The layer of sky above the cruising height of commercial aircraft and below satellite orbits was, until recently, regarded by most people to be empty space.That changed after a gigantic Chinese balloon drifted across North America 20km above the ground, followed by three more mysterious objects with lower altitude flight paths, which were all shot down.
以往大部分人都以为，在商业飞机巡航高度以上、卫星轨道以下的这段天空就是空无一物的空间。
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