(Don’t worry, I’m not asking that in the quasi-philosophical manner that sends anyone familiar with relativistic physics screaming for cover.)
On the surface of the planet Earth, there is Something. Lots of somethings, actually; rock and water and dirt and metal and air and loads more besides. By contrast if you were to go out into interplanetary space you would find Nothing. Or at least, almost Nothing; even the hard vacuum of intergalactic space has trace elements of Something in it – a few particles of hydrogen/helium per cubic metre, electromagnetic radiation, possibly even dark matter/dark energy if no better culprit can be found for their effects on the universe – but in general you’d have to work very hard to find somewhere with less Something than outer space. If you get into a rocket and blast off into the heavens, you will at some point transition from the Something-rich environment of Earth to the large quantity of Nothing that makes up outer space.
But at what point does this happen? When exactly can you say that you’ve gone from Something to Nothing? What, in other words, is the precise boundary which separates the Earth and its atmosphere from space? It’s a complex question because – as with so much in our imperfect universe – such a boundary does not exist in physical terms. There’s no concrete point at which the Earth’s atmosphere stops and the vacuum of space takes over. All there is are1 successively thinner layers of gas, gradually dwindling away to nothing as the particles inside the atmosphere hit the upper bounds of their Maxwell-Boltzmann distribution and escape into space. So while space may indeed be nothing we’re going to have to find some other way of defining it that doesn’t involve its notable lack of content.
Now, when you look at a diagram of the various atmosphere layers it seems as though trying to point at a particular part of it to say “This is where space starts,” would be about as accurate as a game of Pin The Tail On The Donkey. However, we’re human beings and we like to separate things into clearly delineated spheres of influence even if it makes little practical sense, and as it turns out such a boundary between the Earth and space does indeed exist. Remember Spaceship One? That thing is only a spaceship in the most technical sense of the word, but it does qualify. Spaceship One goes up above the 100 km Kármán line where it lingers for just a few minutes before retreating back towards the Earth’s welcoming embrace, and that 100 km altitude just happens to be the point where we’ve decided that the atmosphere gives way to space, which is why it’s called Spaceship One instead of just One.
100 km sounds like the sort of number that somebody arbitrarily selected because it sounds good, but there’s actually a pretty decent reason why it was chosen as the boundary: 100 km is the point where it becomes physically impossible for fixed-wing aircraft to operate, and so rockets have to take over. Aeroplanes generate lift by flying into a thick body of air with their wings at a given angle of attack; the air flowing on either side of the wing will create a lift force as a byproduct that is sufficient to keep the plane in the sky. However, the amount of lift you get for a given aeroplane is directly proportional to both the speed it’s travelling at and the density of the air it’s flying through – and as you ascend upwards towards space the atmosphere gets progressively thinner and thinner. The higher you fly, the faster you have to go in order to generate enough lift to stay airborne.
The Kármán line is born from the logical conclusion that if you keep going up you’ll eventually hit an altitude where the airspeed required for an aeroplane to stay in the sky will match the velocity required to orbit something around the Earth. At that point you don’t need wings any more; the pilot of our hypothetical aircraft could clamber out of their cockpit and saw the wings off, and the aircraft would just keep on going. Except you couldn’t call it an aircraft any more, because it’s no longer reliant on having a thick soup of atmospheric gases around it to fly. It’s made the transition from aeroplane to spaceship, and so 100 km was selected2 as the dividing line between aeronautics and astronautics; the demarcation boundary between the Earth and space.
Of course there’s no such thing as an aeroplane that can fly at 100 km altitude. Jet engines conk out long before they ever get that high, and ramjet engines are still in development (and are likely to be so for a long while to come). The only winged craft that have made it to 100 km are rocket-powered spaceplanes that then glide back down to Earth, so strictly speaking the Kármán line is very much a theoretical boundary rather than a practical one. In practice airplanes are stuck muddling along at altitudes of 20-30 km or so, and above that is the domain of rockets punctuated by an occasional high-altitude balloon flight. Still, if you’re going to have these boundaries you may as well make them scientifically unassailable, and the nice thing about Kármán is that it doesn’t matter how high-tech planes get in the future; they’re still going to be physically unable to function as planes past 100 km, making the Kármán line a very robust point at which to set the defining line between the Earth and space. Aeronautically speaking, anyway.