Who can provide some information about comets?

First, the discovery and naming of comets.

Comet, commonly known as broom star, the word "comet" means broom. The word comet in a foreign language comes from Greek, which means a star with "tail" or "hair". In astronomy, comets are vividly represented by symbols. Ancient people were frightened when they happened to see a strange-looking comet, which was considered as a sign of disaster. In fact, the appearance of comets is only a natural phenomenon, and astronomical observation and research have gradually unveiled the mystery of comets.

There are many records of comets in history, the earliest and most of which are recorded in ancient books in China, and sometimes there are records of comets, comets, demon stars, alien stars, pengxing and long stars. Huainanzi contains "King Wu hits the comet and comes out". According to Yuzhe Zhang, a famous astronomer in China, this is the return of Halley's Comet in BC 1056, and it is an important contribution of astronomy to the textual research of historical age.

Westerners have long been influenced by Aristotle's wrong view that comets are burning phenomena in the earth's atmosphere, and even Copernicus believes that "the so-called comets of the Greeks were born in the upper atmosphere." It was not until the end of 16 that Tycho first observed that 1577 was much farther than the moon. As early as in the Book of Jin, China said that "comets have no light, and wealth makes them light". Therefore, when you see it at night, it means east, and when you see it in the morning, it means west. Japan's north and south all point to the sunshine, and the mountain is frustrated, long or short. "In ancient times, comets only happened by accident. It was not until the17th century that the British astronomer Halley worked out the orbit of the comet. He found that the orbits of comets in 1682, 1607 and 153 1 year were similar, asserted that this was the three regression of the same comet, and predicted that it would be in 168. Although he died in 1742 and didn't see it with his own eyes, the comet did appear in 1759. In memory of him, this comet is called "Halley's Comet". Harley calculated the orbits of comets 24 times in his life.

The realization of Harley's prophecy inspired people to find and discover new comets. According to international practice, the newly discovered comet is named after the first discoverer (three at most, changed to two at most after 1994).

Although bright comets are rarely seen by naked eyes, modern telescopes can see an average of 20-25 comets every year, of which about 1/3 is newly discovered and 2-3 are discovered by amateur naked eyes. More than 1800 comets were observed, and only 1600 comets were excluded from repeated regression. In fact, comets can only be observed when they are close to us, but not when they are far away from the sun. According to statistics, there are 10 12 (trillion)1kloc-0/3 (trillion) comets in the solar system, most of which are outside the solar system.

Second, the orbital characteristics of comets

1, comet orbit

Through many observations, we can deduce the elements of the comet's orbit around the sun, that is, the short distance, the time of perihelion, eccentricity, the inclination of the orbit to the ecliptic plane, the rising point (the point on the ecliptic plane from south to north), and the angular distance between the perihelion and the rising point, and then we can calculate the calendar of the comet, that is, the apparent positions (right ascension and declination) on the celestial sphere at different times. The orbits of many comets are oblate ellipses, parabolas or even hyperbolas. Obviously, comets moving in parabolic or hyperbolic orbits are aperiodic comets, and they will never return and escape from the solar system. So do they really belong to members of the solar system? In fact, these comets were observed near the perihelion, and their kissing orbits were calculated. Moreover, there is little difference between the three orbits during this period, and the observation accuracy is not enough, which will lead to the orbit error of calculation. More importantly, the orbit of the comet has changed due to the gravitational perturbation of the big planet. After correcting these effects, most of the calculated orbits of comets are still oblate ellipses with eccentricity slightly less than 1, so they are members of the solar system, and a few comets will. Even a comet with a very flat elliptical orbit has a very long period of revolution, which takes hundreds or even tens of thousands of years to return to the solar system at a time. In the history of human civilization, only short-period comets (period of revolution is less than 200 years old) have been observed many times. Most short-period comets are in forward orbit (that is, in the same direction as the planetary orbit), and the inclination of their orbital plane relative to the ecliptic plane is less than 45 degrees. A few (such as Halley's comet) are in reverse orbit, while long-period comets are observed.

