Artificial structures visible from Earth orbit without magnification include highways, dams, and cities. The Great Wall of China, often cited as the only human-made structure visible from space, is not visible from low Earth orbit without magnification, and even then can be seen only under perfect conditions. On the other hand, the centimetre-band Spaceborne Imaging Radar of STS-59 and STS-68 was able to detect not only the Great Wall but also invisible buried segments of it. Whether an object is visible depends significantly on the height above sea level from where it is observed. The Kármán line, at 100 kilometres, is accepted by the World Air Sports Federation, an international standard-setting and record-keeping body for aeronautics and astronautics, as the boundary between the Earth's atmosphere and outer space. However, astronauts typically orbit the Earth at several hundreds of kilometres; the ISS, for example, orbits at about 420 km above the Earth, and the Moon orbits at about 381 415 km away.
Examples
From US Space Shuttles, which typically orbited at around, cities were easily distinguishable from surrounding countryside. Using binoculars, astronauts could even see roads, dams, harbors, even large vehicles such as ships and planes. At night, cities are also easily visible from the higher orbit of the ISS. Metropolitan areas are clearly visible at night, particularly in industrialized countries, due to a multitude of street lights and other light sources in urban areas.
The greenhouse complex that covers about 260 km2 in the province of Almería, Andalucía, Spain is visible from space. It is sometimes referred to as the "Plastic sea" due to the high concentration of these greenhouse structures. This area produces much of the fruit and vegetables that are sold in the rest of Spain and Europe. Apart from the area depicted in the photo, other zones of the province of Almería have a big concentration of white-plastic greenhouses too.
The claim that the Great Wall of China is the only man-made object visible from the Moon or outer space has been debunked many times, but remains a common misconceptionin popular culture. According to astronauts Eugene Cernan and Ed Lu, the Great Wall is visible from the lower part of low Earth orbit, but only under very favorable conditions. Different claims were historically made for the factoid that the Great Wall is visible from the Moon. William Stukeley mentioned this claim in his letter dated 1754, and Henry Norman made the same claim in 1895. The issue of "canals" on Mars was prominent in the late 19th century and may have led to the belief that long, thin objects were visible from space. A viewer would need visual acuity 17 000 times better than normal to see the Great Wall from the Moon.
Popular culture
questions and urban legends often claim certain constructed objects or effects are so large they are visible from outer space. For example, a giant beaver dam in Canada was described as "so large it is visible from outer space." Field and Stream, a Canadian Magazine, wrote, "How big? Big enough to be visible... from outer space."
Theoretical calculation of visibility from the ISS
The human naked eye has an angular resolution of approximately 280 microradians, and the ISS targets an altitude of 400 km. Using basic trigonometric relations, this means that an astronaut on the ISS with 20/20 vision could potentially detect objects that are 112 m or greater in all dimensions. However, since this would be at the absolute limit of the resolution, objects on the order of 100 m would appear as unidentifiable specks, if not rendered invisible due to other factors, such as atmospheric conditions or poor contrast. For readability from the ISS, using the same trigonometric principles and a recommended character size of about 18 arcminutes, or about 5,000 μrad, each letter would need to be about 2 km tall for clear legibility in good conditions.
