|Fig. 1: Thermocouple exhibiting Seebeck Effect.|
On August 6, 2012 NASA's Mars Science Laboratory mission made headlines with the successful landing of the Curiosity rover.  Unlike previous rovers, like Spirit and Odyssey, which landed on the red planet in 2004, instead of including solar-powered photovoltaic panels, Curiosity is nuclear-powered.  A radioisotope thermoelectric generator (RTG) using plutonium fuel provides the electricity Curiosity needs to explore the Martian surface, conduct experiments, and send data back to Earth.  This report looks at why the RTG system was adopted, its development, and its operation now and into the future.
The Seebeck Effect is the basis for the RTG. In 1821, Thomas Seebeck found that two metals placed in a temperature gradient would cause the arrow on a nearby compass to move.  Seebeck's associate, Hans Christian Oersted, recognized that this behavior was the result of an electric current creating a magnetic field.  Despite Seebeck's not accepting this explanation, the name "Seebeck Effect" stuck and has come to mean the phenomenon of creating an electric voltage across the cold ends of two different metals (a thermocouple) in a temperature gradient. [4,5] Fig. 1 depicts a heat source on the left side, with p and n-type conducting materials that are joined on the hot end. The temperature gradient causes electrons and holes to conduct across to the heat sink on the right. This conduction creates an electric potential between the top and bottom electrodes, with positive and negative ends on the heat sink side of the thermocouple.  Simply put, a thermocouple allows an electric current to be created when there is a temperature difference.
|Fig. 2: Radioisotope Thermoelectric Generator (RTG). (Courtesy of NASA.)|
Over a hundred years after Seebeck's discovery when the world was gripped by the Cold War, nuclear fuel became available as a heat source. Most modern nuclear weapons have a warhead composed of plutonium-239.  The production of such nuclear weapons produces byproducts, including plutonium-238 (Pu-238) in the form of plutonium dioxide.  Pu-238 is a radioactive isotope that works well in an RTG because this fuel source undergoes alpha decay with a half-life of 87.7 years.  The emission of alpha particles generates heat that is useful in an RTG. That means the fuel source lasts long enough to be useful and can be shielded easily. Alpha particles can be shielded by even just a piece of paper, so there is little worry of the radiation damaging other components of the RTG or neighboring devices.  The heat produced by decaying Pu-238 is used as the heat source with a thermocouple. As can be seen in Fig. 2, fins serve as heat sinks to create a voltage across the thermocouple. Thus, the temperature gradient needed for the Seebeck Effect in Curiosity's RTG, results from hot Pu-238 and the cold atmosphere of Mars.
The first RTG was used as a power source in a US satellite launched in 1961.  While photovoltaic (PV) cells have been used in space, including in the Mars Exploration Rovers (MER) mission prior to Curiosity, nuclear power offers certain advantages. One of the most important factors is distance from the sun. It should be noted that while solar panels are usually lighter than an RTG, deriving power from sunlight is limited for some missions.  The amount of solar flux reduces by a factor of the distance from the sun squared.  This means that a solar panel on Mars produces about half the power that the same PV panel would on Earth.  The RTG is also useful for missions where there is simply less sunlight. For example, the Apollo Lunar Surface Experiments Package (ALSEP) used an RTG because of the lunar night that lasts 350 hours.  In terrestrial applications, dust covering the PV panels reduces power. Lastly, the RTG affords greater durability and is long-lived. While the Curiosity rover is only designed to operate at least two years, the RTG should provide power for about 14 years.  Finally, the RTG produces heat, which helps stabilize conditions for the Curiosity rover and protect it from the Martian cold.  While the MER mission employs PV panels, Pu-238 decay is also employed in order to produce heat.  That heat production also hints at the fact that an RTG has low efficiency of only 6 to 7 percent.  The fundamental reason that nuclear power was used as the primary electrical generation method on Curiosity is due to this rover's size and mass.  Curiosity has 16 times the amount of equipment of the MER mission vehicles.  Just 4.8 kilograms of plutonium dioxide is enough to provide 110W power for the entirety of the two years of planned mission life (though it will last longer). [11,13] Fig. 3 shows Curiosity's RTG outlined against the Martian horizon.
|Fig. 3: Curiosity and its RTG on Mars. (Courtesy of NASA.)|
It appears NASA may have some difficulty securing Pu-238 for future rovers. As the Cold War has ended, not nearly as much Pu-239 is being generated for warheads, and thus there is less Pu-238 available.  In fact, the US stopped producing Pu-238 in 1988 and the fuel in Curiosity came from a Russian warhead factory.  NASA, in 2012, requested $10 million from Congress to finance the future production of Pu-238 in the US because even Russian supplies are dwindling.  That means that the next Mars rover, which NASA plans to have on Mars in 2020 will probably still use Russian-made plutonium, but at some point RTG fuel may be stamped "Made in USA." 
© John Belanger. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
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