Artificial structures visible from space
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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.
Whether an object is visible varies significantly on the height above sea level from where it is observed. The Kármán line, at 100 kilometers (62 mi), 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 394 km (245 miles) above the earth and the Moon orbits at about 381,415 km (237,000 mi) away.
From US Space Shuttles, which typically orbited at around 135 mi (217 km), 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 clear at night, particularly in industrialized countries, due to a multitude of street lights and other light sources in urban areas (see light pollution).
The Greenhouses of Almería
The greenhouse complex that covers almost 50,000 acres (200 km2) in the province of Almería, Andalucía, Spain is visible from space.. It is sometimes referred to as the "Plastic sea" ("Mar de plástico" in Spanish) due to the high concentration of said greenhouse structures.
This area produces a great amount of the fruits 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 (and also the south of Spain) have a big concentration of white-plastic greenhouses too.
Bingham Canyon Mine
The Bingham Canyon Mine, more commonly known as Kennecott Copper Mine, is an open-pit mining operation extracting a large porphyry copper deposit southwest of Salt Lake City, Utah, in the Oquirrh Mountains. The mine is the largest man-made excavation in the world.
The Great Wall of China
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 misconception in 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 are 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.
Trivia 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 0.00028 radians, and the ISS targets an altitude of 400km. Using basic trigonometric relations, this means that an astronaut on the ISS with 20/20 vision could potentially detect objects that are 112m or greater in all dimensions. However, since this would be at the absolute limit of the resolution, objects on the order of 100m would appear as unidentifiable specs, 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 an assumed legibility requirement of 18 arcminutes, each letter would need to be approximately 2km tall.
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- "the wall is only visible from low orbit under a specific set of weather and lighting conditions. And many other structures that are less spectacular from an earthly vantage point—desert roads, for example—appear more prominent from an orbital perspective." The Afsluitdijk in The Netherlands can also be seen from outer space Scientific American found at Is China's Great Wall Visible from Space? at Scientific American website.
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