Researchers and space enthusiasts see helium 3 as the perfect fuel source: extremely potent, nonpolluting, with virtually no radioactive by-product. Proponents claim it's the fuel of the 21st century. The trouble is, hardly any of it is found on Earth. But there is plenty of it on the moon, about 1 million tons with a market value of around $4 billion dollars a ton when compared to the market value of oil.