Kármán line

The Kármán line, or Karman line, lies at an altitude of 100 km (62 mi; 330,000 ft) above Earth's sea level and commonly represents the boundary between Earth's atmosphere and outer space.

The Kármán line lies within the lower thermosphere above 100 kilometers where the air density is about 1/2,200,000 the density on the surface of Earth.

Named after Theodore von Kármán (1881–1963) who calculate that the atmosphere around this altitude becomes too thin to support aeronautical flight, since a vehicle at this altitude would have to travel faster than orbital velocity to derive sufficient aerodynamic lift to support itself.

The line represents a mean or median measurement. It is comparable to such measures used in the law as mean sea level, meander line, tide line; but it is more complex.

At this height above Earth you will experience 'Free fall' a sense of weightlessness even though Earth gravity pulling you down is 97% the same as on Earth.

An orbiting spacecraft only stays in the sky if the centrifugal component of its movement around the Earth is enough to balance the downward pull of gravity. If it goes slower, the pull of gravity gradually makes its altitude decrease. The required speed is called orbital velocity, and it varies with the height of the orbit.

The Karman line is therefore the highest altitude at which orbital speed provides sufficient aerodynamic lift to fly in a straight line that doesn't follow the curvature of the Earth's surface.

Another definition proposed in international law discussions defines the lower boundary of space as the lowest perigee attainable by an orbiting space vehicle, but does not specify an altitude. Due to atmospheric drag, the lowest altitude at which an object in a circular orbit can complete at least one full revolution without propulsion is approximately 150 km (90 mi), whereas an object can maintain an elliptical orbit with perigee as low as about 130 km (80 mi) without propulsion. Above altitudes of approximately 160 km (100 mi) the sky is completely black.

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