divided into three groups: stone, iron-stone and iron. An asteroid is a cold body. But it, like the Moon, reflects sunlight, and therefore we can observe it in the form of a star-shaped object. This is where the name “asteroid” comes from, which in Greek means star-shaped. Since asteroids move around the Sun, their position in relation to the stars is constantly and quite quickly changing. Based on this initial sign, observers discover asteroids.

Comets, or "tailed stars", have been known since time immemorial. Comet is complicated physical phenomenon, which can be briefly described using several concepts. The comet's nucleus is a mixture or, as they say, a conglomerate of dust particles, water ice and frozen gases. The ratio of dust to gas content in cometary nuclei is approximately 1:3. The sizes of cometary nuclei, according to scientists, range from 1 to 100 km. The possibility of the existence of both smaller and larger nuclei is now being discussed. Known short-period comets have nuclei ranging in size from 2 to 10 km. The size of the nucleus of the brightest comet Haley-Bopp, which was observed with the naked eye in 1996, is estimated at 40 km.

A meteoroid is a small body orbiting the Sun. A meteor is a meteoroid that flew into the atmosphere of a planet and became heated to the point of brilliance. And if its remnant fell on the surface of the planet, it is called a meteorite. A meteorite is considered to have “fallen” if there are eyewitnesses who observed its flight in the atmosphere; otherwise it is called "found".

Let us consider the above small bodies of the Solar System in more detail.

1. Asteroids

These cosmic bodies differ from planets primarily in their size. Thus, the largest of the small planets, Ceres, has a diameter of 995 km; the next one (in size): Palada - 560 km, Hygea - 380 km, Psyche - 240 km, etc. For comparison, we can point out that the smallest of the major planets, Mercury, has a diameter of 4878 km, i.e. 5 times larger than the diameter of Ceres, and their masses differ many hundreds of times.

The total number of small planets accessible to observation by modern telescopes is determined to be 40 thousand, but their total mass is 1 thousand times less than the mass of the Earth.

The movement of small planets around the Sun occurs in elliptical orbits, but more elongated (the average eccentricity of their orbits is 0.51) than that of the large planets, and the inclination of their orbital planes to the ecliptic is greater than that of the large planets (the average angle is 9.54) . The bulk of the planets revolve around the Sun between the orbits of Mars and Jupiter, forming the so-called asteroid belt. But there are also small planets whose orbits are closer to the Sun than the orbit of Mercury. The most distant ones are located behind Jupiter and even behind Saturn.

Space researchers have expressed various ideas about the reason for the large concentration of asteroids in the relatively narrow space of the interplanetary medium between the orbits of Mars and Jupiter. One of the most common hypotheses for the origin of the bodies of the asteroid belt is the idea of ​​​​the destruction of the mythical planet Phaethon. The very idea of ​​the existence of a planet is supported by many scientists and even seems to be supported by mathematical calculations. However, the reason for the destruction of the planet remains inexplicable. Various assumptions have been made. Some researchers believe that the destruction of Phaeton occurred as a result of its collision with some large body. According to others, the reasons for the collapse of the planet were explosive processes in its bowels. Currently, the problem of the origin of bodies in the asteroid belt is an integral element of an extensive program of space research at the international and national levels.

Among the small planets, there is a peculiar group of bodies whose orbits intersect with the Earth’s orbit, and therefore, there is a potential possibility of their collision with it. The planets of this group began to be called Apollo objects, or simply Apollo (Wetherill, 1979). The existence of Apollo first became known in the 30s of this century. In 1932, an asteroid was discovered. He was named

Apollo 1932 HA. But it did not arouse much interest, although its name became a household name for all asteroids crossing the earth's orbit.

In 1937, a cosmic body with a diameter of approximately 1 km passed 800 thousand km from the Earth and twice the distance from the Moon. Subsequently he was named Hermes. To date, 31 such bodies have been identified, and each of them has received its own name. Their diameters range from 1 to 8 km, and the inclination of their orbital planes to the ecliptic ranges from 1 to 68. Five of them orbit between Earth and Mars, and the remaining 26 orbit between Mars and Jupiter (Wetherill, 1979). It is believed that out of 40 thousand small planets in the asteroid belt with a diameter of more than 1 km, there may be several hundred Apollos. Therefore, the collision of such celestial bodies with the Earth is quite likely, but at very long intervals.

