What altitude is commonly used as the boundary of space, known as the Kármán line?

The Kármán line is commonly defined as 100 kilometers above mean sea level.

The boundary is named after Theodore von Kármán, a Hungarian-American physicist and aerospace engineer. At roughly this altitude, the atmosphere is so thin that an aircraft would need to travel at orbital speeds to generate enough aerodynamic lift to remain supported by its wings. Flight therefore becomes more like spaceflight than conventional aviation.

The 100-kilometre definition is widely used internationally, including by the Fédération Aéronautique Internationale for astronautics records. The United States has traditionally used 50 miles, about 80 kilometres, as an astronautical boundary, so different sources can cite different thresholds.

The Kármán line is a useful convention rather than a sharp physical edge. Earth’s atmosphere gradually becomes thinner for hundreds of kilometres, and some satellites orbit within the extremely thin upper atmosphere.