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In a report published in Chinese science magazine Science & Technology Review, Dai Changda, Jiang Xiaoguang and Xi Xiaohuan, researchers with the Chinese Academy of Sciences (CAS), argue that the claim, made by American and Russian astronauts, defies the laws of biological science.

"The Great Wall contains some sections that are approximately 10 meters wide. But a 10-meter-wide object can only be seen with a naked eye from a maximum distance of 36 kilometers in extremely good weather conditions," Jiang Xiaoguang told Xinhua News Agency.

"Since the definition of space is widely acknowledged as beginning 100 kilometers from the Earth, the 36 kilometer limit would be reached before a shuttle even entered orbit," he said. "Therefore the Great Wall is indeed invisible from space with the human eye."

Jiang acknowledged the fact that the Great Wall had been photographed from space but said it was not related to whether or not the structure could be seen with the human eye.

"Obviously the human eye is very different from a camera and cannot pick up details that advanced photographic equipment can," Jiang noted.

In March 2004, US astronaut Gene Cernan, the last man to walk on the moon as commander of the Apollo 17 mission, told a Singapore newspaper during an interview that "in Earth's orbit at a height of 160 to 320 km, the Great Wall of China is indeed visible to the naked eye."

However, during China's first manned spaceflight in 2003, Yang Liwei, China's first astronaut, confirmed that he did not see the Great Wall while in orbit.

(Xinhua News Agency March 2, 2007)