SNAP-19 Radioisotope Thermoelectric Generator Fact Sheet by Energy Research & Development Administration (ERDA) Page 2

The SNAP-19 generators were not only critical for the mission longevity, but they were uniquely designed for Viking to provide additional benefits beyond even their power source. The SNAP-19 was redesigned specifically to be internal to the Viking Lander in order to provide heat to the internal systems which would otherwise succumb to the surface temperatures. This was the first instance of internal deployment of RTGs and became one of many Viking designs to influence future missions.