I have blogged in the past about the U.S. shortage of plutonium-238. Plutonium-238 is an radioactive isotope of plutonium which is very useful in the construction of space probes for the exploration of space. It has a half-life of eighty eight years which means that its production of heat will take that long to fall to half its original output. "It is stable at high temperatures, can generate substantial heat in small amounts and emits relatively low levels of radiation that is easily shielded, so mission-critical instruments and equipment are not affected." It is used to create radioisotope thermoelectric generators and radioisotope heater units that can last for years, providing power and heat to a probe as it carries out deep space missions.
During the Cold War, the U.S. nuclear weapons facility at Savannah River in South Carolina produced plutonium-238. This facility was shut down in 1988 as the Cold War was ending and the production of plutonium-238 stopped. The U.S. has been purchasing plutonium-238 from Russia since 1993 after domestic production ceased. Eventually, the Russians also stopped production and their stockpiles are running out.
The U.S. maintains separate stockpiles of plutonium-238 for military and civilian use. The current useful stockpile available to NASA is about thirty seven pounds. This stockpile will be utilized for a multi-mission radioisotope thermoelectric generator (MMRTG) for the 2020 Mars Rover mission and two more MMRTG for a 2024 NASA mission. There will be about nine pounds of useful plutonium-238 left after these two missions. Without a new source of plutonium-238, U.S. deep space missions would have to be seriously curtailed. Fortunately, the U.S. has begun making plutonium-238 again.
NASA began a new project to produce plutonium-238 two years ago at Nuclear Security and Isotope Technology Division at the Department of Energy's Oak Ridge National Laboratory (ORNL). The Lab has just announced the creation of about two ounces of plutonium-238. The reactors at ORNL are smaller than the old Savannah River reactors and had to be modified in order to produce plutonium-238. Now that they have proven that they are able to make plutonium-238, they will work on scaling up the process to manufacture useful amounts of plutonium-238. This new production process will insure that the U.S. has sufficient plutonium-238 to carry out future space missions. The U.S. has some old plutonium-238 that has decayed past the point of usefulness. Fortunately, it can be mixed with newly-produced plutonium-238 to bring it back to "life."
Boeing recently launched a satellite that utilizes a Variable Specific Impulse Magnetoplasma Rocket (VASIMR) engine. These engines are extremely efficient. "They use radio waves to ionize and heat a propellant, and magnetic fields to accelerate the resulting plasma to generate thrust." These engines require energy to operate and plutonium-238 MMRTGs can provide that energy. In addition to being ideal for deep space probes, the VASIMR engines are being considered for launching a manned expedition to Mars. Current rocket engines would take six months or more to reach Mars, exposing the astronauts to unacceptable levels of radiation and requiring huge amounts of fuel. It is estimated that VASIMR engines could get a manned mission to Mars in about forty days which would be much more practical.
