Choose a legend: The Great Wall of China is the one of the few man-made structures visible from orbit. Or, more remarkably, it's the only human artifact on Earth visible from the moon. Both are false, say astronauts and remote-sensing specialists. Although the Great Wall spans some 4,500 miles (7,200 kilometers), it's constructed from materials that make it difficult to discern from space.



The unglamorous truth is that the wall is only visible from low orbit under a specific set of weather and lighting conditions. And many other structures that are less spectacular from an earthly vantage point—desert roads, for example—appear more prominent from an orbital perspective.



Misinformation about the barrier's visibility dates back decades. A 1932 Ripley's Believe It or Not! cartoon claimed that the wall is "the mightiest work of man, the only one that would be visible to the human eye from the moon." The belief persisted into the Space Age. Since Neil Armstrong returned from the moon in 1969, he has been repeatedly asked whether he could see it.



His answer was relayed in a recent NASA Johnson Space Center oral history: He saw continents, lakes and splotches of white on blue. But he could not make out any man-made structures from the lunar surface, which averages a distance of 230,000 miles (370,000 kilometers) from Earth.



So just how visible is the Great Wall from low Earth orbit, at an altitude that begins around 100 miles (160 kilometers) up? Not very. Although sections near Beijing, China's capital, have been restored for tourists, in many areas the structure is crumbling. Where it still stands, the wall's mixture of stone and clay blends into the surrounding land.



"I have spent a lot of time looking at the Earth from space, including numerous flights over China, and I never saw the wall," asserts former NASA astronaut Jeffrey Hoffman, who flew on five space shuttle missions from 1985 to 1996. "The problem is that the human eye is most sensitive to contrast, and the color of the wall is not that different from the ground on either side of it."



Hoffman, now an aerospace engineering professor at the Massachusetts Institute of Technology, failed to make out the Egyptian pyramids for the same reason. But he could identify roads, airport runways and irrigation ditches simply because they stood out in their environments.



Some U.S. astronauts, notably Eugene Cernan and Ed Lu, have said they've seen the wall from low orbit. But it tends to show up only in certain lighting conditions. When the sun is low on the horizon, for example, the wall casts extended shadows that make it possible to discern its silhouette.



In 2004 American astronaut Leroy Chiao snapped a photo from the International Space Station of a swath of Inner Mongolia, around 200 miles (320 kilometers) north of Beijing, while the sun's angle was favorable. NASA experts later confirmed that the photo appears to show the wall. But Chiao admitted that he wasn't sure what he was seeing from space.



Machines can do a better job. Low-orbit satellites have sensors that can penetrate through haze and clouds, making it easier for them to produce clear images. But, as with the naked eye, identifying the wall is hardly a guarantee.



Moderate-resolution satellites, like the U.S. Geological Survey's (USGS) two operating Landsat land observation satellites that orbit 438 miles (705 kilometers) above Earth's surface, can typically only pick up the structure under specific weather conditions, says Ronald Beck, program information specialist with the USGS's Land Remote Sensing Program. "We have satellite images where snow covers the fields near the wall and snow has been cleared on the wall, and that allows us to see the wall," Beck says. "The key is contrast."



Often, identifying the rampart in satellite images requires a degree of sleuth work. In populated areas, Beck says, USGS scientists pinpoint sections of the wall by looking for parking lots and pathways. In more remote areas, they may scan for breaks in the vegetation surrounding the structure. But those techniques are hardly foolproof; at many points, the vegetation grows up and over the wall.



For the Chinese, the wall's visibility from space has long been a point of pride. When "taikonaut" Yang Liwei, China's first man in space, returned from the 14-orbit Shenzhou 5 mission in 2003 and admitted to reporters that he had not seen the Great Wall, online forums exploded with disappointment. The Ministry of Education even moved to revise its elementary school textbooks, which had long claimed the ancient barricade was visible.



Since then, a debate has raged in China, with scholars grasping at evidence that might settle the question of how great the wall really is. Chinese Academy of the Sciences Institute of Remote Sensing Application professor Wei Chengjie, who appeared on a national television special devoted to the issue in 2006, says more research is needed. "We need to carry out more tests and improve astronaut training. Some astronauts have said that they didn't see it, but that doesn't mean it isn't there. A shuttle passes by so quickly."



In the meantime, however, China's search for clarity is coming up against a modern complication. As the country industrializes and its factories belch out noxious gases, the wall further fades from view. "The biggest problem nowadays is the pall of pollution which exists over much of China," Hoffman says. "It effectively makes it impossible to see almost anything."