As depicted in this illustration, Cassini will plunge into Saturn’s atmosphere on Sept. 15, 2017.
Credit: NASA/JPL-Caltech

With NASA’s Cassini spacecraft set to end its exploration of Saturn on September 15, 2017 by plunging into the atmosphere of that ringed world, it is worth noting how that impressive mission was enabled by the use of nuclear power and how, if action isn’t taken soon, the U.S. may lose the capability to do future outer-planet flagship missions like Cassini.

That’s the view of Gary Bennett, now retired from NASA headquarters where he served as Manager of Advanced Space Power Systems. He also worked on the General-Purpose Heat Source Radioisotope Thermoelectric Generator (GPHS-RTG) program as Director of Safety and Nuclear Operations at the U.S. Department of Energy.

Cutaway view of a General Purpose Heat Source RTG.
Credit: NASA

Milestone missions

For almost 20 years, the Cassini spacecraft has been powered by three GPHS-RTGs, Bennett points out, each capable of producing 300 watts of electrical power at the start of the mission.

Originally, the GPHS-RTG was developed for the Ulysses solar-polar mission then GPHS-RTGs were added to the Galileo mission to Jupiter.

Voyager spacecraft.
Credit: NASA/JPL

Most recently, the GPHS-RTG is powering the New Horizons spacecraft which successfully flew past Pluto and is now on its way for a 2019 flyby of the Kuiper Belt Object (KBO) 2014 MU69.

“The GPHS-RTG built on the successful technology employed to power the two Voyager spacecraft which are celebrating 40 years in space this year,” Bennett adds. The Voyager power sources are known as Multi-Hundred Watt RTGs (MHW-RTGs).]

New Horizons spacecraft encountering a Kuiper Belt Object.
Credit: Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute (JHUAPL/SwRI)

Dire situation

“Currently, the U.S. has only a limited supply of ‘old’ plutonium-238, the radioisotope that provides the thermal power for the GPHS-RTG,” Bennett explains. “The situation is so dire that in 2009 the National Research Council’s Aeronautics and Space Engineering Board issued the report Radioisotope Power Systems: An Imperative for Maintaining U.S. Leadership in Space Exploration.”

Credit: NRC

Bennett concludes that, “during the forthcoming celebrations of the successes of the Cassini mission and of the 40th anniversary of the launches of the two Voyager spacecraft it should be noted that these missions were enabled by nuclear power and, unless action is taken, that option may not be available in the future.”

To view that 2009 National Research Council report, go to:

https://www.nap.edu/download/12653

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