Comet (from other Greek hairy, shaggy) - small heavenly body, having a foggy appearance, revolving around the Sun in a conic section with a very stretched orbit. When approaching the Sun, a comet forms a coma and sometimes a tail of gas and dust.
Comets are divided according to the period of revolution into:
1. Short period
On this moment more than 400 short-period comets have been discovered. Of these, about 200 have been observed in more than one perihelion passage. Short-period comets (periods less than 200 years) come from the region of the outer planets, moving in a forward direction along orbits that lie close to the ecliptic. Away from the Sun, comets usually do not have "tails", but sometimes have a barely visible "coma" surrounding the "core"; together they are called the "head" of the comet. As it approaches the Sun, the head enlarges and the tail appears. Many of them are included in the so-called families. For example, most of the shortest period comets (their complete revolution around the Sun lasts 3-10 years) form the Jupiter family. Slightly smaller than the families of Saturn, Uranus and Neptune (the latter, in particular, includes the famous comet Halley).
Families:
- Jupiter family
- Saturn family
- Uranus family
- Neptune family
When a comet passes near the Sun, its core heats up, and the ice evaporates, forming a gaseous coma and tail. After several hundreds or thousands of such passages, no fusible substances remain in the core, and it ceases to be visible. For short-period comets regularly approaching the Sun, this means that in less than a million years their population should become invisible. But we observe them, therefore, replenishment from "fresh" comets constantly arrives.
Replenishment of short-period comets occurs as a result of their "capture" by planets, mainly Jupiter. Long-period comets from the Oort cloud were previously thought to be captured, but they are now thought to originate from a cometary disk called the "inner Oort cloud". In principle, the concept of the Oort cloud has not changed, however, calculations have shown that the tidal influence of the Galaxy and the impact massive clouds interstellar gas should destroy it rather quickly. You need a source to replenish it. Such a source is now considered to be the inner Oort cloud, which is much more resistant to tidal influence and contains an order of magnitude more comets than the outer cloud predicted by Oort. After each approach solar system with a massive interstellar cloud, comets from the outer Oort cloud scatter into interstellar space, and they are replaced by comets from the inner cloud.
The transition of a comet from an almost parabolic orbit to a short-period one occurs if it catches up with the planet from behind. It usually takes several passes through a planetary system to capture a comet into a new orbit. The resulting orbit of a comet typically has a small inclination and a large eccentricity. The comet moves along it in a forward direction, and the aphelion of its orbit (the point farthest from the Sun) lies near the orbit of the planet that captured it. These theoretical considerations are fully confirmed by the statistics of cometary orbits.
2. Long-term
Presumably, long-period comets fly to us from the Oort Cloud, which contains a huge number of cometary nuclei. Bodies located on the outskirts of the solar system, as a rule, consist of volatile substances (water, methane and other ices) that evaporate when approaching the Sun. Long-period comets (with an orbital period of more than 200 years) come from regions located thousands of times farther than the outermost planets, and their orbits are tilted at all sorts of angles.
Many comets belong to this class. Since their periods of revolution are millions of years, only one ten-thousandth part of them appears in the vicinity of the Sun during a century. About 250 such comets were observed in the 20th century; hence there are millions of them. In addition, not all comets approach the Sun enough to become visible: if the perihelion (the point closest to the Sun) of the comet's orbit lies beyond the orbit of Jupiter, then it is almost impossible to notice it.
Considering this, in 1950 Jan Oort suggested that the space around the Sun at a distance of 20-100 thousand a.u. (astronomical units: 1 AU = 150 million km, the distance from the Earth to the Sun) is filled with comet nuclei, the number of which is estimated at 10 12, and the total mass is 1–100 Earth masses. The outer boundary of the Oort "comet cloud" is determined by the fact that at this distance from the Sun, the movement of comets is significantly affected by the attraction of neighboring stars and other massive objects. Stars move relative to the Sun, their perturbing effect on comets changes, and this leads to the evolution of cometary orbits. So, by chance, a comet may be in an orbit passing near the Sun, but on the next revolution its orbit will change slightly, and the comet will pass far from the Sun. However, instead of it, “new” comets will constantly fall from the Oort cloud into the vicinity of the Sun.
Comets coming from the depths of space look like nebulous objects with trailing tails sometimes reaching millions of kilometers in length. The nucleus of a comet is a body of solid particles and ice, wrapped in a foggy shell called a coma. A nucleus with a diameter of several kilometers can have around it a coma 80,000 km across. Streams of sunlight knock gas particles out of the coma and throw them back, pulling them into a long smoky tail that follows her through space.
The brightness of comets depends very much on their distance from the Sun. Of all the comets, only a very small part comes close enough to the Sun and Earth to be seen. naked eye. The most notable ones are sometimes referred to as "great comets".
Many of the meteors we observe (“shooting stars”) are of cometary origin. These are particles lost by a comet that burn up when they enter the atmosphere of planets.
