So far,NASA has found 16,000 pieces of junk that are larger than 10 cm in diameter and more than 500,000 pieces measuring between 1 cm and 10 cm.You can imagine how many even smaller ones in space.
Space junk is dangerous for spacecraft and the International Space Station.Even a very small piece could seriously damage or even destroy a satellite or spacecraft if it collided with the junk.The International Space Station has to regularly change its orbit to avoid space junk.
Space junk might also be a danger to humans.During the last four months,three very large spacecraft have fallen to Earth.Fortunately,no one was hurt because the junk landed in the ocean.
21.According to the article,space junk .
A.travels at 8 kilometers per hour
B.could be cleared away within five years
C.travels at the same speed as satellites
D.will cause a lot of trouble for space stations
解析:细节理解题。从第七段第一句“SpacejunkisdangerousforspacecraftandtheInternationalSpaceStation.”可知D项正确。由第一段第三句中的“Spacejunktravelsat8kilometersasecond,...”可知A项错误;第二段第二句中说“...senda ‘cleanersatellite’ intospacewithinthenextfiveyears.”而不是说太空垃圾能在五年内被清理干净,故B项错误;文中只说到卫星和太空垃圾都在高速飞行,并没有提到彼此的速度是否一样。
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