Nimbus weather satellites. The Nimbus SNAP-19 program was the first demonstration of RTG technology aboard a NASA spacecraft, and, as such, it developed the data and experience to support interplanetary missions using RTGs. Subsequent modifications were made in the SNAP-19B design to power NASA's Pioneer and Viking missions. The Viking SNAP-19 is shown schematically in Fig. 8. For Nimbus III, two 13.4-kg SNAP-19B RTGs were mounted on the spacecraft platform as shown in Fig. 9 to provide a total of 56.4 We at beginning of mission (BOM) to augment the solar power source. During the design lifetime of one year, nuclear power comprised about 20% of the total power delivered to the regulated power bus, allowing a number of extremely important atmospheric-sounder experiments to operate in a full-time duty cycle. Without the RTGs the total delivered power would have fallen below the load line about 2 weeks into the mission [8,9]. Additional improvements were made leading to the SNAP-19s which were built for the Pioneer 10 and 11 spacecraft, the first to fly by Jupiter and Saturn. Figure 10 is an artist's rendition of a Pioneer spacecraft flying past Jupiter. The four RTGs on each
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