Things seen from space: the Great Wall of China

It's quite a claim to make: that such-and-such a 'man-made' thing can be seen from space. The one people are probably most familiar with is the Great Wall of China. There's a rumour that Yuri Gagarin saw it during his epic first orbit of the Earth in 1961. There's also a popular culture myth that it's visible from the Moon.

The Moon story, is, interestingly, pre-Space Age. In the 16th and 17th centuries, Portuguese travellers brought accounts of the Great Wall back to Europe, so people there became aware that this impressive structure existed. 

In 1754, the Reverend William Stukeley, who was one of the first people to study Stonehenge and hence somewhat of an expert on large structures, mentioned the "Chinese wall, which makes a considerable figure upon the terrestrial globe, and may be discerned at the moon" in a letter. (Stonehenge really wasn't of much interest to the English public before Stukeley). 

In 1895, English journalist and MP Henry Norman made the same claim: "Besides its age it enjoys the reputation of being the only work of human hands on the globe visible from the moon." He had at least traveled in China and seen the Great Wall first hand. But he doesn't give any evidence to back this claim up. The debate around canals on Mars in the late 19th century may have made it seem logical that long linear artificial structures would be visible from other planets.

In the 20th century, human eyes went to space for the first time. They were in Yuri Gagarin's head.  But he did not mention the Great Wall at all. 

Of course, it's all relative. Whether it's visible or not depends on where your orbit is, whether it's day or night, and what you are looking with. In general I think people mean visible with the unaided human eye, or with satellites in Low Earth Orbit. Higher up in the orbital column, satellites aren't looking. In certain limited conditions, the Great Wall can be seen by astronauts on the ISS or by Earth observation satellites.

The Great Wall seen by ESA's Proba satellite in 2004. Credit: ESA

There are, however, other human structures which are visible from space, including:

• Dubai's palm islands - these are artificial islands built as residential complexes off the coast of Dubai, in the shape of palm leaves
• Major cities at night - when illuminated, the largest cities are easy to pick out as distinct entities. 
• The Pyramids at Giza - located in Egypt, the Great Pyramid of Giza was one of the seven wonders of the ancient world. It's astonishing to think that it's retained this significance with a new space age spin. The three pyramids at Giza are not visible to the naked eye, but can be seen through a space-based camera.
• The greenhouses of Almeria - 64, 000 acres of greenhouses in Spain. These are visible through cameras because the reflective material of the greenhouses catches the light in daytime. 
• The Bingham Canyon Mine - this open-cut copper mine is the largest human excavation in the world. It extends over 4 km and is over 1 km deep. 

I don't think anything has been designed with the specific aim of being seen from space - yet.

This is Earth taken from lunar orbit - the famous Earthrise photo of 1968.
No human-made features visible. Credit: NASA

From another perspective, it's not just the human artefacts that are visible. It's the impacts of human activities, such as forest loss, desertification and the creep of agriculture. These are also human things seen from space.

Why am I interested in this? 

Firstly, things seen from space may tell us something about what to look for as signs of intelligent life on exoplanets. This is one of the goals of SETI research. 

On the other hand, there are a bunch of people who look for alien structures on the Moon and Mars using remote sensing imagery from orbiting spacecraft, just like archaeologists do to locate and map artificial structures on Earth - and, apparently, they find them!. It comes down to what looks natural and what looks cultural, and this is often because something resembles an Earthly structure. This is called pareidolia, the phenomenon of seeing patterns where none exist.

But here's the real reason. Although you won't find it mentioned in the World Heritage listing, the myth of being visible from space contributes to the heritage value of the Great Wall of China. It is about aesthetic and social value, at least to people in the 'west'. 

The aesthetic value of the Great Wall is about scale - something so long, that required huge amounts of labour over centuries in its different forms (I'm not going to go into it's complex history). It's a stable landscape feature, 20, 000 km long. In 1644, when it's construction finished, it was the world's largest military structure. It follows the top of a ridgeline, so it is visible from a distance. To say that it can be seen from space is a testament to its sheer size and ambition. 

Being seen from space is really a statement about human perceptions of things so large that our eyes can't take them all in at once, that their true form is not comprehensible from the ground. The Nazca lines are like this too. It's about things so enormous from a human scale that they seem almost inhuman. Unlike the Great Pyramid at Giza, though I don't think anyone has claimed that the Great Wall was made by aliens. 

Hold my beer, it seems that they have.

The social phenomenon is about people's beliefs about space. It's like space-flown objects, which are held in museum collections and fetch high prices in the collector's market: the special relationship with space adds value. They could be the most boring, mundane object or material, but when they've come back from space they take on almost magical qualities. They're a talisman, a touchstone, something that brings you spatially closer to that unattainable place (for most of us).

In the 1980s. Frank White put forward the idea of the Overview Effect. The Overview Effect is a semi-mystical experience that involves seeing Earth as a whole planet, a natural object on which human traces are insignificant. The planet is fragile and beautiful, and its delicate ecological balance could be destroyed by human actions. The people of Earth should be united, not torn apart by wars and conflicts: national boundaries are not visible from space. (Although having said that, South Korean astronaut Soyeon Yi reports her sadness at seeing the very obvious differences between North and South Korea when lit up at night - that was a national border visible from space).

This is a different form of the Overview Effect, which involves seeing instead the stability and resilience of human culture, its ability to take raw telluric materials and build them into a human signature on the planet: mark the planet as cultural rather than natural, when seen from space. And potentially when seen by outsiders approaching from space, seeking the evidence of sentient actions. (There might be a few clues before then. I talk about this in 'The Abandoned Solar System')

This brings us to the concept of Outstanding Universal Value, which is the basic criterion for achieving World Heritage status. A natural or cultural place has to have heritage value potentially for the entire human species. It's a big call. But if any cultural feature could be said to have it, it would be something so monumental it could be seen from space - by us, or by another species.

And: at some point in the future, we may have to consider what it means if human-made structures or environmental impacts are visible on the Moon, from Earth.

References
The Family Memoirs of the Rev. William Stukeley (1887) Vol. 3, p. 142. (1754)

Norman, Henry 1895 The Peoples and Politics of the Far East, p. 215.