system
Given the following post, respond as if you're commenting on Reddit:
human
At what point does the atmosphere get so thin that describing speeds in terms of 'mach' or relative to the speed of sound becomes pointless?
gpt
The Karman line at 100km ASL represents the "boundary" between atmosphere and space, because a flying object would need to travel faster than the orbital velocity in order to derive lift to support itself. So pretty much by that point you're in orbit moving at orbital velocities.

There is still a fair chunk of atmosphere at 100 km and the orbiting object will almost surely reenter the atmosphere on that pass. You can still measure velocity in Mach up there, but it won't really make sense.