Space radar reveals secrets of Great Wall

Scientists in China are using radar images taken from space to study parts of the Great Wall of China which have been eroded and buried by centuries of windblown sand.

The pictures can identify different versions of the wall - one of the few manmade structures visible with the naked eye from space. It was first built in the third century BC, to protect the country from northern invaders.

"In the images, we can recognise two different dynasties that built the Great Wall," said Dr Guo Huadong, of the Chinese Academy of Sciences in Peking. "One was built in the Ming dynasty and is about 600 years old. The other was built during the Sui dynasty and is more than 1,000 years old."

The colour picture (top right) shows a 45-mile segment of the wall, which in total is more than 1,860 miles long. This piece lies about 430 miles west of Peking, in a remote part of the north-central China desert, and is visible running from top to bottom as a continuous line.

The radar images are black and white, each showing a section two miles long. The one illustrated here shows the two generations of construction: the bright line on the left is the present-day wall, while just to its right is a discontinuous line - the part built during the Sui dynasty (which lasted from 589-618), which has been intermittently buried by sand dunes blown by winds.

"In this region the wall was made out of loose soil and mud, not bricks and rocks," Dr Huadong said. "Usually you cannot find these segments even if you go there, so the radar data are helping to show us the whole wall."

The different generations of the wall are easy to detect by radar from space, because the steep, smooth sides - between 15 and 25 feet high in its present form - provide a prominent surface which reflects the radar beam.

The radar, called the "synthetic aperture radar", was carried on the Space Shuttle and took these pictures earlier this year.

"Archaeology wasn't one of our original science objectives, but the imaging radar data has been found to be very useful for this type of research," said Dr Diane Evans, a project scientist at the United States space agency, NASA.

The radar system is now being used for archaeological efforts in areas which include Angkor in Cambodia, the Lost City of Ubar, in Oman, and the Silk Road along the north-west desert of China.