Solar system comets names. Comets of the solar system. Asteroids and comets have a number of other differences from each other

Comets are divided into two main classes depending on the period of their revolution around the Sun

Comets with periods of less than 200 years are called short-period, and long-period comets with periods of more than 200 years. More recently, it was possible to observe the bright long-period (with a period of about 4000 years) comet Hale-Bopp, which first appeared in the immediate vicinity of the Sun.

The name of the comet consists of the names of the scientists who discovered it in July 1995. Now about 700 long-period comets have already been discovered, of which about 30 have small perihelion distances and are called “scratching” comets. About a sixth of all known long-period comets are “new,” that is, they were observed only during one approach to the Sun. Obviously, their calculated orbit turns out to be open (parabolic), which is why they are also called parabolic. The orbital inclinations of long-period comets relative to the ecliptic plane are randomly distributed

Dutch astrophysicist Jan Oort, having analyzed the distribution of the orbits of 19 long-period comets known at that time, discovered that the semi-major axes of their primary orbits are grouped towards a region located at distances of more than 200,000 AU. Oort suggested that the Solar System is surrounded by a giant cloud of cometary bodies or icy planetesimals (according to his estimate, numbering up to 10 11 bodies), located at distances from 2 10 4 to 2 10 5 AU. If in 1950 Oort proceeded from the assumption that these bodies were “thrown” over such distances as a result of the explosion of a hypothetical planet (which previously supposedly existed on the site of the modern main asteroid belt), then already in 1951 he switched to the ideas , coinciding with the conclusions of representatives of the Schmidt school, who showed that during the growth of giant planets (primarily Jupiter and Saturn), when they reach a sufficiently large mass, their gravitational disturbances become so strong that they begin to massively eject smaller primary bodies ( planetesimals) from the ring zones closest to their orbits. This process not only affected the asteroid belt and planets terrestrial group, but at the same time could create a reservoir of cometary bodies on the periphery of the Solar system, from which they come now. This cometary cloud later became known as the “Oort cloud”.

More than 200 short-period comets are now known. As a rule, their orbits are located very close to the ecliptic plane. All short-period comets are members of different comet-planet families

The largest such family belongs to Jupiter - these are comets (about 150 of them are known), whose aphelion distances (from the Sun to the point of greatest removal) are close to the semi-major axis of Jupiter’s orbit equal to 5.2 AU. The periods of revolution around the Sun for comets of the Jupiter family range from 3.3 to 20 years (of which the most frequently observed are Encke, Tempel-2, Pons-Winnecke, Fay, etc.). Other's major planets the family of comets is significantly smaller: now about 20 comets of the family are known (Tutl, Neuimin-1, Van Biesbrouck, Gale, etc. with periods of revolution around the Sun of 10-20 years), only a few comets of the Uranian family (Crommelin, Tempel-Tutl, etc. . with orbital periods of 28-40 years) and about 10 - the Neptune family (Halley, Olbers, Pons-Brooks, etc. with orbital periods of 58-120 years). It is believed that all these short-period comets were originally long-period, but as a result of the long-term gravitational influence on them major planets they gradually moved into orbits associated with the corresponding planets and became members of their cometary families.

It has been shown that the predominance in number of comets of the Jupiter family is a consequence of its significantly greater gravitational influence on these bodies compared to other planets (10 times greater than the influence of Saturn and 100 times or more the gravitational influence of any other planet). Of all the known short-period comets, Comet Encke, a member of the Jupiter family, has the shortest period of revolution around the Sun - 3.3 Earth years. This comet has been observed the maximum number of times during its close approaches to the Sun: 57 times over approximately 190 years. But still, the most famous in the history of mankind is Halley’s comet, which is part of the Neptune family. There are records of her sightings dating back to 467 BC. e. During this time, it passed near the Sun 32 times, given that the period of its revolution around the Sun is 76.08 years.

