|Fig. 1: Curiosity self-portrait at 'Okoruso' Drill Hole. Curiosity is powered by a 110W electrical power Radioisotope Thermoelectric Generator. (Courtesy of NASA/JPL)|
Radioisotope Thermoelectric Generators (RTG) are tiny power plants that can be used like very long lasting batteries. The electricity is constantly generated from the heat produced by a decaying radioactive core. RTGs are found to be extremely useful in specific applications, where human interaction is rare or nonexistent. Such applications are as diverse as space probes and rovers (Fig. 1), lighthouses, and even pacemakers with miniaturized RTGs. [1,2] RTGs are indeed very durable and reliable thanks to a very simple and robust architecture. RTGs are a critical component of the future of deep space exploration. However, their future has been uncertain due to the diminishing supply and the lack of production of Plutonium-238, which is the common radioactive element used as a fuel. RTGs have for example been popularized in the movie "The Martian" (2015) by Ridley Scott in which Mark Watney (Matt Damon) uses the waste heat of a RTG to survive his trip across the Martian desert in order to reach the rocket that can let him escape from the planet.
A RTG consists in both a heat source and thermocouples to transform the heat into electricity. The heat comes from a decaying radioactive material emitting particle (ideally alpha particle) whose kinetic energy is captured by the surrounding material. A thermocouple is a device producing a voltage difference, and eventually electricity, from two different joint conductors at different temperatures. The basic principle of RTGs can be found in Jiang and Crerend. [3,4] RTGs are the most common type of Radioisotope Power Systems (RPS). Other technology are being explored by NASA with better electrical power to heat efficiency like the Stirling thermodynamic cycle or thermophotovoltaic power conversion technologies.  These approaches have their own flaws: a Stirling cycle requires a complex architecture and moving parts that are more likely to fail while a thermophotovoltaic approach is still not meeting efficiency and cost requirements compared to conventional RTGs. For all these reasons, RTGs are still the optimal choice for deep space exploration.
Several isotope could, and have been, used to power RPS but it has been shown that Pu-238 is the optimal choice.  The numerous advantages include a long 88 years half-life, a high heat to weight and volume efficiency while being safe and affordable. Recently, the problem was that NASA has been running on a fixed Pu-238 inventory for the past 30 years because the production stopped in the United-States in 1988.  A additional total of 40kg were bought from Russia in order to meet NASA's needs but Russia has itself stopped all production. Besides, the stock of Pu-238 has being degrading and most of the inventory does not meet the missions requirements anymore. While the current inventory could provide sufficient material until 2020, a 2009 study claimed that "If the status quo persists, the United States will not be able to provide RPSs for any subsequent missions." 
As a result of these findings, NASA decided to invest and resume Pu-238 production. In December 2015, Oak Ridge National Laboratory (ORNL) announced the production 50g of Pu-238.  The process involves bombarding Np-237 with neutrons to transform it into Np-238 using the High Flux Isotope Reactor at ORNL. Np-238 then quickly decays to become Pu-238. Np-237 is a by-product of nuclear reactors, both civil and military, as it easily forms from the neutron capture of U-235 and U-238 and has a 2 million years half-life. The Np-237 inventory is stored at the Idaho National Laboratory while the final Pu-238 is shipped Los Alamos National Laboratory and stored there. This new high quality Pu-238 can be mixed with older supplies in order to make them meet future mission's requirements, therefore taking full advantage of the current limited production.
© Jean-Baptiste Ruffio. 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.
 W. J. F. Standring et al., "Environmental, Health and Safety Assessment of Decommissioning Radioisotope Thermoelectric Generators (RTGs) in Northwest Russia," J. Radiol. Prot. 27, 321 (2007).
 F. N. Huffman, et al., "Radioisotope Powered Cardiac Pacemakers," Cardiovasc. Dis., 1, 52 (1974).
 M. Jiang, "An Overview of Radioisotope Thermoelectric Generators," Physics 241, Stanford University, Winter 2013.
 A. Crerend, "Radioisotope Thermoelectric Generators (RTGs)," Physics 241, Stanford University, Winter 2015.
 D. J. Anderson, et al., "NASA Radioisotope Power Conversion Technology NRA Overview," U.S. National Aeronautics and Space Administration, NASA/TM-2005-213981, November 2005.
 Radioisotope Power Systems: An Imperative for Maintaining U.S. Leadership in Space Exploration (National Academies Press, 2009)
 P. Gwynne, "US Restarts Plutonium-238 Production," Physics World 29, No. 4, 7 (2016).