Not seen from space

kw: observations, artifacts

The narrow feature seen down the middle of this image is a small section of the Great Wall of China, on the portion accessible from the Badaling Station northwest of Beijing. Four of the guard towers are visible. The northernmost is at an elevation of 2,550 ft (777 m). The viewpoint by Google Earth's reckoning is at an altitude of 5,322 ft (1,622 m).

If I understand correctly that altitude is measured from the geoid, not from "ground" level, the viewpoint is about a half mile (0.8 km) above the locally highest point of the Wall. At this altitude, the Wall is impressive. Using the "tilting" feature of Google Earth I was able to determine that the Wall extends from horizon to horizon.

The length of the main trunk of the Great Wall is given in several sources as 5,500 miles, or about 8,850 km. As the crow flies, it is probably about half this extent from end to end. Side branches and parallel sections are thought to extend the total to about six times this amount. However it is measured, the Wall is one of the largest of human artifacts. It is truly on a continental scale.

So, is it visible from space? I remember hearing that it is visible from the Moon, or from orbit, and so forth, then also hearing that it is not. What is the real story? Surely a continent-spanning object should be visible from pretty far away…right? I think the images here show, no, not really. It may be long, but it is rather narrow, rarely exceeding 100 feet (30 m) in width.

From an altitude of 21,000 feet (6,400 m) the Great Wall is seen as a thin scratch just left of center. I shifted the view over a bit so as to include a portion of a nearby divided highway, with each section easily twice as wide. There is also a river valley further to the left that is more visible than the Wall. Keep these two features in mind when you look at the third image below.

Let us consider what the unaided eye can see. Two criteria are found in the literature about keenness of vision. Firstly, for discerning two stars close together, either in the sky or in a telescope eyepiece, one arc minute is used, which is a factor of 1/3,400 of the distance to an object. Secondly, the "lines per degree" criterion: when discerning a small object in an image, the criterion is 1/1,000 of the distance to the object.

If a bright light was put on either side of the Great Wall, how high would you have to be before you could not tell there were two lights? Assume the lights are 100' (30 m) apart. 3,400 times this is 340,000 feet (62 mi) or 102 km. That's just high enough for the lowest orbit that won't decay quickly. However, to see the wall as part of an image, without the help of the lights, the more realistic criterion is 1,000x, or 100,000 feet (19 mi) or 30 km. That's only about a tenth of the way to the altitude of the International Space Station.

At an elevation of 15 miles (24 km), by the reckoning of Google Earth, this is what you might see. First, click on the image to see the 800-pixel-size version. Look near the center of the image, between the divided highway on the right and the river valley on the left. The Wall is barely visible between them. In the image on this page, half that size, it is what you might see from twice the height, and you aren't likely to see the Great Wall at all. Double the height yet again, and the divided highway gets rather hard to see.

In this essay, some reports are summarized, showing that nobody has seen the Great Wall from orbit without optical aid, and even then it was found only with difficulty. It may be really, really long, but it simply isn't wide enough to be seen from orbit, let alone from the Moon!

When you are right up next to them, the "works of man" do seem imposing, mighty to behold. But from afar, on the daylit side, the only detectable sign of human life on the planet is air pollution.