In March 1986, the VEGA-1 and VEGA-2 spacecraft (USSR) and the Giotto spacecraft (European Space Agency) approached Halley's Comet. At that moment, the mass of the comet's nucleus was close to 6 1011 tons. Then other extremely interesting results were obtained. It was discovered that the nucleus of Halley's comet was an ice block resembling a worn-out shoe. The size of this body along the major axis was approximately 14 km, and along the two minor axes - approximately 7.5 km. The comet's nucleus rotates around a minor axis passing through the "heel" with a period of 53 hours. The surface temperature of the comet at its distance is 0.8 AU. from the Sun was approximately equal to 360 K or 87° Celsius

Photo of the nucleus of Halley's comet (Giotto's apparatus)

The surface of the comet's nucleus turned out to be very dark and reflects only 4% of the light incident on it. For comparison, recall that the surface of the Moon on average reflects 7%, and the surface of Mars 16% of the incident light. Most likely, the icy body of the comet is covered with a heat-insulating layer of refractory particles (metals, sulfur, silicon, their oxides and other compounds), the existence of which Whipple assumed in his model. Where the ice melts, jets of water vapor, carbon dioxide and other gases, along with dust, escape from under the crust. It was calculated that at the moment of passage of perihelion, the comet loses about 45 tons of gaseous compounds and 5-8 tons of dust every second. According to estimates, the reserves of volatile matter in Halley's comet should last for a hundred thousand years. During this time, it can still make about 1,300 revolutions around the Sun, and then it will probably join the number of extinct comets.

These are former comet nuclei that no longer show any signs of cometary activity and, according to their observable characteristics, are no different from asteroids. Eventually, comets collapse, some of them spawn a swarm meteoroids- ice and dust particles rotating in the same orbit, called meteor showers. In particular, it is believed that the “mother” of the most famous Perseid shower is comet Swift-Tuttle. Another sensational one in 1999 and 1998, the Leonid shower, was generated by comet Tempel-Tuttle.

As the Earth passes through comet tails no effects, even the most minor ones, were noticed. Only comet nuclei can pose a danger to the Earth.



COMET
a small celestial body moving in interplanetary space and abundantly releasing gas when approaching the Sun. A variety of physical processes are associated with comets, from sublimation (dry evaporation) of ice to plasma phenomena. Comets are the remnants of the formation of the Solar System, a transitional stage to interstellar matter. The observation of comets and even their discovery are often carried out by amateur astronomers. Sometimes comets are so bright that they attract everyone's attention. Past appearance bright comets caused fear in people and served as a source of inspiration for artists and cartoonists.
Movement and spatial distribution. All or almost all comets are components of the Solar System. They, like the planets, obey the laws of gravity, but they move in a very unique way. All planets revolve around the Sun in the same direction (which is called “direct” as opposed to “reverse”) in almost circular orbits lying approximately in the same plane (the ecliptic), and comets move in both direct and reverse directions along highly elongated (eccentric) orbits, inclined at different angles to the ecliptic. It is the nature of the movement that immediately gives away the comet. Long-period comets(with an orbital period of more than 200 years) arrive from regions located thousands of times further than the most distant planets, and their orbits are inclined at all sorts of angles. Short-period comets (periods of less than 200 years) come from the region of the outer planets, moving in a forward direction in orbits lying close to the ecliptic. Far from the Sun, comets usually do not have "tails", but sometimes have a barely visible "coma" surrounding the "nucleus"; together they are called the "head" of the comet. As it approaches the Sun, the head enlarges and a tail appears.
Structure. In the center of the coma there is a core - a solid body or a conglomerate of bodies with a diameter of several kilometers. Almost all of the comet's mass is concentrated in its nucleus; this mass is billions of times less than the earth's. According to F. Whipple's model, the comet's nucleus consists of a mixture various ices, mostly water ice mixed with frozen carbon dioxide, ammonia and dust. This model is confirmed by both astronomical observations and direct measurements from spacecraft near the nuclei of comets Halley and Giacobini-Zinner in 1985-1986. 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 with it dust particles and creating a coma. Water molecules destroyed by sunlight form a huge hydrogen corona around the comet's nucleus. In addition to solar attraction, repulsive forces also act on the rarefied matter of a comet, 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 study of comets in 1985-1986. The plasma tail, consisting 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, exhibiting high plasma activity.

