两位作者提出的最大担忧是太空垃圾——环绕地球运行的“垃圾”碎片，如果它们与包括国际空间站在内的运行单位相撞，可能会造成严重损害。
The greatest concern the authors raise is space debris-pieces of "junk" that orbit the earth, with the potential to cause serious damage if they collide with operational units, including the ISS.
环绕地球运行的人造卫星为科学家们提供了大量有关外层空间情况的资料。
Satellites orbiting round the earth have provided scientists with a vast amount of information about conditions in outer space.
因为有卫星的头衔，而且沿椭圆形轨道环绕地球运行，月亮似乎每天平均51分钟返回到天空的同一地点。
Due to the moon's titled and elliptical orbit around Earth, the moon seems to return to the same place in the sky 51 minutes later each day, on average.
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