Sun. Stars. Galaxies. Small bodies of the Solar system. Celestial bodies of the Solar System: new information about Saturn

The house in which we live is our solar system. It is not yet known whether we are alone in the Universe. Celestial bodies are scattered throughout the Cosmos, and life may well exist in its other manifestations not only on Earth. Solar heat gives birth to life on our planet, since the Sun is our only star.

Celestial bodies of our system

The sun is the center of our system. The movement of celestial bodies occurs around the Sun in separate orbits. They don't leak on planets. The sun, thanks to its reactions, heats the planets that revolve around it. All planets are large and have a spherical shape, which they acquired as a result of evolution.

Previously, astrologers assumed that there were only seven planets in the solar system. These are the Sun, Moon, Mercury, Venus, Mars, Jupiter and Saturn.

A long time ago, before the discovery of the solar system, people believed that the Earth was the center of everything and all cosmic celestial bodies, including the Sun, moved around it. Such a system was called geocentric.

In the 16th century, Nicolaus Copernicus proposed a new system for constructing the World, called heliocentric. Copernicus stated that the Sun, not the Earth, is at the center of the World. The change of day and night occurs due to the rotation of our planet around its own axis.

Other solar systems

The invention of the telescope allowed people to see for the first time that comets were moving across the sky, approaching the Earth, and then leaving it. Almost 20 centuries later, scientists have determined that cosmic celestial bodies are capable of rotating not only in orbit around the Earth or the Sun. This conclusion followed when the existence of

Are there other planetary systems around other stars? This is not yet known with absolute certainty, but there is no doubt about their existence.

In 1781, the discovery of the large and distant planet Uranus followed, i.e. There were not seven planets, and the system of cosmic hierarchy was revised.

For a long time, it was believed that the disintegration or formation of some planet between Mars and Jupiter gave birth to all asteroids. Today, scientists have identified more than 15,000 asteroids.

In recent years, celestial bodies have been discovered that are difficult to attribute to any particular class, comets or planets. These objects have very elongated orbits, but there are no signs of tail or comet activity.

Two types of planets

The planets of our system are classified into giants and terrestrials. The difference between the terrestrial planets is their high average density and solid surface. Mercury, compared to other planets, has a higher density due to its iron core, which makes up 60% of the mass of the entire planet. Venus is similar to Earth in mass and density.

The Earth differs from other planets in the rather complex structure of its mantle, the depth of which is 2900 km. Beneath it is a core, presumably metal. Mars has a relatively low density, and the mass of its core is no more than 20%.

Celestial bodies belonging to the group of giant planets have low density and a complex atmospheric chemical composition. These planets are made of gas and their chemical composition is close to that of the sun (hydrogen and helium).

Small bodies of the solar system

Introduction

Asteroids

Meteorites

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. A comet is a complex physical phenomenon that 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.

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 were called small planets or, at the suggestion of V. Herschel, asteroids (from the Greek “aster” - star and “eidos” - type).

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.

Of the indicated number of 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. In recent years, about 10 asteroids have been discovered, the perihelia of which are located near the orbits of the terrestrial planets, and the 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 there are not so many large asteroids. 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, up 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 ( = 3 µm) was discovered 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.

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. It is not possible to describe all possible features of this process in a short story.

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, Antarctic ice fields are an excellent place to look for them, where thousands of meteorites have already been found. 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 ice 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. By studying the damage caused by cosmic rays, we can tell how long the meteorite was 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. The terms “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.

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.

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.

SMALL BODIES OF THE SOLAR SYSTEM

Content

Introduction

Asteroids

Meteorites

Small fragments

5. Search for planets in the solar system

Literature

Introduction

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. Observers use this initial feature to discover asteroids.

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

Asteroids

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

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 space research program at the international and national levels

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. The sizes of their diameters range from 1 to 8 km, and the inclination of the orbital planes to the ecliptic ranges from 1 to 68. Five of them rotate in orbits between Earth and Mars, and the remaining 26 - between Mars and Jupiter (W etherill, 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 Apollo. Therefore, the collision of such celestial bodies with the Earth is quite likely, but at very long intervals of time

On March 28, 1802, the German doctor and astronomer W. Olbers (1758-1840) discovered another planet (8 m) 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 were called small planets or, at the suggestion of V. Herschel, asteroids (from the Greek “aster” - star and “eidos” - type)

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 much more in the Solar System

Of the indicated number of known asteroids, astronomers of the Crimean Astrophysical Observatory discovered about 550, immortalizing the names of famous people in their names

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

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

Icarus rotates the fastest, in 2 hours 16 m

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

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

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

Meteorites

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 car 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:

The fall process of each meteoroid is individual. It is not possible to describe all possible features of this process in a short story.

