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Is China's Great Wall Visible from Space?
Scientific American ^ | February 21, 2008 | Mara Hvistendahl

Posted on 02/21/2008 11:58:42 AM PST by Marc Tumin

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 artifice 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 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 barrier 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."

TOPICS: Foreign Affairs; News/Current Events
KEYWORDS: china; greatwall; space

1 posted on 02/21/2008 11:58:43 AM PST by Marc Tumin
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To: Marc Tumin

Growing up in New York, rumor had it that there were only two man-made structures visible from space: The Great Wall, and the Staten Island landfill.

2 posted on 02/21/2008 12:02:35 PM PST by FR Class of 1998 (the long term solution to corruption is to starve the government of money)
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To: FR Class of 1998

My backyard rows of tomatoes are visable on GoogleEarth.

3 posted on 02/21/2008 12:07:27 PM PST by ASA Vet (I'm on a T-1 line so don't care if you post big.)
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To: Marc Tumin
"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."

IOW, Almighty Beijing mandates Chinese astronauts can and must see the Wall, or else "improve their training".

4 posted on 02/21/2008 12:11:07 PM PST by Scothia ( When something important is going on, silence is a lie.)
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To: Marc Tumin

I didn’t see it.........

5 posted on 02/21/2008 12:13:52 PM PST by Red Badger ( We don't have science, but we do have consensus.......)
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To: Marc Tumin
Google Earth view of the Great Wall
6 posted on 02/21/2008 12:14:11 PM PST by ASA Vet
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7 posted on 02/21/2008 12:14:41 PM PST by evets (beer)
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To: Marc Tumin

This has been settled for a long time. As far as the “visible from the moon” claim, just look at any of the Apollo photos where the Earth is visible from the moon. It occupies maybe twice as much of the sky as the moon does of the Earth’s sky. If the Great Wall were on the moon, could you spot it from here?

As far as visible from LEO, a lot of man-made structures can be seen from that distance. Highways, for starters. You probably couldn’t spot Hoover Dam from LEO, but you could pinpoint the spot between Lake Mead and the Colorado River.

Also worth mentioning is that implicit in the claim is that it can be seen with the naked eye. With a long enough lens and a sharp enough sensor, my house is visible from space — I found it on Google Earth.

8 posted on 02/21/2008 12:16:56 PM PST by ReignOfError
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To: Scothia
Almighty Beijing mandates Chinese astronauts can and must see the Wall, or else "improve their training".

"improve their training"
hmm does that mean in addition to charging his family for the bullet they also train the astronaut to patriotically denounce his failure to see the Wall as being the result of his corruption by the West and then shoot himself to save them the trouble?
9 posted on 02/21/2008 12:20:37 PM PST by verum ago (The Iranian Space Agency: set phasers to jihad!)
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To: ReignOfError
Hoover Dam via Google Earth
10 posted on 02/21/2008 12:24:32 PM PST by ASA Vet
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To: Marc Tumin

I wonder if Hillary’s butt is observable from orbit?


11 posted on 02/21/2008 12:38:01 PM PST by ARE SOLE (Agents Ramos and Campean are in prison at this very moment.. (A "Concerned Citizen".)
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Her butt and ankles keep getting mistaken for giant sequoias.

12 posted on 02/21/2008 12:44:37 PM PST by Hayzo
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To: All
Is the October 2006 President Bush-authorized Mexican border wall visible from anywhere on earth?

"We have a responsibility to secure our borders. We take this responsibility seriously." Sure.

13 posted on 02/21/2008 12:44:44 PM PST by WilliamofCarmichael (If modern America's Man on Horseback is out there, Get on the damn horse already!)
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To: ReignOfError
It's also worth noting that "seen from space," "seen from OUTER space," and "seen from the space shuttle" may not necessarily all have the same connotation. If you see an image taken from the space shuttle that shows the proper perspective of the shuttle relative to the earth (like the one I've shown here), you'll see that the shuttle is really "in orbit" as opposed to "outer space."

A view from "outer space," on the other hand (as seen from Apollo 17 below), gives a much different perspective and clearly does not provide as much visibility for man-made structures.

14 posted on 02/21/2008 12:55:52 PM PST by Alberta's Child (I'm out on the outskirts of nowhere . . . with ghosts on my trail, chasing me there.)
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