# Is it possible to determine whether a spacecraft is in the thermosphere or in the exosphere?

The Earth's thermopause is varying greatly between 500 km (310 mi) and 1000 km (620 mi) altitude. If a craft orbits inbetween these altitudes, or reaches them at some point of their orbit, is there any possibility to determine if it's still in the thermosphere or in the exosphere already? Has this been done? Other than high LEO satellites, examples would include the Space Shuttle flights to Hubble and some Gemini missions as well as the Apollo moon flights.

One option how to determine it might be if a sensitive enough instrument measured a reduced microgravity due to reduced atmospheric drag. The ISS experiences about 3 micro-g due to air drag; I guess there is some value that might determine the exobase from where molecules no longer collide with each other, except by chance.

• There is no hard, well-defined boundary between thermosphere and exosphere, so I'd answer "No" to the question asked in the title. If you are interested in if and how atmospheric density and composition can be measured by a satellite - that's definitely possible and might yield interesting answers. Apr 14 at 15:13
• @asdfex Like about measuring if atoms still collide with each other, if that would be possible. As the exosphere tag puts it, it shouldn't behave like a low viscosity fluid anymore. Apr 14 at 16:33
• It essentially boils down to measure the pressure (which is clearly possible). But that doesn't tell you anything about the theoretic construct of different layers of the atmosphere. Everything changes very slowly with height. There's no defined point where collisions stop to happen or fluid dynamics are not applicable any more. Apr 14 at 18:14
• @asdfex But we can determine or guess how many atoms are there in a cubic foot, can't we? Below a certain number of air atoms there would be little to no collision. Apr 15 at 5:18
• "Little to no collision" is a very broad description. In 500 km height there are still a hundred billion collisions each second in each cubic meter of space. Apr 15 at 8:09