The Great Wall of China is not the only artificial structure that can be seen from low Earth orbit.
Many human-made objects can be seen with the naked eye from the space shuttle.
For example, astronauts can look out their windows and see highways, railroads, and even bridges.
But once space vehicles climb a few thousand miles above Earth, astronauts cannot see any human-made objects on the planet.
Many scientists have argued that due to limitations of the optics of the eye and the spacing of photoreceptors on the retina, it is impossible to see the Great Wall of China with the naked eye, even from low orbit.
Spotting the Great Wall from that height would require eyesight 7.7 times better than normal human vision.
