to less than . Various square lookout towers that characterize the most famous images of the wall have disappeared. Many western sections of the wall are constructed from mud, rather than brick and stone, and thus are more susceptible to erosion. In 2014 a portion of the wall near the border of Liaoning and Hebei province was repaired with concrete. The work has been much criticized. Visibility from space Various factoids in popular culture claim that the Great Wall can be seen (with the naked eye) from space, with questionable degrees of veracity. From the Moon The Great Wall of China cannot be seen by the naked human eye from the Moon. Even though the myth is thoroughly debunked, it is still ingrained in popular culture. The apparent width of the Great Wall from the Moon would be the same as that of a human hair viewed from away. One of the earliest known references to the myth that the Great Wall can be seen from the moon appears in a letter written in 1754 by the English antiquary William Stukeley. Stukeley wrote that, "This mighty wall of four score miles [130 km] in length is only exceeded by the Chinese Wall, which makes a considerable figure upon the terrestrial globe, and may be discerned at the Moon." The claim was also mentioned by Henry Norman in 1895 where he states "besides its age it enjoys the reputation of being the only work of human hands on the globe visible from the Moon." 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. The claim that the Great Wall is visible from the moon also appears in 1932's Ripley's Believe It or Not! strip. From low Earth orbit A more controversial question is whether the wall is visible from low Earth orbit (an altitude of as little as ). NASA claims that it is barely visible, and only under nearly perfect conditions; it is no more conspicuous than many other human-made objects. Veteran US astronaut Gene Cernan has stated: "At Earth orbit of high, the Great Wall of China is, indeed, visible to the naked eye." Ed Lu, Expedition 7 Science Officer aboard the International Space Station, adds that, "It's less visible than a lot of other objects. And you have to