example, the RTG-powered Pioneer 10 and 11 spacecraft continue to send data back to earth after 17 years as they pass beyond the solar system. All of these RTGs have been fueled with the long-lived (87.7 year half-life) Pu-238 a-emitter radioisotope. The fuel form and heat source technology has been steadily improved over the years to operate at higher temperatures and to meet the stringent aerospace nuclear safety requirements with increasingly larger heat sources. The safety designs of the radioisotope space power systems are proven by exhaustive analyses and tests prior to receiving flight approval. They have been amply demonstrated by the three mission aborts experienced in the past in which the heat sources performed precisely as predicted. As power levels of the RTGs have increased over the years, improved heat sources, thermoelectric materials and thermal insulation have been developed by the DOE to increase the performance of the RTGs. Even though operating temperatures have been increased, long-term power stability has been improved so that mission lifetimes of 10 years or more are now routine. At the same time, the specific power of the RTGs launched to date has been improved from 1.5 We/kg to nearly 4 We/kg. Concurrently, the DOE has increased its capabilities for producing and processing greater quantities of Pu-238 fuel and larger heat sources. The outstanding record of the radioisotope space power systems program speaks for itself, but the job is far from over. As mission planners require more power, longer mission durations and/or more resistance to hostile natural or man-made environments, improved radioisotope space power systems will have to be developed and the technology to support these systems will continue to be advanced. General Purpose Heat Source RTGs Since the launches of the multi-hundred watt RTG-powered Voyager spacecraft in 1977, the DOE has developed a new generation of RTGs which are scheduled for launch on the Galileo mission to Jupiter this fall and on the NASA-ESA Ulysses mission in the fall of 1990. This new RTG is designated the GPHS-RTG because it employs the new General Purpose Heat Source (GPHS) modules. The GPHS module is shown in Fig. 1. Each module delivers 250 Wt and weighs 1.442 kg for a specific power of 173.3 Wt/kg. This is a significant improvement over previously flown high temperature heat sources designed for intact reentry from space. The module size and shape were selected to survive reentry through the atmosphere and impact the earth at a modest terminal velocity of 54 m/s. Each GPHS module contains four PuO2 fuel pellets, encapsulated in iridium alloy containment shells which are equipped with frit vents to release the helium formed during decay of the Pu-238 fuel. The cladded fuel pellets are enclosed in graphite impact shells (GISs)—two fueled clads per GIS—designed to limit the damage to the iridium clads during free-fall or explosion fragment impacts. A thermal insulation layer of carbon bonded carbon fibers surrounds the GIS to limit the peak temperatures of the iridium during reentry heating and to maintain its ductility during the subsequent impact. The two GISs are enclosed in a fine weave pierced fabric 3D graphite aeroshell to complete the module. The GPHS module has undergone an exhaustive safety analysis and test program over the last decade and is the first isotope heat source to be qualified for launch on the Space Shuttle. It is our current standard heat source module for use in various radioisotope space power systems. The GPHS-RTG, shown in Fig. 2, is the largest long-lived RTG built for use in
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