Visible from space

I was getting my hair cut this morning when I heard an advert on the radio claim that the only man-made structure visible from space was the Great Wall of China. I chuckled to myself and told the hairdresser that it wasn't true. I told him about the first Chinese astronaut, Yang Liwei, to be launched by the China National Space Administration last October. He orbited for 21 hours but didn't manage to see the Great Wall. Astronaut Eugene Cernan claims you can see it (or possibly the shadow cast by it) while astronauts William Pogue and Jay Apt claim not to have seen it.

So does that mean that astronauts can't see any man-made objects from space? Not quite. It depends on how big something is and whether they are using a pairs of binoculars! To make out details of something it is important to have a high enough resolution, although it is quite possible to see something smaller is there if there is enough contrast with the background. After all, the angular size of stars is extremely small but we can still see that they are there - we just can't make out any details on them.

So to round up, we can see man-made objects from space if there is enough contrast between them and their surroundings or if we use a pair of binoculars or a small telescope. The wall can also be 'seen' by radar. However if we can make out the Great Wall of China, we should also be able to see the M62!

Posted in astro blog by Stuart on Wednesday 26th May 2004 (13:03 UTC) | Permalink
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