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| A .et is a small body in the solar system that orbits the Sun and (at least occasionally) exhibits a .a (or atmosphere) and/or a tail both due primarily to the effects of solar radiation upon the .et’s nucleus, which itself is a minor planet .posed of rock, dust, and ices. Due to their origins in the outer solar system and their propensity to be highly affected (or perturbed) by relatively close approaches to the major planets, .ets’ orbits are constantly changing. Some are moved into sungrazing orbits that destroy the .ets when they near the Sun, while others are thrown out of the solar system forever. Most .ets are believed to originate in a cloud (the Oort cloud) at large distances from the Sun consisting of debris left over from the condensation of the solar nebula; the outer edges of such nebulae are cool enough that water exists in a solid (rather than gaseous) state. Asteroids originate via a different process, but very old .ets which have lost all their volatile materials may .e to resemble asteroids. The word .et came to the English language through Latin .etes. From the Greek word kom, meaning "hair of the head," Aristotle first used the derivation komts to depict .ets as "stars with hair." Physical characteristics Long-period .ets are believed to originate in a distant cloud known as the Oort cloud (after the astronomer Jan Hendrik Oort who hypothesised its existence). They are sometimes perturbed from their distant orbits by gravitational interactions, falling into extremely elliptical orbits that can bring them very close to the Sun. One theory says that as a .et approaches the inner solar system, solar radiation causes part of its outer layers, .posed of ice and other materials, to melt and evaporate, but this has not been proven. The streams of dust and gas this releases form a very large, extremely tenuous atmosphere around the .et called the .a, and the force exerted on the .a by the Sun’s radiation pressure and solar wind cause an enormous tail to form, which points away from the sun. The streams of dust and gas each form their own distinct tail, each pointed in slightly different directions. The tail made of dust is left behind in the .et’s orbit in such a manner that it often forms a curved tail. At the same time, the ion tail, made of gases, always pointing directly away from the Sun, as this gas is more strongly affected by the solar wind than dust is, following magnetic field lines rather than an orbital trajectory. While the solid body of .ets (called the nucleus) is generally less than 50km across, the .a may be larger than the Sun, and the ion tails have been observed to extend over 150 million km (1 Astronomical unit) or more. Both the .a and tail are illuminated by the Sun, and may be.e visible from the Earth when a .et passes through the inner solar system, the dust reflecting sunlight directly and the gases glowing due to ionization. Most .ets are too faint to be visible without the aid of a telescope, but a few each decade be.e bright enough to be visible with the naked eye. Before the invention of the telescope, .ets seemed to appear out of nowhere in the sky and gradually vanish out of sight. They were usually considered bad omens of deaths of kings or noble men, or .ing catastrophes. From ancient sources, such as Chinese oracle bones, it is known that their appearance have been noticed by humans for millennia. One very famous old recording of a .et is the appearance of Halley’s .et on the Bayeux Tapestry, which records the Norman conquest of England in 1066. Surprisingly, .etary nuclei are among the blackest objects known to exist in the solar system. The Giotto probe found that .et Halley’s nucleus reflects approximately 4% of the light that falls on it, and Deep Space 1 discovered that .et Borrelly’s surface reflects only 2.4% to 3% of the light that falls on it; by .parison, asphalt reflects 7% of the light that falls on it. It is thought that .plex organic .pounds are the dark surface material. Solar heating drives off volatile .pounds leaving behind heavy long-chain organics that tend to be very dark, like tar or crude oil. The very darkness of .etary surfaces allows them to absorb the heat necessary to drive their outgassing. In 1996, .ets were found to emit X-rays. These X-rays surprised researchers, because their emission by .ets had not previously been predicted. The X-rays are thought to be generated by the interaction between .ets and the solar wind: when highly charged ions fly through a .etary atmosphere, they collide with .etary atoms and molecules. In these collisions, the ions will capture one or more electrons leading to emission of X-rays and far ultraviolet photons. About the Author: 相关的主题文章:

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