Although the tail and coma contain less than one millionth of the comet's mass, 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 particles lost by the comet move in their orbits and, entering the atmospheres of the planets, cause the formation of meteors ("shooting stars"). Most of the meteors we observe are associated with cometary particles. Sometimes the destruction of comets is more catastrophic. Comet Bijela, discovered in 1826, split into two parts in front of observers in 1845. When this comet was last seen in 1852, the pieces of its nucleus were millions of kilometers away from each other. Nuclear fission usually heralds the complete disintegration of a comet. In 1872 and 1885, when Bijela's comet, if nothing had happened to it, would have crossed the Earth's orbit, unusually heavy meteor showers were observed.
see also
METEOR ;
METEORITE. Sometimes comets are destroyed when approaching planets. On March 24, 1993, at the Mount Palomar Observatory in California, astronomers K. and Y. Shoemaker, together with D. Levy, discovered a comet with an already destroyed nucleus near Jupiter. Calculations showed that on July 9, 1992, the Shoemaker-Levy-9 comet (this is the ninth comet they discovered) passed near Jupiter at a distance of half the radius of the planet from its surface and was torn apart by its gravity into more than 20 parts. Before destruction, the radius of its core was approx. 20 km.

Table 1.
MAIN GAS COMPONENTS OF COMETS

Stretching out in a chain, the fragments of the comet moved away from Jupiter in an elongated orbit, and then in July 1994 approached it again and collided with the cloudy surface of Jupiter.
Origin. 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 not a single such comet has been reliably identified.
Gas composition. In table Table 1 lists the main gas components of comets in descending order of their content. The movement of gas in the tails of comets shows that it is strongly influenced by non-gravitational forces. The glow of the gas is excited by solar radiation.
ORBITS AND CLASSIFICATION
To better understand this section, we recommend that you read the following articles:
CELESTIAL MECHANICS;
CONIC SECTIONS;
ORBIT;
SOLAR SYSTEM .
Orbit and speed. The movement 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. The average speed of a body is inversely proportional to the square root of its average distance to 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 circular speed (vc) at a distance a. The speed of escape from the gravitational field of the Sun along a parabolic orbit (vp) is times greater than the circular speed at this distance. If the comet's speed is less than vp, then it moves around the Sun in an elliptical orbit and never leaves the Solar System. But if the speed exceeds vp, then it moves around the Sun in an elliptical orbit and never leaves the Solar System. But if the speed exceeds vp, then the comet passes by the Sun once and leaves it forever, moving along a hyperbolic orbit. The figure shows the elliptical orbits of the two comets, as well as the nearly 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 Earth, the speed of Comet Encke is 37.1 km/s, and the speed of Comet Halley is 41.6 km/s; This is why Comet Halley goes much further from the Sun than Comet Encke.

Classification of cometary orbits. 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; along them, comets move away from the Sun very far and for a long time. It is customary to divide the elliptical orbits of comets into two main types: short-period and long-period (almost parabolic). The orbital period is considered to be 200 years.
SPATIAL DISTRIBUTION AND ORIGIN
Almost parabolic comets. Many comets belong to this class. Since their orbital periods are millions of years, only one ten-thousandth of them appears in the vicinity of the Sun over the course of a century. In the 20th century observed approx. 250 such comets; therefore, there are millions of them in total. In addition, not all comets come close enough to the Sun 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. Taking this into account, in 1950 Jan Oort suggested that the space around the Sun at a distance of 20-100 thousand AU. (astronomical units: 1 AU = 150 million km, distance from the Earth to the Sun) is filled with comet nuclei, the number of which is estimated at 1012, 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 influenced by the attraction of neighboring stars and other massive objects (see below). Stars move relative to the Sun, their disturbing influence on comets changes, and this leads to the evolution of cometary orbits. So, by chance, a comet may end up in an orbit passing close to the Sun, but on the next revolution its orbit will change slightly, and the comet will pass away from the Sun. However, instead of it, “new” comets will constantly fall from the Oort cloud into the vicinity of the Sun.
Short-period comets. When a comet passes near the Sun, its core heats up and the ice evaporates, forming a gas coma and tail. After several hundreds or thousands of such flights, there are no fusible substances left in the core, and it ceases to be visible. For short-period comets that regularly approach the Sun, this means that their populations should become invisible in less than a million years. But we observe them, therefore, replenishment from “fresh” comets is constantly arriving. Replenishment of short-period comets occurs as a result of their “capture” by planets, mainly Jupiter. It was previously thought that long-period comets coming from the Oort cloud were captured, but it is now believed that their source is a cometary disk called the “inner Oort cloud.” In principle, the idea of ​​the Oort cloud has not changed, but calculations have shown that the tidal influence of the Galaxy and the influence of massive clouds of interstellar gas should destroy it quite quickly. A source of replenishment is needed. Such a source is now considered to be the inner Oort cloud, which is much more resistant to tidal influences and contains an order of magnitude more comets than the outer cloud predicted by Oort. After each approach of the Solar System to 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 orbit occurs when it catches up with the planet from behind. Typically, capturing a comet into a new orbit requires several passes through the planetary system. The resulting orbit of a comet typically has low inclination and high eccentricity. The comet moves along it in a forward direction, and the aphelion of its orbit (the point farthest from the Sun) lies close to the orbit of the planet that captured it. These theoretical considerations are fully confirmed by the statistics of cometary orbits.
Non-gravitational forces. Gaseous sublimation products exert 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 solar rays and solar gravity. If the axial rotation of the nucleus and its orbital revolution 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 rotation and circulation occur in opposite directions, then the comet’s movement is slowed down and the orbit is shortened. If such a comet was initially captured by Jupiter, then after some time its orbit is entirely in the region of the inner planets. This is probably what happened to Comet Encke.
Comets touching the Sun. A special group of short-period comets consists of comets that “graze” the Sun. They were probably formed thousands of years ago as a result of the tidal destruction of a large core, at least 100 km in diameter. After the first catastrophic approach to the Sun, fragments of the nucleus made approx. 150 revolutions, continuing to fall apart. Twelve members of this family of Kreutz comets were observed between 1843 and 1984. Their origins may be related to big comet, which Aristotle saw in 371 BC.

