Can you see the Great Wall of China from space, and how long is it?

From low Earth orbit you can make out airports, highway interchanges, reservoirs and, at night, entire urban networks. The Great Wall of China does not fit that pattern: it is only a few metres wide, follows the terrain through countless curves, and is built from earth and stone the same colour as the landscape it crosses. In photographs taken from the space station, some stretches can be located, but only with a telephoto lens, raking light and, almost always, only if you already know where to look. From the Moon, even the entire continent is no more than a smudge.

The wall is not a single continuous structure but a system of walls, ramparts, ditches, towers and garrisons from several dynasties, built up over two millennia; some stretches run in parallel, others have been reduced to simple earthen mounds. China's official 2012 survey puts the combined length of all mapped sections at 21,196 km. The walled, crenellated appearance with watchtowers that most people picture comes mainly from the Ming era and represents only part of that total length.

The claim that the structure is visible from the Moon was already circulating decades before the first spaceflight, back then as a mere assumption based on its enormous length. Spaceflight has not confirmed it, but disproved it: China's first astronaut, Yang Liwei, said after his 2003 flight that he had not been able to see the wall. What crews often do mistake for it are rivers, roads and irrigation canals. And that is precisely what makes the question interesting: visibility does not depend on length, but on contrast.