**4.3 Viking Landers 1 and 2**

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

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: NASA/JPL/Caltech/ERDA/Teledyne)

The success of the Viking SNAP-19 RTGs was a key factor in the selection of the telluridethermoelectric-based Multi-Mission Radioisotope Thermoelectric Generator (MMRTG) for the upcoming MSL mission (see Section 8).
