Advanced Stirling radioisotope generator
The advanced Stirling radioisotope generator (ASRG) is a radioisotope power system under development at NASA's Glenn Research Center. It uses a Stirling power conversion technology to convert radioactive-decay heat into electricity for use on spacecraft. The energy conversion process used by an ASRG is about four times more efficient than in previous radioisotope systems to produce a similar amount of power, and allows it to use about one quarter of the plutonium-238 as other similar generators.
Development was undertaken in 2000 under joint sponsorship by the United States Department of Energy (DoE), Lockheed Martin Space Systems, and the Stirling Research Laboratory at NASA's Glenn Research Center (GRC) for potential future space missions.
In 2012, NASA chose a solar-powered mission (InSight) for Discovery 12 interplanetary mission, which would have otherwise needed a radioisotope power system for the planned 2016 launch (which as it happens has been delayed until 2018).
The DOE cancelled the Lockheed contract in late 2013, after the cost had risen to over $260 million, $110 million more than originally expected. It was also decided to make use of remaining program hardware in constructing and testing a second engineering unit (for testing and research), which was completed in August 2014 in a close-out phase and shipped to GRC. NASA also needed more funding for continued plutonium-238 production (which will be used in existing MMRTGs for long-range probes in the meantime) and decided to use the savings from the ASRG cancellation to do so rather than take funding from science missions.
Despite termination of the ASRG flight development contract, NASA continues a small investment testing Sterling converter technologies developed by Sunpower Inc. and Infinia Corporation, in addition to the unit supplied by Lockheed and a variable-conductance heat pipe supplied by Advanced Cooling Technologies, Inc. Flight-ready ASRG units are not expected until 2028.
The higher conversion efficiency of the Stirling cycle compared with that of radioisotope thermoelectric generators (RTGs) used in previous missions (Viking, Pioneer, Voyager, Galileo, Ulysses, Cassini, New Horizons, and Mars Science Laboratory) would have offered an advantage of a fourfold reduction in PuO2 fuel, at half the mass of an RTG. It would have produced 140 watts of electricity using a quarter of the plutonium an RTG or MMRTG needs.
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