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]

Benefits of RTGs

Problems with solar

While solar arrays have long powered spacecraft within the inner solar system, solar intensity decreases according to the inverse-square law.[1] That is, a craft twice as far from the sun will only receive one quarter the energy: around Earth, sunlight produces 1,374 Watts/m², but this falls to 50 Watts/m² near Jupiter, and 1 Watt/m² at Pluto.[9] This means that the farther from the sun a solar-powered spacecraft travels, the larger the solar panels it must carry.

Juno’s three solar arrays are able to power the spacecraft out as far as Jupiter, but come at a cost in terms of size and mass. Credits: NASA/JPL-Caltech

The Juno mission to Jupiter, for example, holds the record as having traveled the farthest from the sun while sustained by solar power.[12] In order to accomplish this feat, it is equipped with three 2.7 by 8.9 meter (8.9 by 29.2 feet) solar arrays[1][13]—fully extended, these arrays cover roughly the size of a regulation basketball court. The arrays themselves total 340 kilograms (750 pounds), over three times the weight of an RTG system.[14]

2007 NASA report demonstrating hypothetical power generation of next-generation solar array technology in the outer solar system.

While advances in photovoltaic technology may allow for solar-powered probes to function as far out as Saturn in the near future,[15] the required increases in probe size and mass will correspondingly add to the cost of launch.

Indeed, an all-solar configuration was considered for the Cassini mission to Saturn.[16] Sporting four solar arrays, each five times larger than those of the Hubble Space Telescope, this design increased the spacecraft’s dry mass by 1,337 kg (2,948 lbs) and would have exceeded the maximum launch capacity of the Titan IV (SMRU)/Centaur by almost a ton. The weight of the panels on another proposed design dor the mission greatly increased the spacecraft’s moment of inertia and thereby the difficulty of turning and maneuvering the probe. This would have significantly limited the mission’s scientific output due to factors such as reduced target tracking capability leading to lower image quality, and a loss of observation time while rotating to communicate with Earth.

In addition to the design challenges which face solar-powered systems, complications may arise over the course of their mission: fragile solar arrays are vulnerable to debris, particularly as size increases, and no sunlight will be available while on the far side of a planetary body.

This latter consideration is particularly relevant for rovers and landers who may need to spend long periods of time in obscured regions.[1] Notably, ESA’s Rosetta mission successfully landed a probe on the comet 67P/Churymov–Gerasimenko in 2014, but it bounced into the shadow of a cliff where its solar panels were unable to generate additional charge. The lander’s on-board batteries were only able to sustain the lander for 64 hours, a much shorter time period than the anticipated mission duration.[7]

Similar complications can arise from atmospheric conditions which reduce the amount of sunlight available. Although both rovers of the solar-powered MERs program long outlived their three month lifespan target, they were both dependent on ‘cleaning events,’ when small dust devils clear accumulations off of solar panels.[17] These chance-reliant cleaning events risk dangerously low levels of power generation. The Spirit rover, for example, at one point had as low as 25 percent dust-free solar arrays after a global dust storm.[18]

General benefits

References

  1. 1.0 1.1 1.2 1.3 1.4 1.5 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 7.3 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. 9.0 9.1 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/
  12. News | NASA’s Juno Spacecraft Breaks Solar Power Distance Record. (2016, January 13). https://www.jpl.nasa.gov/news/news.php?feature=4818
  13. Juno Solar Panels Complete Testing. (2016, June 24). NASA. http://www.nasa.gov/mission_pages/juno/news/juno20110527.html
  14. Brakels, R. (2016, July 6). Why NASA Chose Solar Power Over Nuclear For The Juno Space Probe. Solar Quotes Blog. https://www.solarquotes.com.au/blog/nasa-chose-solar-power-nuclear-juno-space-probe/
  15. Benson, S. W. (2007, November 8). Solar for Outer Planets Study. Outer Planets Assessment Group.
  16. Cassini Final Environmental Impact Statement | NASA Solar System Exploration. (1997, September 24). https://solarsystem.nasa.gov/resources/17808/cassini-final-environmental-impact-statement/
  17. O’Neill, I. (2014, April 21). Opportunity: The Amazing Self-Cleaning Mars Rover (Photos). Space.Com. https://www.space.com/25577-mars-rover-opportunity-solar-panels-clean.html
  18. Final Environmental Impact Statement for the Mars 2020 Mission. (2014). Retrieved from https://mars.nasa.gov/mars2020/files/mep/Mars2020_Final_EIS.pdf