2. Comet family

About two-thirds of short synchronous comets are called "Jupiter family comets" because their apohelion distance is less than 7 astronomical units, that is, they approach Jupiter's orbit at apohelion. It is generally believed that when a comet with a parabolic orbit (eccentricity e is about equal to 1) approaches Jupiter, it is captured as a short-period comet due to the great disturbance of Jupiter's gravity and the change of its orbit. A typical example is Comet Brooks (1889V). After it approached Jupiter, period of revolution changed from about 29 years to 7 years. In addition, some comets are called "Saturn family comets", "Uranus family comets" and "Neptune family comets" respectively, but the number is small, and it is doubtful whether they come from "capture".

3. Comets

Except perihelion time, some comets whose other five orbital elements are very close are called "comets". 10 groups of comets have been identified, and the number of comets in each group is more or less. There is a view that the same group of comets is split by a big comet. Some examples of comet splitting have indeed been observed. The most famous is the "sun-skimming comet group", which has at least 16 comets, and their nearest distance is less than 0.0 1 astronomical unit, which can pass through the corona. Among them, Comet Ikeya-Guan Yi (65,438+0965VIII) split into three within two weeks after passing through the perihelion of 65,438+0965+00. 1993 comet shoemaker -Levy9 discovered by E.Shoemaker and C.Shoemaker and D.Levy may split many times when/kloc-0 approached Jupiter in July, 1992. 1993, five subnuclei were observed at first, and then increased to 165433.

4. Oort Cloud and Kuiper Belt

1950, Dutch astronomer J.H.Oort made a statistical study on the orbits of comets, and found that many comets had orbital radii ranging from 30,000 to 65,438+million astronomical units. He estimated that there was an approximately spherical comet library with hundreds of billions of comets in it. As early as 1932, E.Opiek also put forward a similar view, so this comet library was called "Oort Cloud" or "Oort-Aupic Cloud", in which the period of the comet's revolution around the sun was as long as millions of years. According to more detailed research in recent years, there are trillions to 10 trillion comets in Oort cloud. Of course, most of these distant comets cannot be directly observed. Only when the gravitational photography of stars or comets collide with each other will the orbits of some comets change greatly, and only when they enter the inner solar system along a flat orbit can they become? Quot A new comet has been observed.

195 1 year, American astronomer G.Kuiper studied the properties and formation process of comets, and thought that the volatiles in the cold outer layer of the primitive nebula in the solar system condensed into ice bodies-comets. When the outer planets grow up in the ice, the gravitational dispersion of the outer planets drives some comets into the Oort cloud, but no planets form outside Pluto. He proposed that there is a comet belt outside Pluto, namely Coco. 1964, F. Whipple and others proposed that the extraterrestrial comet belt would cause gravitational perturbation of exoplanets and comets. If this belt is in 40 astronomical units, the total mass of comets is about 80% of the mass of the earth. If it is in 50 astronomical units, the total mass is 0.3 times that of 65438+ Earth. 1988, M.Duncan proved that the kuiper belt is the main source of short-period comets, but Oort cloud is not their source area. (Right: It is said to be a Kuiper Belt star)

As mentioned earlier, the newly discovered extraterrestrial objects 1992QB 1 (smiling face) and 1993FW should be comets in the inner boundary area of the Kuiper Belt (although they are now named asteroids), while 1993RO and 1993RP are far from the sun. The Kuiper Belt extends from 40 astronomical units to hundreds of astronomical units (unknown to the outside world) from the sun. It is estimated that there are tens of thousands of comets in this belt, which are the remnants of primitive ice bodies during the formation of the solar system. These comets preserve information about the primitive materials in the solar system. In 2003, the European Space Agency will launch the Rosetta spacecraft to rendezvous with the short-period comet brought by Kuiper, revealing the nature of the comet and the mystery of the formation of the solar system.

Third, the morphological changes of comets.

1, the shape of a bright comet

Bright comets seen by naked eyes can be divided into three parts according to their morphological characteristics (as shown in the left picture): nucleus, coma and tail. The bright spot in the center of the comet's head is called the comet nucleus. A coma is a hazy atmosphere that largely surrounds the core of a comet. The tail of the comet is a faint band of light, which extends from the head of the comet to the direction away from the sun. Tails can be divided into three types: I, II and III. Class I tail is long and straight, slightly blue, mainly composed of gas ions, and now it is often called "plasma tail" (plasma is a mixture of positive and negative ions, which is electrically neutral on average on a large scale). Class ⅱ tail is curved and bright, and Class ⅲ tail is more curved. These two kinds of tails are slightly yellow, both of which are composed of dust particles, but the dust particles of Class III tail are larger than those of Class II tail, and now they are often collectively called "dust tails".