It can be assumed that once a century one of these cosmic bodies may pass near the Earth at a distance less than from us to the Moon, and once in 250 thousand years it may collide with our planet. The impact of such a body releases energy equal to 10 thousand. Hydrogen bombs each with a power of 10 Mt. This should form a crater with a diameter of about 20 km. But such cases are rare and unknown in human history. Hermes belongs to class III asteroids, but there are many such bodies of larger size - classes II and I. The impact of their collision with the Earth, naturally, will be even more significant.

When Uranus was discovered in 1781, its average heliocentric distance turned out to correspond to the Titius-Bode rule, then in 1789 the search began for a planet which, according to this rule, should have been located between the orbits of Mars and Jupiter, at an average distance a = 2, 8 a.u. from the sun. But scattered surveys of the sky did not bring success, and therefore on September 21, 1800, several German astronomers, led by K. Zach, decided to organize a collective search. They divided the entire search for zodiac constellations into 24 sections and distributed them among themselves for thorough research. But before they had time to begin a systematic search, on January 1, 1871. Italian astronomer G. Piazii (1746-1826) discovered through a telescope a star-shaped object of the seventh magnitude, slowly moving across the constellation Taurus. The orbit of the object calculated by K. Gaus (1777-1855) turned out to be a planet corresponding to the Titius-Bode rule: semimajor axis a = 2.77 AU. and eccentricity e=0.080. Piatsi named the newly discovered planet Ceres.

On March 28, 1802, the German doctor and astronomer W. Olbers (1758-1840) discovered another planet (8m) near Ceres, called Pallas (a = 2.77 AU, e = 0.235). On September 2, 1804, the third planet, Juno (a=2.67 AU), was discovered, and on March 29, 1807, the 4th, Vesta (a=2.36 AU). All newly discovered planets had a star-shaped appearance, without disks, indicating their small geometric dimensions. Therefore these celestial bodies called small planets or, at the suggestion of V. Herschel, asteroids (from the Greek “aster” - star and “eidos” - species).

By 1891, about 320 asteroids had been discovered by visual methods. At the end of 1891, the German astronomer M. Wolf (1863-1932) proposed a photographic search method: with a 2-3 hour exposure, the images of stars on the photographic plate were dotted, and the trace of a moving asteroid was in the form of a small dash. Photographic techniques have led to a dramatic increase in asteroid discoveries. Particularly intensive studies of small planets are now being carried out at the Institute of Theoretical Astronomy (in St. Petersburg) and at the Crimean Astrophysical Observatory of the Russian Academy of Sciences.

Asteroids whose orbits are reliably determined are given a name and a serial number. There are now over 3,500 such asteroids known, but there are many more in the Solar System.

From the specified number known asteroids astronomers of the Crimean Astrophysical Observatory discovered about 550, immortalizing the names of famous people in their names.

The vast majority (up to 98%) of known asteroids move between the orbits of Mars and Jupiter, at average distances from the Sun from 2.06 to 4.30 AU. (circulation periods from 2.96 to 8.92 years). However, there are asteroids with unique orbits, and they are given masculine names, usually from Greek mythology.

The first three of these minor planets move outside the asteroid belt, and at perihelion Icarus approaches the Sun twice as close as Mercury, and Hermes and Adonis as close as Venus. They can approach the Earth at a distance of 6 million to 23 million km, and Hermes in 1937 passed close to the Earth even at a distance of 580 thousand km, i.e. only one and a half times further than the Moon. At aphelion, Hidalgo goes beyond the orbit of Saturn. But Hidalgo is no exception. Behind last years About 10 asteroids have been discovered, the perihelia of which are located near the orbits of the planets terrestrial group, and aphelion - near the orbits of Jupiter. Such orbits are characteristic of Jupiter-family comets and indicate a possible common origin of asteroids and comets.

In 1977, a unique asteroid was discovered that revolves around the Sun in an orbit with a semi-major axis a = 13.70 AU. and eccentricity e=0.38, so that at perihelion (q=8.49 AU) it enters the orbit of Saturn, and at aphelion (Q=18.91 AU) it approaches the orbit of Uranus. He is named Chiron. Apparently, there are other similar distant asteroids, the search for which continues.

