ESA's Proba satellite here shows a winding segment of the 7240-km long
Great Wall of China situated just northeast of Beijing. The Great Wall's
relative visibility or otherwise from orbit has inspired much recent
The 21 hours spent in space last October by Yang Liwei - China's first
ever space traveller - were a proud achievement for his nation. The only
disappointment came as Liwei informed his countrymen he had not spotted
their single greatest national symbol from orbit.
"The Earth looked very beautiful
from space, but I did not see our Great Wall," Liwei told reporters
after his return.
cherished for decades the idea that the Wall was just about the only
manmade object visible to astronauts from space, and the news
disappointed many. A suggestion was made that the Wall be lit up at
night so it can definitely be seen in future, while others called for
school textbooks to be revised to take account of Liwei's finding.
However such revisions may be
unnecessary, according to American astronaut Eugene Cernan, speaking
during a visit to Singapore: "In Earth's orbit at a height of 160 to 320
kilometres, the Great Wall of China is indeed visible to the naked eye."
Artist's impression of Proba in orbit
well have been unlucky with the weather and local atmospheric or light
conditions – with sufficiently low-angled sunlight the Wall's shadow if
not the Wall itself could indeed be visible from orbit.
What is for sure is that what the
human eye may not be able to see, satellites certainly can. Proba's High
Resolution Camera (HRC) acquired this image of the Wall from 600 km away
in space. The HRC is a black and white camera that incorporates a
miniature Cassegrain telescope, giving it far superior spatial
resolution to the human eye.
So while the HRC resolves
man-made objects down to five square metres, astronauts in low Earth
orbit looking with the naked eye can only just make out such large-scale
artificial features as field boundaries between different types of crops
or the grid shape formed by city streets. They require binoculars or a
zoom lens to make out individual roads or large buildings.
Source: ESA - European Space