The Fédération aéronautique internationale established a boundary at 100 kilometers above mean sea level in the 1960s. This specific altitude serves as the Kármán line for international record-keeping purposes.
Theodore von Kármán presented a paper on aerothermal limits to flight in 1956 at Berkeley, California. He calculated speeds and altitudes where continuous flight became possible without overheating due to friction.
Andrew G. Haley later invented the term Kármán line in a 1959 paper based on these calculations. Haley acknowledged the limit was theoretical and dependent on current aircraft technology like the Bell X-2.
The United States Armed Forces define an astronaut as someone who has flown higher than 80 kilometers above mean sea level. NASA formerly used the FAI figure before changing its standard in 2005 to eliminate inconsistencies.
Isidoro Martínez arrived at 30 kilometers for Mars and 110 kilometers for Venus in his research on space environments. Nicolas Bérend proposed alternative calculations reaching 40 kilometers for Mars and 90 kilometers for Venus during a 2022 conference in Paris.