View source for Astronomy:Multi-mission radioisotope thermoelectric generator

{{Short description|Nuclear thermal source whose heat is converted into electricity}}
[[File:MMRTG schematic - english labels.png|thumb|Diagram of a MMRTG.]]
The '''multi-mission radioisotope thermoelectric generator (MMRTG)''' is a type of [[Physics:Radioisotope thermoelectric generator|radioisotope thermoelectric generator]] (RTG) developed for [[Organization:NASA|NASA]] space missions<ref>{{cite web |url=http://www.jpl.nasa.gov/news/fact_sheets/radioisotope-power-systems.pdf |title=Radioisotope Power Systems for Space Exploration |website=[[Organization:Jet Propulsion Laboratory|Jet Propulsion Laboratory]] |date=March 2011 |access-date=2015-03-13 |archive-date=2019-05-18 |archive-url=https://web.archive.org/web/20190518173305/https://www.jpl.nasa.gov/news/fact_sheets/radioisotope-power-systems.pdf |url-status=dead }}</ref> such as the [[Organization:Mars Science Laboratory|Mars Science Laboratory]] (MSL), under the jurisdiction of the United States Department of Energy's Office of Space and Defense Power Systems within the Office of Nuclear Energy. The MMRTG was developed by an industry team of [[Company:Aerojet Rocketdyne|Aerojet Rocketdyne]] and [[Company:Teledyne Technologies|Teledyne Energy Systems]].

==Background==
Space exploration missions require safe, reliable, long-lived power systems to provide electricity and heat to spacecraft and their science instruments. A uniquely capable source of power is the radioisotope thermoelectric generator (RTG) – essentially a nuclear battery that reliably converts heat into electricity.<ref name="doe"/> Radioisotope power has been used on eight Earth orbiting missions, eight missions to the outer planets, and the [[Astronomy:Apollo program|Apollo]] missions after [[Engineering:Apollo 11|Apollo 11]] to the Moon. The outer Solar System missions are the [[Astronomy:Pioneer program|''Pioneer 10'' and ''11'']], [[Astronomy:Voyager program|''Voyager 1'' and ''2'']], ''[[Engineering:Ulysses (spacecraft)|Ulysses]]'', ''[[Engineering:Galileo (spacecraft)|Galileo]]'', ''[[Astronomy:Cassini–Huygens|Cassini]]'' and ''[[Astronomy:New Horizons|New Horizons]]'' missions. The RTGs on ''[[Astronomy:Voyager 1|Voyager 1]]'' and ''[[Astronomy:Voyager 2|Voyager 2]]'' have been operating since 1977.<ref>{{cite web|last=Bechtel|first=Ryan|title=Radioisotope Missions|url=http://www.jpl.nasa.gov/msl/pdf/MMRTG_RyanBechtel_DOE.pdf|publisher=US Department of Energy|url-status=dead|archive-url=https://web.archive.org/web/20120201232852/http://www.jpl.nasa.gov/msl/pdf/MMRTG_RyanBechtel_DOE.pdf|archive-date=2012-02-01}}</ref> In total, over the last four decades, 26 missions and 45 RTGs have been launched by the United States.

==Function==
[[Physics:Semiconductor device|Solid-state]] [[Physics:Thermoelectric effect|thermoelectric]] couples convert the [[Physics:Heat|heat]] produced by the [[Physics:Radioactive decay|natural decay]] of the radioisotope [[Physics:Plutonium-238|plutonium-238]] to [[Physics:Electricity|electricity]].<ref name="osti.gov">[http://www.osti.gov/cgi-bin/display_accomp_bib?specfile=%2Fweb%2Fosti%2Fdocs%2Faccomplishments%2Fopentext%2Faccomplishments.p2w&act=recquery&index=1439480 SNAP-19: Pioneer F & G, Final Report] {{Webarchive|url=https://web.archive.org/web/20180401075331/https://www.osti.gov/cgi-bin/display_accomp_bib?specfile=%2Fweb%2Fosti%2Fdocs%2Faccomplishments%2Fopentext%2Faccomplishments.p2w&act=recquery&index=1439480 |date=2018-04-01 }}, Teledyne Isotopes, 1973</ref> The physical conversion principle is based on the Seebeck effect, obeying one of the [[Physics:Onsager reciprocal relations|Onsager reciprocal relations]] between flows and gradients in thermodynamic systems. A [[Earth:Temperature gradient|temperature gradient]] generates an [[Physics:Electron|electron]] flow in the system. Unlike [[Chemistry:Photovoltaics|photovoltaic]] solar arrays, RTGs are not dependent upon [[Physics:Solar energy|solar energy]], so they can be used for deep space missions.

