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[[File:ASRG Labeled Cutaway (English).jpg|thumb|Cutaway diagram of the advanced Stirling radioisotope generator]]

The '''advanced Stirling radioisotope generator''' ('''ASRG''') is a [[radionuclide|radioisotope]] power system first developed at NASA's [[Glenn Research Center]]. It uses a [[Stirling cycle|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==
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<ref name=lab>{{cite web|url=https://tec.grc.nasa.gov/rps/stirling-research-lab/ |archive-url=https://web.archive.org/web/20141226074801/https://tec.grc.nasa.gov/rps/stirling-research-lab/ |url-status=dead |archive-date=2014-12-26 | access-date=2016-08-12 |title= Stirling Research Lab / Thermal Energy Conversion}}</ref> at NASA's [[Glenn Research Center]] (GRC) for potential future [[Space exploration|space missions]].

In 2012, NASA chose a solar-powered mission ([[InSight]]) for the [[Discovery Program#Discovery 12|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.<ref>[http://futureplanets.blogspot.com/2013/12/the-asrg-cancellation-in-context.html The ASRG Cancellation in Context] Future Planetary Exploration</ref><ref>[http://www.planetary.org/blogs/casey-dreier/2014/0123-lockheed-closes-out-the-asrg-program-and-ships-hardware-to-nasa-glenn.html Closing out the ASRG program]. Author: Casey Dreier. 23 January 2014.</ref><ref name='support 2015'>[https://ntrs.nasa.gov/search.jsp?R=20150008244 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.</ref><ref>{{cite web |url=https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20150006753.pdf |title=Testing of the Advanced Stirling Radioisotope Generator Engineering unit at Glenn Research Centre |date=August 2012 |access-date=2016-05-20 }}</ref> 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.<ref>{{cite web |url=https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20150004104.pdf |title=Advanced Stirling Radioisotope Generator Engineering Unit 2 (ASRG EU 2) Final Assembly |date=23 February 2015 |access-date=2016-05-20 }}</ref><ref name=spacenews>{{cite web|url=http://spacenews.com/39124lockheed-shrinking-asrg-team-as-closeout-work-begins/|title=Lockheed Shrinking ASRG Team as Closeout Work Begins - SpaceNews.com|author=|date=January 16, 2014|access-date=August 31, 2016}}</ref> Testing done in 2015 showed power fluctuations after just 175 hr of operation, becoming more frequent and larger in magnitude.<ref>[https://ntrs.nasa.gov/search.jsp?R=20180002070 Advanced Stirling Radioisotope Generator Engineering Unit 2 Anomaly Investigation]. NASA. Lewandowski, Edward J., Dobbs, Michael W., and Oriti, Salvatore M. Published 30 March 2018.</ref>

NASA also needed more funding for continued plutonium-238 production (which will be used in existing [[MMRTG]]s 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.<ref name=spacenews />

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.<ref name="lab" /><ref>[https://ntrs.nasa.gov/search.jsp?R=20160004351 Optimized Heat Pipe Backup Cooling System Tested with a Stirling {{sic|Conve|rtor|nolink=y}}]. (PDF) NASA GRC. March 1, 2016.</ref>  Flight-ready units based on Stirling technology are not expected until 2028.<ref name='Meeting 2015'>{{cite web|url=https://prod.nais.nasa.gov/eps/eps_data/165279-OTHER-005-001.pdf |title=Stirling Technical Interchange Meeting |access-date=2016-04-08 |url-status=dead |archive-url=https://web.archive.org/web/20160420084629/https://prod.nais.nasa.gov/eps/eps_data/165279-OTHER-005-001.pdf |archive-date=2016-04-20 }}</ref>

