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Radioactive Thermoelectric Generator (RTG)
Radioactive Thermoelectric Generator uses heat 
generated spontaneously from radioactive substances. The technology 
requires a large space to capture escaping heat inside semiconductors 
effectively. The shortcomings of RTG technology are its poor efficiency 
of 6%, its low power density, and its large size. [ 2]
NASA calls their technology Multi-Mission 
Radioisotope Thermoelectric Generator (MMRTG), and in 2016, NASA 
announced the next generation Enhanced Multi-Mission Radioisotope 
Thermoelectric Generator (eMMRTG). As Fig. 1 illustrates, eMMRTG 
improves the original MMRTG with a new thermoelectric technology called 
Thermoelectric Couple Assembly. eMMRTG's improved efficiency will also 
help NASA save plutonium which is in extreme shortage in the United 
States. Betavoltaic Cells
Betavoltaic cells, also known as betavoltaic devices, 
are a nuclear battery technology used in small devices that cannot use 
Radioactive Thermoelectric Generators. Betavoltaic cells utilize 
beta-decay of isotopes such as tritium. Tritium is a byproduct of 
nuclear power plants, so manufacturing betavoltaic cells with tritium is 
an excellent way to turn nuclear wastes into useful goods. [