You know that while airplanes fly through the air, they’re still technically on planet Earth. You also know that at some point, a vehicle could get so high up that it would be in outer space. That all seems a bit vague, but it turns out, space has a definite on-paper start point, and it’s precisely 327,360 feet up.

That point is known as the Kármán Line, and “327,360" is not just an unwieldy number, it’s also in the wrong Imperial system, so it’s better to just go with its official definition of 100 kilometers, or 62 miles.

(NASA has a different line for the beginning of space at 50 miles up, but we’ll get to that one in just a second.)

I’m a big dummy, so I thought that “100 km” number was just pulled out of thin air by scientists. I mean, a nice, even “100" is just a bit too convenient, isn’t it?

But it turns out, it isn’t. The 100-kilometer figure based on some genuine scientific reasoning, as the points out:

This is where an aircraft would have to travel at a speed greater than orbital velocity to get enough lift from its wings to stay in the air.

Here’s an example of what the Institute is talking about, in layman’s terms. Take everyone’s second-favorite spy plane, the U-2 Dragon Lady. At its maximum altitude somewhere above 70,000 feet, the air is incredibly thin. It’s so thin, in fact, that pilots have to wear a space suit, despite 70,000 feet being 257,360 lower than the official limit of “outer space.”

To fly at 70,000 feet, the U-2 has very long, slender wings to cling to that very thin air. It also has to be going pretty quickly, too, so that what air there is is moving fast enough over those wings to generate lift. Any slower than that, and it’ll stall, even if it’s moving at 350 mph.

But! If the U-2 moves too quickly, it will start encountering shock waves as it approaches the speed of sound, something known as Mach Buffet. When it hits that point, it’s said to be at its Critical Mach Number. And that little bit of space between the stall speed and that Critical Mach Number is colloquially called “coffin corner,” as this person on helpfully explains:

At the coffin corner, the margin between the Critical Mach Number and the Stall Speed is so small, that they almost coincide. So any decrease in speed will cause the wing to lose lift and stall. And any increase in speed with cause Mach Buffet and the pilot will lose control of the aircraft. And the U2 is probably the only aircraft in the world DESIGNED to operate IN the coffin corner. The speed margin for a U2, between a 1G stall and Mach buffet, is around 4 knots. 

What the Kármán Line denotes is a similar idea to the coffin corner, but instead of the risk of losing control of the vehicle at a Critical Mach Number, the real problem is the risk that you’ll just start zooming off into deep space.

Essentially, once you get to about 62 miles up, the air is so thin that your theoretical airplane has to be traveling fast enough just to stay aloft that you would escape the clutches of Earth’s gravity anyway based on speed alone (for those wondering, it’s a little above 17,000 mph, give or take a few thousand here or there).

Speed up, you go blasting off into space. Slow down, you fall out of the sky. That’s the Kármán Line.

(You might still be having a problem with how nice and easy and convenient it is that the Kármán Line still equates to precisely 100 km, but the FAI, the international sanctioning body, that the real Kármán Line is somewhere “around” 100 km, and they just say 100 km to make things easy on everyone. You and I both know you weren’t going to build a special rocket plane just to test it out, anyway.)

So why does the U.S. government award astronaut wings at 50 miles up, instead of at the 62-mile mark of the Kármán Line?

It turns out, that’s just what the U.S. Air Force felt like defining space as back in the 1960s, .

Truth be told, while we like to draw bright lines defining “outer space” and “not outer space,” Earth’s atmosphere is a bit fuzzy. It just sort of thins out for a while. It’s more of a you-know-it-when-you-see-it sort of thing. And the Institute of Physics points out that for practical purposes, NASA pretty much ignores the 50-mile point:

NASA use 76 miles as their re-entry altitude, the point at which atmospheric drag is noticeable and the space shuttle can switch from steering with thrusters to maneuvering like a conventional aircraft.

So it turns out that space doesn’t just start where the sky goes all black, or where you start to get a space-y feeling. It’s much more than that.