==History==
In June 2003, the Department of Energy (DOE) awarded the MMRTG contract to a team led by Aerojet Rocketdyne. Aerojet Rocketdyne and Teledyne Energy Systems collaborated on an MMRTG design concept based on a previous thermoelectric converter design, SNAP-19, developed by Teledyne for previous space exploration missions.<ref name="jpl">{{cite web |url=http://trs-new.jpl.nasa.gov/dspace/bitstream/2014/38246/1/04-0191.pdf |title=Archived copy |access-date=2011-11-21 |url-status=dead |archive-url=https://web.archive.org/web/20111216101915/http://trs-new.jpl.nasa.gov/dspace/bitstream/2014/38246/1/04-0191.pdf |archive-date=2011-12-16 }}</ref> SNAP-19s powered ''[[Astronomy:Pioneer 10|Pioneer 10]]'' and ''[[Astronomy:Pioneer 11|Pioneer 11]]'' missions<ref name="osti.gov"/> as well as the ''[[Astronomy:Viking 1|Viking 1]]'' and ''[[Astronomy:Viking 2|Viking 2]]'' landers.

==Design and specifications==
The MMRTG is powered by eight Pu-238 dioxide [[Engineering:General-purpose heat source|general-purpose heat source]] (GPHS) modules, provided by the US Department of Energy (DOE). Initially, these eight GPHS modules generate about 2&nbsp;kW thermal power.

The MMRTG design incorporates [[Chemistry:Lead telluride|PbTe]]/TAGS thermoelectric couples (from [[Company:Teledyne Technologies|Teledyne Energy Systems]]), where TAGS is an acronym designating a material incorporating [[Chemistry:Tellurium|tellurium]] (Te), [[Chemistry:Silver|silver]] (Ag), [[Chemistry:Germanium|germanium]] (Ge) and [[Chemistry:Antimony|antimony]] (Sb). The MMRTG is designed to produce 125 W electrical power at the start of mission, falling to about 100 W after 14 years.<ref>{{Cite web |url=http://pdf.aiaa.org/preview/CDReadyMIECEC06_1309/PV2006_4187.pdf |title=Archived copy |access-date=2009-05-12 |archive-date=2012-08-09 |archive-url=https://web.archive.org/web/20120809003144/http://pdf.aiaa.org/preview/CDReadyMIECEC06_1309/PV2006_4187.pdf |url-status=dead }}</ref> With a mass of 45&nbsp;kg<ref>{{Cite web |url=http://solarsystem.nasa.gov/docs/MMRTG_Jan2008.pdf |title=Archived copy |access-date=2013-04-22 |archive-url=https://web.archive.org/web/20140202100907/http://solarsystem.nasa.gov/docs/MMRTG_Jan2008.pdf |archive-date=2014-02-02 |url-status=dead }}</ref> the MMRTG provides about 2.8 W/kg of electrical power at beginning of life.

The MMRTG design is capable of operating both in the vacuum of space and in planetary atmospheres, such as on the surface of Mars. Design goals for the MMRTG included ensuring a high degree of safety, optimizing power levels over a minimum lifetime of 14 years, and minimizing weight.<ref name="doe"/>

==Usage in space missions==
[[File:MMRTG for the MSL.jpg|thumb|The multi-mission radioisotope thermoelectric generator of Mars Science Laboratory.]]
''Curiosity'', the MSL rover that was successfully landed in [[Astronomy:Gale (crater)|Gale Crater]] on August 6, 2012, uses one MMRTG to supply heat and electricity for its components and science instruments. Reliable power from the MMRTG will allow it to operate for several years.<ref name="doe">{{NASA |article=Space Radioisotope Power Systems Multi-Mission Radioisotope Thermoelectric Generator |url=http://mars.nasa.gov/mars2020/files/mep/MMRTG_FactSheet_update_10-2-13.pdf  |accessdate=2016-07-05}} (pdf) October 2013</ref>

On February 20, 2015, a NASA official reported that there is enough plutonium available to NASA to fuel three more MMRTGs like the one used by the ''Curiosity'' rover.<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><ref name='blastr'>{{cite news |last=Moore |first=Trent |url=http://www.blastr.com/2015-3-12/nasa-can-only-make-three-more-batteries-one-powers-mars-rover |title=NASA can only make three more batteries like the one that powers the Mars rover |work=Blastr |date=12 March 2015 |access-date=2015-03-13 |archive-date=2015-03-14 |archive-url=https://web.archive.org/web/20150314214647/http://www.blastr.com/2015-3-12/nasa-can-only-make-three-more-batteries-one-powers-mars-rover |url-status=dead }}</ref> One is already committed to the [[Astronomy:Mars 2020|Mars 2020]] and its [[Engineering:Perseverance (rover)|''Perseverance'' rover]].<ref name="Plutonium stockpile"/> The other two have not been assigned to any specific mission or program,<ref name='blastr'/> and could be available by late 2021.<ref name="Plutonium stockpile"/>

