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The Best Science Writing Online 2012
Showcasing more than fifty of the most provocative, original, and significant online essays from 2011, The Best Science Writing Online 2012 will change the way...
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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."
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10 Comments
Add CommentThis is idiotic and incorrect. Using Google Earth you can zoom in on Harvard Square and clearly see a gray car with a sunroof. This sunroof is large enough to be seen from space! Anything you can see from Google Earth is something that can be "seen from space," as all the images were taken from satellites with cameras. If you are raising the question of what can be seen by the naked eye from space, given 20-20 vision, that's an entirely different question.
Reply | Report Abuse | Link to thisI cannot agree more. Archives of classified photographs will not be opened to answer a question about the Great Wall.
Reply | Report Abuse | Link to thisSuch photographs would give China information about capabilities of US recognition from space, i.e., types of sensors used, and their resolution.
Anyone here who earnestly thinks that covering installations with a veneer of soil, rock or vegetation would make them invisible?
The assumption alone appears grotesque.
And - by the way - why should Scientific Ameridan bother to bring this article?
All of the separate reports in the article may be true (why shouldn't they?), but without quantitative information of the limitations e.g. resolution for respective parameters, its results can neither be verified nor falsified.
=>=>=> A statement which does not include its limitations cannot be scientific. <=<=<=
Thus the conclusions on visibility of the Great Wall remain in the realm of belief, not of science.
Sorry to say: The article is Balderdash.
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Edited by pletti at 02/22/2008 7:52 AM
You guessed it, we're talking naked eye, not technologically advanced satellite screening. With that, heck, you can see individual saplings in China's anti-desertification scheme. Remember, the legend isn't about enhanced vision, it's about astronauts picking out the works of man while orbiting. And apparently the Great Wall is not one of them. (Which doesn't make it any less great in my view.)
Reply | Report Abuse | Link to thisWhat a dumb article. The first sentence declares the Great Wall can't be seen from space, then the rest of the article goes on to talk about the Great Wall being seen from space.
Reply | Report Abuse | Link to thisThe fact is, you can see it from space when the lighting is right.
Myth confirmed.
But, where's the sensationalist headline in that?
To dbiello:
Reply | Report Abuse | Link to thisFirst I have to apologize for so blunt a response. in my first comment above.
Unfortunately your reply does't make things better.
You eplained that in the discussion you meant to refer to the naked eye - sure enough the article does include references to remote sensors as well.
The root cause for problems like that is our inadvertent use of tacit assumptions which may not be shared by listeners or readers:
Let me try to illustrate how a scientific contribution might read.
"The naked human eye generally has a resolution of... (degrees, minutes, seconds). Some eyes will do better, but we will take the typical resolution for further discussions.
At which distance from earth's surface wold the natural limit for visibility by the human eye ne reached?
The Great Wall has a diameter of nnn meters. With growing distance the angle of observation becomes narrower. At a distance of nnn kilometers the angle of vision becomes too small for detection.
(2 more parts to follow)
(part 2of 3)
Reply | Report Abuse | Link to thisA structure with the diameter of the Great Wall cannot be differentiated from the background any more. Further constraints (= less favorable conditions) would reduce the limit of visibility to shorter distances.
A sharp contrast in colors visible to our eyes between the Great Wall and its surroundings would be helpful. However, the Great Wall is built from locally available rock and soil, and thus has their very hue.
On the other hand, the observation of a continuous line does help to come nearer to the general limit.
We may be more lucky under especially favorable condition: When the sun is low on the horizon, for example, the wall casts extended shadows that make it possible to discern its silhouette.
A shadow ten times as broad as theheight of the Great Wall means a dark line along the structure with a cross-section of nnn meters: This extends the maximum distance for visibilty with the human eye under ideal conditions to nnn kilometers.
(part 3 of 3)
Reply | Report Abuse | Link to thisA telescope (or similar sensor) in space can do better. With a resolution of an angle of nnn (minutes, seconds), the wall itself might be visible from nnn kilometers and a broad shadow line from nnn kilometers.
As an example, the Great Wall itself has been teased out from its surroundings by (& specified satellites &) under ideal conditions (clear sky and high contrast) from 705 kilometers above earth's surface&.."
& and so on.
All articles on scientific issues in Scientific American should be pleasant to read (not as cumbersome as the example above!), for the general public as well as for scientists from other disciplins, easy to comprehend and correct in the facts.
Moreover, it should convey a differential contribution to a better understanding what science is all about.
One of the fundamental tenets is that each and every scientific statement has to include its own limitation here that would especially be different specifications of various observations.
Is Chinas Great Wall visible from space? Yes and no, granted that space means where the orbits are. Yes, it is visible by special space cameras. No, it is not visible by the naked eyes of astronauts.
Reply | Report Abuse | Link to thisI am pleased to note that the article has reflected my views during a discussion on the issue with my science colleagues more than a decade ago. Precisely, the narrow Great Wall may only be seen by human eyes at low orbits from certain opportune angles when the sun is casting a very wide shadow of the long wall in a contrasting background.
SciAm has the case officially closed. Thank you. (Tan Boon Tee)
It has long been a myth that the Great Wall of China is the only artifact visible from space. It and several variations on the theme are great fodder for water cooler arguments. In fact, many human structures can be seen from orbit.
Reply | Report Abuse | Link to thishttp://www.historicaltravelguide.com/great-wall-of-china-facts.html
The Great Wall of China was initially built as separated walls by different states before the unification of China to protect their land from invasions. After the Qin Dynasty proved to be the most powerful and took over most of the lands, Emperor Qin Shi Huang connected several walls and built the first Great Wall of China between 220–206 BC, and, thus, called “10,000 li Wall.” The great wall of China was established to prevent any invasion attempts and savage attacks by the northern nomadic tribes and dynasties. However, Genghis Khan, a known leader who united northern nomadic tribes, was able to penetrate the wall and able to conquer northern China, which spread all over the country later on. After the Mongol Empire, China was once again back to the leadership of their countrymen; this time, the Ming Dynasty emerged to be the most powerful. During the Ming Dynasty, the concept of the Great Wall was revived once again. The Ming Dynasty revived, reconstruct, and further elongated the Great Wall of China under its 10 year project. They adopted the strategies used by Mongols to penetrate the walls. They further extended it to the desert where the nomadic tribes used to came from. Unlike the earlier walls, Ming used bricks and stones instead of rammed earth to strengthen it, and devoted on reconstruction and repair of the walls as Mongols continued their attempts. Learn more interesting facts here http://www.great-wall-of-china-facts.com
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