I doubt that this is a NASA definition.  It's probably an Air Force definition 
so that they could call their pilots astronauts.  The boundary between the 
mesosphere and the ionosphere is about 90 km up.  But then the ionosphere 
extends for 200-300 km higher.  Even at an altitude of about 400 km, the 
space station needs an occasional boost to counteract the drag of the earth's 
atmosphere.  Remember Skylab?  So as far as I'm concerned, 50 miles up is too 
low to reasonably be called "space."

That said, the answer to your question is probably statute miles, since some 
pilots earned wings for X-15 flights lower than 50 nautical miles, e.g. Pete 
Knight for an altitude of 280,500 feet.

Someone wrote:
>       Does anyone know if NASA's intended definition of the edge of space
> at 50 miles is 50 miles statute or nautical?  One converts to 80.5 km and
> the other to 92.6 km.  Of course the average person on the street does not
> the difference between the two or that there is even a difference.  But I
> was under the impression that NASA's use of miles was nautical in nature
> and thus the edge of space as defined by NASA should be about 93 km.  The
> nautical definition puts the boundary much closer to the accepted metric
> definition.

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