The brightness of most known asteroids during opposition is from 7 m to 16 m, but there are also fainter objects. The brightest (up to 6 m) is Vesta.

The diameters of asteroids are calculated by their brightness and reflectivity in visual and infrared rays. It turned out that large asteroids not so much. The largest are Ceres (1000 km across), Pallas (610 km), Vesta (540 km) and Hygia (450 km). Only 14 asteroids have diameters of more than 250 km, while the rest have smaller diameters, down to 0.7 km. Bodies of such small sizes cannot have a spheroidal shape, and all asteroids (except, perhaps, the largest) are shapeless blocks.

The masses of asteroids are extremely different: the largest is close to 1.5 . 10 21 kg (i.e. 4 thousand times less than the mass of the earth), Ceres has. The total mass of all asteroids does not exceed 0.001 Earth masses. Of course, all these celestial bodies are devoid of atmosphere. Axial rotation has been detected in many asteroids based on regular changes in their brightness.

In particular, the rotation period of Ceres is 9.1 hours, and that of Pallas is 7.9 hours.

Icarus rotates the fastest, in 2 hours 16 meters.

The study of the reflectivity of many asteroids made it possible to combine them into three main groups: dark, light and metallic. The surface of dark asteroids reflects only up to 5% of the sunlight falling on it and consists of substances similar to black basalt and carbonaceous rocks. These asteroids are often called carbonaceous. Light asteroids reflect from 10% to 25% of sunlight, which makes their surface similar to silicon compounds - these are rocky asteroids. Metallic asteroids (their absolute minority) are also light, but in their reflective properties their surface is similar to iron-nickel alloys. This division of asteroids is also confirmed by the chemical composition of meteorites falling on Earth. A small number of studied asteroids do not belong to any of the three main groups.

It is significant that the absorption band of water (l = 3 µm) was detected in the spectra of carbonaceous asteroids. In particular, the surface of the asteroid Ceres consists of minerals similar to earthly clays and containing about 10% water.

With small sizes and masses of asteroids, the pressure in their interiors is low: even for the largest asteroids it does not exceed 7 10 5

8 10 5 GPa (700 - 800 atm) and cannot cause heating of their cold solid interior. Only the surface of asteroids is very slightly heated by the distant Sun, but even this insignificant energy is radiated into interplanetary space. The surface temperature of the vast majority of asteroids, calculated according to the laws of physics, turned out to be close to 150 - 170 K (-120...-100°C).

And only a few asteroids that pass close to the Sun have a very hot surface during such periods. Thus, the surface temperature of Icarus rises to almost 1000 K (+730 ° C), and with distance from the Sun it sharply decreases again.

The orbits of the remaining asteroids are subject to significant disturbances from the gravitational influence of large planets, mainly Jupiter. Small asteroids experience especially strong disturbances, which leads to collisions of these bodies and their fragmentation into fragments of a wide variety of sizes - from hundreds of meters in diameter to dust particles.

Currently, the physical nature of asteroids is being studied because it can be used to trace the evolution (development) of the matter from which the Solar System was formed.

2. Meteorites

A variety of meteoroids (cosmic fragments of large asteroids and comets) move in near-Earth space. Their speeds range from 11 to 72 km/s. It often happens that their paths of movement intersect with the Earth’s orbit and they fly into its atmosphere.

Meteorites are stone or iron bodies falling to Earth from interplanetary space. The fall of meteorites to Earth is accompanied by sound, light and mechanical phenomena. A bright fireball called a fireball rushes across the sky, accompanied by a tail and flying sparks. After the fireball disappears, a few seconds later there are explosion-like impacts called shock waves, which sometimes cause significant shaking of the ground and buildings.

The phenomena of intrusion of cosmic bodies into the atmosphere have three main stages:

1. Flight in a rarefied atmosphere (up to altitudes of about 80 km), where the interaction of air molecules is carpuscular in nature. Air particles collide with the body, stick to it or are reflected and transfer part of their energy to it. The body heats up from the continuous bombardment of air molecules, but does not experience noticeable resistance, and its speed remains almost unchanged. At this stage, however, the outer part of the cosmic body is heated to a thousand degrees or more. Here, the characteristic parameter of the problem is the ratio of the mean free path to the size of the body L, which is called the Knudsen number K n. In aerodynamics, it is customary to take into account the molecular approach to air resistance at K n >0.1.

