At NASA, hopes for a new planetary mission to Saturn had been in the works since the early 1980s. Scientists had long sought to visit the second-largest planet in the solar system, with its fascinating system of rings, numerous moons, and unique magnetic field. 11 140 Visiting Saturn 11 The Cassini Mission s the 1980s drew to a close, the DOE Office of Special Applica- tions had its hands full with space nuclear power system work. Although assembly and testing of four GPHS-RTGs (including Aone spare) for the Galileo and Ulysses missions were complete, other projects filled the time. Ongoing assessment and development of DIPS, begun under SDI, continued on a limited basis under SEI. e SP- 100 space reactor program and TFE verification program were in the midst of ongoing development and testing. DOE also continued supporting DoD in development of a space nuclear thermal propulsion system that had begun under the auspices of SDI. At NASA, hopes for a new planetary mission to Saturn had been in the works since the early 1980s. Scientists had long sought to visit the second-largest planet in the solar system, with its fascinating system of rings, numerous moons, and unique magnetic field. Flybys of Saturn by the RTG-powered Pioneer 11 spacecraft in 1979 and the Voyager 1 and Voyager 2 spacecraft in 1980 and 1981, respectively, provided information that further piqued that interest. Efforts to acquire a Saturn mission finally came to fruition in 1989 with the authorization of Congressional funding. Conceived as an international partnership with the ESA and Italian Space Agency, the Cassini-Huygens mission (alternately the Cassini mission) began in 1990 and consisted of an orbiter (Cassini) and a probe (Huygens). e Cassini orbiter was designed to circle the planet and several of its moons over a four-year period. e mission of the Huygens probe was to pass through the atmosphere of Saturn’s largest moon, Titan, and briefly survey its surface during a short-lived mission that was less than one hour. With 12 instruments on the orbiter and six on the probe, the deep space planetary scouts were set up to gather an abundance of information about the planet, its ring system, and its moons. Because Saturn is almost 10 times farther from the sun than is the Earth, it receives only approximately one percent of the sunlight per square meter as does Earth. us, solar power for the new NASA mission was never really an option—the size and weight of the panels would have made their launch unfeasible.1 erefore, NASA turned to DOE to provide A seven-year journey to the ringed planet Saturn began with the lifto of a Titan IVB/Centaur carrying the Cassini orbiter and its attached Huygens probe. (Photo: NASA/JPL/KSC) Photo Credit 141 Visiting Saturn The Cassini Mission What’s In a Name Powering and Heating Mound, the effort necessitated Cassini the duplication of tooling designs, While Galileo Galilei was the rst to tooling, processing steps, and observe Saturn through a telescope With the experience gained during inspection processes that had been in 1609, limitations of the optics he assembly and testing of the GPHS- successfully used at Mound to used precluded his ability to discern RTGs and RHUs for the Galileo and fabricate flight-certified iridium the planet’s rings. Discovery of the Ulysses missions, it would seem hardware for the Galileo and planet’s rings is attributed to Dutch that repeating the effort for the Ulysses missions. Once operational, scientist Christian Huygens who, Cassini mission would be relatively but before any iridium components with the use of improved optics, straightforward. As time would were produced for mission use, the observed the ring system in 1659. tell, however, that would not be the new manufacturing processes at Huygens also discovered Titan, the case. In preparing for the planned ORNL were subjected to a rigorous planet’s largest moon. Several years 1997 Cassini launch, the DOE space review and demonstration process later, Italian French astronomer nuclear power system group faced to ensure the final product would Jean-Dominique Cassini discovered several challenges in meeting the meet the exacting requirements several additional Saturn moons as needs of NASA. Challenges came for flight-qualified hardware. well as a narrow gap that separates in the form of several firsts for its e process included a series of the ring system into two parts. The Federal contractors. For example, qualification tests and studies gap has since been known as the production of the plutonium followed by a pilot production Cassini Division.1 fuel pellets and subsequent effort to provide assurance that the encapsulation (activities that had ORNL team could reliably produce been performed at SRS for the the iridium components for use in Galileo and Ulysses missions) the Cassini RTGs.2 three