Cassini – A Standard for RTG Performance

In the radioisotope thermoelectric generator (RTG) community, the NASA-European Space Agency Cassini mission to Saturn is often considered a standard for RTG performance. Cassini was the highest-power RTG mission ever flown at nearly 900 W
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. It used the largest number (3) of the most advanced space rated RTGs ever flown, the General Purpose Heat Source RTG (GPHS-RTG). There is considerable interest in understanding the lifetime performance of the GPHS-RTG, especially since the primary goal of NASA’s Next Generation RTG project is to recapture GPHS-RTG technology. While the Next Gen RTG will have some notable differences, such as a lower quantity of fuel, a better understanding of the performance of GPHS-RTG will provide insight to the potential behavior and variability of the Next Gen RTG. Previously, only the aggregate performance from Cassini was available. Recently the individual RTG performance has been released. Analysis of that data will help us have a better understanding and appreciation for variations in unit-to-unit performance. Using methods that have successfully analyzed the Multi-Mission RTGs on Curiosity and Perseverance, as well as the GPHS-RTG on New Horizons, the Cassini RTGs have also been analyzed. While some very small differences in performance have been observed, the consistency and low variability suggest that the future Next Gen RTG should also be very reliable. Analysis of the data shows the Cassini RTGs all exhibit some individual electrical characteristics. RTG 1 was observed to produce about 2-3% less power, which is expected to be due to unit-to-unit variations in fabrication and assembly. RTG 2 showed more later life degradation in performance due to the thermal characteristics of the generator. RTG 3 experienced a change in its performance due to changes in its electrical system, which is not a common problem in RTG technology. Despite these minor differences, the overall lifetime behavior of all three RTGs was nearly identical. These results should be considered a showcase for the consistency and reliability of the modern RTG.