FYI: Is China's Great Wall really the only man-made object visible from space?

It's undoubtedly big, and a very impressive feat of engineering, but contrary to established wisdom, the Great Wall of China isn't the only human achievement that can be seen all the way from the heavens - in fact, it's not even visible from space.

The origins of this belief are murky but are probably rooted in a 1932 newspaper cartoon produced by Ripley's Believe It or Not!, a venerable US media franchise that specialises in the dissemination of fantastical facts. The Great Wall is 'the mightiest work of man, the only one that would be visible to the human eye from the moon', Ripley's gushed. A few years later, respected adventurer Richard Halliburton made a similar assertion in his popular Second Book of Marvels, which detailed the wonders of the Orient, and the claim soon began appearing as gospel truth in school textbooks, at a time when it could be neither verified nor disproved.

It wasn't until man actually went to the moon that the Great Wall theory was seriously questioned. Neil Armstrong, commander of the Apollo 11 lunar mission in 1969, remarked that as far as he could tell there were no man-made objects visible from Earth's orbit - and that he couldn't find a single colleague who would testify to spotting the wall from a spacecraft.

Armstrong's comments were backed by Alan Bean, an astronaut on Apollo 12, who said the only thing the human eye picks up when gazing at Earth from the moon is a 'beautiful sphere', swathed in white clouds with patches of blue (oceans), green (vegetation) and yellow (deserts). There's also basic optic theory to contend with - to be visible to the unaided eye from the moon, an object on Earth would have to be thousands of kilometres in diameter and stand in contrast to its surroundings. Continents are about the only things that qualify.

If, by deciding what's visible from 'space', people are talking about positions closer to Earth, however, the picture is slightly different. From about 100km up, the verdict is that the wall just might be spotted, if conditions are perfectly clear and the observer is blessed with excellent vision. But that's no real claim to fame - plenty of human creations can be picked out at these heights, including Egypt's pyramids, China's Grand Canal, sections of the Kennedy Space Centre, even some motorways and dams.

The wall also regularly puts in (barely noticeable) appearances in radar and photographic images from facilities such as the International Space Station (ISS). In fact, a shot taken by Chinese-American astronaut Leroy Chiao in which the wall can be seen recently led the China Daily to insist Ripley's original assertion was true. Old beliefs, like old walls, are hard to knock down.

Some things that are visible from space should give people cause to worry. The devastation wrought by the increasingly common fishing technique of bottom trawling is already apparent on a galactic scale. New satellite images clearly show cloud-like plumes - massive quantities of sediment stirred up by the huge nets - beneath the surface of the oceans.

Frequent visitors to the ISS, such as US astronaut Frank Culbertson, have also commented on a decline in the visibility of Earth in recent years due to smog, a consequence of the planet's rising consumption of fossil fuels and shrinking forest cover. So even if the Great Wall really had been a sight to behold from the moon 70 years ago, it wouldn't be now.