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, Antarctic ice fields are an excellent place to look for them, where thousands of meteorites have already been found. 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 ice 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 distinguishable 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, it has been established that some meteorites containing white fragments embedded in darker parent rock are 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. By studying the damage caused by cosmic rays, we can tell how long the meteorite was in orbit before it came under the protection of the Earth's atmosphere.

Small fragments.

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

Comets

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

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, is 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 determined the orbits of 24 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, the 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 that year

Only a small part of comets observed annually are periodic, i.e. known from their previous appearances. Most 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 the opposite direction. Among the short-period (i.e., having periods of 3–10 years) comets, the “Jupiter family” stands out, a large group of comets whose aphelions are removed from the Sun at the same distance 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. Apart from 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 for the 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 a meteor shower was repeatedly observed, the orbit of which coincided with the orbit of Comet Biely

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 the chemical composition of the region 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, CH, CH, MH, OH and others, 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. The first slit spectrogram of the head of the Great Comet 1881 was obtained by the Englishman Heggins, who discovered in the spectrum the radiation of the chemically active cyanide radical C N

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 N H3 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 C N. 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 the planet or be thrown out of the solar system

From time to time, theoretical predictions about the existence of large planets beyond the orbit of Pluto are published, but so far they have not been confirmed. Analysis of cometary orbits shows that up to a distance of 75 AU. There are no planets larger than Earth beyond Pluto. 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 correct to call Pluto not “the smallest planet,” but “the largest body in the Kuiper belt.”

Literature

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

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

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

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

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

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

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

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

20. Small bodies of the solar system

1. Asteroids

Minor planets, or asteroids, mostly orbit between Mars and Jupiter and are invisible to the naked eye. The first minor planet was discovered in 1801, and according to tradition it was called one of the names of Greco-Roman mythology - Ceres. Soon other small planets were found, called Pallas, Vesta And Juno. With the use of photography, fainter asteroids began to be discovered. Currently, more than 3,000 asteroids are known. Over billions of years, asteroids collide with each other from time to time. This idea is suggested by the fact that a number of asteroids are not spherical, but irregular in shape. The total mass of asteroids is estimated at only 0.1 Earth masses.

The brightest asteroid, Vesta, is no brighter than 6th magnitude. The largest asteroid is Ceres. Its diameter is about 800 km, and beyond the orbit of Mars, even with the strongest telescopes, nothing can be seen on such a small disk. The smallest known asteroids have diameters of only about a kilometer (Fig. 56). Of course, asteroids have no atmosphere. In the sky, small planets look like stars, which is why they were called asteroids, which translated from ancient Greek means “star-like.” They have a loop-like movement characteristic of planets against the background of the starry sky. The orbits of some asteroids have unusually large eccentricities. As a result, at perihelion they approach the Sun closer than Mars and Earth, and Icarus- closer than Mercury (Fig. 57). In 1968, Icarus approached the Earth at a distance of less than 10 million kilometers, but its insignificant gravity had no effect on the Earth. From time to time, Hermes, Eros and other minor planets come close to the Earth.

New asteroids are discovered every year. The discoverer has the right to choose the name of the planet he discovers, which is then approved by an international committee. Most often, asteroids are named after famous scientists, heroes, and artists. Thus, in 1978, an asteroid was discovered, which later received the name Voronvelia in honor of the author of this textbook.

2. Fireballs and meteorites

A fireball is a rather rare phenomenon - a fireball flying across the sky (Fig. 58). This phenomenon is caused by the intrusion of large solid particles called meteoroids into the dense layers of the atmosphere. Moving in the atmosphere, the particle heats up due to braking and an extensive luminous shell consisting of hot gases forms around it. Fireballs often have a noticeable angular diameter and are visible even during the day. Superstitious people mistook such fireballs for flying dragons with fire-breathing mouths. Due to strong air resistance, the meteor body often splits and falls to Earth in the form of fragments with a roar. The remains of meteoroids that fall to Earth are called meteorites.

A meteoroid body, which is small in size, sometimes evaporates entirely in the Earth's atmosphere. In most cases, its mass decreases greatly during the flight and only the remnants reach the Earth, usually having time to cool down when the escape velocity has already been extinguished by air resistance. Sometimes a whole meteor shower falls. During flight, meteorites melt and become covered with a black crust. One such “black stone” in Mecca is embedded in the wall of the temple and serves as an object of religious worship.

Three types of meteorites are known: stone, iron (Fig. 59) and iron-stone. Sometimes meteorites are found many years after they fell. Especially many iron meteorites have been found. In the USSR, a meteorite is the property of the state and must be submitted to scientific institutions for study. The age of meteorites is determined by the content of radioactive elements and lead. It varies, but the oldest meteorites are 4.5 billion years old. Some of the largest meteorites have a large crater and form meteorite craters, reminiscent of the moon. The largest well-preserved crater is located in Arizona (USA) (Fig. 60). Its diameter is 1200 m and its depth is 200 m. This crater apparently appeared about 5000 years ago. Traces of even larger and more ancient meteorite craters have been found. All meteorites are members of the solar system.

