Chinese scientists have reopened the debate on whether or not the Great Wall is visible from space with the human eye, labeling it "impossible".

In a report published in Chinese science magazine Science & Technology Review, Dai Changda, Jiang Xiaoguang and Xi Xiaohuan, all 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.

"The distance of 36 kilometers is well below the widely-acknowledged distance of space from the earth, which is at least 100 kilometers," 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 can not pick up details that advanced photographic equipment can," said Jiang.

In March, 2004, U.S. 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.

Source: Xinhua