6000 mph @ 20,000’? That’s roughly mach 10. An F-15 going mach 1.5 at 20,000 feet might break windows on the ground. The space shuttle is going sub sonic by that altitude upon reentry because the air is getting so thick. An alien space ship the size of an apartment building doing Mach 10 at low altitude would be leveling forests with it’s sonic boom pressure. Just like the asteroid did in that part of the world 100 years ago or so. Yet there are no reports of sonic booms or damage?