Question

What atmospheric layer lies above the highest altitude an airplane can go and below the lowest altitude a spacecraft can orbit?

Answer 1

mesosphere

Not so fast. The mesosphere is the least known layer of the atmosphere. The mesosphere lies above the highest altitude an airplane can go. It lies below the lowest altitude a spacecraft can orbit. Maybe that's just as well. If you were in the mesosphere without a space suit, your blood would boil! This is because the pressure is so low that liquids would boil at normal body temperature.