Orbit and speed
The motion of the comet's nucleus is completely determined by the attraction of the sun. The shape of a comet's orbit, like any other body in the solar system, depends on its speed and distance from the Sun. average speed body is inversely proportional square root from its average distance from the Sun (a). If the speed is always perpendicular to the radius vector directed from the Sun to the body, then the orbit is circular, and the speed is called the circular speed (υc) at a distance a. The speed of escape from the gravitational field of the Sun along a parabolic orbit (υp) is √2 times greater than the circular velocity at this distance. If the comet's speed is less than υp, then it moves around the Sun in an elliptical orbit and never leaves the solar system. But if the speed exceeds υp, then the comet passes the Sun once and leaves it forever, moving along a hyperbolic orbit. Most comets have elliptical orbits, so they belong to the solar system. True, for many comets these are very elongated ellipses, close to a parabola; According to them, comets leave the Sun very far and for a long time.
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COMETS IN THE SOLAR SYSTEM |
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The figure shows the elliptical orbits of two comets, as well as almost circular orbits of the planets and a parabolic orbit. At the distance that separates the Earth from the Sun, the circular speed is 29.8 km/s, and the parabolic speed is 42.2 km/s. Near the Earth, the speed of Encke's comet is 37.1 km/s, and the speed of Halley's comet is 41.6 km/s; that is why Halley's comet goes much farther from the Sun than Encke's comet.
The gaseous products of sublimation exert a reactive pressure on the comet's nucleus (similar to the recoil of a gun when fired), which leads to the evolution of the orbit. The most active outflow of gas occurs from the heated “afternoon” side of the core. Therefore, the direction of the pressure force on the core does not coincide with the direction of the sun's rays and solar gravity. If the axial rotation of the nucleus and its orbital circulation occur in the same direction, then the pressure of the gas as a whole accelerates the movement of the nucleus, leading to an increase in the orbit. If the rotation and reversal occur in opposite directions, then the comet's motion is slowed down, and the orbit is reduced. If such a comet was originally captured by Jupiter, then after some time its orbit is wholly in the region of the inner planets. This is probably what happened to Comet Encke.
Comet nomenclature
Over the past centuries, the rules for naming comets have been repeatedly changed and refined. Until the beginning of the 20th century, most comets were named after the year they were discovered, sometimes with additional clarifications regarding the brightness or season of the year if there were several comets that year. For example, "The Great Comet of 1680", "The Great September Comet of 1882", "The Daytime Comet of 1910" ("The Great January Comet of 1910").
After Halley proved that the comets of 1531, 1607 and 1682 were the same comet and predicted its return in 1759, the comet became known as Halley's Comet. Also, the second and third known periodic comets were named by Encke and Biela in honor of the scientists who calculated the orbit of comets, despite the fact that the first comet was observed by Méchain, and the second by Messier in the 18th century. Later, periodic comets were usually named after their discoverers. Comets that were observed in only one passage of perihelion continued to be named after the year of appearance.
At the beginning of the 20th century, when comet discoveries became a frequent event, a comet naming convention was developed that remains relevant to this day. A comet gets a name only after it is discovered by three independent observers. IN last years, many comets are being discovered with the help of instruments that serve large teams of scientists. In such cases, comets are named after their instruments. For example, comet C/1983 H1 (IRAS - Araki - Alcock) was independently discovered by the IRAS satellite and amateur astronomers Genichi Araki and George Alcock. In the past, if one group of astronomers discovered several comets, a number was added to the names (but only for periodic comets), such as Comets Shoemaker-Levy 1-9. Now a number of comets are opening near the instruments, which has made such a system impractical. Instead, a special comet naming system is used.
Prior to 1994, comets were first given temporary designations consisting of the year of their discovery and a lowercase Latin letter that indicates the order in which they were discovered in that year (for example, Comet 1969i was the ninth comet discovered in 1969). After a comet passed perihelion, its orbit was securely established, after which the comet received a permanent designation, consisting of the year of passage of perihelion and a Roman number indicating the order of passage of perihelion in a given year. Thus comet 1969i was given the permanent designation 1970 II (the second comet to pass perihelion in 1970).
As the number of discovered comets increased, this procedure became very inconvenient. In 1994 International astronomical union approved new system comet symbols. Now the name of the comet includes the year of discovery, the letter indicating the half of the month in which the discovery took place, and the number of the discovery in that half of the month. This system is similar to the one used for naming asteroids. Thus, the fourth comet, discovered in the second half of February 2006, receives the designation 2006 D4. The comet is preceded by a prefix indicating the nature of the comet. The following prefixes are used:
P/ is a short period comet (that is, a comet whose period is less than 200 years, or which has been observed in two or more perihelion passages);
C/ - long-period comet;
X/ - comet for which a reliable orbit could not be calculated (usually for historical comets);
D/ - comets collapsed or were lost;
A/ - objects that were mistaken for comets, but actually turned out to be asteroids.
For example, Comet Hale-Bopp was designated C/1995 O1. Usually, after the second observed passage of perihelion, periodic comets receive a serial number. So, Halley's Comet was first discovered in 1682. Its designation in that appearance by modern system- 1P/1682 Q1. Comets, which were first discovered as asteroids, retain letter designation. For example, P/2004 EW38 (Catalina-LINEAR).
The structure of comets
The comet is:
1. Core
2. Coma
3. Tail
The nucleus is in the center of the coma solid or a conglomerate of bodies with a diameter of several kilometers. Almost the entire mass of a comet is concentrated in its nucleus; this mass is billions of times smaller than the earth. According to F. Whipple's model, the nucleus of a comet consists of a mixture various ice, mostly water ice mixed with frozen carbon dioxide, ammonia and dust. This model is confirmed by both astronomical observations and direct measurements with spacecraft near the nuclei of comets Halley and Giacobini – Zinner in 1985–1986.
