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  3. Aristotle believed that the heavens were perfect. If they ever were, they are no longer. The skies above Earth are now littered

Aristotle believed that the heavens were perfect. If they ever were, they are no longer. The skies above Earth are now littered

admin2011-03-11  33
问题     Aristotle believed that the heavens were perfect. If they ever were, they are no longer. The skies above Earth are now littered with the debris (残骸) of dead satellites, bits of old rockets and the odd tool dropped by a spacewalking astronaut. Such is the extent of the detritus that the first accidental collision between two satellites has already taken place. It happened in February 2009, when a defunct (废弃的) Russian Cosmos smashed into a functioning American Iridium, destroying both and creating even more space junk.
    To stop this sort of thing happening again Vaios Lappas of the University of Surrey, in England, has designed a system that will remove satellites from orbit at the end of their useful lives--and as a bonus will scour part of the sky clean as it does so. Dr. Lappas’s satellite-removal system employs a solar sail. As light from the sun hits the sail, it imparts a minuscule but continuous acceleration. When a satellite is first launched, the sail is angled in a way that causes this acceleration to keep the satellite in orbit. (Orbits gradually decay as a result of collisions with the small number of air molecules found even at altitudes normally classified as "outer space". )
    Solar sails have yet to be used widely to propel spacecraft in this way--several earlier versions came unstuck when the sails failed to unfurl properly-but doing so is not a novel idea in principle The novelty Dr. Lappas envisages is to change the angle of the sail when the satellite has become defunct. Instead of keeping the derelict craft in orbit, it will, over the course of a couple of years, drag it into the atmosphere and thus to a fiery end. Not only that, but the sail will also act like a handkerchief, mopping up microscopic orbital detritus such as flecks of paint from previous launches. A fleck of paint may not sound dangerous, but if travelling at 27 000kph (17 000mph), as it would be in orbit, it could easily penetrate an astronaut’s spacesuit.
    A prototype of Dr. Lappas’s design, called CubeSail, will be launched late next year. It weighs just 3kg and, when folded up, measures 30cm (12 inches) by 10era by 10era. Once unfurled, however, the sail will have an area of 25 square metres. If this prototype, which is paid for by EADS, a European aerospace company, proves successful, solar sails might be added to many future satellites. That would enable them to be removed rapidly from orbit when they became useless and would restore to the skies some measure of Aristotelian perfection.
Orbits decay due to ______.
选项 A、the acceleration of the spacecraft
B、the crash with small amounts of air molecules
C、the invention and application of solar sails
D、the existence of air molecules
答案B
解析 本题考查轨道渐渐衰减的原因。第二段括号中as a result of引出了其原因，即与为数甚少的空气分子相互碰撞所致，只有[B]陈述的是与微粒碰撞，符合题意。第二段倒数第一句谈到加速是为了把卫星保持在轨道之上，并非轨道衰减的原因，故[A]理解有误。从第二段第三、四句可以看出，太阳能帆板的作用是给卫星加速，所以[C]不正确。[D]属于断章取义，空气分子的存在只是轨道衰减的一个前提条件，并不是充分条件，必须碰撞才会使轨道逐渐衰减。  
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