Pioneer spacecraft provided over 160 We at BOM. The Pioneer RTGs performed so well that Pioneer 11 was retargeted for a flyby of Saturn [10]. Both spacecraft are still operating 16-17 years after their launches, well beyond their 3-year design life requirement, and are providing valuable information about the heliosphere. Pioneer 10 is presently the most distant man-made object, having traveled beyond the orbit of Pluto, the outermost known planet [11]. The spacecraft should have sufficient power to provide useful data through at least 1996 [12]. The SNAP-19 design was further modified for the Viking Mars Landers to accommodate high-temperature (400 K.) sterlization, storage during the spacecraft's cruise to Mars, and, on the surface of Mars, the thermal cycling caused by the rapid and extreme temperature changes of the Martian day-night cycle. As shown in Fig. 11 each Viking Lander carried two of the 15.2-kg RTGs which produced a total power of over 85 We at BOM. The RTGs were to produce a total of 70 We for the primary mission of 90 days on the surface of Mars. All four RTGs met the 90-day requirement and they were still operating 4-6 years later when the Landers were separately and inadvertently shutdown on commands from Earth [13,14]. Based on their power performance, it had been estimated that the RTGs on Viking Lander 1 were capable of providing sufficient power for operation until 1994—18 years beyond the mission requirement [15]. Both the Pioneer and Viking RTGs demonstrated the operability and usefulness of RTGs in interplanetary spacecraft. All of these RTGs performed beyond their mission requirements.
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