Well, according to the Fédération Aéronautique Internationale, the world governing body for this sort of thing, outer space begins 100 kilometers (roughly 62 miles) above sea level. This barrier is called the Kármán Line. Above that, the air is too thin for a vehicle to maintain altitude. So…once you’ve crossed Kármán, you’ve been to space.Ah, but now the USAF weighs in. By military standards, Space starts 12 miles sooner, or 50 miles above sea level. For Americans, crossing that threshold makes a pilot an astronaut.
All of which gave NASA a bit of a headache. During the 1960s, 8 American test pilots flew the experimental X-15 above the 50-mile mark, but only 5 of them passed Kármán. Astronauts all by American military definition, but to the FAI, 3 hadn’t even been to space.For nearly 40 years, NASA waffled on whether to recognize these X-15 pilots as astronauts. Then in 2005 the agency relented, finally awarding astronaut wings to the remaining three.
This info, including the illustration, was shamelessly copped from the March/April 2021 issue of Mental_Floss.