2. Flight in the atmosphere in the mode of continuous flow of air around the body, that is, when the air is considered a continuous medium and the atomic-molecular nature of its composition is clearly not taken into account. At this stage, a head shock wave appears in front of the body, followed by a sharp increase in pressure and temperature. The body itself is heated due to convective heat transfer, as well as due to radiation heating. Temperatures can reach several tens of thousands of degrees, and pressures up to hundreds of atmospheres. When braking sharply, significant overloads appear. Deformations of bodies, melting and evaporation of their surfaces, and mass entrainment by the incoming air flow (ablation) occur.

3. When approaching the surface of the Earth, the air density increases, the resistance of the body increases, and it either practically stops at some height, or continues its path until it directly collides with the Earth. In this case, large bodies are often divided into several parts, each of which falls separately to the Earth. With strong deceleration of the cosmic mass above the Earth, the accompanying shock waves continue their movement to the Earth's surface, are reflected from it and produce disturbances in the lower layers of the atmosphere, as well as the Earth's surface.

The fall process of each meteoroid is individual. There is no possibility in short story describe all possible features of this process.

There are significantly more “found” meteorites than “fallen” ones. They are often found by tourists or peasants working in the fields. Since meteorites are dark in color and easily visible in the snow, great place The Antarctic ice fields, where thousands of meteorites have already been found, are used to search for them. The meteorite was first discovered in Antarctica in 1969 by a group of Japanese geologists studying glaciers. They found 9 fragments lying nearby, but belonging to four different types of meteorites. It turned out that meteorites that fell on the ice in different places, gather where glacial fields moving at a speed of several meters per year stop, resting against mountain ranges. The wind destroys and dries the upper layers of ice (dry sublimation occurs - ablation), and meteorites concentrate on the surface of the glacier. Such ice has a bluish color and is easily visible from the air, which is what scientists use when studying places that are promising for collecting meteorites.

An important meteorite fall occurred in 1969 in Chihuahua (Mexico). The first of many large fragments was found near a house in the village of Pueblito de Allende, and, following tradition, all the found fragments of this meteorite were united under the name Allende. The fall of the Allende meteorite coincided with the start of the Apollo lunar program and gave scientists the opportunity to develop methods for analyzing extraterrestrial samples. In recent years, some meteorites containing white debris embedded in darker parent rock have been identified as lunar fragments.

The Allende meteorite is a chondrite, an important subgroup of stony meteorites. They are called so because they contain chondrules (from the Greek chondros, grain) - the oldest spherical particles that condensed in a protoplanetary nebula and then became part of later rocks. Such meteorites make it possible to estimate the age of the Solar System and its original composition. The calcium- and aluminum-rich inclusions of the Allende meteorite, the first to condense due to their high boiling point, have a radioactive decay age of 4.559 ± 0.004 billion years. This is the most accurate estimate of the age of the solar system. In addition, all meteorites carry “historical records” caused by the long-term influence of galactic cosmic rays, solar radiation and solar wind. Having examined the damage caused cosmic rays, we can say how long the meteorite remained in orbit before it came under the protection of the earth's atmosphere.

The direct connection between meteorites and the Sun follows from the fact that the elemental composition of the oldest meteorites - chondrites - exactly repeats the composition of the solar photosphere. The only elements whose contents differ are volatile ones, such as hydrogen and helium, which evaporated abundantly from meteorites during their cooling, as well as lithium, which was partially “burned” in the Sun in nuclear reactions. Concepts " solar composition" and "chondrite composition" are used interchangeably when describing the above-mentioned "recipe for solar matter". Stony meteorites whose composition differs from that of the sun are called achondrites.

