|MadSci Network: Astronomy|
The first thing to say is that comets are mostly water - they're best described as "dirty snowballs". The water that some scientists believe is to be found at the Moon's South Pole, deep within craters that never see sunlight, is believed to have been brought there by occasionally cometary collisions. So it would certainly be possible to collect water from comets. The problem comes from the feasibility of such a scheme. Comets are small, with nuclear diameters of a couple of kilometers at the most, and also quite rare. No more than 50 are ever discovered in a year, and few of these pass close to the Earth/Moon system. So almost certainly it will always prove to be cheaper to bring up water from the Earth, rather than go comet chasing!
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