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Content: However, the electric 
conversion principles employed distinguish atomic batteries into two 
categories: thermal and non-thermal. Power output from a thermal atomic 
battery is dependent on temperature whereas a non-thermal battery is 
independent of temperature. Thermal Conversion
NASA and the Department of Energy have extensively 
studied radioisotope power systems to produce heat and electricity for 
space missions lasting over a decade and extending into regions of the 
solar system where sunlight is too faint to permit solar energy 
conversion as a viable power source. As of 2005, the United States had 
launched 44 radioisotope thermoelectric generators (RTGs) on 25 
different missions, including the Cassini mission to Saturn and the 
Galileo mission to Jupiter. [ 2]
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.