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As there is no clear physical limit between the atmosphere and the outer space, the Fédération Aéronautique Internationale (an international organisation that defines standards for aeronautics and astronautics) has defined this limit at the altitude of 100 km above the Earth's sea level. At that altitude,
- the density of the atmosphere is so low that the speeds required to make an aerodynamic object fly tend actually make it orbit around the Earth.
- the density of the atmosphere is however high enough for objects requiring a lot of power to keep them orbiting around the Earth more than a couple of revolutions.
Humans going above the limit are named astronauts. In order to survive, they must be well prepared and wear special equipment because they face challenges such as radiations, microgravity and vacuum.