Kármán line

The Kármán line is a conventionally defined boundary between Earth’s atmosphere and outer space, commonly set at 100 kilometers above mean sea level.

Federation Aeronautique Internationale: The international organization that governs air sport records and certifies aviation and astronautical achievements. It adopted 100 kilometers as the boundary for classifying spaceflight records.
Theodore von Kármán: A Hungarian-American engineer and physicist known for foundational work in aerodynamics and fluid mechanics. His name became attached to the altitude where aerodynamic lift becomes impractical for an aircraft.
50-mile-high boundary: A boundary at roughly 80 kilometers above Earth used in some definitions of spaceflight. It is the principal lower-altitude alternative to the 100-kilometer Kármán line.
Suborbital spaceflight: A trajectory that reaches space but lacks the speed to complete an orbit around Earth. Many flights cross the line briefly before returning without entering orbit.
United States astronaut badge: A U.S. military badge awarded to personnel who meet defined astronaut qualification criteria. U.S. military criteria have used a lower altitude threshold than the Kármán line.
Atmospheric density: The mass of atmospheric gas per unit volume, which decreases with increasing altitude. Thinning air is the physical reason ordinary wings lose effectiveness near the line.
Atmosphere of Earth: The layers of gases surrounding Earth, held by gravity and gradually thinning with altitude. The atmosphere has no sharp upper edge, unlike the neat boundary implied by the line.
Orbital spaceflight: Spaceflight in which a vehicle travels fast enough to remain in orbit around a celestial body. Crossing the line alone does not provide the velocity needed to achieve orbit.
Space tourism: Commercial travel beyond Earth’s atmosphere for recreation or other nonprofessional purposes. Providers use different altitude thresholds when describing suborbital passengers as having reached space.
Dynamic pressure: The kinetic energy per unit volume of a moving fluid, proportional to density and speed squared. At high altitude, maintaining lift requires speed as dynamic pressure falls with air density.

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MesosphereCompared with: It lies within the mesosphere, showing that space-boundary conventions do not match atmospheric layers.