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The edge of space 'starts' 100 km (62 miles) up, at a mark we call the Kármán line. This represents the 'end' of earth's atmosphere, and the beginning of space.

The Earth's atmosphere is made up of air, but this air gets thinner the higher you are from Earth's surface. Airplanes can't fly out of earth's atmosphere, because they need air for lift. Rockets, however, can go where there is no air, above the Kármán line, and into space, because their thrust is produced differently.