Where Does Space Begin?

If Star Trek has taught us anything, it’s that space is “the final frontier.” But where exactly does that frontier lie? It depends on who you ask.

According to the Fédération Aéronautique Internationale, the world governing body for aeronautics records, outer space begins 100 kilometers (roughly 62 miles) above sea level. This barrier, known as the Kármán Line, represents the height at which air is too thin to give a vehicle sufficient aerodynamic lift to maintain its altitude. In the FAI’s eyes, once you’ve crossed the Kármán Line, you’ve been to space.

Easy enough, right? Not if you’re the United States Air Force. By the military’s reckoning, space starts 12 miles sooner, at 50 miles above sea level. So, for Americans, that’s the threshold a pilot has to cross to become an astronaut.

Over the years, the debate has caused some strife. During the 1960s, eight American test pilots (including three civilians) flew the experimental X-15, a rocket-powered plane, above the 50-mile mark. While some of these flights crossed the Kármán Line, others only crossed the 50-mile barrier without making it up to 100 km.

By the American military definition, the pilots were all bona fide astronauts. This middle ground put NASA in an odd spot, though. Because some flyboys hadn’t crossed the Kármán Line, by international standards they’d never been to space. For nearly 40 years, NASA waffled on whether to recognize these X-15 pilots as official astronauts, but in 2005 the agency finally relented, awarding the three civilian pilots their astronaut wings.

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This story originally appeared in mental_floss magazine in 2014 and has been updated for 2024.