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  3. COMETS (1) Comets are among the most interesting and unpredictable bodies in the solar system. They are made of frozen gases

COMETS (1) Comets are among the most interesting and unpredictable bodies in the solar system. They are made of frozen gases

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问题                                                 COMETS
    (1) Comets are among the most interesting and unpredictable bodies in the solar system. They are made of frozen gases (water vapor, ammonia, methane, carbon dioxide, and carbon monoxide) that hold together small pieces of rocky and metallic materials. Many comets travel in very elongated orbits that carry them far beyond Pluto. These long-period comets take hundreds of thousands of years to complete a single orbit around the Sun. However, a few short-period comets (those having an orbital period of less than 200 years), such as Halley’s Comet, make a regular encounter with the inner solar system.
    (2) When a comet first becomes visible from Earth, it appears very small, but as it approaches the Sun, solar energy begins to vaporize the frozen gases, producing a glowing head called the coma. The size of the coma varies greatly from one comet to another. Extremely rare ones exceed the size of the Sun, but most approximate the size of Jupiter. Within the coma, a small glowing nucleus with a diameter of only a few kilometers can sometimes be detected. As comets approach the Sun, some develop a tail that extends for millions of kilometers. Despite the enormous size of their tails and comas, comets are relatively small members of the solar system.
    (3) The observation that the tail of a comet points away from the Sun in a slightly curved manner led early astronomers to propose that the Sun has a repulsive force that, pushes the particles of the coma away, thereby forming the tail. Today, two solar forces are known to contribute to this formation. One, radiation pressure, pushes dust particles away from the coma. The second, known as solar wind, is responsible for moving the ionized gases, particularly carbon monoxide. Sometimes a single tail composed of both dust and ionized gases is produced, but often two tails—one of dust, the other, a blue streak of ionized gases—are observed.
    (4) As a comet moves away from the Sun, the gases forming the coma recondense, the tail disappears, and the comet returns to distant space. Material that was blown from the coma to form the tail is lost from the comet forever. Consequently, it is believed that most comets cannot survive more than a few hundred close orbits of the Sun. Once all the gases are expelled, the remaining materials—a swarm of tiny metallic and stony particles—continue the orbit without a coma or a tail.
    (5) Comets apparently originate in two regions of the outer solar system. Most short-period comets are thought to orbit beyond Neptune in a region called the Kuiper belt, in honor of the astronomer Gerald Kuiper. During the past decade over a hundred of these icy bodies have been discovered. Most Kuiper belt comets move in nearly circular orbits that lie roughly in the same plane as the planets. A chance collision between two comets, or the gravitational influence of one of the Jovian planets—Jupiter, Saturn, Uranus, and Neptune—may occasionally alter the orbit of a comet in these regions enough to send it to the inner solar system and into our view.
    (6) Unlike short-period comets, long-period comets have elliptical orbits that are not confined to the plane of the solar system. These comets appear to be distributed in all directions from the Sun, forming a spherical shell around the solar system, called the Oort cloud, after the Dutch astronomer Jan Oort. Millions of comets are believed to orbit the Sun at distances greater than 10,000 times the Earth-Sun distance. The gravitational effect of a distant passing star is thought to send an occasional Oort cloud comet into a highly eccentric orbit that carries it toward the Sun. However, only a tiny portion of the Oort cloud comets have orbits that bring them into the inner solar system.
    (7) The most famous short-period comet is Halley’s Comet, named after English astronomer Edmond Halley. [A] Its orbital period averages 76 years, and every one of its 30 appearances since 240 B.C. has been recorded by Chinese astronomers. [B] When seen in 1910, Halley’s Comet had developed a tail nearly 1.6 million kilometers (I million miles) long and was visible during daylight hours. [C] Its most recent approach occurred in 1986. [D]
An introductory sentence for a brief summary of the passage is provided below. Complete the summary by selecting the THREE answer choices that eocpress the most important ideas in the passage. Some sentences do not belong in the summary because they eocpress ideas that are not presented in the passage or are minor ideas in the passage. This question is worth 2 points.
Short-period comets complete their orbits in less than 200 years while long-period comets may take hundreds of thousands of years to complete an orbit.
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Answer Choices
(A) The size of a comet’s nucleus is determined by two solar forces, radiation pressure and solar wind.
(B) Comets accelerate as they move away from the Sun in part because they lose material from the coma each time they pass near the Sun.
(C) Comets are at times sent into the inner solar system as a result of a collision or a gravitational disturbance.
(D) Although a comet’s coma and tail may be quite large, these features do not develop until a comet is close enough to the Sun for solar energy to vaporize its frozen gases.
(E) While short-period comets have circular orbits that lie close to the plane of the planets, long-period comets have more elliptical orbits that form a spherical shell around the solar system.
(F) Although studies of Halley’s Comet and other short-period comets provide some clues, astronomers do not yet fully understand how and when comets formed.
选项
答案C,D,E
解析 本题属于文章总结题。C项“彗星有时由于碰撞或引力干扰而被送入内太阳系”是第5段最后一句和第6段倒数第2句的整合。D项“尽管彗发和彗尾可能很大，但只有彗星离太阳足够近，它们才能形成，因为太阳能会蒸发其冻结的气体”是对第2段第1句和最后两句的整合。E项“短周期彗星的圆形轨道与行星几乎在同一个平面，而长周期彗星有更多的椭圆轨道，形成了围绕着太阳系的球壳”是对第5段倒数第2句和第6段第1、2句的整合。A项“彗星内核的大小取决于两个太阳力，即辐射压力和太阳风”，原文并未提及彗星内核大小的问题，故A项错误。B项“彗星会在远离太阳的过程中加速，而导致这种情况发生的部分原因是彗星每次经过太阳附近时，彗发都会失去物质”，原文并未提及彗星在远离太阳的过程中会加速，属无中生有，故排除B项。F项“尽管对哈雷彗星和其他短周期彗星的研究提供了一些线索，但是天文学家尚未完全了解彗星的形成方式和时间”，原文并未提及天文学家是否了解彗星的形成方式和时间，属无中生有项。
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