**5. Transit RTG**

480 Radioisotopes – Applications in Physical Sciences

In 1972, NASA began its exploration of the outer Solar System with the launch to Jupiter of Pioneer 10 powered by four SNAP-19 RTGs which produced a total of 161.2 We at BOM. The next year Pioneer 10 was followed by the Pioneer 11 spacecraft which was also powered by four SNAP-19 RTGs. In the cold, dark, radiation-rich environment of the Jovian system, nuclear power was the only viable option at that time. Because the SNAP-19 RTGs performed so well, NASA was able to retarget Pioneer 11 to go to Saturn after its flyby of Jupiter. Again, the RTGs performed very well, providing steady power to the spacecraft and its scientific instruments, thus allowing scientists their first close-up measurements of

In anticipation of the 200th anniversary in 1976 of the signing of the U.S. Declaration of Independence, NASA launched the two Viking missions in 1975, each launch carrying an Orbiter and a Lander. Each Lander was powered by two SNAP-19 RTGs specially modified to work on the surface of Mars (see Fig. 6). The 35-We Viking SNAP-19 RTGs contained a special dome allowing an interchange of internal gases (initial fill 90:10 helium-argon; reservoir fill 95:5 argon-helium) during operation on the surface of Mars. This allowed for reduced pre-launch temperatures and maximum power output on Mars. All four SNAP-19 RTGs easily met the 90-day operating requirement of the Landers and went on to power the Landers for up to six years giving scientists their first extraordinary in-situ views of the

Fig. 6. Viking Lander model showing the location of the two SNAP-19 RTGs. The average power per RTG was 42.7 We at BOM. The overall RTG diameter (across fins) was 58.7 cm

and the overall length was 40.4 cm. The mass was 15.2 kg. (Image credit:

the second largest planet in the Solar System (Bennett, et al., 1984).

**4.2 Pioneers 10 and 11** 

**4.3 Viking Landers 1 and 2** 

surface of Mars (Bennett, et al., 1984).

NASA/JPL/Caltech/ERDA/Teledyne)

The successful use of the SNAP-9A RTGs on the Transit 5BN series of Navy navigational satellites led JHU/APL to use a new telluride-based RTG called the "Transit RTG" on its TRIAD navigational satellite. The Transit RTG was based on the SNAP-19 radioisotope heat source design although in this case radiatively coupled to a telluride-based thermoelectric converter instead of being conductively coupled as in the SNAP-19 and SNAP-9A RTGs.

The Transit RTG, which was designed to be modular, produced over 35 We at BOM within a mass of about 13.6 kg. The use of a lower hot-junction temperature (~674 K for the Transit RTG versus ~790+ K for the SNAP-19 RTGs) in a vacuum environment eliminated the SNAP-19 practice of using hermetic sealing and a cover gas to inhibit the sublimation degradation that could cause a reduction in cross section and subsequent increase in electrical resistance of the thermoelectric material. (Lowering the hot junction temperature is also one of the strategies adopted for the MMRTG.) While the TRIAD spacecraft had various problems, the Transit RTG operated well beyond its five-year requirement (Bennett, et al., 1984).
