|Fig. 1: Artist's rendering of New Horizons spacecraft at Pluto. RTG shown in bottom left of figure with radiator fins. (Source: Wikimedia Commons. Courtesy of NASA)|
Nearly all artificial satellites in near Earth orbit are powered by solar energy from photovoltaic arrays. NASA's International Space Station alone produces 75-90 kW of power from nearly an acre of high-efficiency photovoltaic arrays in low Earth orbit. However, the Sun's irradiance rapidly falls with distance as a function of an inverse square law. On Mars, this yields a solar irradiance approximately 40% of that near Earth and, at Saturn, only 1%. Recent advances in thermophotovoltaics (TPV), namely photovoltaic cells that operate using primarily thermal photons, or heat, have demonstrated efficiencies of 2.5 times greater than existing radioisotope thermoelectric generators and may offer significant advantages in future deep space exploration missions. 
At present, missions to explore the solar system's outer planets, moons and Kupier Belt objects have used radioisotope thermoelectric generators (RTG) to generate electrical power, as well as heat, over long periods of time, from the radioactive decay of Pu238, or other isotopes. Voyagers 1 and 2, launched in 1977, are powered by such RTGs and are still returning valuable data from the limits of our solar system.  The New Horizon's spacecraft that completed a flyby of Pluto July 4, 2015 used a similar RTG as its main power source (Fig. 1).  For the Mars Science Laboratory (MSL), such RTGs also provide excess thermal heat which can be used to keep sensitive electronics and batteries from icy temperatures. 
RTGs function by converting heat (thermal photons), produced by the radioactive decay of isotopes, to electrical energy using thermocouples. However, such systems operate with efficiencies of only 3-7% and much of the power is lost via excess heat through radiation fins. Recent advances in TPV radioisotope generators, however, that convert thermal photons directly to electricity with InGaAs cells, have shown operating module efficiencies of up to 19%. 
TPV and radioisotopes thus may offer a next-generation solution for future deep space robotic missions. As the availability of radioisotope fuels dwindles and reduced mass requirements and increased power levels for future deep space missions become dominant mission design criteria, TPV may be one of available technologies to provide efficient power conversion from radioisotope fuels.
© Ved Chirayath. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
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