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The radioisotope of primary interest to NASA is 
plutonium-238 (Pu238), a radioisotope not used for nuclear weapons. Plutonium decay products not only serve to generate electricity to power 
the spacecraft, but the heat generated by radioactive decay fuel 
radioisotope heater units which warm the instruments aboard the 
spacecraft. Keeping electronics and instruments warm is essential in the 
frigid climate of space since most electronic equipment has a relatively 
narrow operating temperature range. NASA and the DOE continue to fund 
research on RTGs with goals to increase power conversion efficiency 
above 35% while maintaining the reliability of electricity generation in 
space. More than ten government contracts have been awarded to private 
companies relating to various aspects of RTGs in an effort to produce 
more cost-effective long-term science missions for NASA. Non-Thermal Conversion
A non-thermal atomic battery that generates 
electricity from electron/beta-particle emission is known as a 
betavoltaic. A viable radioisotope for use in betavoltaics is an isotope 
of hydrogen with two additional protons, known as tritium. Since tritium 
loses half its radioactive energy in 12.3 years (its 
half-life) betavoltaics made with tritium can last well over a decade. [ 3] Furthermore, beta particles emitted from tritium do not penetrate 
human skin, alleviating potential public health concerns. These two 
characteristics of tritium make it well-suited for medical devices.