Kármán Line

The Kármán line is a reference altitude for describing where space begins. It is most commonly placed at 100 kilometres (about 62 miles) above mean sea level, a convention used by the Fédération Aéronautique Internationale (FAI) in its astronautical record rules. The line is useful for classifying flights, but it does not mark a sudden change in the atmosphere. Air density decreases gradually with altitude, and the boundary between atmosphere and space depends partly on what is being measured and the purpose of the definition.

The line is named for Theodore von Kármán, an engineer who examined the limits of aerodynamic flight. As air becomes thinner, an aircraft must move faster to generate enough lift; at sufficiently high altitude, orbital speeds become relevant to the question of how a vehicle can remain aloft. Encyclopaedia Britannica gives von Kármán's estimate as roughly 84 kilometres, while the FAI's historical account explains that a round value of 100 kilometres was adopted as a workable division between aeronautics and astronautics. Accounts of the line's derivation and original intended height are not entirely consistent, so the name should not be taken to mean that von Kármán established a single, universally binding physical boundary.

The FAI's 100-kilometre convention provides a clear threshold for its astronautical records. Some U.S. agencies use 80 kilometres (50 miles) instead. In a 2018 paper in Acta Astronautica, astrophysicist Jonathan C. McDowell argued that a boundary near 80 kilometres better reflects the transition to conditions in which sustained orbital motion becomes possible; this is a scholarly argument, not a globally adopted replacement for the FAI rule. Because atmospheric density and flight behaviour change continuously, different communities can use different limits without there being a sharp natural seam in the sky.

The Kármán line is a useful shorthand in discussions of astronautics, aerospace records and the popular idea of the edge of space. It helps distinguish conventional atmospheric flight from spaceflight for particular purposes, while making a complicated transition easier to communicate. It should be understood as a human-defined convention grounded in physical considerations, rather than as the altitude where the atmosphere abruptly ends.