althoughCometThe comet tail is very long and large, but they contain very little material. When the comet or comet tail occults (occultation is when the comet passes in front of other stars and blocks the starlight), the starlight weakens very little.The vast majority of cometary matter is concentrated in the small solid nucleus. The matter of the comet and tail comes from the nucleus in the final analysis, so the nucleus is the body of the comet.How big is the comet nucleus?It is also difficult to distinguish the size of cometary nuclei from telescopes on Earth.In 1927, when the comet Pons Winnecke approached the Earth to 0.037 AU, the telescope could not distinguish the size of its comet nucleus, and it was estimated that the diameter of its comet nucleus did not exceed 1 Km.Indirect estimation from observation data shows that the diameter of most comet nuclei ranges from several hundred meters to more than ten kilometers.A few comets may have larger nuclei, for exampleSun grazing cometThe diameter of the nucleus of the original mother comet of the group A is up tofiftyKm, Schwassmann Washmann, (2060) chironninetyKm (previously thought it was an asteroid, but now tends to think it is a comet).The diameter of the nucleus of most comets is considered to be about or less thantenMiles（sixteenKm), but we already know that the comet diameter is fromone hundredMeters tofortyEvery kilometer.
shape
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What shape is the comet nucleus?This is more difficult to observe.In the past, it was generally believed that the comet nucleus was spherical.Now there are some evidences that the cometary nucleus is often not spherical, but more likely to be a flat ball with a three-axis ratio of 2:1:1.The most reliable thing is the arrival of the spaceshipHalley's CometA series of video cameras revealed the true face of its comet nucleus. It is roughly a three axis 16 × 8 × 8 km flat ball, more like a flat peanut. Its surface is uneven, with several shallow pits (about 1 km in diameter), hills and valleys. The surface is covered with uneven dark dust. The albedo is very low (0.02-0.04), dark as coal, rather than as bright as ice and snow.Halley's CometIts core is like potato (16 X 8 X 8), which is composed of the same amount of ice and dust. 80% of the ice is water ice, 15% is carbon monoxide, and the rest is almost allcarbon dioxide, methane and ammonia.Scientists believe that the chemical composition of other comets is similar to Halley's Comet.Halley's comet nucleus is extremely dark. Astronomers believe that maybe other comet nuclei are also dark. Covering most of the ice core is a black shell composed of dust and rock. Only when the holes on the comet shell face the sun, the inside will be heated by the sun, and the gas will be released.
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In 2001, whenShenkong 1The spaceship flew byBorrelliWhen he was a comet, his nucleus (8 X 4 km) was about half the size of Halley's Comet.The shape of Comet Paulelli's nucleus is also like a potato, and its surface is also dark.Like Harley,Comet BorrellOnly when the holes in the housing are exposed to the sun will a small part of the area release gas.
Comet Hale BoppThe diameter of his cometary nucleus is estimated to be 30-40 km. Because his cometary nucleus is very large, it can release a large amount of gas and dust, which makes Hale Pope look particularly bright under the naked eye.
Wilt IIThe comet's nucleus is about 5 kilometers in diameter,
P/2007 R5The diameter of the cometary nucleus is about 100-200 meters,
quality
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What is the mass of the comet nucleus (also representing the comet)?This is also difficult to measure accurately.According to the relevant observational data, the mass of cometary nuclei is generally in the range of 1013-1019 grams, and there are also as many as 1020-1022 grams and as few as 1010-1011 grams.The mass of Halley's Comet is 1.5 × 1017 g.
internal structure
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What is the material composition and internal structure of the comet nucleus?Not very clear.From the mass and size of the comet nucleus, we can preliminarily calculate its average density. For example, the average density of Halley's comet is about 0.3g/cm3, which is smaller than the expected density of H2O ice dust mixed solid (about 1g/cm3), indicating that the comet nucleus is porous inside.According to the comet spectrum and the spacecraft's detection of dust particles in Halley's Comet, it can be inferred from these materials from the nucleus that the comet nucleus is mainly composed of ice (water ice, carbon dioxide ice, etc.) and dust materials, of which the largest component is water. It is estimated that the relative content (abundance) of other elements in the comet nucleus is basically the same as that of the sun and the universe, except for a few chemical elements such as hydrogen.In the past, there has been a long debate about whether the comet nucleus is a loose collection of solid particles (gravel model) or a whole solid ice block (compact core model). In 1950, Whipple proposed that the comet nucleus is an "Ice conglomerate model" of ice and dust freezing, or commonly known as "dirty snowball", which perfectly explains many observational facts.Later, this model has been developed a lot. Some people think that there are nuclear, slow and shell structures in the comet nucleus, which are similar to those in the planet interior. Some people think that the comet nucleus is more uniform inside.From the observation fact that the sub nuclei of the comet nucleus split roughly have the same spectral characteristics, it can be considered that the comet nucleus is relatively uniform on a large scale, but may be uneven on a small scale, while the surface layer (i.e. shell) of the comet nucleus is different from the interior, because the surface layer is affected by cosmic raysHigh-energy particleThe bombardment, evaporation, chemical reaction and other processes have changed, forming dark organic substances such as asphalt, and the areas on the surface of the comet nucleus are very uneven.Some properties of cometary nuclei can also be deduced from the near nuclear phenomena.The near nucleus phenomenon of many comets is asymmetric, the important reason is that the comet nucleus has rotation and its surface is uneven.The rotation periods of about 50 comets have been deduced from the near core phenomenon, and some have also calculated the spatial direction of the rotation axis. The rotation periods of comets are less than 5 hours, and some are as long as several days, with an average of about 15 hours. The direction of the rotation axis is randomly distributed.The surface of the comet nucleus is covered with dark dust, and its thermal conductivity is very small, so the interior of the comet nucleus can remain very cold without melting.The surface layer of the cometary nucleus is uneven, and some small areas (active areas) often discharge substances, forming near nuclear phenomena such as jet.[1]
