I recall that one of the reasons frequently given for returning to the moon is that it is "rich" in Helium-3 (the commonest isotope is Helium-4) and it can be u

I recall that one of the reasons frequently given for returning to the moon is that it is "rich" in Helium-3 (the commonest isotope is Helium-4) and it can be used in nuclear fusion[1]. If wikipedia is to be believed "rich" is relative here and corresponds to 0.01 ppm. He-4 is slightly better at 28 ppm. However as He-4 is 5ppm [2] of the atmosphere here on Earth the Moon seems a long way to go to achieve just a 6-fold enrichment. [1] http://en.wikipedia.org/wiki/Helium-3 http://en.wikipedia.org/wiki/Helium-3 [2] http://en.wikipedia.org/wiki/Atmosphere_of_Earth#Composition http://en.wikipedia.org/wiki/Atmosphere_of_Earth#Composition