Space: Boundary

There is no clear boundary between Earth's atmosphere and space, as the density of the atmosphere gradually decreases as the altitude increases. There are several standard boundary designations, namely:

The Fédération Aéronautique Internationale has established the Kármán line at an altitude of 100 km (62 mi) as a working definition for the boundary between aeronautics and astronautics. This is used because at an altitude of about 100 km (62 mi), as Theodore von Kármán calculated, a vehicle would have to travel faster than orbital velocity to derive sufficient aerodynamic lift from the atmosphere to support itself.[8][9]
Up until 2021, the United States designated people who travel above an altitude of 50 mi (80 km) as astronauts.[62] Astronaut wings are now only awarded to spacecraft crew members that "demonstrated activities during flight that were essential to public safety, or contributed to human space flight safety".[63]
NASA's Space Shuttle used 400,000 ft, or 75.76 miles (120 km), as its re-entry altitude (termed the Entry Interface), which roughly marks the boundary where atmospheric drag becomes noticeable, thus beginning the process of switching from steering with thrusters to maneuvering with aerodynamic control surfaces.[64]
In 2009, scientists reported detailed measurements with a Supra-Thermal Ion Imager (an instrument that measures the direction and speed of ions), which allowed them to establish a boundary at 118 km (73.3 mi) above Earth. The boundary represents the midpoint of a gradual transition over tens of kilometers from the relatively gentle winds of the Earth's atmosphere to the more violent flows of charged particles in space, which can reach speeds well over 268 m/s (880 ft/s).[65][66]