Comet nuclei are the remnants of the primary matter of the solar system, which made up the protoplanetary disk. Therefore, their study helps to restore the picture of the formation of planets, including the Earth. In principle, some comets could come to us from interstellar space, but so far no such comet has been reliably identified.
When a comet approaches the Sun, its core heats up and the ice sublimates, i.e. evaporate without melting. The resulting gas scatters in all directions from the nucleus, taking dust particles with it and creating a coma. Destroyed by sunlight, water molecules form a huge hydrogen corona around the comet's nucleus. In addition to solar attraction, the rarefied matter of the comet is also affected by repulsive forces, due to which a tail is formed. Neutral molecules, atoms and dust particles are affected by the pressure of sunlight, while ionized molecules and atoms are more strongly affected by the pressure of the solar wind.
The behavior of the particles that form the tail became much clearer after direct research comets in 1985–1986. The plasma tail, which consists of charged particles, has a complex magnetic structure with two regions of different polarity. On the side of the coma facing the Sun, a frontal shock wave is formed, which exhibits high plasma activity.
Although less than one millionth of the mass of a comet is contained in the tail and coma, 99.9% of the light comes from these gas formations, and only 0.1% from the nucleus. The fact is that the core is very compact and also has a low reflection coefficient (albedo).
The main gas components of comets are listed in descending order of their content. The movement of gas in comet tails shows that it is strongly influenced by non-gravitational forces. The glow of the gas is excited by solar radiation. |
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atoms |
molecules |
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GAS COMPONENTS OF A COMET |
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The particles lost by the comet move along their orbits and, falling into the atmospheres of the planets, become the cause of meteors (“shooting stars”). Most of the meteors we observe are associated precisely with cometary particles. Sometimes the destruction of comets is more catastrophic. Biela's comet, discovered in 1826, split into two parts in front of observers in 1845. When this comet was last seen in 1852, pieces of its nucleus were millions of kilometers apart. The fission of the nucleus usually heralds the complete disintegration of the comet. In 1872 and 1885, when Biela's comet, if nothing had happened to it, should have crossed the orbit of the Earth, unusually abundant meteor showers were observed.
Let's talk in more detail about each element of the structure of the comet:
CORE
The nucleus is the solid part of a comet, in which almost all of its mass is concentrated. Comet nuclei are currently inaccessible to telescopic observations, since they are hidden by continuously formed luminous matter.
According to the most common Whipple model, the core is a mixture of ice interspersed with particles of meteoric matter (the “dirty snowball” theory). With such a structure, layers of frozen gases alternate with dust layers. As the gases heat up, they evaporate and carry clouds of dust with them. This makes it possible to explain the formation of gas and dust tails in comets.
According to studies conducted with the help of the American automatic station Deep Impact launched in 2005, the core consists of very loose material and is a lump of dust with pores occupying 80% of its volume.
Comet nuclei are made of ice with the addition of space dust and frozen volatile compounds: carbon monoxide and dioxide, methane, ammonia.
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COMETS IN THE SOLAR SYSTEM |
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The core has a rather low albedo, about 4%. According to the main hypothesis, this is due to the presence of a dust matrix formed during the evaporation of ice, and the accumulation of dust particles on the surface, similar to how a layer of surface moraine grows during the retreat of glaciers on Earth. A study of Comet Halley by the Giotto probe revealed that it reflects only 4% of the light falling on it, and Deep Space 1 measured the albedo of Comet Borelli, which was only 2.5-3.0%. There are also assumptions that the surface is covered not with a dust matrix, but with a matrix of complex organic compounds, dark, like tar or bitumen. Hypothetically, on some comets, over time, activity may come to naught, with the cessation of sublimation.
On currently there are few comets whose nuclei have been observed directly. The use of spacecraft made it possible to study their coma and nuclei directly, and to obtain close-up images.
ENCOUNTER WITH THE COMET |
- Comet Halley was the first comet to be explored by spacecraft. On March 6 and 9, 1986, Vega-1 and Vega-2 passed at a distance of 8890 and 8030 km from the comet's nucleus. They transmitted 1500 images of the inner halo and, for the first time in history, photographs of the core, and made a number of instrumental observations. Thanks to their observations, it was possible to correct the orbit of the next spacecraft - the Giotto probe of the European Space Agency, thanks to which it was possible to fly even closer on March 14, at a distance of 605 km. Two Japanese vehicles also contributed to the study of the comet: Suisei (flight on March 8, 150 thousand km) and Sakigake (March 10, 7 million km, was used to guide the previous apparatus). All of these 5 spacecraft that explored Halley's comet during its passage in 1986 received the unofficial name "Halley's Armada". |
ENCOUNTER WITH THE COMET |
The size of the comet's nucleus can be estimated from observations at a time when it is far from the Sun and is not shrouded in a gas and dust shell. In this case, the light is reflected only by the solid surface of the nucleus, and its apparent brilliance depends on the cross-sectional area and the reflection coefficient (albedo).