The plasma tail is not always long and straight, but often has some complex structures and changes, such as folded umbrella beam, kink, cloud and spiral wave structure. More interestingly, the old tail was disconnected from the head, moved backward, and a new tail appeared from the head.

Sometimes there will be a long needle-like tail extending from the head of the comet to the sun, which is called "abnormal tail" or "anti-Japanese tail". In fact, this is not the real comet tail facing the sun in space, but the projection effect of the dust comet tail seen from the earth when the sun-comet-earth is in a special relative position.

Since 1970, ultraviolet observations from space (satellites) have found that comets also have "hydrogen (atomic) clouds" much larger than coma, also known as "H coma", with a volume of tens of millions of kilometers and larger than the sun. Sometimes in the telescope, we can see the "jet" and "cladding" of matter thrown from the comet nucleus, which are collectively called "near-nuclear phenomenon".

2. Morphological changes of comets in revolution.

When a comet revolves around the sun, its brightness and shape change with its distance from the sun (heliocentric distance). When the comet is far away from the sun (more than 4 astronomical units), it is only a very dark star shape, mainly a naked core, and may have a well-developed coma.

As the comet approaches the sun, its brightness increases. When it is about 3 astronomical units away from the sun, the coma begins to develop. As it gets closer to the sun, the coma becomes bigger and brighter. When it is about 1.5 astronomical unit from the sun, the coma radius can reach 10- 1 10,000 kilometers. When you get close to the sun, the coma will be slightly smaller. After passing the perihelion, the comet's coma gradually becomes smaller and disappears as it moves away from the sun.

When comets are about 2 astronomical units away from the sun, they begin to produce tails. As the comet approaches the sun, its tail becomes longer and brighter. After passing the perihelion, the comet's tail gradually decreases until it disappears as it moves away from the sun. The longest tail reaches hundreds of millions of kilometers, and the tail of some comets (such as comet 1842c) reaches 320 million kilometers, which exceeds the distance from the sun to Mars.

What is said above is only the general situation of comet form. In fact, the morphological structure of each comet is quite different. Some comets have no developed coma, while others have no developed tail. For example, comet schwassmann-washmann (1925 ii) runs between the orbits of Jupiter and Saturn, and its coma and tail are always weak; When comet Baade( 1955VII) is far away from the sun (4-5 astronomical units), a dust tail appears, but no plasma tail appears.

It should be pointed out that, after all, the observation of comet morphology is carried out from the earth, and the apparent projection effect is related to the distance between the comet and the earth and the observation method. For example, Halley's Comet 19 10 was very close to the Earth when it returned. When the observation conditions are favorable, it can even be seen that its tail crosses the sky at the angle of 100 degrees, which is very spectacular. However, during the return of 1986, the observation conditions were not very favorable. Few people in the northern hemisphere see its richness with naked eyes, and the spacecraft passing through its coma photographed the true face of the nucleus for the first time.

Fourth, the nature of comets.

1, comet nucleus

Although comets are huge and have long tails, they contain very little matter. When a comet or tail covers a star (occultation means that a comet passes in front of other stars and blocks the starlight), the starlight is extremely weakened. Most of the matter in a comet is concentrated in the small solid nucleus, and the matter in the coma and tail comes from the comet nucleus in the final analysis, so the comet nucleus is the body of the comet.

How big is the nucleus? It is also difficult to tell the size of the comet nucleus from the telescope on earth. 1927, when comet Pang Si-Winick approached the earth to 0.037 astronomical unit, the telescope could not distinguish the size of its nucleus, and it was estimated that its nucleus diameter was less than 1Km. According to the indirect estimation of observation data, the diameter of the nucleus of most comets ranges from several hundred meters to more than ten kilometers. A few comets may have larger nuclei. For example, it is estimated that the diameter of the original mother comet of the solar-grazing comet family is 50Km, that of Schwassmann-Washmann is 20Km, and that of Chiron is 90Km (previously considered as an asteroid, but now it tends to be considered as a comet).

