But while higher is better from a fuel conservation perspective, there is an upper limit on how high it is safe to orbit humans for long periods.

越高耗费的燃料越少，而载人飞船能够长时间安全飞行的轨道高度就有上限。

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This is a best-practice time-limit adopted by the world's space agencies for the removal of their equipment from orbit once it has completed its mission.

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