MARK GARLICK / SCIENCE PHOTO LIBRARY MARK GARLICK / SCIENCE PHOTO LIBRARY
Conceptual illustration showing the relative distance to the beginning of space from the surface of Earth. Space does not have a definitive, hard boundary. The atmosphere gradually thins until at a somewhat arbitrary point called the Karman line, space is considered to begin. The small stack of coins on the map of the USA indicates the height of the the Karman line, on the same scale as the map. The larger stack shows the height at which the International Space Station orbits (roughly 400 km). The single coin shows the height of a typical intercontinental commercial aircraft, at just 10 km. These coin stacks show just how close outer space is in relative terms.
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