How many times have you seen this trivia question? What is the only manmade object visible from space? Ans: The Great Wall of China.
Personally, I've seen it dozens of times and it drives me crazy every time. Why? Because it is emphatically, absolutely, positively not true. The factoid even appears on a Trivial Pursuit question. But, as Cecil Adams and the Straight Dope put it so eloquently, "Those wankers at Trivial Pursuit have screwed up again." (For another example, read about the Philip Colombo fiasco.)
It is my mission in life to eradicate this false fact from quizzing circles everywhere, so you will have to forgive this rant, which is going to see strangely obsessive, so I apologize in advance.
The Great Wall is a long ribbon of stone, and one that is not only broken in many places, but is made of materials the same colour as the surrounding territory. From space, it would have a hair-like appearance, at best, and would be impossible to pick out, unless you were close enough to pick out other objects, notably the Pentagon. It certainly can't be seen from the Moon.
This isn't to say that you can't see anything on Earth from space. It's all a question of what you mean by "space." Space.com explains it this way.
Shuttle astronauts can see highways, airports, dams and even large vehicles from an Earth orbit that is about 135 miles (217 kilometers) high. Cities are clearly distinct from surrounding countryside, and that's true even from the higher perch of the International Space Station, which circles the planet at about 250 miles (400 kilometers) up.
As NASA says, "The Great Wall can barely be seen from the Shuttle, so it would not be possible to see it from the Moon with the naked eye."
Space Shuttle astronaut Jay Apt is quoted in National Geographic Magazine, November 1996, as follows: "We look for the Great Wall of China. Although we can see things as small as airport runways, the Great Wall seems to be made largely of materials that have the same color as the surrounding soil. Despite persistent stories that it can be seen from the moon, the Great Wall is almost invisible from only 180 miles up!"
And remember, we're talking naked eye, here. When you add instrumentation, things change because, ironically, a lot of the Wall isn't even visible from Earth anymore. But space-based radar is mapping out parts of the structure that have long been buried.
The following is extracted from Snopes.com, and is probably as close to the final word as you're going to get on this. (This is an item they posted on my suggestion, too!)
If we take "space" to mean a low Earth orbit such as the one travelled by the Space Shuttle (roughly 160 to 350 miles above Earth), the Great Wall claim fails twice. First of all, it's not the only object visible from that distance: NASA's Earth from Space photographic archive (particularly the Human Interactions section) shows that pictures taken from low orbit reveal human-built structures such as highways, airports, bridges, dams, and components of the Kennedy Space Center. Secondly, even though other objects are visible at this distance, according to Shuttle astronaut Jay Apt, the Great Wall is barely discernable, if not invisible:
We look for the Great Wall of China. Although we can see things as small as airport runways, the Great Wall seems to be made largely of materials that have the same color as the surrounding soil. Despite persistent stories that it can be seen from the moon, the Great Wall is almost invisible from only 180 miles up!
An object that can barely be seen from a height of 180 miles up is obviously not going to be visible from the moon (roughly 237,000 miles away), as confirmed by Apollo 12 astronaut Alan Bean:
"The only thing you can see from the moon is a beautiful sphere, mostly white (clouds), some blue (ocean), patches of yellow (deserts), and every once in a while some green vegetation. No man-made object is visible on this scale. In fact, when first leaving earth's orbit and only a few thousand miles away, no man-made object is visible at that point either."
(The Great Wall of China can be discerned in radar images taken from space, but not in ordinary photographs.)
Weirdly, the story seems to predate the existence of satellites, and explorer Richard Halliburton was hawking the idea back in 1938!
In 2004, Red China sent its first astronaut into space, and even Yang Liwei confirmed that he could not see the Great Wall. According to the BBC, "For decades, elementary schoolbooks have maintained that the Great Wall of China could be seen from space - but now the books are being rewritten."
The Beijing Times added, ""Having this falsehood printed in our elementary school textbooks is probably the main cause of the misconception being so widely spread."