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The Great Wall of China from space

"The Earth looked very beautiful from space, but I did not see our Great Wall," lamented China's first astronaut, Yang Liwei, after 21 hours in orbit last October. The comment triggered a round of news stories that implied the structure could not be seen by any astronaut, disappointing many Chinese who thought it was the only manmade structure visible from space.

Sure, spotting the Great Wall of China from space is easy with the right telescope and camera. But why couldn't China's new hero see it? He just didn't have enough time or the right conditions, it would seem.

"In Earth's orbit at a height of 160 to 320 kilometers [100-200 miles], the Great Wall of China is indeed visible to the naked eye," says astronaut Eugene Cernan.

A low angle of sunlight casting long shadows can help.

"You can see the Great Wall," confirms astronaut Ed Lu, who was the science officer of Expedition Seven on the International Space Station. The station circles Earth higher than Yang Liwei's orbit.

The misconception is wrapped up in broader myths about what is and what is not visible from space. For the record: No manmade structures on Earth can be seen with the unaided astronaut's eye from the Moon. But many things -- highways, dams and even large vehicles -- are easily spotted from Earth-orbit with no optical aids.

What's Really Visible from Space

There is a longstanding myth that the Great Wall of China is the only manmade object visible from space. It and several variations on the theme are great fodder for water cooler arguments. In reality, many human constructs can be seen from Earth orbit.

Shuttle astronauts can see highways, airports, dams and even large vehicles from an Earth orbit that is about 135 miles (217 kilometers) high. Cities are clearly distinct from surrounding countryside, and that's true even from the higher perch of the International Space Station, which circles the planet at about 250 miles (400 kilometers) up.

"You can see an awful lot from space," says astronaut Ed Lu, the science officer of Expedition Seven aboard the station. "You can see the pyramids from space, especially with a pair of binoculars. They are a little difficult to pick out with just your eyes."

The naked eye can tell the difference between cities and countryside from space. And with a digital camera and 800mm lens, this view of Manhattan was obtained from the Space Station on April 28, 2001.

Egyptian pyramids have been photographed from space several times with standard digital cameras and high-powered lenses. The largest pyramid at Giza, on the outskirts of Cairo, is 745 feet (227 meters) wide and 449 feet (137 meters) tall.

"With binoculars you can see an awful lot of things," Lu wrote via e-mail in fielding a question from an Earthbound space fan. "You can see roads. You can see harbors. You can even see ships; very large tankers on the ocean we can see using the binoculars."

There are some surprises, too.

"You can see airplane contrails, and occasionally at the end of an airplane contrail, you will see a glint of sunlight off the airplane," Lu says. "And very occasionally, you do see other satellites go by. It is kind of a neat thing to see."

There are of course places in space from which you can't notice how humans have sculpted the planet. Apollo astronauts could not make out manmade features from the Moon, for example. And from Mars, Earth would appear to the naked eye as nothing but a bright "star" in the night sky.

So what about the Great Wall of China?

"You can see the Great Wall," Lu says. But it's less visible than a lot of other objects. And you have to know where to look.

In fact stretches of the wall aren't even visible from China. They've been buried by sand for centuries. NASA has used space-based radar to map out hidden parts of the ancient structure. Lu is trying to get a picture of it, too, with a digital camera.

"The weather hasn't cooperated," he says. "There has been a lot of clouds and haze over that area since I've been trying. But I hope to be successful before I come back down."

This shot of the Badaling section of the Great Wall was taken by space station astronaut Leroy Chiao on the morning of February 20 when he flew over Beijing at an altitude of 360 kilometres. The photograph, the first ever released by Chiao, was studied by Professor Wei Chengjie, an expert in remote sensing, who added the markings and toned down the blue colour so that it closer resembles the view one would have from an aeroplane.   　　First, the good news: The Great Wall of China can, indeed, be seen from outer space with the naked eye. 　　So education officials, who fretted about changing textbooks - which have long held that the Great Wall and the dykes in the Netherlands were the only man-made structures visible from space - can breathe easy. 　　Yang Liwei, the nation's first spaceman, disappointed a lot of Chinese when he said he did not see the wall during his orbit of less than 24 hours in October 2003, prompting calls for revisions in textbooks. 　　But there is a rider: There are many, many structures which can also be seen, such as the Pyramids of Egypt, airports, highways - and even Beijing's Third Ring Road. 　　Apparently, it requires an atmospheric "perfect storm" in outer space, defined as an altitude of about 350 kilometres and you have to be in the right place at the right time, amid the right conditions to spot a particular structure. 　　Chinese-American astronaut Leroy Chiao, who has been on three space flights and is currently coming to the end of his six-month stint on the joint US-Russian space station, has provided the first photographic evidence of sections of the Great Wall using commercially available equipment. 　　The photos have been authenticated by Professor Wei Chengjie of the Institute of Remote Sensing Applications at the Chinese Academy of Sciences. 　　However, Chiao is himself not certain: "It is hard to say whether or not I have seen it. That's because from our altitude, I cannot distinguish between the Wall and roads." He described his picture, taken on February 20, as a "region northwest of Beijing." 　　While the layman might have difficulty in telling the highways, railways and mountain ridges apart, Wei easily identifies all of them. 　　Also, a group of snow-enhanced shots Chiao took in November last year have also been confirmed by the US National Aeronautical Space Agency as the "first verifiable views of the Great Wall of China ever identified in astronaut photography." 　　Chiao describes the near-impossible odds of spotting the wall or getting the perfect picture. 　　The space station orbits the earth every 90 minutes at an altitude of 360 kilometres and travels at a speed of 8 kilometres a second. 　　And the flight path is different for each orbit. 　　So the chances of shooting the Great Wall area are "maybe once every few days and only during daylight every three months." 　　Since late March, Chiao has had just one opportunity to fly over the Great Wall, but the weather was dodgy. 　　He has one last chance before he is scheduled to land in Kazakhstan on April 25 - and he will be keeping a sharp lookout.

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