There is a saying that the Great Wall of China, an earthen fortifications stretching more than 6000km, is the only human-made structure visible from space. Is this a true claim? Unfortunately space tourism has not arrived yet, so we cannot fly over to verify it. But the next best thing, the Google Earth, is already here for free. Better yet, anyone can do an aerial survey simply from a home computer.
Let's go to Badaling(八達嶺), a mountain pass north of Beijing where all the tourists flock to see the restored section of the Great Wall. I have captured three pictures from altitude of 1km, 12km and 100km.
From the 1km picture, you can clearly see the wall and two square towers. The next picture is from 12km above, the cruising altitude of a 747 plane. The gray dot at the center is the park's entrance and a parking lot. The curvy line going from the lower side to upper left is a highway, not the Great Wall! If you look really carefully you can spot the Great Wall as a faint line along the ridge running from the lower left side to upper right side through the visitor's entrance. The Great Wall is already hardly visible from a 747.
Moving on to the third picture from 100km above, not quite from the space yet but from a low earth orbit. Not a trace of the wall nor the visitor's center is visible. The grayish area at the lower right hand corner is part of the Beijing metropolitan.
For comparison, let's do another survey on Beijing's Forbidden City. From 1km we can clearly see the imperial halls and courts. From 12km, the rectangular Forbidden City surrounded by a moat is a distinctly visible feature. Even from 100km, the Forbidden City still appear as a small pod in the middle next to a dark shape of the Beihai lake.
All these are really very logical. The Great Wall is perhaps 5 to 10 meters wide, most of them are now crumbling earth. The Forbidden City on the other hand, have a dimension of about 1000 meters by 800 meters. It is not a surprise that the Forbidden City is far more visible than the Great Wall.
Seeing is believing. The saying that the Great Wall is visible from space is unfortunately only an urban legend.