Judging by the fact that many small asteroids have been discovered that cross the orbit of Mars, one can think that meteorites are fragments of those asteroids that cross the orbit of the Earth. The structure of some meteorites suggests that they were subjected to high temperatures and pressures and, therefore, could exist in the depths of a collapsed planet or large asteroid.

A significantly smaller number of minerals were found in meteorites than in terrestrial rocks. This indicates the primitive nature of the meteorite substance. However, many of the minerals that make up meteorites are not found on Earth. For example, stony meteorites contain round grains - chondrules, the chemical composition of which is almost identical to that of the Sun. This most ancient substance provides information about the initial stage of the formation of the planets of the solar system.

3. Comets. Their opening and movement

Being in space far from the Sun, comets look like very faint, blurry, light spots, in the center of which is the nucleus. Only those comets that pass relatively close to the Sun become very bright and “tailed.” The appearance of a comet from the Earth also depends on the distance to it, the angular distance from the Sun, the light of the Moon, etc. Large comets - nebulous formations with a long pale tail - were considered harbingers of various misfortunes, wars, etc. Back in 1910 In Tsarist Russia, prayer services were served to ward off “God’s wrath in the form of a comet.”

For the first time, I. Newton calculated the orbit of a comet from observations of its movement against the background of stars and was convinced that, like the planets, it moved in the solar system under the influence of the gravity of the Sun. His contemporary, the English scientist E. Halley(1656-1742), having calculated the orbits of several previously appearing comets, suggested that in 1531, 1607 and 1682. the same comet was observed,

periodically returning to the Sun, and for the first time predicted its appearance. In 1758 (16 years after Halley's death), as predicted, the comet actually appeared and was named Halley's Comet. At aphelion it goes beyond the orbit of Neptune (Fig. 61) and after 75-76 years it returns to the Earth and the Sun. In 1986, it again passed at the shortest distance from the Sun. For the first time, automatic interplanetary stations equipped with various scientific equipment were sent to meet the comet.

Halley's Comet is one of the periodic comets. Many short-period comets are now known with orbital periods of three ( Comet Encke) up to ten years. Their aphelions lie near the orbit of Jupiter. The approach of comets to the Earth and their future apparent path across the sky are calculated in advance with great accuracy. Along with this, there are comets moving in very elongated orbits with long orbital periods. We mistake their orbits for parabolas, although in reality they appear to be very elongated ellipses, but it is not easy to distinguish these curves, knowing only a small segment of the path of comets near the Earth and the Sun. Most comets do not have a tail and are only visible through a telescope.

Every year, information appears about the discovery of several previously unknown comets, which are named after the name of the scientist who discovered them. About a thousand observed comets are cataloged.

4. Physical nature of comets

The small core, a fraction of a kilometer in diameter, is the only solid part of the comet, and almost all of its mass is concentrated in it. The mass of comets is extremely small and does not in any way affect the movement of the planets. The planets produce great disturbances in the movement of comets.

The comet's nucleus appears to consist of a mixture of dust grains, solid pieces of matter and frozen gases such as carbon dioxide, ammonia, and methane. As the comet approaches the Sun, the nucleus warms up and gases and dust are released from it. They create a gas shell - the head of the comet. The gas and dust that make up the head, under the influence of the pressure of solar radiation and corpuscular flows, form the tail of the comet, always directed in the direction opposite to the Sun (Fig. 62).

The closer a comet comes to the Sun, the brighter it is and the longer its tail due to its greater irradiation and intense release of gases. Most often it is straight, thin, and streamy. Large and bright comets sometimes have a wide, fan-curved tail (Fig. 63). Some tails reach the distance from the Earth to the Sun, and the head of a comet is the size of the Sun. With o removal from the Sun, the appearance and brightness of the comet change in the opposite order and the comet disappears from view, reaching the orbit of Jupiter.

The spectrum of the comet's head and tail usually has bright bands. Analysis of the spectrum shows that the comet's head consists mainly of carbon and cyanogen vapor, and its tail contains ionized molecules of carbon (II) monoxide (carbon monoxide). The spectrum of a comet's nucleus is a copy of the solar spectrum, i.e., the nucleus glows with reflected sunlight. The head and tail glow with a cold light, absorbing and then re-emitting solar energy (a type of fluorescence). At the Earth's distance from the Sun, the comet is no hotter than the Earth.

