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NMT's Contributions to the Cassini Saturn Mission Follow Division's Space Exploration Tradition

Some of NMT Division's handiwork will be soaring across the solar system on its way to Saturn in the near future. Many NMT members, primarily in Actinide Ceramics and Fabrication (NMT-9), have produced special heat and energy sources to help keep things running aboard the Cassini orbiter and Huygens probe, which will be launched in October of 1997 toward the Saturnian system to explore the gas giant, its mysterious rings, and some of its frigid moons.

Because Saturn is so far away from the sun, solar rays there are only a small fraction as strong as they are on earth. To gain enough electrical energy and warmth to operate, Cassini would need solar panels so large that it would be nearly impossible to launch and maneuver. Hence, the orbiter and probe must carry their own thermal and electrical generators to run the scientific experiments and other equipment aboard. A conventional battery cannot be used because of the mission's long duration and the prohibitive weight of such an energy source. Long-lasting energy without excess weight can be provided by the natural radio-active decay of plutonium-238. With a half-life of approximately 87 years, this material will reliably produce a constant flow of energy for at least 25 years and for even longer thereafter at reduced levels. For the Cassini mission, radioisotope thermoelectric generators (RTGs) will provide the electrical energy needed, while lightweight radioisotope heating units (LWRHUs, or RHUs for short), will keep equipment warm enough to function.

Figure 1: 100-watt plutonium-238 heat source used in the 1970s space missions. The source is about 250 g and about 3 cm in diameter.


Cassini will use three RTGs, which convert thermal energy from plutonium decay into electrical energy. Each RTG contains 72 small pellets of plutonium-238 dioxide, each about the size of a marshmallow and weighing 150 grams. Each pellet is encased in many layers of protective materials, and the complete unit is called a general purpose heat source (GPHS) module. All together these 72 heat sources put out 4400 watts of thermal energy at an operating temperature of 1200°C to 1300°C. Using a set of thermocouples, the RTG converts this heat energy to about 285 watts of electricity.

To maintain operating temperatures, the Cassini orbiter and Huygens probe will use 157 RHUs distributed in various locations. Each unit contains a 2.7-gram pellet of PuO2 encased in a platinum- rhodium alloy, which, in turn, is protected inside a graphite shell. This multilayer casing, as well as that for the GPHSs, is designed to keep the PuO2 safely contained when subjected to accidental impacts or the heat of reentering the earth's atmosphere. Each RHU weighs only 40 grams and generates one watt of thermal energy through radioactive decay, operating at about body temperature (35°C-40°C). That may not seem like much, but it is a vast difference from the subzero temperatures of space.

Originally, Savannah River was to produce the GPHSs for the Cassini mission, but since their plutonium fuel forms facility was not operational, Los Alamos was selected by the DOE in 1990 to produce the heat sources. After performing the necessary operational readiness reviews, NMT Division began producing plutonium-238 heat sources in 1993. All 157 RHUs necessary for Cassini and the Huygens probe, plus 23 spares, were completed in about two years. Just recently, the 216 GPHSs were also completed. During the last few months of production, workers were on the project seven days a week and were able to complete approximately 25 GPHSs in one month. NMT-9 will make from 20 to 30 more GPHSs for spares, safety testing, etc. In July GPHSs were sent to EG&G/Mound Technologies, where they will be installed on the three RTGs for the Cassini orbiter. The RHUs will be shipped directly to the Kennedy Space Center in April 1997.

A large group of scientists, engineers, and technicians has been working on this project. Beside the 45 people dedicated to the project from NMT-9, many others throughout NMT and other Laboratory divisions have contributed to the effort. NMT-9 Group Leader Tim George and Deputy Group Leader Liz Foltyn have headed up the GPHS project, and Gary Rinehart is project leader for the LWRHU Program. Egan McCormick and Mike Lopez have been leading the fabrication effort of processing the PuO2 and hot-pressing it into pellet form. Bob Mathews has been in charge of welding the special cladding and shipping containers for the heat sources. Becky Guillen leads the nondestructive safety testing of RHUs and GPHSs, such as checking for leakage and correct thermal output, Mary Ann Reimus oversees the nondestructive safety testing, and Jim Jones provides critical systems maintenance support.

Cassini will not be the first space mission for which Los Alamos has made heat and/or electricity sources. In the mid-70s, before TA-55 or NMT Division existed, Los Alamos developed and tested heat sources for Voyager I and II. The heat source used for these missions was known as the multi-hundred watt, or "MHW," which was about the size of a golf ball. The new RHUs and GPHSs reflect a move toward a more modular approach; these heat sources are more like small building blocks that can be used in varying quantities depending on specific needs. These newer heat sources (both RHUs and GPHSs) were designed here at TA-55 in the late 70s for the Galileo mission to Jupiter, and while the GPHSs were fabricated at Savannah River, the RHUs were made here. RTGs were also used in the Ulysses mission to orbit the poles of the sun.

The future holds still more opportunities for Los Alamos to contribute heat and energy sources to space missions. The upcoming Mars Pathfinder mission, set to launch in December of 1996, will be using three RHUs from TA-55 on a small rover that will explore the Mars surface, and a Pluto fast fly-by mission set to launch around the year 2000 will probably use around 50 RHUs, which NMT Division plans to produce. Los Alamos scientists and engineers from NMT and other divisions are also considering the possibilities of other energy sources beside the RTG that is standard today. Some of these possibilities include the conversion of PuO2 heat to electricity through a dynamic engine with moving parts, such as a Brayton or Stirling motor. Such motors would greatly increase the efficiency of Pu energy sources (from about 7% efficiency to 20%), but the use of moving parts presents problems in the event of breakdown. Other possibilities include the use of infrared photovoltaic cells or alkali metal thermoelectric conversion.

The Cassini project is nearing its close at TA-55, but NMT scientists and engineers will continue to play their traditional role in space exploration, advancing mankind's efforts to understand our solar system and the universe beyond.

This article was contributed by Laura Linford


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