http://www.planetary.org/blogs/casey-dreier/2013/20130905-no-asrgs-for-europa.html

NASA's Europa Mission Concept Rejects ASRGs
Untested technology is too risky

Posted by Casey Dreier
2013/09/05 06:41 CDT
Topics: ESA Budget, Plutonium-238, Europa

The Europa Clipper team has rejected the use of the Advanced Stirling Radioisotope Generator (ASRG), NASA's next-generation plutonium power source, due to reliability issues within the the extreme Jovian radiation environment.

ASRGs use Plutonium-238 as a heat source to drive a stirling engine which generates electricity. ASRGs can provide the same amount of electricity as NASA's workhorse Multi-Mission Radioisotope Thermoelectric Generators (MMRTG), but using only a quarter of the plutonium.

A dwindling supply of plutonium has led NASA to invest heavily in ASRG technology over the past ten years. The promised efficiency gains would extend the utility of the existing plutonium supply and reduce the amount the Department of Energy would need to create every year, saving NASA millions of dollars.

But the ASRG program has faced constant reliability issues throughout its development. Unlike the MMRTG, which has no moving parts and can last for decades, the ASRG must maintain a smooth motion of a piston throughout the lifetime of a deep-space mission. A planned long-duration mock mission of the ASRG was canceled this summer due to budget cuts related to the sequester.

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The team behind Europa Clipper mission concept is now exploring regular MMRTGs and large solar panels to power the spacecraft.

Since MMRTGs are expensive and would eat up most of the remaining U.S. supply of Plutonium, solar panels are looking increasingly likely. According to Goldstein, solar panels represent the cheapest option for the mission. Preliminary analysis suggests they would have the same surface area as NASA's Juno mission to Jupiter (9m x 2m) and provide about 150W of continuous energy while exposed to the Sun. A battery would be required for the times when the Clipper spacecraft would pass into Jupiter's shadow.

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