Nobody can agree where space actually begins.
The usual answer is 100 kilometres up, a boundary called the Kármán line. But the atmosphere does not stop anywhere, the US uses 80 kilometres to award astronaut wings, and there is no international legal definition at all. The edge of space is a convention, not a physical wall.
The air does not end; it just thins out. So any boundary between 'atmosphere' and 'space' has to be chosen. The most widely used is the Kármán line at 100 kilometres above sea level, adopted by the Fédération Aéronautique Internationale, the body that keeps aviation and spaceflight records, in the 1960s. Below it is aeronautics; above it, astronautics. Many international bodies, including the United Nations, accept it or something close.
The name honours the engineer Theodore von Kármán, whose reasoning ran like this: the thinner the air, the faster a plane must fly to generate lift. Go high enough and the required speed is so great that the craft is effectively in orbit, held up by its motion rather than by its wings. His own 1956 calculations actually put a limit nearer 84 kilometres, based on how hot aircraft would get at such speeds. The term 'Kármán line' was coined by the space lawyer Andrew Haley in 1959.
Other lines are drawn elsewhere. The US armed forces award astronaut status for flights above 50 miles, about 80 kilometres; NASA switched to that figure in 2005, and three X-15 rocket-plane pilots from the 1960s were later given astronaut wings retroactively. In 2018 the astronomer Jonathan McDowell and Thomas Gangale argued that 80 kilometres makes more physical sense, since satellites dipping lower rarely complete another orbit and meteors usually burn up between 70 and 100 kilometres. If you counted the thinnest outer layers as atmosphere, the edge could be 10,000 kilometres up.
It matters mainly for law, to separate aircraft from spacecraft. Von Kármán himself suggested that the air below such a line should belong to each country and the space above it be free. Yet there is still no international legal definition of where one ends and the other begins, and the US government has resisted fixing a precise boundary.
Source: Wikipedia — Kármán line · Text summarised from Wikipedia (CC BY-SA 4.0)