A MMRTG was successfully launched into space on July 30, 2020, aboard the [[Astronomy:Mars 2020|Mars 2020]] mission, and is now being used to supply the scientific equipment on the [[Engineering:Perseverance (rover)|Perseverance]] rover with heat and power. The MMRTG used by this mission is the F-2 built by Teledyne Energy Systems, Inc. and Aerojet Rocketdyne under contract with the US Department of Energy (DOE) with a lifespan of up to 17 years.<ref name="Mars2020MMRTG">{{cite web |last1=Campbell |first1=Colin |title=NASA's Mars 2020 rover, Perseverance, set to launch into space Thursday with power source built in Hunt Valley |url=https://www.baltimoresun.com/business/bs-md-co-mars-2020-teledyne-20200729-3kithx5jjzgvxlw4mlktrkflky-story.html |website=Baltimore Sun |access-date=16 February 2021}}</ref>

The upcoming [[Organization:NASA|NASA]] ''[[Engineering:Dragonfly (spacecraft)|Dragonfly]]'' mission to Saturn's moon [[Astronomy:Titan (moon)|Titan]] will use one of the two MMRTGs for which the Aerojet Rocketdyne/Teledyne Energy Systems team has recently received a contract.<ref name="F4-F5">{{cite news |title=Aerojet Rocketdyne Receives Contract for up to Two More MMRTGs for Future Deep Space Exploration Missions |url=https://www.bloomberg.com/press-releases/2021-02-12/aerojet-rocketdyne-receives-contract-for-up-to-two-more-mmrtgs-for-future-deep-space-exploration-missions |newspaper=Bloomberg.com |date=12 February 2021 |access-date=16 February 2021}}</ref> The MMRTG will be used to charge a set of lithium ion batteries, and then use this higher-power-density supply to fly a quad helicopter in short hops above the surface of Titan.<ref name="quad">{{cite web |url=https://beyondnerva.com/2019/07/09/dragonfly/  |title="Dragonfly: NASA's Newest Nuclear Powered Spacecraft" |author=<!--Not stated--> |date=July 9, 2019 |website=Beyond NERVA |access-date=2020-10-28}}</ref>

==Cost==
The MMRTG cost an estimated {{currency|109M|US}} to produce and deploy, and {{currency|83M|US}} to research and develop.<ref>{{cite report |title=Cost Comparison in 2015 Dollars for Radioisotope Power Systems -- Cassini and Mars Science Laboratory |date=July 2016 |doi=10.2172/1364515 |osti=1364515 |last1=Werner |first1=James Elmer |last2=Johnson |first2=Stephen Guy |last3=Dwight |first3=Carla Chelan |last4=Lively |first4=Kelly Lynn |doi-access=free }}</ref> For comparison the production and deployment of the [[Engineering:GPHS-RTG|GPHS-RTG]] was approximately {{currency|118M|US}}.

==See also==
* [[Astronomy:Advanced Stirling radioisotope generator|Advanced Stirling radioisotope generator]]
* [[Physics:Nuclear power in space|Nuclear power in space]]
* [[Physics:Radioisotope thermoelectric generator|Radioisotope thermoelectric generator]] (RTG)
* [[Physics:Thermoelectric effect|Thermoelectric effect]]:
** Seebeck effect: generating an electrical current from a temperature gradient
** Peltier effect: generating a temperature gradient from an electrical current  
** Thomson effect: heating or cooling of a current-carrying conductor with a temperature gradient

==References==
{{reflist}}

== External links ==
* [https://web.archive.org/web/20120807005925/http://solarsystem.nasa.gov/rps/rtg.cfm NASA Radioisotope Power Systems website – RTG page]
* [http://www.inl.gov/research/mars-science-laboratory/ Idaho National Laboratory MMRTG page with photo-based "virtual tour"]
* [http://spacenews.com/doe-to-crank-out-new-plutonium-238-in-2019/ DOE to crank out new plutonium-238 in 2019]

{{Nuclear technology}}
{{MSL}}
{{Mars 2020}}

[[Category:Nuclear power in space]]
[[Category:Mars Science Laboratory]]
[[Category:Thermoelectricity]]
[[Category:Plutonium]]

{{Sourceattribution|Multi-mission radioisotope thermoelectric generator}}