Except when far away from the sun, the long, bright and sparse tail of a comet gives the impression that it is very close to the earth, even in our atmosphere. 1577 tycho pointed out that comets do not show different orientations from different places on the earth: therefore, he correctly concluded that they must be far away.
Whenever a comet approaches the sun, its brightness will increase rapidly. Observations of comets that are quite far from the sun show that they move along a highly elongated ellipse, and the sun is located at a focus of this ellipse, which is consistent with Kepler's first law. Comets spend most of their time away from the sun, where they are invisible. They can only be seen when they are near the sun.
There are about 40 comets whose period of revolution is quite short (less than 100 years), so they will appear one after another as the same celestial body. Halley's comet is the first celestial body observed in history. Newton's friend and donor Harley (1656- 1742) realized that it was periodic in 1705. Its cycle is 76 years. Historical records show that it will be observed whenever it passes through the sun from 240 BC, possibly from 466 BC. It was last adopted in 1986.
The brightness of a comet is very low when it is far away from the sun, and its spectrum is simply the spectrum of reflected sunlight. When the comet enters within 8 astronomical units from the sun, its brightness begins to increase rapidly and its spectrum changes dramatically. Scientists saw several bright spectral lines belonging to known molecules. This change occurs because the solid matter (comet nucleus) that makes up a comet suddenly becomes hot enough to evaporate, and a gas cloud called coma surrounds the comet nucleus. The ultraviolet rays of the sun make this gas glow.
The diameter of coma is usually about 105 km, but the tail is often very long, reaching 108 km or 1 astronomical unit. Tail is considered to be composed of gas and dust; Four combined effects blow it out of the comet: (1) the initial momentum obtained when gas and accompanying dust evaporate from the comet nucleus. (2) The radiation pressure of sunlight pushes dust away from the sun. (3) The solar wind blows charged particles away from the sun. (4) the attraction to the sun. The interaction of these effects makes each tail look different. Of course, matter evaporates into the coma and tail, consuming the matter in the nucleus. Sometimes it appears in an explosive way, such as comet Bila; It broke into two pieces when it crossed the sun in 1846, and all disappeared after it crossed in 1852. Scientists estimate that comets usually only a few astronomical units away from the sun will disintegrate within thousands of years.
A.D. 1066, on the eve of Norman invasion of England, Comet Halley returned. At that time, people looked at this strange celestial body with a long tail in the night sky with mixed feelings and thought it was a war warning and omen given by God. Later, the Normans conquered England, and Commander Norman's wife embroidered the scene of Halley's Comet's return on the tapestry as a souvenir. China folks call comets "bad luck" and "disaster". In this way, the appearance of comets is associated with human wars, famines, floods, plagues and other disasters.
Comet is a small cloud-like celestial body with low mass, which orbits the sun in a flat orbit (very few in a near-circular orbit).
The orbit of a comet
There are three kinds of orbits of comets: ellipse, parabola and hyperbola. Comets with elliptical orbits are also called periodic comets, and comets with other two orbits are also called aperiodic comets. Periodic comets are divided into short-period comets and long-period comets. Generally speaking, a comet consists of a head and a tail. The comet head includes two parts: the nucleus and the coma, and some have comet clouds. Not all comets have structures such as nucleus, coma and tail. In ancient China, the shape of comets has been studied a lot. There are 29 pictures of comets on the silk book unearthed from Mawangdui ancient tomb in Changsha. It is clearly stated in Tian Wenzhi in the Book of Jin that comets don't shine because they reflect sunlight and are seen by us. Their tails are facing away from the sun. Comets are huge, but their mass is pitifully small, even the mass of big comets is less than one tenth of that of the earth. Because a comet is composed of frozen impurities and dust, it is only a small cloud-like spot when it is far away from the sun; When it is close to the sun, the comet tail is produced because of the evaporation, gasification, expansion and eruption of the solidified body. The tail is huge, hundreds of millions of kilometers long. It has different shapes, some more than one, and generally extends away from the sun, and the closer it is to the tail of the sun, the longer it is. There are many comets in the universe, but only about 1600 has been observed so far.
The orbit of a comet is very different from that of a planet. It is an extremely flat ellipse, and some even have parabolic or hyperbolic orbits. Comets with elliptical orbits can return to the sun regularly, and such comets are called periodic comets. A comet with a parabolic or hyperbolic orbit can only approach the sun once in its life, and once it leaves, it will never come back. They are called aperiodic comets. These comets may not be members of the solar system, but passers-by from outside the solar system, inadvertently broke into the solar system, and then returned to the depths of the vast universe without hesitation. Periodic comets can be divided into short-period comets (period of revolution around the sun is less than 200 years) and long-period comets (period of revolution around the sun is more than 200 years). The orbits of more than 600 comets have been calculated so far. The orbit of a comet may be affected by planets and changes. When the comet is accelerated by the influence of the planet, its orbit will flatten, even become a parabola or hyperbola, thus making the comet leave the solar system; When the comet slows down, the eccentricity of the orbit will become smaller, which will make the long-period comet become a short-period comet, or even change from an aperiodic comet to a periodic comet and be "captured".
