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.
NASA claims that the first flight-ready ASRG will be flight ready by 2016, but there is no planned mission use it. Two small mission concepts that would have depended on ASRGs – the Titan Mare Explorer and Comet Hopper – were rejected last year.
The news was announced at the National Research Council's Committee on Astrobiology and Planetary Science meeting earlier this week by Barry Goldstein, the Europa Clipper mission project manager at JPL.
According to the presented slides, the lack of any previous missions using ASRGs, as well as reliability questions of the moving piston within harsh radiation environment around Europa created an unacceptable risk engineering and cost risk for the mission.
The team behind Europa Clipper mission concept is now exploring regular MMRTGs and large solar panels to power the spacecraft.
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 – equivalent to an MMRTG power source. A battery (required for any power source configuration) would provide power when the Clipper's orbit passes into shadow. While solar panels are the heaviest of all options, they are well within the mass constraints of the mission.
Goldstein emphasizes that MMRTGs are still the current baseline for the Clipper. But since MMRTGs are expensive, would eat up most of the remaining U.S. supply of plutonium-238, and the fact that current space policy dictates the use of solar panels unless the the use of RTGs is absolutely necessary or significantly increases the chances of mission success, my bet would be on solar panels for the Clipper.
Of course, all of this is hypothetical. The Europa Clipper mission is not accepted or advocated by NASA headquarters. All current activity is funded by specific earmarks in congressional budgets which provide just enough money to continue advanced concept studies and reduce the overall technical risk of the mission.
Should a mission to Europa be accepted by the White House and NASA, an official budget request would have to be submitted in FY2015. The mission is currently estimated at around $2 billion.
Note: this post was updated slightly on 2013-09-06 to clarify that MMRTGs are still the current baseline of the mission.