UPDATED: 12:39, May 14, 2004
"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.
Great Wall of China from Space
This photo, released yesterday, was taken by the European Space Agency's Proba satellite on March 25. It shows a short stretch of the wall atop hills northeast of Beijing. The wall is highlighted in the upper right. (The lower left of the image is purposely washed out; it shows a stretch of engineered waterways called the Da Yunhe, or Grand Canal, a marvel all its own.)
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."
China's Great Wall Less Great in View from Space
It has become a space-based myth. The Great Wall of China, frequently billed as the only man-made object visible from space, generally isn't, at least to the unaided eye in low Earth orbit. It certainly isn't visible from the Moon.
You can, though, see a lot of other results of human activity.
The visible wall theory was shaken after China's own astronaut, Yang Liwei, said he couldn’t see the historic structure. There was even talk about rewriting textbooks that espouse the theory, a formidable task in the Earth's most populous nation.
Image: This photo of central Inner Mongolia, about 200 miles north of Beijing, was taken on Nov. 24, 2004, from the International Space Station. The yellow arrow points to an estimated location of 42.5N 117.4E where the wall is visible. The red arrows point to other visible sections of the wall. Credit: NASA.
The issue surfaced again after photos taken by Leroy Chiao from the International Space Station were determined to show small sections of the wall in Inner Mongolia about 200 miles north of Beijing.
Taken with a 180mm lens and a digital camera last Nov. 24, it was the first confirmed photo of the wall. A subsequent Chiao photo, taken Feb. 20 with a 400mm lens, may also show the wall.
The photos by Chiao, commander and NASA ISS science officer of the 10th Station crew, were greeted with relief and rejoicing by the Chinese. One was displayed prominently in the nation's newspapers. Chiao himself said he didn't see the wall, and wasn't sure if the picture showed it.
Kamlesh P. Lulla, NASA's chief scientist for Earth observation at Johnson Space Center in Houston, directs observation science activities from the Space Shuttle and the International Space Station. He says that generally the Great Wall is hard to see and hard to photograph, because the material from which it is made is about the same color and texture as the area surrounding it.
"The interpretation of this (Nov. 24) ISS photo," Lulla said, "seems to be good. It appears that the right set of conditions must have occurred for this photograph to capture the small segment of the wall." It was a sunny day and a recent snowfall had helped make the wall more visible.
The theory that the wall could be seen from the Moon dates back to at least 1938. It was repeated and grew until astronauts landed on the lunar surface.
"The only thing you can see from the Moon is a beautiful sphere, mostly white, some blue and patches of yellow, and every once in a while some green vegetation," said Alan Bean, Apollo 12 astronaut. "No man-made object is visible at this scale."
From space you can see a lot of things people have made, Lulla said. Perhaps most visible from low Earth orbit are cities at night. Cities can be seen during the day too, as can major roadways and bridges, airports, dams and reservoirs.
Of the wall visibility theories, Lulla said: "A lot has been said and written about how visible the wall is. In fact, it is very, very difficult to distinguish the Great Wall of China in astronaut photography, because the materials that were used in the wall are similar in color and texture to the materials of the land surrounding the wall -- the dirt."
It's questionable whether you can see it with the unaided eye from space. "The shape, the age of the structure, the resolution of the camera, the condition of the atmosphere -- all these factors affect the ability to detect an object from space." But, he added, "you can see the wall in radar images taken from space."