The Kármán line lies at an altitude of 100 km (62.1 miles) above the Earth's surface, and is commonly used to define the boundary between the Earth's atmosphere and outer space. This definition is accepted by the Fédération Aéronautique Internationale (FAI), which is an international standard setting and record-keeping body for aeronautics and astronautics.

The line was named after Theodore von Kármán, a Hungarian- American engineer and physicist who was active primarily in the fields of aeronautics. He first calculated that around this altitude the Earth's atmosphere becomes too thin for aeronautic purposes (because any vehicle at this altitude would have to travel faster than orbital velocity in order to derive sufficient aerodynamic lift from the atmosphere to support itself). Also, there is an abrupt increase in atmospheric temperature and interaction with solar radiation ( more...)