The Wall That Astronauts Could Not Find

In 2003 China sent its first astronaut into orbit, and the country waited to hear about the Great Wall. Yang Liwei circled the Earth fourteen times aboard Shenzhou 5 and, after he landed, reportedly told interviewers that he had looked for the wall and could not see it. The answer caused a stir at home, where schoolbooks had long taught that the wall is the only human construction visible from space. Textbook editors were later reported to have reviewed the claim.

The claim is much older than the space age. A version is credited to a Ripley's Believe It or Not cartoon from 1932, and the traveller Richard Halliburton is said to have repeated a similar boast in a book of marvels in 1938. Both dates lie decades before any human left the atmosphere. The idea survived because it was flattering and impossible for most people to test, and it grew more impressive as the wall grew longer in the telling.

The physics is simple. The wall is long but narrow, in most places only a few metres across, and it is built of local stone and earth whose colours blend into the hills it follows. At the height of the International Space Station, roughly four hundred kilometres up, the unaided eye cannot separate so thin a line from its background. Astronauts have reported spotting cities at night, airport runways and large highways with ease, because these are wider, brighter or sharper against their surroundings. Photographs taken from orbit with powerful lenses have shown stretches of the wall, but that is a different achievement from seeing it with the eye alone.

The myth also teaches a lesson about how facts spread. Correction rarely travels as fast as the original claim. Schoolbooks and quiz shows had carried the legend for decades, and a single astronaut's answer had to compete with all of them.

Correction rarely travels as fast as the original claim.

Yet the story has a second layer. The wall that cannot be seen from orbit can be seen clearly on the ground, and its real history, a patchwork of walls built by different states and dynasties over some two thousand years, is more interesting than the legend of one unbroken line.