Advanced Stirling radioisotope generator

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Cutaway diagram of the advanced Stirling radioisotope generator

The advanced Stirling radioisotope generator (ASRG) is a radioisotope power system first developed 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 significantly more efficient than previous radioisotope systems, using one quarter of the plutonium-238 to produce the same amount of power.

Despite termination of the ASRG flight development contract in 2013, NASA continues a small investment testing by private companies. Flight-ready Stirling-based units are not expected before 2028.


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[1] at NASA's Glenn Research Center (GRC) for potential future space missions.

In 2012, NASA chose a solar-powered mission (InSight) for the Discovery 12 interplanetary mission, negating the need for a radioisotope power system for the 2018 launch.

The DOE cancelled the Lockheed contract in late 2013, after the cost had risen to over $260 million, $110 million more than originally expected.[2][3][4][5] 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.[6][7] Testing done in 2015 showed power fluctuations after just 175 hr of operation, becoming more frequent and larger in magnitude.[8]

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.[7]

Despite termination of the ASRG flight development contract, NASA continues a small investment testing Stirling 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.[1][9] Flight-ready units based on Stirling technology are not expected until 2028.[10]


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, Mars Science Laboratory, and Mars 2020) 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.[11]

The two finished units had these expected specifications:[12]

  • ≥14-year lifetime
  • Nominal power: 130 W
  • Mass: 32 kg (71 lb)
  • System efficiency: ≈ 26%
  • Total mass of plutonium-238-dioxide: 1.2 kg (2.6 lb)
  • Plutonium housed in two General Purpose Heat Source (“Pu238 Bricks”) modules
  • Dimensions: 76 cm × 46 cm × 39 cm (2.5 ft × 1.5 ft × 1.3 ft)

Flight proposals[edit]

ASRGs could be installed on a wide variety of vehicles, from orbiters, landers and rovers to balloons and planetary boats. A spacecraft proposed to use this generator was the TiME boat-lander mission to Titan, the largest moon of the planet Saturn, with a launch intended for January 2015,[13][14] or 2023.[15] In February 2009 it was announced that NASA/ESA had given Europa Jupiter System Mission (EJSM/Laplace) mission priority ahead of the Titan Saturn System Mission (TSSM), which could have included TiME.[16][17] In August 2012, TiME also lost the 2016 Discovery class competition to the InSight Mars lander.[18]

The Herschel Orbital Reconnaissance of the Uranian System (HORUS) mission was proposing to use three ASRGs to power an orbiter for the Uranian system.[19] Another Uranus probe concept using the ASRG was MUSE which has been evaluated as both an ESA L-Class mission and New Frontiers enhanced mission.[20] The Jupiter Europa Orbiter mission proposed using four ASRG to power an orbiter in the Jovian system. Another possibility was the Mars Geyser Hopper.

It was proposed in 2013 to fly three ASRG units on board the FIRE probe to study Jupiter's moon Io for the New Frontiers program Mission 4.[21][22]

See also[edit]


  1. ^ a b "Stirling Research Lab / Thermal Energy Conversion". Archived from the original on 2014-12-26. Retrieved 2016-08-12.
  2. ^ The ASRG Cancellation in Context Future Planetary Exploration
  3. ^ Closing out the ASRG program. Author: Casey Dreier. 23 January 2014.
  4. ^ NASA Glenn Research Center Support of the Advanced Stirling Radioisotope Generator Project. (PDF) Wilson, Scott D. NASA Glenn Research Center. April 1, 2015. Accessed April 8, 2016.
  5. ^ "Testing of the Advanced Stirling Radioisotope Generator Engineering unit at Glenn Research Centre" (PDF). August 2012. Retrieved 2016-05-20.
  6. ^ "Advanced Stirling Radioisotope Generator Engineering Unit 2 (ASRG EU 2) Final Assembly" (PDF). 23 February 2015. Retrieved 2016-05-20.
  7. ^ a b "Lockheed Shrinking ASRG Team as Closeout Work Begins -". January 16, 2014. Retrieved August 31, 2016.
  8. ^ Advanced Stirling Radioisotope Generator Engineering Unit 2 Anomaly Investigation. NASA. Lewandowski, Edward J., Dobbs, Michael W., and Oriti, Salvatore M. Published 30 March 2018.
  9. ^ Optimized Heat Pipe Backup Cooling System Tested with a Stirling Convertor [sic]. (PDF) NASA GRC. March 1, 2016.
  10. ^ "Stirling Technical Interchange Meeting" (PDF). Archived from the original (PDF) on 2016-04-20. Retrieved 2016-04-08.
  11. ^ Leone, Dan (11 March 2015). "U.S. Plutonium Stockpile Good for Two More Nuclear Batteries after Mars 2020". Space News. Retrieved 2015-03-12.
  12. ^ Reckart, Timothy A. (January 22, 2015). "Advanced Stirling Radioisotope Generator". Glenn Research Center. NASA. Archived from the original on 2016-03-30. Retrieved 2016-04-08.
  13. ^ Stofan, Ellen (25 August 2009). "Titan Mare Explorer (TiME): The First Exploration of an Extra-Terrestrial Sea" (PDF). Archived (PDF) from the original on 24 October 2009. Retrieved 2009-11-03.
  14. ^ Titan Mare Explorer (TiME) Archived 2009-10-24 at the Wayback Machine: The First Exploration of an Extra-Terrestrial Sea
  15. ^ Titan Mare Explorer: TiME for Titan. (PDF) Lunar and Planetary Institute (2012).
  16. ^ "NASA and ESA Prioritize Outer Planet Missions". NASA. February 18, 2009.
  17. ^ Rincon, Paul (February 18, 2009). "Jupiter in space agencies' sights". BBC News.
  18. ^ Vastag, Brian (August 20, 2012). "NASA will send robot drill to Mars in 2016". Washington Post.
  19. ^ Smith, R.M.; Yozwiak, A.W.; Lederer, A.P.; Turtle, E.P. (2010). "HORUS—Herschel Orbital Reconnaissance of the Uranian System". 41st Lunar and Planetary Science Conference (1533): 2471. Bibcode:2010LPI....41.2471S.
  20. ^ A NEW FRONTIERS MISSION CONCEPT FOR THE EXPLORATION OF URANUS (PDF). 45th Lunar and Planetary Science Conference. Universities Space Research Association. 17 March 2014. Archived (PDF) from the original on 3 March 2022. Retrieved 28 October 2022.{{cite conference}}: CS1 maint: date and year (link)
  21. ^ Flyby of Io with Repeat Encounters: A conceptual design for a New Frontiers mission to Io. Terry-Ann Suer, Sebastiano Padovan, Jennifer L. Whitten, Ross W.K. Potter, Svetlana Shkolyar, Morgan Cable, Catherine Walker, Jamey Szalay, Charles Parker, John Cumbers, Diana Gentry, Tanya Harrison, Shantanu Naidu, Harold J. Trammell, Jason Reimuller, Charles J. Budney, Leslie L. Lowes. Advances in Space Research, Volume 60, Issue 5, 1 September 2017, Pages 1080-1100
  22. ^ Flyby of Io with Repeat Encounters (FIRE): A New Frontiers Mission Designed to Study the Innermost Volcanic Body in the Solar System. (PDF) R. W. K. Potter, M. L. Cable, J. Cum-bers, D. M. Gentry, T. N. Harrison, S. Naidu, S. Padovan6, C. W. Parker, J. Reimuller, S. Shkolyar, T-A. Su-er, J. R. Szalay, H. J. Trammell, C. C. Walker, J. L. Whitten and C. J. Budney. 44th Lunar and Planetary Science Conference (2013).

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