Actually, it takes a moderately good telescope from the distance of Mars just to equal naked eye from the moon. Even at conjunction, Mars is 200 times farther away.
But the thing that really makes the whole "the Great Wall can be seen from outer space" claim silly is that people assume that the length is what counts. A square the length of the Great Wall on a side of a contrasting color would be easily visible from the Moon with the naked eye. But seeing something that's what, 50 feet wide? from so far away is ludicrous no matter how long it is. It's like saying you should be able to see a fine thread 12" long from a mile away because a dinner plate is visible from that distance.
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But the thing that really makes the whole "the Great Wall can be seen from outer space" claim silly is that people assume that the length is what counts. A square the length of the Great Wall on a side of a contrasting color would be easily visible from the Moon with the naked eye. But seeing something that's what, 50 feet wide? from so far away is ludicrous no matter how long it is. It's like saying you should be able to see a fine thread 12" long from a mile away because a dinner plate is visible from that distance.