Halley's Comet. This is the most famous of all comets. It has been observed 30 times since 239 BC. Named in honor of E. Halley, who, after the appearance of the comet in 1682, calculated its orbit and predicted its return in 1758. The orbital period of Halley's comet is 76 years; it last appeared in 1986 and will next be observed in 2061. In 1986, it was studied at close range by 5 interplanetary probes - two Japanese (Sakigake and Suisei), two Soviet (Vega-1 and Vega-1). 2") and one European ("Giotto"). It turned out that the comet's nucleus is potato-shaped, approx. 15 km and width approx. 8 km, and its surface is “blacker than coal.” It may be covered with a layer of organic compounds, such as polymerized formaldehyde. The amount of dust near the core turned out to be much higher than expected. See also HALLEY, EDMUND.

Comet Encke. This faint comet was the first to be included in the Jupiter family of comets. Its period of 3.29 years is the shortest among comets. The orbit was first calculated in 1819 by the German astronomer I. Encke (1791-1865), who identified it with comets observed in 1786, 1795 and 1805. Comet Encke is responsible for meteor shower Taurids, observed annually in October and November.

Comet Giacobini-Zinner. This comet was discovered by M. Giacobini in 1900 and rediscovered by E. Zinner in 1913. Its period is 6.59 years. It was with it that on September 11, 1985, the space probe "International Cometary Explorer" first approached, which passed through the tail of the comet at a distance of 7800 km from the nucleus, thanks to which data was obtained on the plasma component of the tail. This comet is associated with the Jacobinids (Draconids) meteor shower.
PHYSICS OF COMETS
Core. All manifestations of a comet are somehow connected with the nucleus. Whipple suggested that the comet's nucleus was a solid body consisting mainly of water ice with dust particles. This “dirty snowball” model easily explains the multiple passages of comets near the Sun: with each passage, a thin surface layer (0.1-1% of the total mass) evaporates and the inner part of the nucleus is preserved. Perhaps the core is a conglomerate of several “cometesimals,” each no more than a kilometer in diameter. Such a structure could explain the disintegration of nuclei, as observed with Comet Biela in 1845 or Comet West in 1976.
Shine. Observed brilliance illuminated by the Sun celestial body with a constant surface varies inversely with the squares of its distances from the observer and from the Sun. However, sunlight is scattered mainly by the comet's gas and dust shell, the effective area of ​​which depends on the rate of ice sublimation, and that, in turn, on the heat flux incident on the nucleus, which itself varies inversely with the square of the distance to the Sun. Therefore, the brightness of the comet should vary in inverse proportion to the fourth power of the distance to the Sun, which is confirmed by observations.
Kernel size. The size of the comet's nucleus can be estimated from observations at a time when it is far from the Sun and not shrouded in a gas and dust shell. In this case, light is reflected only by the solid surface of the core, and its apparent brightness depends on the cross-sectional area and reflectance (albedo). The albedo of the nucleus of Comet Halley turned out to be very low - approx. 3%. If this is typical for other nuclei, then the diameters of most of them lie in the range from 0.5 to 25 km.
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 mainly water. This was confirmed by studying the comets Halley and Giacobini-Zinner. The gases observed first as the comet approaches the Sun (CN, C2) 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. Streams of gas and small and fluffy dust particles picked up by them leave the core, since the attraction at its surface is extremely weak. But the gas flow does not carry away dense or interconnected heavy dust particles, and a dust crust is formed. Then the sun's rays heat the dust layer, the heat passes in, the ice sublimates, and gas flows break through, breaking the dust crust. These effects became apparent during the observation of Halley's comet in 1986: sublimation and outflow of gas occurred only in a few regions of the comet's nucleus illuminated by the Sun. It is likely that ice was exposed in these areas, while the rest of the surface was covered with crust. The released gas and dust form the observable structures around the comet's nucleus.