3. Small fragments.

The near-solar space is filled with small particles, the sources of which are the collapsing nuclei of comets and collisions of bodies, mainly in the asteroid belt. The smallest particles gradually approach the Sun as a result of the Poynting-Robertson effect (it consists in the fact that the pressure of sunlight on a moving particle is not directed exactly along the Sun-particle line, but as a result of light aberration is deflected back and therefore slows down the movement of the particle). The fall of small particles on the Sun is compensated by their constant reproduction, so that in the ecliptic plane there is always an accumulation of dust that scatters the sun's rays. On the darkest nights, it is noticeable in the form of the zodiacal light, stretching in a wide strip along the ecliptic in the west after sunset and in the east before sunrise. Near the Sun, the zodiacal light turns into a false corona ( F-corona, from false - false), which is visible only during a total eclipse. With increasing angular distance from the Sun, the brightness of the zodiacal light quickly decreases, but at the antisolar point of the ecliptic it intensifies again, forming counterradiance; this is caused by the fact that small dust particles intensely reflect light back.

From time to time, meteoroids enter the Earth's atmosphere. The speed of their movement is so high (on average 40 km/s) that almost all of them, except the smallest and largest, burn up at an altitude of about 110 km, leaving long luminous tails - meteors, or shooting stars. Many meteoroids are associated with the orbits of individual comets, so meteors are observed more often when the Earth passes near such orbits at certain times of the year. For example, many meteors are observed around August 12 each year as Earth crosses the Perseid shower, associated with particles lost by comet 1862 III. Another shower, the Orionids, around October 20 is associated with dust from Comet Halley.

Particles smaller than 30 microns can slow down in the atmosphere and fall to the ground without burning up; such micrometeorites are collected for laboratory analysis. If particles of several centimeters or more in size consist of a fairly dense substance, then they also do not burn entirely and fall to the surface of the Earth in the form of meteorites. More than 90% of them are stone; Only a specialist can distinguish them from earthly rocks. The remaining 10% of meteorites are iron (they are actually an alloy of iron and nickel).

Meteorites are considered to be asteroid fragments. Iron meteorites were once part of the cores of these bodies, destroyed by collisions. It is possible that some loose, volatile-rich meteorites originated from comets, but this is unlikely; Most likely, large particles of comets burn up in the atmosphere, and only small ones are preserved. Considering how difficult it is for comets and asteroids to reach Earth, it is clear how useful it is to study meteorites that independently “arrived” to our planet from the depths of the solar system.

4. Comets

Comets are the most effective celestial bodies in the solar system. Comets are a kind of cosmic icebergs consisting of frozen gases, a complex chemical composition, water ice and refractory mineral matter in the form of dust and larger fragments.

Although comets, like asteroids, move around the Sun in conical curves, their appearance is strikingly different from asteroids. If asteroids shine with reflected sunlight and in the field of view of a telescope resemble slowly moving faint stars, then comets intensively scatter sunlight in some of the most characteristic parts of the spectrum for comets, and therefore many comets are visible to the naked eye, although the diameters of their nuclei rarely exceed 1 - 5 km .

Comets are of interest to many scientists: astronomers, physicists, chemists, biologists, gas dynamics, historians, etc. And this is natural. After all, comets told scientists that the solar wind was blowing in interplanetary space; perhaps comets are the “culprits” for the emergence of life on Earth, since they could have introduced complex organic compounds into the Earth’s atmosphere. In addition, comets apparently carry valuable information about the initial stages of the protoplanetary cloud from which the Sun and planets also formed.

When first meeting a bright comet, it may seem that the tail is the most important part of the comet. But if in the etymology of the word “comet” the tail was the main reason for such a name, then from a physical point of view the tail is a secondary formation that developed from a rather tiny nucleus, the most important part of the comet as a physical object. Comet nuclei are the root cause of the rest of the complex of cometary phenomena, which are still not accessible to telescopic observations, since they are veiled by the luminous matter surrounding them, continuously flowing from the nuclei. Using high magnifications, you can look into the deeper layers of the gas-dust shell glowing around the core, but what remains will still be significantly larger in size than the true size of the core. The central condensation visible in the diffuse atmosphere of the comet visually and in photographs is called the photometric nucleus. It is believed that in its center there is the actual nucleus of the comet, i.e. The center of mass of the comet is located.

The hazy atmosphere surrounding the photometric core and gradually fading away, merging with the background of the sky, is called coma. The coma and nucleus make up the head of the comet. Far from the Sun, the head looks symmetrical, but as it approaches the Sun, it gradually becomes oval, then the head lengthens even more, and a tail develops from it on the side opposite to the Sun.