Sublimation - the transition of matter from a solid to a gaseous state is important for the physics of comets. Measurements of the brightness and emission spectra of comets have shown that the melting of the main ices begins at a distance of 2.5–3.0 AU, as it should be if the ice is mostly water. This was confirmed by the study of comets Halley and Giacobini-Zinner. The gases observed first during the comet's approach to the Sun (CN, C 2) are probably dissolved in water ice and form gas hydrates (clathrates). How this "composite" ice will sublimate depends largely on the thermodynamic properties of the water ice. Sublimation of the dust-ice mixture occurs in several stages. Gas flows and small and fluffy dust particles picked up by them leave the core, since the attraction near its surface is extremely weak. But dense or heavy dust grains fastened together are not carried away by the gas flow, and a dust crust is formed. Then the sun's rays heat the dust layer, the heat passes inside, the ice sublimates, and gas flows break through, breaking the dust crust. These effects manifested themselves when Halley's comet was observed in 1986: sublimation and outflow of gas occurred only in a few regions of the comet's nucleus illuminated by the Sun. Probably, ice was exposed in these areas, while the rest of the surface was covered with crust. Escaped gas and dust form observable structures around the comet's nucleus.
COMA
Dust grains and gas from neutral molecules form an almost spherical comet coma. Usually a coma stretches from 100 thousand to 1 million km from the nucleus. Light pressure can deform the coma, stretching it in the antisolar direction.
A coma is a cup-shaped cloudy shell of light gases and dust. The coma, together with the nucleus, makes up the head of the comet. Most often, a coma consists of three main parts:
- Internal coma(molecular, chemical and photochemical). Here the most intense physical and chemical processes take place.
- Visible coma(radical coma).
- ultraviolet coma(atomic).
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Comet C/2001 Q4 (NEAT) |
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COMETS IN THE SOLAR SYSTEM |
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Since the ices of the core are mostly water, the coma also contains mostly H 2 O molecules. Photodissociation breaks H 2 O into H and OH, and then OH into O and H. Fast hydrogen atoms fly far from the core before they are ionized, and form a hydrogen corona, the visible size of which often exceeds the solar disk.
TAIL
The tail of a comet is an elongated plume of dust and gas of cometary matter, formed when a comet approaches the Sun and is visible due to the scattering of sunlight on it. Usually directed away from the Sun.
When a comet approaches the Sun, volatile substances with a low boiling point, such as water, monoxide, carbon monoxide, methane, nitrogen, and possibly other frozen gases, begin to sublimate from the surface of its nucleus. This process leads to the formation of a coma. Evaporation of this dirty ice releases dust particles that are carried by gas from the core. Gas molecules in a coma absorb sunlight and then re-emit it at different wavelengths (this phenomenon is called fluorescence), and dust particles scatter sunlight in different directions without changing the wavelength. Both of these processes lead to the fact that the coma becomes visible to an outside observer.
Despite the fact that less than one millionth of the comet's mass is concentrated in the tail and coma, almost 99.9% of the glow that we observe when the comet passes through the sky comes from these gas formations. The fact is that the core is very compact and has a low reflection coefficient (albedo).
Comet tails vary in length and shape. Some comets have them stretching across the sky. For example, the tail of a comet that appeared in 1944 was 20 million km long. Comet C/1680 V1 had a tail stretching for 240 million km. Cases of separation of the tail from the comet have also been recorded (C/2007 N3 (Lulin)).
The tails of comets do not have sharp outlines and are practically transparent - stars are clearly visible through them - as they are formed from an extremely rarefied substance (its density is much less than the density of gas released from a lighter). Its composition is diverse: gas or the smallest dust particles, or a mixture of both. The composition of most of the dust particles is similar to the asteroid material of the solar system, which was revealed as a result of the study of comet 81P / Wild by the Stardust spacecraft. In essence, it is "visible nothing": a person can observe the tails of comets only because the gas and dust glow. At the same time, the glow of the gas is associated with its ionization by ultraviolet rays and streams of particles ejected from the solar surface, and the dust simply scatters sunlight.
The theory of tails and forms of comets developed in late XIX century Russian astronomer Fyodor Bredikhin. He also owns the classification of comet tails, which is used in modern astronomy.
Bredikhin proposed to classify comet tails into three main types:
- I type. Straight and narrow, directed directly from the Sun;
- II type. Wide and slightly curved, deviating from the Sun;
- III type. Short, strongly deviated from the central luminary.
Astronomers explain such different forms of comet tails as follows. The particles that make up comets have different compositions and properties and respond differently to solar radiation. Thus, the paths of these particles in space "diverge", and the tails of space travelers take on different shapes.
The speed of a particle emitted from the nucleus of a comet is the sum of the speed acquired as a result of the action of the Sun - it is directed from the Sun to the particle, and the speed of the comet, the vector of which is tangent to its orbit, therefore, the particles emitted by a certain moment, in the general case, will not be located on a straight line, but on a curve called a syndynam. Syndynam will represent the position of the comet's tail at that moment in time. With separate sharp ejections, the particles form segments or lines on the syndynam at an angle to it, called synchronous. How far the comet's tail will differ from the direction from the Sun to the comet depends on the mass of the particles and the action of the Sun.
