The Kármán line is an artificial boundary in Earth's atmosphere that divides space missions from aeronautics missions — passing the line turns a run-of-the-mill pilot into an astronaut. And this obscure boundary may become extremely important as we enter the era of space tourism.
A plane, aircraft, or other object relies on the density of the atmosphere to keep it aloft. The atmosphere thins as the distance from Earth increases, forcing a craft to increase velocity in order to stay airborne. This push and pull can be illustrated by an equation proposed by Theodore von Kármán, with the eponymous line becoming the distance from the planet at which a craft must meet or exceed orbital velocity in order to prevent a loss of altitude.
Kármán rounded his initial calculated result 100 kilometers, and, in the process, selected the boundary by which pilots and passengers become astronauts.
Theodore von Kármán is an interesting character whose legacy often takes a back seat to the exotic boundary that carries his name. A Jewish immigrant who served as a founder of NASA’s Jet Propulsion Laboratory and as an advisory to the U.S. military, Kármán dedicated the last four decades of his life to advancing U.S. rocket design and laying the foundation for the space race.
Fruit flies and pilots passing the Kármán line
The Kármán line exists 100 km above the Earth in the thermosphere, a layer of Earth's atmosphere that you probably haven't thought of since an elementary physical science class. In this layer, solar radiation is absorbed, leading to temperatures of 2000 °C or higher. To put the height of the Kármán line in perspective, Felix Baumgartner's recent record-breaking space jump took place from an altitude of 40 km, while commercial aircraft cruise at approximately 12 km.