Pluto Flyby Mission Powered by Plutonium

07.07.15 | 2 min read | Text by Steven Aftergood

News of the Earth these days is such that one welcomes news from elsewhere, especially when it concerns a prospect as spectacular as the impending flyby of Pluto by the NASA spacecraft New Horizons that will take place on July 14.

In reality, of course, New Horizons also represents news from Earth, having been built by humans and launched from Cape Canaveral in January 2006. Moreover, the New Horizons probe is not simply a technological artifact; it is the result of a political process and a policy debate. At issue were not only the parameters of the mission — its scope, timing, budget, and so on — but also the fact it uses a nuclear power source fueled, appropriately if controversially, by plutonium.

The plutonium-238 isotope used by New Horizons is an exceptionally hazardous material that is dangerous to produce, manufacture into suitable form, handle, transport and launch. The hazards are sufficient, in the eyes of some, to preclude its use altogether.

NASA and Department of Energy engineers did not dismiss public concerns about the safety of plutonium-fueled power sources, but they argued that the risks could be mitigated to an acceptably low level by proper design.

“Safety was the principal design driver for the [plutonium-fueled General-Purpose Heat Source used aboard New Horizons],” according to a 2006 retrospective account of its development. “The main safety objective was to keep the fuel contained or immobilized to prevent inhalation or ingestion by humans.” See “Mission of Daring: The General-Purpose Heath Source Radioisotope Thermoelectric Generator” by Gary L. Bennett, et al.

In effect, the design of the plutonium power source was predicated on the assumption that a launch accident or other mishap would occur, and that any resulting health and safety impact had to be minimized. Simulations were performed to validate the design, but fortunately no real-world test of the safety of the device under extreme conditions ever came to pass.

The GPHS plutonium power source has been used successfully on some of the boldest and most productive missions of space exploration ever undertaken, including Galileo, Ulysses, Cassini, and New Horizons.

For the most part, these missions were conducted with commendable openness, especially in earlier years. When one young critic raised questions about the use of plutonium power sources and the hazards of high-velocity Earth flybys in the Galileo mission prior to its 1989 launch, project manager John Casani of NASA Jet Propulsion Laboratory forthrightly invited him to come inspect the spacecraft in its clean room at JPL and to discuss the alternatives.

“Pluto is going to change us,” wrote analyst Dwayne Day last month, anticipating the possible consequences of the New Horizons mission for science, art, culture, politics, and space policy. See “Deep in space, corner of No and Where,” The Space Review, June 15, 2015.