The action of solar radiation on a coma leads to the formation of a comet's tail. But here, too, dust and gas behave differently. Ultraviolet radiation from the sun ionizes some of the gas molecules, and the pressure of the solar wind, which is a stream of charged particles emitted by the Sun, pushes the ions, pulling the coma into a long tail that can be more than 100 million kilometers long. Changes in the solar wind flux can lead to observed rapid change the type of tail and even a complete or partial break. Ions are accelerated by the solar wind to speeds of tens and hundreds of kilometers per second, much greater than the speed of the comet's orbital motion. Therefore, their movement is directed almost exactly in the direction from the Sun, as is the Type I tail they form. The ion tails have a bluish glow due to fluorescence. The solar wind has almost no effect on cometary dust, it is pushed out of the coma by the pressure of sunlight. Dust is accelerated by light much weaker than ions by the solar wind, so its movement is determined by the initial orbital velocity of movement and acceleration under the action of light pressure. The dust lags behind the ion tail and forms tails II or III type. Type II tailings are formed by a uniform flow of dust from the surface. Type III tailings are the result of a short-term release of a large cloud of dust. Due to the spread of accelerations acquired by dust grains of different sizes under the action of the light pressure force, the initial cloud is also stretched into a tail, usually curved even more than a type II tail. Dust tails glow with diffused reddish light.
The dust tail is usually homogeneous and stretches for millions and tens of millions of kilometers. It is formed by dust grains pushed back by the pressure of sunlight from the nucleus in an anti-solar direction, and is yellowish in color because the dust grains simply scatter sunlight. The structures of the dust tail can be explained by the uneven eruption of dust from the core or the destruction of dust grains.
A plasma tail tens and even hundreds of millions of kilometers long is a visible manifestation of the complex interaction between a comet and the solar wind. Some molecules that have left the nucleus are ionized by solar radiation, forming molecular ions (H 2 O+, OH+, CO+, CO 2 +) and electrons. This plasma impedes the movement of the solar wind penetrated by the magnetic field. When hitting a comet, the field lines wrap around it, taking the shape of a hairpin and forming two regions of opposite polarity. Molecular ions are trapped in this magnetic structure and form a visible plasma tail in the central, densest part of it, which has a blue color due to the spectral bands of CO +. The role of the solar wind in the formation of plasma tails was established by L. Birman and H. Alven in the 1950s. Their calculations confirmed measurements from spacecraft that flew through the tails of comets Giacobini-Zinner and Halley in 1985 and 1986.
Other phenomena of interaction with the solar wind, which hits the comet at a speed of about 400 km/s and forms in front of it, also occur in the plasma tail. shock wave, in which the matter of the wind and the head of the comet is compacted. The process of "capture" plays an essential role; its essence is that the comet's neutral molecules freely penetrate into the solar wind stream, but immediately after ionization they begin to actively interact with the magnetic field and are accelerated to significant energies. True, very energetic molecular ions are sometimes observed, which are inexplicable from the point of view of the indicated mechanism. The capture process also excites plasma waves in the giant volume of space around the nucleus. The observation of these phenomena is of fundamental interest for plasma physics.
A remarkable spectacle is the “tail break”. As is known, in the normal state, the plasma tail is connected to the head of the comet by a magnetic field. However, often the tail comes off the head and lags behind, and a new one forms in its place. This happens when a comet passes through the boundary of solar wind regions with oppositely directed magnetic fields. At this point, the magnetic structure of the tail is rearranged, which looks like a break and the formation of a new tail. Complex topology magnetic field leads to acceleration of charged particles; Perhaps this explains the appearance of the above-mentioned fast ions.
anti-tail is a term used in astronomy to describe one of the three kinds of tails that a comet has when it approaches the sun. The peculiarity of this tail is that, unlike the other two tails, dust and gas, it is directed towards the Sun, and not away from it, therefore it is geometrically opposite to other tails. The anti-tail consists of large dust particles, which, due to their mass and size, are weakly affected by the solar wind and, as a rule, remain in the plane of the comet's orbit, eventually taking the form of a disk. Due to the rather low concentration of dust particles, it is almost impossible to see this disk under normal conditions. Therefore, it can only be fixed edge-on when it is bright enough for observation. This becomes possible in a short period of time when the Earth crosses the plane of the comet's orbit. As a result, the disk becomes visible in the form of a small tail directed away from the Sun.
Since dust particles take the form of a disk, it is quite natural that the anti-tail exists not only in front, but also behind and on the sides of the comet. But on the sides of the comet, it is not visible due to the cometary nucleus, and behind it is lost behind denser and brighter dust and gas tails.
Most passing comets are too small to have an anti-tail, but there are comets large enough for this, such as Comet C/1995 O1 (Hale-Bopp) in 1997.
degenerate comet
A degenerate comet is a comet that has lost most of its volatiles and therefore no longer forms a tail or coma as it approaches the Sun. All the volatiles have already evaporated from the comet's nucleus, and the remaining rocks consist mainly of relatively heavy non-volatile elements, similar to those that are common on the surface of asteroids. Extinct comets are small dark celestial bodies that are very difficult to detect even with the most powerful telescopes.
