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The Boundary of Space

The boundary of space is not a fixed idea. As the altitude increase, the atmosphere gets thinner and thinner, but we do not define space as starting at the height at which there is no atmosphere. In fact, the height at which space starts is drawn from aeronautics.
As a plane flies higher, the speed of the plane must increase so that the plane can be controlled, since a thinning atmosphere results in less air flowing over the control surfaces. At a certain altitude the speed will be higher the theoretical orbital speed of a satellite, ignoring air resistance.
This height, called the 'Karman Line', is taken by most scientists to be the height at which space begins, and is calculated to be about 100 km. Any object reaching an altitude of 100km will not in fact orbit the Earth. Air resistance s still significant at this altitude, and any object orbiting the Earth at this altitude will soon in fact, fall to Earth.