Double Great Wall spied from space.

Despite the claims of an oft-repeated rumor, the Apollo astronauts could not spot the Great Wall of China from the moon, 400,000 kilometers away (SN: 12/24&31/94, p. 432). Crews on the space shuttle, however, orbiting only a few hundred kilometers above Earth, can pick out China's best-known monument when the lighting is right. But a trio of radars on the space shuttle demonstrates the best vision. The radar images show not only the current wall but also a previous edition running parallel to it, report NASA scientists.

The triple-radar instrument, known as SIR-C/X-SAR, operates at three different frequencies, and it flew on the space shuttle twice in 1994. Although archaeology was not an intended part of those missions, images from the radars can reveal hidden structures and help map man-made features.

The SIR-C/X-SAR team is investigating a section of the Great Wall running across the desert about 430 miles west of Beijing. The radar image shows two parallel lines separated by a span of only a few meters. According to Chinese scientists participating in the experiment, the main line represents the current wall, dating to the Ming Dynasty 600 years ago. A fainter line is an older section built during the Sui Dynasty, more than 1,000 years ago, says Jeffrey J. Plaut, the SIR-C experiment scientist at NASA's Jet Propulsion Laboratory in Pasadena, Calif.

In the location imaged by the radar, the walls were constructed out of dirt to form raised berms much simpler than the well-photographed sections of wall near Beijing. Although some segments of the older version are visible at the surface, dirt and sand have covered the wall in other places. Archaeologists may be able to use images from the spaceborne radars to track these earlier editions of the wall, says Plaut.