If you could invest in commodity futures for lunar helium-3, you would have reaped a windfall last month in the wake of President Bush’s call for America to return to the moon. Helium-3 (as opposed to the garden-variety helium-4 that fills all those balloons) is one of the rarest substances on Earth, produced primarily as the result of processing tritium for nuclear weapons. But it’s known to exist naturally within lunar soil, and theoretically, it’s one of the cleanest fuels around for nuclear fusion.