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[GMAT阅读题讨论] PREP2012阅读第2篇第1题

icetong123 发表于 2015-1-8 17:22:20 | 显示全部楼层 |阅读模式
本帖最后由 icetong123 于 2015-1-9 10:17 编辑

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.

The author states that the research described in the first paragraph was undertaken in order
A. determine the age of an actual meteor stream
B. dentify the various structural features of meteor streams
C. explore the nature of a particularly interesting meteor stream
D. test the hypothesis that meteor streams become broader as they age
E. show that a computer model could help in explaining actual astronomical data




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