By Roland Piquepaille
When Yang Liwei, the first taikonaut, orbited 14 times around Earth in October 2003, he didn't see the Great Wall of China from his Shenzhou V capsule. He was disappointed, and so were his countrymen. China said for decades that the Great Wall was the only manmade object visible from space. So, the Chinese government was tempted to modify the schoolbooks to remove this reference. But now, Proba, the polyvalent satellite launched in 2001 by the European Space Agency (ESA), was able to capture images of the Great Wall from space taken at an altitude of 600 km. As a practical consequence, millions of books will not have to be printed again, saving probably a large forest.China has cherished for decades the idea that the Wall was just about the only manmade object visible to astronauts from space, and the news disappointed many. A suggestion was made that the Wall be lit up at night so it can definitely be seen in future, while others called for school textbooks to be revised to take account of Liwei's finding.
However such revisions may be unnecessary, according to American astronaut Eugene Cernan, speaking during a visit to Singapore: "In Earth's orbit at a height of 160 to 320 kilometres, the Great Wall of China is indeed visible to the naked eye."
What is for sure is that what the human eye may not be able to see, satellites certainly can. Proba's High Resolution Camera (HRC) acquired this image of the Wall from 600 km away in space. The HRC is a black and white camera that incorporates a miniature Cassegrain telescope, giving it far superior spatial resolution to the human eye.
Here is an image acquired by Proba's High Resolution Camera on March 25, 2004. It shows a short stretch of the 7240-km-long Great Wall of China snaking along hilltops northeast of Beijing, running from the top middle of the image down to bottom right.
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The white watercourse that meanders from the middle of the left side down to the bottom of the image is the initial part of the 1500-km-long Da Yunhe or Grand Canal, a linked series of natural and man-made waterways that represents an engineering achievement on a par with the Great Wall (Credit for image and legend: ESA). |
Here is a link to a larger version of the same image (1,019 x 1,026 pixels).
And for more information about the Great Wall, you can look at this page on Wikipedia.
Update: The ESA said on May 19, 2004, that Proba didn't see the Grat Wall of China. In "Correction: 'Great Wall of China' image," the agency gives the following explanation.
The story published 11 May 2004 'Great Wall of China seen from space' featured image interpretation errors. The feature that was claimed to be the Great Wall is actually a river running into the Miyun Reservoir, northeast of Beijing. Further acquisitions of the region are being planned with other ESA instruments to investigate the subject further.
Source: European Space Agency news release, May 11, 2004
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