So, the nucleus is the most important part of the comet. However, there is still no consensus on what it actually is. Even in the times of Bessel and Laplace, there was an idea of ​​the comet's nucleus as a solid body consisting of easily evaporating substances such as ice or snow, which quickly transform into the gas phase under the influence of solar heat. This icy classical model of the cometary nucleus has been significantly expanded and developed recently. The model of the nucleus developed by Whipple, a conglomerate of refractory rocky particles and frozen volatile components (CH4, CO2, H2O, etc.), is the most widely recognized among comet researchers. In such a core, ice layers of frozen gases alternate with dust layers. As the sun's heat warms it, gases such as evaporating "dry ice" burst out, carrying clouds of dust with it. This allows, for example, to explain the formation of gas and dust tails in comets, as well as the ability of small comet nuclei to actively release gases.

The heads of comets take on a variety of shapes as comets move in orbit. Far from the SUN, the heads of comets are round, which is explained by the weak influence of solar radiation on the particles of the head, and its outlines are determined by the isotropic expansion of the cometary gas into interplanetary space. These are tailless comets that resemble globular star clusters in appearance. As it approaches the Sun, the comet's head takes the shape of a parabola or chain line. The parabolic shape of the head is explained by the “fountain” mechanism. The formation of heads in the form of a chain line is associated with the plasma nature of the cometary atmosphere and the influence of the solar wind on it and the magnetic field transferred by it.

Sometimes the comet's head is so small that the comet's tail appears to emerge directly from the nucleus. In addition to changing outlines, various structural formations appear and disappear in the heads of comets: tacks, shells, rays, outpourings from the nucleus, etc.

Large comets with tails stretching far across the sky have been observed since ancient times. It was once assumed that comets were atmospheric phenomena. This misconception was refuted by Brahe, who discovered that the comet of 1577 occupied the same position among the stars when observed from different points, and, therefore, was further from us than the Moon.

The movement of comets across the sky was first explained by Halley (1705), who found that their orbits were close to parabolas. He identified 24 orbits bright comets, and it turned out that the comets of 1531 and 1682 have very similar orbits. From this, Halley concluded that this is the same comet, which moves around the Sun in a very elongated ellipse with a period of about 76 years. Halley predicted that it should appear again in 1758, and in December 1758 it was actually discovered. Halley himself did not live to see this time and could not see how brilliantly his prediction was confirmed. This comet (one of the brightest) was named Halley's Comet.

Comets are designated by the names of the people who discovered them. In addition, a newly discovered comet is assigned a provisional designation based on the year of discovery, with the addition of a letter indicating the sequence of the comet's passage through perihelion in a given year.

Only a small part of comets observed annually are periodic, i.e. known from their previous appearances. Most of comets move in very elongated ellipses, almost parabolas. Their periods of revolution are not precisely known, but there is reason to believe that they reach many millions of years. Such comets move away from the Sun at distances comparable to interstellar ones. The planes of their almost parabolic orbits are not concentrated towards the ecliptic plane and are randomly distributed in space. The forward direction of movement occurs as often as the reverse.

Periodic comets move in less elongated elliptical orbits and have completely different characteristics. Of the 40 comets observed more than once, 35 have orbits inclined less than 45° to the ecliptic plane. Only Halley's comet has an orbit with an inclination greater than 90^ and, therefore, moves in reverse direction. Among short-period (i.e., having periods of 3 - 10 years) comets, the “Jupiter family” stands out. large group comets whose aphelions are the same distance from the Sun as the orbit of Jupiter. It is assumed that the “Jupiter family” was formed as a result of the planet’s capture of comets that had previously moved in more elongated orbits. Depending on the relative position of Jupiter and the comet, the eccentricity of the comet's orbit can either increase or decrease. In the first case, there is an increase in the period or even a transition to a hyperbolic orbit and the loss of the comet by the Solar System; in the second, a decrease in the period.

The orbits of periodic comets are subject to very noticeable changes. Sometimes a comet passes near the Earth several times, and then, by the attraction of the giant planets, it is thrown into a more distant orbit and becomes unobservable. In other cases, on the contrary, a comet that has never been observed before becomes visible because it passed near Jupiter or Saturn and abruptly changed its orbit. In addition to such abrupt changes, known only for a limited number of objects, the orbits of all comets experience gradual changes.

