First, the good news: The Great Wall of China can, indeed, be seen from outer space with the naked eye.

So education officials, who fretted about changing textbooks -- which have long held that the Great Wall and the dykes in the Netherlands were the only man-made structures visible from space -- can breathe easy.

Yang Liwei, the nation's first spaceman, disappointed a lot of Chinese when he said he did not see the wall during his orbit of less than 24 hours in October 2003, prompting calls for revisions in textbooks.

But there is a rider: There are many, many structures which can also be seen, such as the Pyramids of Egypt, airports, highways -- and even Beijing's Third Ring Road.

Apparently, it requires an atmospheric "perfect storm" in outer space, defined as an altitude of about 350 kilometers and you have to be in the right place at the right time, amid the right conditions to spot a particular structure.

Chinese-American astronaut Leroy Chiao, who has been on three space flights and is currently coming to the end of his six-month stint on the joint US-Russian space station, has provided the first photographic evidence of sections of the Great Wall using commercially available equipment.

This shot of the Badaling section of the Great Wall was taken by space station astronaut Leroy Chiao on the morning of February 20 when he flew over Beijing at an altitude of 360 kilometers. The photograph, the first ever released by Chiao, was studied by Professor Wei Chengjie, an expert in remote sensing, who added the markings and toned down the blue colour so that it closer resembles the view one would have from an aeroplane.

The photos have been authenticated by Professor Wei Chengjie of the Institute of Remote Sensing Applications at the Chinese Academy of Sciences.

However, Chiao is himself not certain: "It is hard to say whether or not I have seen it. That's because from our altitude, I cannot distinguish between the Wall and roads." He described his picture, taken on February 20, as a "region northwest of Beijing."

While the layman might have difficulty in telling the highways, railways and mountain ridges apart, Wei easily identifies all of them.

Also, a group of snow-enhanced shots Chiao took in November last year have also been confirmed by the US National Aeronautical Space Agency as the "first verifiable views of the Great Wall of China ever identified in astronaut photography."

Chiao describes the near-impossible odds of spotting the wall or getting the perfect picture.

The space station orbits the earth every 90 minutes at an altitude of 360 kilometres and travels at a speed of 8 kilometres a second.

And the flight path is different for each orbit.

So the chances of shooting the Great Wall area are "maybe once every few days and only during daylight every three months."

Since late March, Chiao has had just one opportunity to fly over the Great Wall, but the weather was dodgy.

He has one last chance before he is scheduled to land in Kazakhstan on April 25 -- and he will be keeping a sharp lookout.

(China Daily April 19, 2005)