Coma. Dust grains and gas of neutral molecules (Table 1) form an almost spherical coma of the comet. Usually the coma stretches from 100 thousand to 1 million km from the nucleus. Light pressure can deform the coma, stretching it in an anti-solar direction.
Hydrogen corona. Since the core ices are mainly water, the coma mainly contains H2O molecules. Photodissociation breaks down H2O into H and OH, and then OH into O and H. The fast-moving hydrogen atoms fly far from the nucleus before they become ionized, and form a corona, the apparent size of which often exceeds the solar disk.
Tail and related phenomena. The tail of a comet may consist of molecular plasma or dust. Some comets have both types of tails. The dust tail is usually uniform and stretches for millions and tens of millions of kilometers. It is formed by dust grains thrown away from the core in the antisolar direction by the pressure of sunlight, and has a yellowish 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. The plasma tail, tens or even hundreds of millions of kilometers long, is a visible manifestation of the complex interaction between the comet and the solar wind. Some molecules that leave the nucleus are ionized by solar radiation, forming molecular ions (H2O+, OH+, CO+, CO2+) and electrons. This plasma prevents the movement of the solar wind, which is permeated by a magnetic field. When the comet hits the comet, the field lines wrap around it, taking the shape of a hairpin and creating two areas of opposite polarity. Molecular ions are captured in this magnetic structure and form a visible plasma tail in its central, densest part, 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. Bierman and H. Alfven in the 1950s. Their calculations were confirmed by 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 approx. 400 km/s and forming a shock wave in front of it, 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 neutral molecules of the comet freely penetrate the solar wind flow, but immediately after ionization they begin to actively interact with the magnetic field and are accelerated to significant energies. True, sometimes very energetic molecular ions are observed that are inexplicable from the point of view of the indicated mechanism. The capture process also excites plasma waves in the gigantic volume of space around the nucleus. The observation of these phenomena is of fundamental interest for plasma physics. The “tail break” is a wonderful sight. As is known, in the normal state the plasma tail is connected to the comet's head by a magnetic field. However, often the tail breaks away from the head and lags behind, and a new one is formed in its place. This happens when a comet passes through the boundary of regions of the solar wind with an oppositely directed magnetic field. At this moment, the magnetic structure of the tail is rearranged, which looks like a break and the formation of a new tail. The complex topology of the magnetic field leads to the acceleration of charged particles; This may explain the appearance of the fast ions mentioned above.
Collisions in the Solar System. From the observed number and orbital parameters of comets, E. Epic calculated the probability of collisions with the nuclei of comets of various sizes (Table 2). On average, once every 1.5 billion years, the Earth has a chance to collide with a core with a diameter of 17 km, and this can completely destroy life in an area equal to the area North America. Over the 4.5 billion years of Earth's history, this could have happened more than once. Smaller disasters occur much more often: in 1908, a core probably entered the atmosphere and exploded over Siberia small comet, causing forest lodging over a large area.