I have blogged in the past about problems that NASA has with obtaining sufficient plutonium for nuclear batteries to supply power to space probes launched into deep space. Those problems have not been solved and NASA is facing a shortage of plutonium.
        Space probes sent beyond Earth orbit require long-lived steady sources of power for instruments. Solar panels can be used when probes are in the inner solar system but for mission to the outer planets, another source of power is needed.
        For decades, our outer space missions have been powered by the human-made isotope: plutonium-238. (Also known as Pu-238) This isotope is extremely radioactive and has a half-life of eighty-eight years. It can emit over five hundred watts of power for every kilogram that is present in a space probe.
        Pu-238 can be stored safely in the form of plutonium oxide (PuO2). This compound is highly resistant to many different kinds of potential problems. It forms a crystalling lattice which means that chunks do not break or chip off. It has a very high melting point and will remain a solid at temperatures over forty-nine hundred degrees Fahrenheit. It is very insoluable in water which means that if a satellite falls to earth in a body of water, it will not dissolve.
       Pu-238 has two uses on space probes. Radioisotope Heater Units are used to prevent instruments from freezing in space. Radioisotope Thermoelectric generators are small power sources that emit heat continuously which is used to generate electricity.
        Many in the space industry feel that Pu-238 should be the standard for space missions to the outer solar system. They have been successfully used by probes such as the Pioneer 10 and 11 and Voyager 1 and 2. Power sources utilizing Pu-238 are light weight, consistent, reliable, long lasting and self warming. They are immune to such problems as dust, shadowing and surface damage.
       The U.S. used to produce more than forty five pounds of Pu-238 per year as part of the production of nuclear weapons during the Cold War. The U.S. stopped producing Pu-238 in the 1980s as the Cold War drew to a close with the collapse of the Soviet Union. There were plans to produce about a hundred pounds of Pu-238 at the Savannah River Site starting in 1987 but those plans were abandoned.
      Many analysts believe that the dangers association with the nuclear arms race have prevented the production Pu-238 for other uses in the U.S. The U.S. stockpile of Pu-238 is at the lowest level it have ever been. There is enough left to power the 2020 Mars Rover and one other deep space mission such as the Europa Clipper which is scheduled for the mid-2020s.
       For the past twenty-five years, almost all of the thirty six pounds of Pu-238 used by NASA has been purchased from Russia. There have been a few attempts to restart Pu-238 production in the U.S. during that time but most of them were not implemented. The Oak Ridge National Laboratory started producing Pu0238 in 2013. Less than a pound of Pu-238 is being produced each year. They hope to be producing a little over three pounds a year by 2023. The Ontario Power Generation in Canada has also started producing Pu-238 which will provide a backup for NASA.
      NASA has ambitions for many future space probes but without Pu-238 as a power source, NASA will not be able to accomplish them. With respect to safety, efficiency, weight, power output and design optimization, no other power source can equal Pu-238.