题目内容

A meteor stream is composed of dust particles that have been ejected from a parent comet at a variety of velocities. These particles follow the same orbit as the parent comet, but due to their differing velocities they slowly gain on or fall behind the disintegrating comet until a shroud of dust surrounds the entire cometary orbit. Astronomers have hypothesized that a meteor stream should broaden with time as the dust particles' individual orbits are perturbed by planetary gravitational fields. A recent computer-modeling experiment tested this hypothesis by tracking the influence of planetary gravitation over a projected 5,000-year period on the position of a group of hypothetical dust particles. In the model, the particles were randomly distributed throughout a computer simulation of the orbit of an actual meteor stream, the Geminid. The researcher found, as expected, that the computer-model stream broadened with time. Conventional theories , however, predicted that the distribution of particles would be increasingly dense toward the center of a meteor stream. Surprisingly, the computer-model meteor stream gradually came to resemble a thick-walled, hollow pipe.


Whenever the Earth passes through a meteor stream, a meteor shower occurs. Moving at over 1,500,000 miles per day around its orbit, the Earth would take, on average, just over a day to cross the hollow, computer-model Geminid stream if the stream were 5,000 years old. Two brief periods of peak meteor activity during the shower would be observed, one as the Earth entered the thick-walled "pipe" and one as it exited. There is no reason why the Earth should always pass through the stream's exact center, so the time interval between the two bursts of activity would vary from one year to the next.


Has the predicted twin-peaked activity been observed for the actual yearly Geminid meteor shower? The Geminid data between 1970 and 1979 show just such a bifurcation, a secondary burst of meteor activity being clearly visible at an average of 19 hours (1,200,000 miles) after the first burst. The time intervals between the bursts suggest the actual Geminid stream is about 3,000 years old.

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第一题 第二题 第三题 第四题 第五题

The passage suggests that which of the following is a prediction concerning meteor streams that can be derived from both the conventional theories mentioned in the highlighted text and the new computer derived theory?

  • A.

    Dust particles in a meteor stream will usually be distributed evenly throughout any cross section of the stream.

  • B.

    The orbits of most meteor streams should cross the orbit of the Earth at some point and give rise to a meteor shower.

  • C.

    Over time the distribution of dust in a meteor stream will usually become denser at the outside edges of the stream than at the center.

  • D.

    Meteor showers caused by older meteor streams should be, on average, longer in duration than those caused by very young meteor streams.

  • E.

    The individual dust particles in older meteor streams should be, on average, smaller than those that compose younger meteor streams.

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网友讨论

  • hannahyz_pku

    第一段:两个观点的对立。宇航员认为stream会随着时间扩大,一个电脑模型证明了这一点。但是传统的模型却认为越到meteor的中心密度越高。
    第二段:地球穿过mateor stream时,会有流星雨。两个活动的高峰期。
    第三段:两个活动高峰期真的在电脑模型中被观测到了,可以推论Geminid的年龄。



    A: 错误,两个观点对于粒子的分布是不同的
    B:没有提到的most
    C:错误,对于分布的观点是不同的。
    D:随着stream的age增长,传统观点认为中心密度会越来越高;新观点认为会越来越广,这都会增加地球穿过stream的时间,因此选D。
    E:没提到每个粒子的大小

科目:RC阅读

来源:Prep12

难度:650-700

考点:推理