Research is being carried out in the United States into a different type of fusion reaction, which produces even less radioactive waste, though it operates at an even higher temperature. This is the fusion reaction of helium-3 (an isotope of helium) with deuterium. One problem with this fuel technology is that there are only tint amounts of helium-3 on Earth. However, in their paper “ Astrofuel – An Energy Source for the 21st Century”, J.F. Santarius and G.L. Kulcinski estimate that the moon has about 1m tons of helium-3,enough to meet the world’s primary energy needs for over 1,000 years. How mineral deposits on the moon might be used to generate electricity on Earth is beyond the scope of this book. But it does illustrate that alternative sources of energy are available – even if the technology has not yet been developed to exploit them.