Orbital changes are not the only possible reason disappearance of comets. It has been reliably established that comets are quickly destroyed. The brightness of short-period comets fades over time, and in some cases the destruction process has been observed almost directly. A classic example is Comet Biely. It was discovered in 1772 and observed in 1813, 1826 and 1832. In 1845, the size of the comet turned out to be increased, and in January 1846. Observers were surprised to find two very close comets instead of one. The relative movements of both comets were calculated, and it turned out that Comet Biely split into two about a year ago, but at first the components were projected on top of each other, and the separation was not immediately noticed. Comet Biely was observed one more time, with one component much fainter than the other, and it could not be found again. But it was observed several times meteor shower, whose orbit coincided with the orbit of Comet Biele.

When deciding the question of the origin of comets, one cannot do without knowledge of the chemical composition of the substance from which the cometary nucleus is composed. It would seem, what could be simpler? We need to photograph more spectra of comets, decipher them - and the chemical composition of cometary nuclei will immediately become known to us. However, the matter is not as simple as it seems at first glance. The spectrum of the photometric core can be simply the reflected solar spectrum or the emission molecular spectrum. The reflected solar spectrum is continuous and does not reveal anything about chemical composition the area from which it was reflected - the core or the dust atmosphere surrounding the core. The emission gas spectrum carries information about the chemical composition of the gas atmosphere surrounding the core, and also does not tell us anything about the chemical composition of the surface layer of the core, since molecules emitting in the visible region, such as C2, CN, CH, MH, OH, etc. are secondary, daughter molecules - “fragments” of more complex molecules or molecular complexes that make up the cometary nucleus. These complex parent molecules, evaporating into the perinuclear space, are quickly exposed to the destructive action of solar wind and photons, or decay or dissociate into simpler molecules, the emission spectra of which can be observed from comets. The parent molecules themselves produce a continuous spectrum.

The Italian Donati was the first to observe and describe the spectrum of the comet's head. Against the background of the faint continuous spectrum of comet 1864, he saw three wide luminous bands: blue, green and yellow. As it turned out, this confluence belonged to C2 carbon molecules, which found themselves in abundance in the cometary atmosphere. These emission bands of C2 molecules are called Swan bands, named after the scientist who studied the spectrum of carbon. First slit spectrogram of the head Great Comet 1881 was obtained by the Englishman Heggins, who discovered the radiation of the chemically active cyanogen radical CN in the spectrum.

Far from the Sun, at a distance of 11 AU, the approaching comet appears as a small nebulous speck, sometimes with signs of the beginning formation of a tail. The spectrum obtained from a comet located at such a distance, and up to a distance of 3-4 AU, is continuous, because at such large distances the emission spectrum is not excited due to weak photon and corpuscular solar radiation.

This spectrum is formed as a result of reflection of sunlight from dust particles or as a result of its scattering on polyatomic molecules or molecular complexes. At a distance of about 3 AU. from the Sun, i.e. When the cometary nucleus crosses the asteroid belt, the first emission band of the cyanogen molecule appears in the spectrum, which is observed in almost the entire head of the comet. At a distance of 2 AU The radiation of triatomic molecules C3 and NH3 is already excited, which are observed in a more limited region of the comet's head near the nucleus than the ever-increasing radiation of CN. At a distance of 1.8 AU carbon emissions appear - Swan stripes, which immediately become noticeable throughout the entire head of the comet: both near the nucleus and at the boundaries of the visible head.

The mechanism of the glow of cometary molecules was deciphered back in 1911. K. Schwarzschild and E. Kron, who, studying the emission spectra of Halley's comet (1910), came to the conclusion that the molecules of cometary atmospheres resonantly re-emit sunlight. This glow is similar to the resonant glow of sodium vapor in the famous experiments of Auda, who was the first to notice that when illuminated with light having the frequency of the yellow sodium doublet, the sodium vapor themselves begin to glow at the same frequency with a characteristic yellow light. This is a mechanism of resonant fluorescence, which is a frequent case of the more general mechanism of luminescence. Everyone knows the glow of fluorescent lamps above shop windows, in fluorescent lamps, etc. A similar mechanism causes gases in comets to glow.