==Specifications==
The higher conversion efficiency of the [[Stirling cycle]] compared with that of [[radioisotope thermoelectric generator]]s (RTGs) used in previous missions ([[Viking program|Viking]], [[Pioneer program|Pioneer]], [[Voyager program|Voyager]], [[Galileo (spacecraft)|Galileo]], [[Ulysses (spacecraft)|Ulysses]], [[Cassini–Huygens|Cassini]], [[New Horizons]], [[Mars Science Laboratory]], and [[Mars 2020]]) would have offered an advantage of a fourfold reduction in [[Plutonium(IV) oxide|PuO<sub>2</sub>]] 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.<ref name="Plutonium stockpile">{{cite news |last=Leone |first=Dan |url=http://spacenews.com/u-s-plutonium-stockpile-good-for-two-more-nuclear-batteries-after-mars-2020/ |title=U.S. Plutonium Stockpile Good for Two More Nuclear Batteries after Mars 2020 |work=Space News |date=11 March 2015 |access-date=2015-03-12 }}</ref>

The two finished units had these expected specifications:<ref name="flight units">{{cite news|last=Reckart |first=Timothy A. |url=https://spaceflightsystems.grc.nasa.gov/sspo/asrg/ |title=Advanced Stirling Radioisotope Generator |work=Glenn Research Center |publisher=NASA |date=January 22, 2015 |access-date=2016-04-08 |url-status=dead |archive-url=https://web.archive.org/web/20160330212653/https://spaceflightsystems.grc.nasa.gov/SSPO/ASRG/ |archive-date=2016-03-30 }}</ref>
*≥14-year lifetime
*Nominal power: 130 [[Watt#Conventions in the electric power industry|W]]
*Mass: {{convert|32|kg|abbr=on}}
*System efficiency: ≈ 26%
*Total mass of [[plutonium-238]]-dioxide: {{convert|1.2|kg|abbr=on}} 
*Plutonium housed in two [[General Purpose Heat Source]] (“Pu<sup>238</sup> Bricks”) modules 
*Dimensions: 76&nbsp;cm × 46&nbsp;cm × 39&nbsp;cm  (2.5&nbsp;ft × 1.5&nbsp;ft × 1.3&nbsp;ft)

==Flight proposals==
{{redirect|HORUS|the ancient Egyptian deity|Horus||Horus (disambiguation)}}
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 (spacecraft)|TiME]] boat-lander mission to [[Titan (moon)|Titan]], the largest moon of the planet [[Saturn]], with a launch intended for January 2015,<ref>{{cite web|url=http://www.spacepolicyonline.com/pages/images/stories/PSDS%20Sat1%20Stofan-TIME.pdf |title=Titan Mare Explorer (TiME): The First Exploration of an Extra-Terrestrial Sea |access-date=2009-11-03 |last=Stofan |first=Ellen |author-link=Ellen Stofan |date=25 August 2009 | archive-url= https://web.archive.org/web/20091024224613/http://www.spacepolicyonline.com/pages/images/stories/PSDS%20Sat1%20Stofan-TIME.pdf| archive-date= 24 October 2009 | url-status= live}}</ref><ref>[http://www.spacepolicyonline.com/pages/images/stories/PSDS%20Sat1%20Stofan-TIME.pdf Titan Mare Explorer (TiME)] {{Webarchive|url=https://web.archive.org/web/20091024224613/http://www.spacepolicyonline.com/pages/images/stories/PSDS%20Sat1%20Stofan-TIME.pdf |date=2009-10-24 }}: The First Exploration of an Extra-Terrestrial Sea</ref> or 2023.<ref>[http://www.lpi.usra.edu/opag/mar2012/presentations/Friday/1_TiME_opag.pdf Titan Mare Explorer: TiME for Titan]. (PDF) Lunar and Planetary Institute (2012).</ref> 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.<ref>{{Cite news |url=http://www.nasa.gov/topics/solarsystem/features/20090218.html |title=NASA and ESA Prioritize Outer Planet Missions |date=February 18, 2009 |publisher=NASA}}</ref><ref>{{Cite news |last=Rincon |first=Paul |url=http://news.bbc.co.uk/1/hi/sci/tech/7897585.stm |title=Jupiter in space agencies' sights |work=BBC News |date=February 18, 2009}}</ref> In August 2012, TiME also lost the 2016 Discovery class competition to the [[InSight]] Mars lander.<ref name=washington>{{Cite news |url=https://www.washingtonpost.com/national/health-science/nasa-will-send-robot-drill-to-mars-in-2016/2012/08/20/43bf1980-eaef-11e1-9ddc-340d5efb1e9c_story.html |title=NASA will send robot drill to Mars in 2016 |newspaper=Washington Post |first=Brian |last=Vastag |date=August 20, 2012}}</ref>