For a comet to become extinct, it does not have to lose all of its volatiles: it is enough that they are sealed under a layer of sedimentary non-volatile compounds. Such layers can form if the composition of the comet's surface contains non-volatile compounds. When gases and other volatile substances evaporate, non-volatile compounds settle down and, accumulating, form a crust several centimeters thick, which, in the end, completely blocks access solar energy into the deep layers. As a result, solar heat can no longer break through this crust and heat them up to a temperature at which they would begin to evaporate - the comet becomes extinct. These types of comets are sometimes also called hidden or dormant comets. An example of such a body is the asteroid (14827) Hypnos.
The term sleeper comet is also used to describe inactive comets that can become active if they get close enough to the Sun. For example, during the passage of perihelion in 2008, the cometary activity of the asteroid (52872) Okiroya significantly intensified. And the asteroid (60558) Echeclus, after the appearance of a coma was recorded, also received the comet designation 174P / Echeclus.
When asteroids and comets were separated into two different classes, the main differences between these classes from each other were not formulated for a long time. It was possible to resolve this issue only in 2006 at the 26th General Assembly in Prague. The main difference between an asteroid and a comet was recognized to be that the comet, in the process of approaching the Sun, forms a coma around itself due to the sublimation of ice near the surface under the influence of solar radiation, while the asteroid never forms a coma. As a result, some objects received two designations at once, since at first they were classified as asteroids, but then, when cometary activity was detected, they also received a comet designation. Another difference is that comets tend to have more elongated orbits than most asteroids - hence "asteroids" with large orbital eccentricities are most likely the nuclei of extinct comets. Another important indicator is the proximity of the orbit to the Sun: it is assumed that most objects moving in orbits close to the Sun are also extinct comets. Approximately 6% of all near-Earth asteroids are extinct comets, which have already completely depleted their reserves of volatile substances. It is quite possible that all comets sooner or later lose all their volatiles and turn into asteroids.
A comet is a celestial body of small size, consisting of ice interspersed with dust and stone fragments. As it approaches the sun, the ice begins to evaporate, leaving a tail behind the comet, sometimes stretching for millions of kilometers. The tail of a comet is made up of dust and gas.
comet orbit
As a rule, the orbit of most comets is an ellipse. However, circular and hyperbolic trajectories along which ice bodies move in outer space are also quite rare.
Comets passing through the solar system
Many comets pass through the solar system. Let's focus on the most famous space wanderers.
Comet Arend-Roland was first discovered by astronomers in 1957.
Comet Halley passes near our planet every 75.5 years. Named after the British astronomer Edmund Halley. The first mention of this celestial body is found in Chinese ancient texts. Perhaps the most famous comet in the history of civilization.
Comet Donati was discovered in 1858 by the Italian astronomer Donati.
Comet Ikeya-Seki was noticed by Japanese amateur astronomers in 1965. Differed in brightness.
Comet Lexell was discovered in 1770 by the French astronomer Charles Messier.
Comet Morehouse was discovered by American scientists in 1908. It is noteworthy that photography was used for the first time in its study. Distinguished by the presence of three tails.
Comet Hale-Bopp was visible in 1997 to the naked eye.
Comet Hyakutake was observed by scientists in 1996 at a small distance from the Earth.
Comet Schwassmann-Wachmann was first noticed by German astronomers in 1927.
"Young" comets have a bluish tint. This is due to the presence a large number ice. As the comet rotates around the sun, the ice melts and the comet takes on a yellowish tint.
Most comets originate from the Kuiper Belt, a collection of frozen bodies near Neptune.
If the tail of a comet is blue and turned away from the Sun, this is evidence that it consists of gases. If the tail is yellowish and turned towards the Sun, then there is a lot of dust and other impurities in it that are attracted to the luminary.
Study of comets
Scientists obtain information about comets visually through powerful telescopes. However, in the near future (in 2014), the launch of the ESA Rosetta spacecraft is planned to study one of the comets. It is assumed that the device will be near the comet for a long time, accompanying the space wanderer on her way around the Sun.
Note that earlier NASA launched the Deep Impact spacecraft to collide with one of the solar system comets. Currently, the device is in good condition and is used by NASA to study icy space bodies.
The fear of a comet impacting the Earth will always live in the hearts of our scientists. In the meantime, they will be afraid, let's remember the most sensational comets that have ever excited mankind.
Comet Lovejoy
In November 2011, Australian astronomer Terry Lovejoy discovered one of the largest comets of the near-solar Kreutz group, about 500 meters in diameter. She flew through the solar corona and did not burn out, was clearly visible from Earth and even photographed from the International Space Station.
Source: space.com
Comet McNaught
The first brightest comet of the 21st century, also called the "Big Comet of 2007". Discovered by astronomer Robert McNaught in 2006. In January and February 2007 it was perfectly visible to the naked eye to the inhabitants of the southern hemisphere of the planet. The next return of the comet is not soon - in 92,600 years.
Source: www.wyera.com
Comets Hale-Bopp and Hyakutake
Appeared one after another - in 1996 and 1997, competing in brightness. If the Hale-Bopp comet was discovered back in 1995 and flew strictly “on schedule”, Hyakutake was discovered only a couple of months before its approach to the Earth.
Source: website
Comet Lexell
In 1770, comet D/1770 L1, discovered by Russian astronomer Andrei Ivanovich Leksel, passed at a record close distance from the Earth - only 1.4 million kilometers. This is about four times farther than the Moon is from us. The comet was visible to the naked eye.