What is the shape of the nucleus? This is difficult to observe. In the past, it was generally believed that the comet nucleus was spherical. Now there is some evidence that the comet nucleus is often not spherical, but more likely to be a oblate sphere with a triaxial ratio of 2: 1: 1. The most reliable thing is that the spacecraft visited a series of cameras taken by Halley's comet and revealed the true face of its nucleus. Roughly a flat ball, triaxial 16×8×8 km, more like a flat peanut. Its surface is rugged, with several shallow pits (about 1 km in diameter) and hills and valleys. The surface is covered with uneven dark dust, and the albedo is very low (0.02).

What is the mass of the comet nucleus (also representing a comet)? This is also difficult to measure accurately. According to relevant observation data, the mass of the comet nucleus is generally between1013-10/9g, and some are as small as 1020- 1022g. The mass of Halley's comet is1.5×1017g.

What is the material composition and internal structure of the comet nucleus? It is not clear at present. From the mass and size of the comet nucleus, we can initially calculate its average density. For example, the average density of Halley's comet is about 0.3 g /cm3, which is less than the expected solid density of H2O ice-dust mixture (about 1 g/cm3), indicating that the interior of comet nucleus is porous. According to the comet spectrum and the spacecraft's detection of dust particles in Harley's coma, it is inferred from these substances from the nucleus that the nucleus is mainly composed of ice (water ice, carbon dioxide ice, etc.). ) and dust substances, the most important component of which is water. It is estimated that except for a few chemical elements such as hydrogen, the relative content (abundance) of other elements in the comet nucleus is basically the same as that of the sun and the universe.

In the past, it has been argued for a long time whether the comet nucleus is a loose solid particle aggregate (gravel model) or a complete solid ice block (dense core model). 1950, whipple proposed that the comet nucleus is an "ice conglomerate model" of ice and dust freezing, or commonly known as "dirty snowball", which completely explained many observation facts. This model has developed a lot since then. Some people think that there is a planet-like core, slow and shell structure inside the comet nucleus, while others think that the interior of the comet nucleus is more uniform. From the observation that the sub-nuclei split from the comet nucleus have roughly the same spectral characteristics, it can be considered that the comet nucleus is relatively uniform on a large scale, but it may be uneven on a small scale, and the surface layer (that is, the shell layer) of the comet nucleus is different from the inside. This is because the surface layer has changed due to the bombardment, evaporation and chemical reaction of cosmic rays, forming dark organic matter like asphalt, and the surface area of the comet nucleus is very uneven.

Some properties of the comet nucleus can also be deduced from the phenomenon of near nucleus. The near-nucleus phenomenon of many comets is asymmetric, and the important reason is that the comet nucleus rotates and its surface layer is uneven. About 50 comets' rotation periods are derived from the near-nucleus phenomenon, and some have calculated the spatial direction of their rotation axes. The rotation period of comets is less than 5 hours, some as long as several days, with an average of about 15 hours, and the direction of the rotation axis is randomly distributed. The surface of the nucleus is covered with dark dust, and its thermal conductivity is very small, so the interior of the nucleus can be kept very cold without melting. The surface of the comet nucleus is uneven, and some small areas (active areas) often discharge substances, forming near-nuclear phenomena such as jets.

2. Coma

The spectral characteristics of coma are that there are many emission lines or emission bands of molecules, atoms and ions on the background of continuous spectrum, which shows that coma is composed of dust (scattering sunlight and presenting continuous spectrum) and some molecules, atoms and ions (emission lines or emission bands). The coma aberration consists of the following components: H, CCC2, C3, O, S, Na, K, Ca, V, Cr, Mn, Fe, CO, Ni, Cu, OH, CN, Co, CS, S2, NH, NH2, H2CO, H2S, CH4, HCN+, CH3OH.

