Where does space actually begin?
There is no wall and no line painted in the sky. Different countries even use different heights — which children find far more interesting than a single tidy answer.
4 min read
Children ask this expecting a number, and there is one — 100 kilometres — but the honest answer is more interesting than the number, and it is a rare chance to show that grown-ups sometimes disagree about facts.
The air does not stop. It thins.
Going up, the atmosphere does not end at a boundary. It gets steadily thinner, roughly halving every five and a half kilometres or so near the surface, and it keeps on thinning for hundreds of kilometres. There are still enough stray molecules at the space station’s altitude to slow it down, which is why the station has to be nudged back up periodically.
So any line we draw is a decision, not a discovery.
The Kármán line
The line most of the world uses sits at 100 km and is named after Theodore von Kármán, who worked out roughly where an aircraft would have to fly so fast to generate lift that it might as well be in orbit. The Fédération Aéronautique Internationale, which ratifies aviation and astronautics records, settled on 100 km as a round number in that region.
Except in the United States
NASA and the US Air Force use 50 miles — about 80 km — instead. That is the height above which the US has historically awarded astronaut wings. Some researchers argue 80 km is actually the better physical boundary, on the grounds that satellites with a low point below it are unlikely to survive another orbit.
A sense of the distances
- A tall building: about 0.03 km
- A passenger aircraft: about 11 km
- A weather balloon: about 40 km
- Space, if you are American: 80 km
- Space, if you are almost anyone else: 100 km
- The International Space Station: about 420 km
Put like that, space is close. It is a shorter journey than most people drive at the weekend — it is just straight up, and getting there is the easy part. Staying there means going sideways at 27,600 km per hour, and that is what actually costs the fuel.
Common questions
How high up is space?
Most of the world uses the Kármán line at 100 km above sea level. NASA and the US Air Force use 50 miles, about 80 km. There is no physical boundary — the atmosphere simply thins with height.
Why do the USA and the rest of the world disagree?
The 100 km Kármán line was adopted by the Fédération Aéronautique Internationale as a round figure near where aerodynamic flight stops working. The US uses 50 miles, the historical threshold for awarding astronaut wings.
Is there air at the space station?
A very small amount. There are still enough molecules at about 420 km to create drag, which is why the station’s orbit has to be raised from time to time.