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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.
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."
"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."
[Courtesy of space.com]