Source: solarviews.com
1948 eclipse comet
November 1, 1948 during the full solar eclipse astronomers suddenly discovered a bright comet near the sun. Officially named C/1948 V1, it was the last "sudden" comet of our time. It could be seen with the naked eye until the end of the year.
Source: philos.lv
Great January Comet 1910
Appeared in the sky a couple of months before Halley's comet, which everyone was waiting for. The first new comet was noticed by miners from the diamond mines of Africa on January 12, 1910. Like many superbright comets, it was visible even during the day.
Source: arzamas.academy
Great March Comet of 1843
It is also a member of the Kreutz family of near-solar comets. It flew only 830 thousand kilometers from the center of the Sun and was clearly visible from the Earth. Its tail is one of the longest of all known comets = 2 astronomical units (1 astronomical unit equals the distance between the Earth and the Sun).
Presumably, long-period comets fly to us from the Oort Cloud, which contains millions of cometary nuclei. Bodies located on the outskirts of the solar system, as a rule, consist of volatile substances (water, methane and other ices) that evaporate when approaching the Sun.
More than 400 short-period comets have been discovered so far. Of these, about 200 have been observed in more than one perihelion passage. Many of them are included in the so-called families. For example, approximately 50 of the shortest period comets (their full revolution around the Sun lasts 3-10 years) form the Jupiter family. Slightly smaller than the families of Saturn, Uranus and Neptune (the latter, in particular, includes the famous comet Halley).
Comets emerging from the depths of space look like nebulous objects, behind which a tail stretches, sometimes reaching a length of millions of kilometers. The nucleus of a comet is a body of solid particles and ice, shrouded in a foggy shell called a coma. A nucleus with a diameter of several kilometers can have around it a coma 80,000 km across. Streams of sunlight knock particles of gas out of the coma and throw them back, pulling them into a long smoky tail that drags behind her in space.
The brightness of comets depends very much on their distance from the Sun. Of all the comets, only a very small part approaches the Sun and the Earth enough to be seen with the naked eye. The most notable ones are sometimes referred to as the "Great Comets".
The structure of comets
Comets move in elongated elliptical orbits. Notice the two different tails.
As a rule, comets consist of a "head" - a small bright clot-core, which is surrounded by a light foggy shell (coma), consisting of gases and dust. In bright comets, as they approach the Sun, a “tail” is formed - a weak luminous band, which, as a result of light pressure and the action of the solar wind, is most often directed in the opposite direction from our luminary.
The tails of celestial wanderers of comets differ in length and shape. Some comets have them stretching across the sky. For example, the tail of a comet that appeared in 1944 [ specify], was 20 million km long. Comet C/1680 V1 had a tail stretching for 240 million km.
The tails of comets do not have sharp outlines and are practically transparent - stars are clearly visible through them - as they are formed from an extremely rarefied substance (its density is much less than the density of gas released from a lighter). Its composition is diverse: gas or the smallest dust particles, or a mixture of both. The composition of most of the dust grains is similar to the asteroid material of the solar system, which was revealed as a result of the study of Comet Wild (2) by the Stardust spacecraft. In essence, it is "visible nothing": a person can observe the tails of comets only because the gas and dust glow. At the same time, the glow of the gas is associated with its ionization by ultraviolet rays and streams of particles ejected from the solar surface, and the dust simply scatters sunlight.
The theory of tails and shapes of comets was developed at the end of the 19th century by the Russian astronomer Fyodor Bredikhin (-). He also owns the classification of comet tails, which is used in modern astronomy.
Bredikhin suggested that the tails of comets be classified into three main types: straight and narrow, directed directly from the Sun; wide and slightly curved, deviating from the sun; short, strongly deviated from the central luminary.
Astronomers explain such different forms of comet tails as follows. The particles that make up comets have different compositions and properties and respond differently to solar radiation. Thus, the paths of these particles in space "diverge", and the tails of space travelers take on different shapes.
Comets up close
What are comets themselves? Astronomers got an exhaustive idea of them thanks to the successful "visits" to Halley's comet by the spacecraft "Vega-1" and "Vega-2" and the European "Giotto". Numerous instruments installed on these vehicles transmitted to Earth images of the comet's nucleus and various information about its shell. It turned out that the nucleus of Halley's comet consists mainly of ordinary ice(with small inclusions of carbon dioxide and methane ice), as well as dust particles. It is they that form the shell of the comet, and as it approaches the Sun, some of them - under the pressure of the sun's rays and the solar wind - pass into the tail.
The dimensions of the nucleus of Halley's comet, as scientists correctly calculated, are equal to several kilometers: 14 in length, 7.5 in the transverse direction.
The nucleus of Halley's comet has an irregular shape and rotates around an axis, which, as the German astronomer Friedrich Bessel (-) suggested, is almost perpendicular to the plane of the comet's orbit. The rotation period turned out to be 53 hours - which again agreed well with the calculations of astronomers.
