Geoff, Cottenham asked:
Is it true that the Great Wall of China is the only man-made object visible from space ?
This is a myth ! Although the Great Wall is extremely long it is only about 5 metres or so wide. At distances greater than about 20 km up (about twice the height of Mt. Everest), a person even with perfect eyesight, but without the aid of a telescope or binoculars, cannot physically resolve something that small on the ground. Allowing for the shadow cast by the Great Wall (making it wider) or sand accumulating on the windward side you might be able to see it from 60 km up, in places, but since this is still too low for a spacecraft to orbit, it's safe to say that you can't see it from space. Indeed, Neil Armstrong and other astronauts from the Apollo missions have all declared that it cannot be seen from space with the Naked Eye. Other man-made objects can though, such as cities because of their bright lights.