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A meteor stream (meteor stream: 流星群) 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 (gain on: v.逼近, 超过, 侵蚀) 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 positions of a group of hypothetical dust particles. In the model, the particles were randomly distributed throughout a computer simulation (computer simulation: 计算机模拟) 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 (meteor shower: n.[天]流星雨) occurs. Moving at a little 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（分歧）, 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. 85.
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 line 18 and the new computer-derived theory? (A) Dust particles in a meteor stream will usually be distributed evenly throughout any cross section of the steam. (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.（D） (E) The individual dust particles in older meteor streams should be, on average, smaller than those that compose younger meteor streams. 答案是D而我选的A,其实我根本没找到原文中可以解释它的部分，请NN们帮帮忙吧。。（如果可以把整篇都讲一下就更好了~文章中提到的多少year多少mile的信息我都整合不起来。。
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发表于 2008-1-23 14:20:00
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