The Herschel Orbital Reconnaissance of the Uranian System (HORUS) mission was proposing to use three ASRGs to power an orbiter for the [[Uranus|Uranian]] system.<ref name=Smith2010>{{cite journal| last=Smith| first=R.M.| author2=Yozwiak, A.W.| author3= Lederer, A.P.|author4= Turtle, E.P.| title=HORUS—Herschel Orbital Reconnaissance of the Uranian System| year=2010 |page=2471| journal=41st Lunar and Planetary Science Conference| issue=1533| bibcode=2010LPI....41.2471S}}</ref> Another Uranus probe concept using the ASRG was [[MUSE (spacecraft)|MUSE]] which has been evaluated as both an ESA L-Class mission and New Frontiers enhanced mission.<ref>{{cite conference |url=https://www.hou.usra.edu/meetings/lpsc2014/pdf/2406.pdf |title=A NEW FRONTIERS MISSION CONCEPT FOR THE EXPLORATION OF URANUS |date=17 March 2014 |conference=45th Lunar and Planetary Science Conference |conference-url=https://www.hou.usra.edu/meetings/lpsc2014/ |publisher=Universities Space Research Association |archive-url=https://web.archive.org/web/20220303032110/https://www.hou.usra.edu/meetings/lpsc2014/pdf/2406.pdf |archive-date=3 March 2022|url-status=live |format=PDF |access-date=28 October 2022 }}</ref> 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 [[Flyby of Io with Repeat Encounters|FIRE]] probe to study Jupiter's moon Io for the [[New Frontiers program]] Mission 4.<ref name='FIRE 2017'>[http://www.sciencedirect.com/science/article/pii/S0273117717303484 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</ref><ref name='FIRE 2013'>[http://www.lpi.usra.edu/meetings/lpsc2013/pdf/2874.pdf 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)''.</ref>

==See also==
* [[Nuclear power in space]]
* [[Radioisotope heater unit]]
* [[Stirling engine]]
* [[Stirling radioisotope generator]]

==References==
{{Reflist|30em}}

==External links==
*[http://esto.nasa.gov/conferences/nstc2007/papers/Shaltens_Richard_D2P1_NSTC-07-0138.pdf Advanced Stirling Technology Development at NASA Glenn Research Center] {{Webarchive|url=https://web.archive.org/web/20170513230005/https://esto.nasa.gov/conferences/nstc2007/papers/Shaltens_Richard_D2P1_NSTC-07-0138.pdf |date=2017-05-13 }} at NASA, 2015 (PDF)
*[https://web.archive.org/web/20160420084629/https://prod.nais.nasa.gov/eps/eps_data/165279-OTHER-005-001.pdf ASRG Technical Interchange Meeting] at NASA, 2015 (PDF)
*[https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20150011443.pdf Advanced Stirling {{sic|Conv|ertor|nolink=y}} Development for NASA Radioisotope Power Systems]. Sunpower, Inc. & GRC. April 1, 2015.

[[Category:Advanced Stirling radioisotope generator| ]]
[[Category:Nuclear technology]]
[[Category:Electrical generators]]
[[Category:Nuclear power in space]]
[[Category:Stirling engines]]