Notes
comet explorers
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See what "Comets" are in other dictionaries:
Celestial bodies that occasionally appear in the solar system. They are bright nebulae with a brilliant core inside; most often a light trail, or, as it is called, a tail stretches behind them; it is always turned in the opposite direction of the sun ... ... Dictionary of foreign words of the Russian language
- (Greek, sg. kometes, lit. long-haired) small bodies of the Solar System with extended (up to hundreds of millions of km) non-stationary atmospheres. Physical structures also differ from other small bodies. chem. And orbital characteristics. Observed from Earth... Physical Encyclopedia
- (Comet) celestial bodies, having the form of a hazy spot with a more or less bright core in the middle; most of them are accompanied, in addition, by a rather bright hazy band called the comet's tail. Some of them appear on the vault ... ... Marine Dictionary
comets- The celestial bodies of the solar system, moving in highly elongated orbits, consisting of an ice core and a gas "tail" stretched for millions of kilometers. [Glossary of geological terms and concepts. Tomsk State University] Topics… … Technical Translator's Handbook
- (from the Greek kometes a star with a tail, a comet; literally long-haired) the bodies of the solar system, which look like nebulous objects, usually with a light clot core in the center and a tail. General information about comets. K. are observed when ... Big soviet encyclopedia
- (from the Greek komētēs, literally long-haired), the bodies of the solar system move along highly elongated orbits, at considerable distances from the Sun they look like faintly luminous oval spots, and as they approach the Sun they appear ... ... encyclopedic Dictionary
Comets are one of the most mysterious celestial bodies that appear in the sky every now and then. Today, scientists believe that comets are a by-product left over from the formation of stars and planets billions of years ago. They consist of a core various kinds ice (frozen water, carbon dioxide, ammonia and methane mixed with dust) and a large cloud of gas and dust surrounding the core, often referred to as "coma". Today, more than 5260 of them are known. The brightest and most impressive ones are collected here.
Great Comet of 1680
This magnificent comet, discovered by the German astronomer Gottfried Kirch on November 14, 1680, became one of the brightest comets of the seventeenth century. She was remembered for being visible even in the daytime, as well as for her spectacular long tail.
Mrkos (1957)
Comet Mrkos was photographed by Alan McClure on August 13, 1957. Photo produced great impression on astronomers, since for the first time a double tail was seen on a comet: a straight ion tail and a curved dust tail (both tails are directed in opposite side from the sun).
De Cock-Paraskevopoulos (1941)
This strange but beautiful comet is best remembered for its long but faint tail, and for being visible at dawn and dusk. So strange name The comet received because it was simultaneously discovered by an amateur astronomer named De Kock and the Greek astronomer John S. Paraskevopoulos.
Skjellerup - Maristani (1927)
Comet Skjellerup-Maristani was a long-period comet whose brightness suddenly increased greatly in 1927. It could be observed with the naked eye for about thirty-two days.
Mellish (1917)
Mellish is a periodic comet that has been observed mainly in the southern hemisphere. Many astronomers believe that Mellish will return to the earth's sky again in 2061.
Brooks (1911)
This bright comet was discovered in July 1911 by astronomer William Robert Brooks. She was remembered for her unusual blue color, which was the result of the emission of carbon monoxide ions.
Daniel (1907)
Comet Daniel was one of the most famous and widely observed comets of the early twentieth century.
Lovejoy (2011)
Comet Lovejoy is a periodic comet that comes extremely close to the sun at perihelion. It was discovered in November 2011 by Australian amateur astronomer Terry Lovejoy.
Bennet (1970)
The next comet was discovered by John Caister Bennett on December 28, 1969, when it was at a distance of two astronomical units from the Sun. It was notable for its radiant tail, consisting of plasma compressed into filaments by the action of magnetic and electric fields.
Seki Lines (1962)
Initially visible only in the southern hemisphere, the Seki Lines became one of the brightest objects in the night sky on April 1, 1962.
Arend-Roland (1956)
Visible only in the southern hemisphere during the first half of April 1956, Comet Arend-Roland was first observed on November 8, 1956 by Belgian astronomers Sylvain Arend and Georges Roland in photographic images.
Eclipse (1948)
Eclipse is an exceptionally bright comet that was discovered during a solar eclipse on November 1, 1948.
Viscara (1901)
The large comet of 1901, sometimes called Comet Viscar, became visible to the naked eye on April 12. It was visible as a second magnitude star with a short tail.
McNaught (2007)
Comet McNaught, also known as the Great Comet of 2007, is a periodic celestial body discovered on August 7, 2006 by British-Australian astronomer Robert McNaught. It was the brightest comet in the past forty years and was clearly visible to the naked eye in the southern hemisphere in January and February 2007.
Hyakutake (1996)
Comet Hyakutake was discovered on January 31, 1996, during its closest passage to Earth. It was named the "Big Comet of 1996" and is remembered for being the celestial body that approached the Earth at the minimum distance in the last two hundred years.
Vesta (1976)
Comet West was arguably the most spectacular and attention-grabbing comet of the last century. She was visible to the naked eye, and her two huge tails stretched across the sky.
Ikeya-Seki (1965)
Also known as the "Great Comet of the Twentieth Century", Ikeya-Seki was the brightest comet of the last century and appeared in daylight even brighter than the sun. According to Japanese observers, it was about ten times brighter than the full moon.
Halley's Comet (1910)
Despite the appearance of much brighter long-period comets, Halley is the brightest short-period (it returns to the Sun every 76 years) comet that is clearly visible to the naked eye.
Great Southern Comet (1947)
In December 1947, a huge comet was seen near the setting sun, the brightest in decades (since Halley's comet in 1910).
