article What is the Kármán line? (Draft) ~3 minute read

You may have heard people refer to the “edge of space” – 100km – as the Kármán line. But what does that mean, and what’s so significant about it?

Simply put, the Kármán line is loosely defined as the altitude at which a fixed-wing aircraft would need to travel faster than orbital velocities to produce enough lift. This is a bit of a complex concept to understand, so I’ll break it down.

What is space?

Let’s fall back to a question that’s easier to answer. Or is it? “Space” typically means “outside of the atmosphere”; for example, we consider the ISS, at 400 km249 miles, to be “in space”. It’s obviously not in the Earth’s atmosphere anymore; anything in the atmosphere is subject to drag, and with the ISS traveling at 7.8 km/s17,448 mph, the drag would be very apparent, to say the least.

Now, let’s talk about the Earth’s atmosphere. At sea level, it’s rather dense; in fact, it’s dense enough to generate lift by moving an airfoil through the air. (This is how airplanes and helicopters work.) As you move up, though, the air gets thinner. This is because the limited amount of atmosphere is pulled down to the surface via Earth’s gravity.

The curve actually isn’t linear, either. The atmosphere quickly thins off; at 20 km12 miles, the atmospheric pressure is less than half of sea level pressure; and at 50 km31 miles, the pressure is about 10% of sea level pressure.

This means that, theoretically, Earth’s atmosphere will never end. It just gets less and less dense as you ascend, making it impossible to even have a line where one side is definitively “space” and the other side definitively “atmosphere”. This is why the Kármán line exists.

But first, let’s take a look at atmospheric flight.

Atmospheric flight

Given the fact that the atmosphere gradually thins out as altitude increases, aircraft need to increase their speed at higher altitudes to keep the total lift constant. This is an important concept: if it was going fast enough, a 747 would be able to fly at 50 km31 miles, several times higher than normal. Unfortunately, the speeds required to fly straight and level at that altitude are well in excess of Mach 1, and thus impossible, for the 747.

It turns out that at high altitudes, the loss of lift due to the thinner air does not directly define the maximum altitude of an aircraft. The real limitation is how much power the engine can produce. Since engines directly consume atmospheric oxygen to mix with their fuel, the thin air at higher altitudes reduces engine power, thus indirectly limiting top speed and therefore the maximum amount of lift that can be generated.

This establishes a relationship between atmospheric density and the minimum velocity needed to maintain level flight.

Space flight

Anything in orbit around the Earth is essentially falling around the earth. The ISS orbits at about the lowest feasible altitude for a large, very high-drag object: 400 km249 miles, and it still needs to be reboosted every few months. So there isn’t no drag at the ISS; there’s just very little. Now, the ISS is traveling at about 7.8 km/s17,448 mph; for a 747 to produce enough lift for level flight at that altitudes, it would need to fly far faster than the orbital velocity of the ISS.

That’s exactly what the Kármán line is: the