The brightness of coma decreases from inside to outside, indicating that the density of matter is dense inside and thin outside. As mentioned above, the size and brightness of coma vary with the distance from the sun. The distribution of various components in coma is also different. The distribution of a component in coma can be understood by observing the narrow-band filter or spectrum in its emission band, which is expressed by terms such as CN (cyano) coma, OH (hydroxyl) coma, H coma (i.e. hydrogen cloud) and dust coma. The typical size of CN coma can reach one million kilometers, C2 coma can reach hundreds of thousands of kilometers, OH coma and C3 coma can generally reach tens of thousands of kilometers, and hydrogen clouds can reach tens of millions of kilometers. Various gas components flow out at a speed of several hundred meters to several thousand meters per second. When the comet is away from the sun 1 astronomical unit, the outflow rate of matter is about105-107g per second. Many molecules, atoms and ions in coma are often not the original components (parent molecules) evaporated from the surface of coma nucleus, but the daughter molecules whose parent molecules are dissociated or ionized by solar radiation. For example, parent molecule H2O dissociates into OH and H, and CO2 ionizes into CO2+.

3. Comet tail

The spectral observation and analysis of the comet tail show that the dust comet tail is mainly composed of dust particles, which are usually called "comet dust", and the size of dust particles ranges from several tenths to hundreds of microns. Comet dust is not only repelled by the solar gravity (the comet nucleus gravity is very small), but also by the solar radiation pressure (light pressure). The ratio of repulsive force Fr to attractive force Fg is 5.7× 10-5/(AP), where a and p are the radius and density of dust particles respectively, and the value of Fr/Fg can reach 2.2.

The plasma tail is composed of many kinds of gas ions, the most of which is CO+, followed by H2O+. The plasma tail is long and straight, which indicates that the repulsion of ions is greater (the repulsion is dozens of times of the solar gravity to more than 100 times), which is the repulsion generated by the solar wind (high speed from the sun and plasma flow) and its magnetic field acting on comet ions. The changes of solar wind and its magnetic field lead to the phenomena of rays, kinks, clouds, spiral waves and tail breakage at the plasma tail.

Verb (abbreviation of verb) physical and chemical process of comet

Based on the observation and research results of comets and the characteristics of comets' dust and gas, various forms and phenomena of comets depend on the properties of comets on the one hand, and on the other hand, they are also related to the effects of solar radiation and solar wind. When the comet nucleus with H2O ice as the main component is irradiated by solar radiation, it will reflect some solar radiation energy. The solar radiation absorbed by the nucleus can be used to heat and evaporate the surface layer of the nucleus and convert it into (infrared) thermal radiation. When the comet is about 2 astronomical units away from the sun, the surface temperature of the comet nucleus reaches 200K, H2O ice sublimation is more effective, dust particles and ice particles are extracted, and coma begins to develop. "parent molecules" (H2O, HCN, carbon dioxide, etc. ) comes out of the comet nucleus and is dissociated by solar radiation ("photolysis") or chemically reacts to generate "sub-molecules", such as H2O dissociated into H+OH. The daughter molecules of comets are often unstable "free radical" molecules (such as OH, CN, ch, NH3, etc. Under the conditions of the earth (the air density is much higher than that in a comet). Excited by solar radiation, these molecules emit fluorescent radiation, which is manifested as spectral emission lines or spectral ionization of comets, or chemical reactions, or charge exchange reactions with solar wind ions to generate comet ions. For example, CO2 is ionized into CO++O+e (electron), and CO+ reacts with H2O to generate H2O+ and CO, and E (electron) reacts with CO2 to generate CO+, O and 2e. These comet gases interact with the solar wind and its magnetic field, forming a structure similar to the planetary magnetosphere on the side facing the sun. There is an arcuate shock surface at a distance of 0/05-106 km, and a discontinuous surface at a distance of 0/03-104 km (pure comet gas inside and mixed solar wind outside). The magnetic lines of the solar wind magnetic field are blocked by comets, hanging and folding in the direction of the comet tail, repelling the comet ions from moving away from the sun, forming the plasma comet tail and its rays, tail knots, waves, tail breakage and other phenomena.

As the gas comes out of the nucleus, the dust particles form a dust particle coma. Comet dust not only scatters sunlight, but also emits continuous infrared radiation and silicate characteristics, with wavelengths of 18 micron and 18 micron respectively. The pressure of solar radiation repels dust particles and forms dust tails. Comet dust can also be dissociated by solar radiation to produce molecules and atoms. In fact, the physical and chemical process of comets is much more complicated than this.