GPHS-RTGs for the Cassini were transferred to LANL in 1990. spacecraft to meet its almost 900- Production of iridium cladding and Despite rigorous preparations, watt power requirement, and over frit vent components (called a clad- production of flight quality 100 small one-watt RHUs to keep vent set) in which the fuel pellet hardware wasn’t without its bumps. scientific instruments and other was encapsulated was moved from Although initial production of the equipment warm aboard Cassini Mound Laboratory to ORNL in iridium alloy components started and Huygens during their nearly 1987. Finally, DOE would usher in a in 1989, concerns eventually eight-year journey to Saturn and new transportation system to ship arose related to the metallurgical follow-on missions. the assembled GPHS-RTGs from integrity of the frit vent assemblies. Mound Laboratory to KSC e integrity of the frit vent is in Florida. critical in that it allows the helium gas produced from the decay of At ORNL, the Materials plutnonium-238 to vent from the Engineering Department began fueled clad so as to preclude the a multi-year effort to establish buildup of pressure that could the capability to produce the rupture the cladding. e concerns iridium cladding cups and frit vent resulted in a six-month shutdown assemblies. With assistance from of operations beginning in 142 Atomic Power in Space II Chapter 11 GPHS Fueled Clad September 1992, during which time Frit Vent the ORNL manufacturing processes and product were scrutinized “The vent technology is really pretty and reviewed. After successfully elegant...It’s what’s called a frit. You start demonstrating the rigor of out with...iridium powder. You compress the manufacturing processes, it into a tablet... and...re that at a high production resumed the following temperature to the point where some of year.3 e clad vent set production those individual grains of the power begin campaign would eventually result to fuse together... so what you end up in the production of 425 flight- with is a kind of porous media that you quality sets, over 500 weld shields can pass gas through but you can’t pass (used to protect the fuel pellet particles through... and they sandwich during welding of the iridium [the frit] between...thin layers of... iridium alloy cladding cups), and other metal... and then that assembly gets supporting hardware.4 e flight- welded together and then the whole ready hardware was subsequently assembly gets welded into a capsule.” sent to LANL, where new fuel –Tim George, LANL pellet production and encapsulation operations were being established Iridium and Tungsten to support the Cassini mission. “Tungsten and iridium are the two Some expected difficulties came highest-melting-point metals on the as a result of the fuel pellet table of elements. The challenge that production transfer. Like any we had was to develop a metal that… good mass-production process, could contain all of the plutonium-238 the goal is to produce widgets and that would… survive both a launch that are identical. e processes pad explosion and a reentry into the by which fuel pellet production earth’s atmosphere… so we had to have operations had been performed a material that was… able to withstand at SRS had been perfected during great temperatures and was also ductile, the work to support Galileo and so when it hit the earth rather than Ulysses. If performed exactly the shatter it would … deform without same way time after time, DOE breaking. It is a very highly specialized Shield cup assemblies (top) and knew what the result would be. metal…” vent cup assemblies with its frit Such is the nature of a rigorous vent (middle) are matched after all manufacturing process. In the –Gordon Michaels, ORNL fabrication steps are completed (bottom). (Photos: ORNL) 143 Visiting Saturn The Cassini Mission early 1990s, LANL was largely a Production of the heat sources and LANL preparing for production research and development facility; and heater units landed within the and encapsulation of fuel pellets, production operations were not Actinide Ceramics and Fabrication DOE awarded a contract to GE their forte. While researchers and Group of the Nuclear Materials Aerospace in 1991 for production of developers strive for consistency Technology Division at LANL. the thermoelectric generator units and repeatability, there is also Following two years of operational to be used for the Cassini mission. a tendency to experiment and preparations and one year of Although the Cassini GPHS- try to make things better. As a internal and independent readiness RTG program began under GE result, with the transfer of fuel reviews, production of LWRHUs Aerospace, changes at the corporate pellet production operations to and GPHS fueled clads began in level soon followed. In 1993, GE LANL, DOE had to ensure that the 1993.