The U.S. is running out of plutonium 238 (238Pu) which is the only practical isotope for fueling the power systems of NASA planetary missions. As a result, "NASA is already making mission-limiting decisions based on the short supply of 238Pu," as revealed in a prepublication version of a report by the Space Sciences Board of the National Research Council. UA geosciences professor Spencer Titley is one of the co-authors of the study.
Plutonium-238 [right, 238Pu pellet glowing from its own heat. Credit, US Dept. of Energy] has been produced in quantity only for the purpose of fueling Radioisotope Power Systems for spacecraft (RPSs). "In the past, the United States had an adequate supply of 238Pu, which was produced in facilities that existed to support the U.S.nuclear weapons program. The problem is that no 238Pu has been produced in the United States since the Department of Energy (DOE) shut down those facilities in the late 1980s. Since then, the U.S. space program has had to rely on the inventory of 238Pu that existed at that time, supplemented by the purchase of 238Pu from Russia. However, Russian facilities to produce 238Pu were also shut down many years ago, and the DOE will soon take delivery of its last shipment of 238Pu from Russia."
The NRC committee "does not believe that there is any additional 238Pu (or any operational 238Pu production facilities) available anywhere in the world."
"If the status quo persists, the United States will not be able to provide RPSs for any subsequent missions." The report concludes that the cost of reestablishing domestic production of 238Pu will likely exceed $150 million.
Radioisotope Power Systems: An Imperative for Maintaining U.S. Leadership in Space Exploration; National Research Council, Aeronautics and Space Engineering Board (ASEB), Space Studies Board (SSB)