The Beginning of Space
The Mesosphere is the name of the layer of the atmosphere from 40 miles up to 60 miles up where the atmosphere is very thin but still dense enough to produce sufficient drag to pull a satellite down. Just beyond the Mesosphere is where “Space” starts.
In the real world almost nothing flies to that height, or it goes through it so fast that it's ignored.
But the mesosphere has some interesting and not-well-understood-even-now phenomena such as
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In the Voidships universe ships fly through these areas slowly so the phenomena would be seen regularly, dangerous and not understood at all.