What is a comet?

Comets are small celestial bodies composed of ice and a small amount of rocks, with an average material density of only 10- 1000 kg/m3. Astronomers vividly call comets "dirty snowballs". In general, comets are all at the edge of the solar system, and even if they are observed, they are similar to extremely weak stars, and no detailed structure can be seen. However, as it gradually approached the sun, due to the increase of solar radiation, solar wind and solar pressure, especially after entering the orbital region of Mars, the surface substances volatilized to form a comet tail, showing its unique structure.

Comet head: Comet has a hazy and bright "head" part-comet head. The head of coma consists of two parts, the center of which is a dense and bright nucleus and a foggy coating-coma.

Comet nucleus: Comet nucleus is usually very small, with a diameter of 0. 1- 100km, and rarely exceeds 100km. The mass of almost all comets is concentrated in the nucleus.

Coma: Coma is the evaporation of nucleus, and its shape and size are closely related to the distance from the sun. Generally speaking, the closer to the sun, the brighter and bigger the coma, with a diameter of hundreds of thousands of kilometers, sometimes equivalent to or even exceeding the diameter of the sun.

Comet Cloud: After human beings entered the space exploration, they found that there was a larger envelope outside the coma, with a diameter of about100-1100,000 kilometers-comet cloud. Because it is almost entirely composed of hydrogen atoms, it is also called hydrogen cloud. The density of hydrogen clouds is extremely thin, which is generally not observed on the ground.

Comet tail: The long tail is one of the main features of comets visible to the naked eye and the longest celestial body in the solar system. It always has its back to the sun. Because the comet tail is the result of the material in the head of the comet under the action of the sun, it is not only related to the mass of the comet, but also depends on the distance from the sun. At perihelion, it reaches the longest; Some planetary comets have only fuzzy tails in telescopes. The ion tail, which is mainly composed of gas, is straight, while the dust tail containing many dust substances is generally curved. In addition, a small number of comets have abnormal tails at a certain stage-tail-like substances protruding towards the sun, which are called anti-tails, and some people call them "comet tails". Some comets have more than one tail, and there are many examples of double tails or even three tails in history. The most famous comet is 1744, which has six tails. The material of the tail is very thin, and the light of the star can pass freely without any influence. The density of thin tail is much lower than that of gas in laboratory vacuum.

Every time a comet visits the sun, it will lose some volatile substances and eventually become another rocky celestial body in the solar system. So for the universe, comets can be said to be very short-lived Many scientists believe that some asteroids are extinct comets, and these comets have lost all their volatile substances.

More than 1800 comets have been discovered so far, and most of them have only met us once. After they run around the sun, they will never return along a parabola or hyperbola. Only comets in closed orbits can go back and forth. This is a periodic comet. Scientists set 200 years as the dividing line between short-period comets and long-period comets.

Scientists have always been interested in comets, because comets are considered to be the oldest and most primitive celestial bodies in our solar system, and their material composition is similar to that of nebulae before the formation of the solar system. This nebula later collapsed to form the sun and planets, so it contained dust and gas when the sun and planets formed 4.6 billion years ago. Scientists believe that the original material that formed life on the earth was probably brought to the earth when the comet hit the earth, which provided a window for scientists to study the formation of life in the solar system and on the earth.