It is a familiar modern or urban myth that the Great Wall of China is the only man-made object that can be seen from space, but this is in fact a very vague concept. Assuming that this means ‘with the naked eye’ it is still necessary to ask from how high an orbit such observations should be made and under what conditions. On its website, NASA shows how some segments of the Great Wall can be made out in photographs taken by astronauts but that the human eye cannot see it from low earth orbit. If it is that difficult for a large structure like that, Hadrian’s Wall stands no chance!
Nowadays, of course, remote sensing technology means that civilian satellites can routinely image down to 0.5m resolution and military satellites probably supply even finer detail. In that respect, Hadrian’s Wall is no different from any other monument in the degree of its visibility from space.
Further reading: Man 2008