Cassini: Near Term Use of RTGs

The only planned use of RTGs in the US space program in the near term is the unmanned, 1997 Cassini mission to explore Saturn. The Cassini spacecraft will be powered by three General Purpose Heat Source Radioisotope Thermal Generators (GPHS RTGs) each designed to provide 276 W of electrical power at the beginning of the mission.

Currently, the mission is planned to last for 10.75 years, at which time the RTGs will be capable of providing 216 W each. There is a possibility of extending the mission to 16 years, when each RTGs will provide 199 W.

Thermocouple Production

The GPHS RTGs planned for use on Cassini are nearly identical in design to those used on Voyager 1 and 2 and Lincoln Experimental Satellites (LES) 8 and 9. These RTGs have been in continuous operation for 21 years (LES 8/9) and 20 years (Voyager 1/2). Nearly all of the degradation that these devices have experienced during their lives is due to the decay of the plutonium heat source.

The same contractor – Lockheed Martin (previously known as Martin Marietta) – and the same facility at Martin Marietta Astro Space in Valley Forge Pennsylvania won the contract to supply the thermoelectric elements (known as unicouples) for the Cassini RTGs.

The production line had been shut down for about 8 years before it was restarted in 1993. It was necessary to train new production people, install new quality assurance programs, and to refurbish some of the production equipment. Following the production of the Cassini elements, the facility will again be shut down to await another mission.

GPHS RTG Description

The three RTGs that will be on the Cassini spacecraft each weigh 56 kilograms, have a diameter of 42.2 cm and a length of 114 cm. Like the SNAP-27 that served as a power source on the moon during the Apollo mission, the GPHS RTG consists of a cylindrical fuel supply surrounded by rings of thermocouples. Again, there are cooling fins attached to the cold shoes of the thermocouples.

Plutonium-238 is the fuel source. The fuel elements are modularized with each module producing approximately 250 watts of thermal power. The fuel modules are encased in a heat and impact resistant shell designed to prevent any postulated vehicle accident from releasing plutonium. Sufficient testing has been done to show that the modules will remain intact, even in a Challenger type launch explosion or a reentry accident.

Some Opposition

Despite a long history of successful space missions using RTGs, there is some opposition to the Cassini mission. Some anti-nuclear groups have attempted to spread fear among the residents of the area near the Kennedy Space Center. Their only tool is to loudly claim that plutonium might be released.

The potential hazard is essentially zero. The fuel modules are unlikely to be breeched in any accident, but even if all of the coatings and containers were to fail, there is little chance that any person would consume enough material to cause any health consequences. Plutonium oxide is a dense and relatively non-reactive material; it is most likely that it would rapidly fall out of the air and sink to the bottom of the ocean.