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Great Wall of China from Space
(space.com)
Updated: 2004-05-14 11:43
"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. [Space.com] |
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.
The latest astronaut photo of the pyramids was
taken in August 2003 with a Kodak DCS760 digital camera equipped with an
800mm lens. [space.com] |
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.
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. [space.com] |
"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]
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