thank you both. sorry archie, i thought (well you did) you were just trolling me.
so just to wrap it up can use 60 miles at the end? if a helium balloon falls/pops at 20 miles its fair to say its getting less dense and even if the stars were in an atmosphere at 3100 miles it would be so thin it would be undetectable. to be fair it probably wouldnt be detectable at 100 miles then. this is all work towards my work on atmospheric density and the distances we can see. its slow progress but its something that interests me