Great Wall of China from Space :
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>"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.
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>Great Wall of China from Space
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>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.)
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>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.
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>"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
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>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.
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>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.
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>"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?
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>"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."
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>Source: space.com
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