To explain the glow of the green and red oxygen lines (similar lines are also observed in the spectra of auroras), various mechanisms were used: electron impact, dissociative recombination and photodissipation. Electron impact, however, cannot explain the higher intensity of the green line in some comets compared to the red line. Therefore, more preference is given to the photodissociation mechanism, which is supported by the brightness distribution in the comet's head. However, this issue has not yet been completely resolved and the search for the true mechanism of luminescence of atoms in comets continues. The question of the parent, primary molecules that make up the cometary nucleus still remains unresolved, and this question is very important, since it is the chemistry of the nuclei that predetermines the unusually high activity of comets, capable of developing gigantic atmospheres and tails from very small nuclei in size. the size of all known bodies in the solar system.

5. Search for planets in the solar system.

Suggestions have been made more than once about the possibility of the existence of a planet closer to the Sun than Mercury. Le Verrier (1811–1877), who predicted the discovery of Neptune, examined anomalies in the perihelion motion of Mercury's orbit and, on the basis of this, predicted the existence of a new unknown planet within its orbit. Soon a message about her observation appeared and the planet was even given a name - Vulcan. But the discovery was not confirmed.

In 1977, American astronomer Cowell discovered a very faint object, which was dubbed the “tenth planet.” But the object turned out to be too small for a planet (about 200 km). It was named Chiron and was classified among the asteroids, among which it was then the most distant: the aphelion of its orbit was removed at 18.9 AU. and almost touches the orbit of Uranus, and the perihelion lies just beyond the orbit of Saturn at a distance of 8.5 AU. from the sun. With an orbital inclination of only 7°, it can actually get close to Saturn and Uranus. Calculations show that such an orbit is unstable: Chiron will either collide with a planet or be thrown out of the solar system.

From time to time theoretical predictions about the existence of major planets beyond the orbit of Pluto, but so far they have not been confirmed. Analysis of cometary orbits shows that up to a distance of 75 AU. planets larger than Earth beyond Pluto, no. However, it is quite possible that there are a large number of small planets in this area, which are not easy to detect. The existence of this cluster of trans-Neptunian bodies has been suspected for a long time and even received a name - the Kuiper belt, after the famous American planetary explorer. However, it was only recently that the first objects were discovered in it. In 1992–1994, 17 minor planets were discovered beyond the orbit of Neptune. Of these, 8 move at distances of 40–45 AU. from the Sun, i.e. even beyond the orbit of Pluto.

Due to their great distance, the brightness of these objects is extremely weak; Only the largest telescopes in the world are suitable for searching for them. Therefore, only about 3 square degrees of the celestial sphere have been systematically examined so far, i.e. 0.01% of its area. Therefore, it is expected that beyond the orbit of Neptune there may be tens of thousands of objects similar to those discovered, and millions of smaller ones, with a diameter of 5–10 km. Judging by estimates, this cluster of small bodies is hundreds of times more massive than the asteroid belt located between Jupiter and Mars, but is inferior in mass to the giant Oort comet cloud.

Objects beyond Neptune are still difficult to classify as any class of small bodies in the Solar System - asteroids or comet nuclei. The newly discovered bodies are 100–200 km in size and have a rather red surface, indicating its ancient composition and the possible presence of organic compounds. Kuiper belt bodies have recently been discovered quite often (by the end of 1999, about 200 of them had been discovered). Some planetary scientists believe that it would be more accurate to call Pluto not “the smallest planet,” but “the largest body in the Kuiper belt.”

Literature

1. V.A. Brashtein “Planets and their observation” Moscow “Science” 1979.

2. S. Dole “Planets for People” Moscow “Science” 1974.

3. K.I. Churyumov “Comets and their observation” Moscow “Science” 1980.

4. E.L. Krinov “Iron Rain” Moscow “Science” 1981.

5. K.A. Kulikov, N.S. Sidorenkov “Planet Earth” Moscow “Science”

6. B.A. Vorontsov - Velyaminov “Essays on the Universe” Moscow “Science”

7. N.P. Erpyleev “Encyclopedic Dictionary of a Young Astronomer” Moscow “Pedagogy” 1986.

8. E.P. Levitan “Astronomy” Moscow “Enlightenment” 1994