 In the mesosphere, 80 km is nearly equal to 50 mi, the altitude used by the United States to confer astronaut status on pilots, as in the X-15 program itself.It also differs from the internationally accepted Kármán line altitude of 100 km, used by the Fédération Aéronautique Internationale for the same purpose.In 2018 McDowell published a refereed journal paper in Acta Astronautica making detailed physical arguments for the 80 km value.