The outstanding Russian scientist F.A. Bredikhin (1831-1904) developed a method for determining the force acting on its particles by the curvature of the tail. He established a classification of comet tails and explained a number of phenomena observed in them on the basis of the laws of mechanics and physics. In recent years, it has become clear that the movement of gases in straight tails and kinks are caused by the interaction of ionized molecules of tail gases with an incident stream of particles (corpuscles) flying from the Sun, which is called the solar wind. The influence of the solar wind on the ions of the comet's tail exceeds their attraction by the Sun by thousands of times. Increased short-wave radiation from the Sun and corpuscular flows cause sudden bursts of brightness in comets.

And in our time, fears are sometimes expressed among the population that the Earth will collide with a comet. In 1910, the Earth passed through the tail of Comet Halley, which contains carbon monoxide. However, its admixture in the surface air could not be detected, since even in the head of the comet the gases are extremely rarefied. A collision between the Earth and a comet's nucleus is an extremely unlikely event. Perhaps such a collision was observed in 1908 as the fall of the Tunguska meteorite. At the same time, a powerful explosion occurred at an altitude of several kilometers, the air wave of which felled a forest over a huge area.

5. Meteors and meteor showers

It has long been noted that the nuclei of periodic comets are depleted, with each revolution they glow less and less. The division of cometary nuclei into parts has been observed more than once. This destruction was produced either by solar tides or by collisions with meteorite bodies. The comet, discovered by the Czech scientist Biela back in 1772, was observed during repeated returns with a seven-year period. In 1846, its core disintegrated and it turned into two faint comets, which were not observed after 1852. When in 1872, according to calculations, the disappeared comets were supposed to pass near the Earth, a shower of “falling stars” was observed. Since then, on November 27, this phenomenon has been repeated annually, although less spectacularly. Small solid particles of the disintegrated nucleus of the former comet Biela stretched along its orbit (Fig. 64), and when the Earth crosses their stream, they fly into its atmosphere. These particles cause the phenomenon of meteors in the atmosphere and are completely destroyed before reaching the Earth. A number of other meteor showers are known, the width of which, as a rule, is immeasurably greater than the size of the cometary nuclei that generated them.

Halley's Comet is associated with two meteor showers, one observed in May and the other in November.

By photographing the path of the same meteor in the starry sky, as it is projected for observers 20-30 km apart, the altitude at which the meteor appeared is determined. Most often, meteor bodies begin to glow at an altitude of 100-120 km and completely evaporate already at an altitude of 80 km. Their spectra show bright lines of iron, calcium, silicon, etc. Studying the spectra of meteors makes it possible to determine the chemical composition of solid particles that left the comet's nucleus. By photographing the flight of a meteor with a camera whose lens is blocked by a rotating shutter, an intermittent trace is obtained, from which one can evaluate the braking of the meteor by air.

The mass of meteoroids is on the order of milligrams, and the size is fractions of a millimeter. It is likely that meteoroids are porous particles filled with cometary ice that evaporates first.

It is also possible to determine the speed of meteors. Meteor bodies catching up with the Earth have speeds with which they fly into the atmosphere of at least 1 km/s, and those flying towards the Earth - up to 60-70 km/s.

Think about why the minimum and maximum speeds at which meteoroids meet the Earth have exactly these values.

The hot gases left by the meteor body form a luminous trail. A meteor particle ionizes the air along its path. A trail of ionized air reflects radio waves. This made it possible to use radar to study meteors.

Meteors sometimes appear to emerge from an area in the sky called the radiant of a meteor shower (Figure 65). This is the perspective effect. The paths of meteors flying in parallel directions, when continued, appear to converge in the distance, like railroad tracks. The radiant is located in the sky in the direction from which these meteoroids are flying. Each radiant occupies a certain position among the constellations and participates in the daily rotation of the sky. The position of the radiant determines the name of the meteor shower. For example, meteors observed on August 10-12, whose radiant is in the constellation Perseus, are called Perseids.

Observing meteor showers is an important scientific task that is quite feasible for schoolchildren. They contribute to the study of our atmosphere and the substance of disintegrated comets.

Exercise 17

1. After sunset there is a comet in the west. How is its tail directed relative to the horizon?

2. What is the major axis of the orbit of Comet Halley if its orbital period is 76 years?

3. How can you prove that stars really don’t fall from the sky?

4. The fireball, noticed at a distance of 0.5 km from the observer, had a visible disk half the size of the lunar one. What was its actual diameter? 5. Can a comet, periodically returning to the Sun, forever maintain its appearance unchanged?

Task 10

Suppose that Figure 63 is a tenfold enlargement of a photograph taken by a camera with a lens focal length of 10 cm. Estimate the length of the direct ray in the comet's tail in degrees, knowing that the images of the Moon and Sun (0.5°) on photographic film are equal to 1/114 of the focal length lens distance.