Structure of comet
Comets have no fixed volume. They are very small when they are far away from the sun. When approaching the sun, the coma becomes bigger and bigger, the tail becomes longer and the volume becomes very huge. The longest tail can reach more than 200 million kilometers. The mass of comets is very small, and most of them are concentrated in the nucleus. The average density of the nucleus is per cubic centimeter1g. The material in coma and tail is extremely thin, accounting for only 1%-5% of the total mass, or even less. Comet material is mainly composed of water, ammonia, methane, cyanide, nitrogen, carbon dioxide and so on. The comet nucleus is a "dirty snowball" composed of water, carbon dioxide (dry ice), ammonia and dust particles condensed into ice.
The origin of comets
The origin of comets is an unsolved mystery. It has been suggested that there is a large comet area outside the solar system, where there are about 654.38+000 billion comets, called Oort Cloud. Due to the gravity of other stars, some comets enter the solar system. Due to the influence of Jupiter, some comets escape from the solar system, while others are "captured" as short-period comets. Others believe that comets are formed near Jupiter or other planets; Others believe that comets formed in remote areas of the solar system; Some people even think that comets are visitors from outside the solar system.
Because periodic comets have been disintegrating, there must be a way to produce new comets to replace the old ones. One possible method is to store billions of comets orbiting the sun in various possible directions on the radius from the sun 105 astronomical unit. This concept is supported by observations, that is, aperiodic comets approach the sun in random directions along a long elliptical orbit. With the passage of time, due to the slight gravity given by the passing stars, the orbits of distant comets can be disturbed until the distance of their perihelion becomes less than several astronomical units. When this comet subsequently enters the solar system, the gravity of the planets in the solar system can transform this aperiodic comet into a new periodic comet (it will exist for thousands of years before it collapses). On the other hand, these forces can completely throw it out of the comet cloud. If this statement is correct, then about 1000 comet records in the past few centuries are only a few samples in the huge comet cloud and have not been directly observed so far. This comet cloud associated with a single star may be all over our galaxy. So far, no method has been found to detect a large number of comets that may form a group with the sun, let alone those that form a group with other stars.
The total mass of comet clouds is not clear, not only the total number of comets is difficult to determine, but also the mass of a single comet is very uncertain. It is estimated that the mass of comet cloud is between 10- 13 and 10-3 earth mass.
Characteristics of comets
The nature of the comet is not exact, because it is hidden in the coma and cannot be directly observed, but we can guess some of its properties from the spectrum of the comet. Usually, these spectral lines indicate the existence of gases with OH, NH and NH2 groups, which can be easily explained as the result of decomposition of the most common stable hydrogen compounds of elements C, N and O, namely CH4, NH3 and H2O, and the ice frozen by these compounds may be the main component of the comet nucleus. Scientists believe that all kinds of ice and silicate particles are scattered in the core of the comet in a loose structure, and some of them are like dirty snowballs with a density of about 0. 1 g/cm3. After the ice is heated and evaporated, it leaves loose rock materials, and the size of individual particles varies from 104 cm to 105 cm. When the earth passes through the orbit of a comet, we regard the observed particles as meteors. There is reason to believe that comets may be part of the matter in nebulae that gather to form the sun and planets. Therefore, people want to try to get samples of comet material for analysis, so as to learn more about the origin of the solar system. This plan can be done in theory, such as trying to rendezvous with periodic comets in space. This plan is currently under study.
Comets and life
Comet is a very special star, which may have an important connection with the origin of life. Comets contain many gases and volatile components. According to spectral analysis, there are mainly C2, CN, C3 and other atoms and atomic groups such as OH, NH, NH2, ch, Na, C and O .. This shows that comets are rich in organic molecules. Many scientists have noticed this phenomenon: maybe life originated from comets!
At 1990, Kevin. J. Zahule and Daid Grinspoon of NASA explained the organic dust in the strata near the Cretaceous-Tertiary boundary as follows: one or several comets swept the earth, leaving amino acids and forming this organic dust; It is pointed out that in the early days of the formation of the earth, comets can also sprinkle organic matter on the earth like light rain in this way-this is the source of life on the earth.