Radioisotope Thermoelectric Generators: Advantages and Disadvantages

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This article was written by Stefan DuBois,
volunteer for The Mars Society

It is licensed under Creative Commons BY-SA 3.0 and may be freely shared, but must include this attribution.
Operational envelope for different power conversion technologies

Spacecraft have three main options for power generation: chemical, solar, and nuclear. To the general public, the last of these sources may conjure images of reactors using fission processes, and many probes (particularly those launched by Russia[1]) have successfully employed such systems. Most nuclear-powered probes traveling beyond Earth orbit, however, have instead utilized radioisotope thermoelectric generators (RTGs), which harness the heat produced by radioactive decay rather than a nuclear chain reaction. RTGs offer an alternative to the more typical solar power when conducting missions where sunlight is scarce, as occurs when traveling to the outer solar system or the dusty atmosphere of Mars.

How RTGs Work

Although RTGs use radioactive fuel to generate electricity, they should not be confused with nuclear reactors. The latter harness the energy produced by controlled fission or fusion processes, but no chain reaction takes place in RTGs.

Electricity production in an RTG. Radioactive decay heats one side of a thermocouple (1). An electric current is produced on the opposite, cooler side (2), then tapped from terminals connected to the thermocouple (3), producing power (4).

Instead, unstable radioactive materials known as radioisotopes produce heat as a by-product of their radioactive decay as emitted particles transfer their energy into surrounding atoms.[2] This decay takes place within a shell of semiconductors which generates an electric current when each end is exposed to differing temperatures due to the thermoelectric effect. Simply put, charge carriers diffuse away from a heat source and build up at the cold end of a material; in semiconductors, these charge carriers can be electrons (“n-type semiconductors”) or electron holes (i.e. a position where an electron could exist; these semiconductors are called “p-type”). By connecting n-type and p-type semiconductors with a metallic strip, electrons flow between the two once heat is applied, generating an electric current. This connection of n- and p-type semiconductors is called a thermocouple.[3][4]

In RTGs, the thermocouple’s heat source comes from the radioactive decay of the RTG’s fuel source heating the interior of its thermoelectric shell, while the exterior is kept cool by the surrounding atmosphere or vacuum. As long as a constant temperature gradient is maintained, electricity will be produced.[1][5][6][7]

Previous Missions Utilizing RTGs

Diagram depicting some of the missions carrying RTGs beyond Earth orbit as of 2014.

Beginning with the navigational satellite Transit 4A in 1961, RTGs have long served as power sources in spacecraft.[1] Successful missions include the following:[6][8][9][10]

  • Earth orbit: the Transit program, Nimbus III, LES 8 & 9, Russian Cosmos navigation satellites
  • Lunar surface: China’s Chang’e landers; Apollos 12 and 14-17 successfully deployed RTGs to power their Lunar Surface Experiments Package (ALSEP)
  • Interplanetary: Pioneer 10 & 11, Voyager 1 & 2, Galileo, Ulysses, Cassini, New Horizons
  • Martian surface: Viking 1 & 2, Curiosity

Future missions such as Mars 2020 and Exomars are also scheduled to employ RTGs as power sources.[7][11]

Other missions such as the Mars Exploration Rovers (MERs) Spirit and Opportunity have employed radioactive heater units (RHUs) which are similarly based on the decay or radioisotopes. As their name implies, however, RHUs are used for heating rather than power generation; for this, both MERs relied on solar arrays.[7]

References

  1. 1.0 1.1 1.2 Karacalıoğlu, G. (2014, January 16). Energy Resources for Space Missions –. Space Safety Magazine. http://www.spacesafetymagazine.com/aerospace-engineering/nuclear-propulsion/energy-resources-space-missions/
  2. Decay heat. (2020). In Wikipedia. https://en.wikipedia.org/w/index.php?title=Decay_heat&oldid=951406419
  3. Alfred. (2018, October 25). How Thermoelectric Generators Work. Applied Thermoelectric Solutions LLC. https://thermoelectricsolutions.com/how-thermoelectric-generators-work/
  4. James, L., & Granath, E. (2020, February 19). Understanding the difference between n- and p-type semiconductors. Power & Beyond. https://www.power-and-beyond.com/understanding-the-difference-between-n-and-p-type-semiconductors-a-905805/
  5. U.S. Plutonium Stockpile Good for Two More Nuclear Batteries after Mars 2020. (n.d.). Retrieved April 19, 2020, from https://spacenews.com/u-s-plutonium-stockpile-good-for-two-more-nuclear-batteries-after-mars-2020/
  6. 6.0 6.1 Nerlich, S. (2010, October 9). Astronomy Without A Telescope—Solar Or RTG? Universe Today. https://www.universetoday.com/74755/astronomy-without-a-telescope-solar-or-rtg/
  7. 7.0 7.1 7.2 Nuclear Reactors for Space—World Nuclear Association. (2020, April). https://www.world-nuclear.org/information-library/non-power-nuclear-applications/transport/nuclear-reactors-for-space.aspx
  8. Robinson, David Gerald. Mars Science Laboratory Launch Risk Analysis Summary. United States.
  9. Allison, P. R. (2016, January 21). What will power tomorrow’s spacecraft? https://www.bbc.com/future/article/20160119-what-will-power-tomorrows-spacecraft
  10. Siegel, E. (2018, December 13). NASA Doesn’t Have Enough Nuclear Fuel For Its Deep Space Missions. Forbes.Com. https://www.forbes.com/sites/startswithabang/2018/12/13/nasa-doesnt-have-enough-nuclear-fuel-for-its-deep-space-missions/#634152fe1c18
  11. Leone, D. (2014, November 17). EPA Finds No Show-stoppers with Radioactive Battery for Mars 2020. SpaceNews.Com. https://spacenews.com/42588epa-finds-no-show-stoppers-with-radioactive-battery-for-mars-2020/