The Kármán line, at 100 kilometres (62 mi), is accepted by the World Air Sports Federation, an international standard-setting and record-keeping body for aeronautics and astronautics, as the boundary between the Earth's atmosphere and outer space. Whether an object is visible depends significantly on the height above sea level from where it is observed. On the other hand, the centimetre-band Spaceborne Imaging Radar of STS-59 and STS-68 was able to detect not only the Great Wall but also invisible buried segments of it. The Great Wall of China, often cited as the only human-made structure visible from space, is not visible from low Earth orbit without magnification, and even then can be seen only under perfect conditions. Human-made things that can be seen from spaceĪrtificial structures visible from space without magnification include highways, dams, and cities.
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