SNAP-19 Radioisotope Thermoelectric Generator Fact Sheet by Energy Research & Development Administration (ERDA) Page 3

One of the unique derivations resulting from the internal use of the RTGs, was the thermal switches. These were developed in response to the need to control the temperature within the internal Lander systems to prevent overheating from the power source (RTGs). Because the RTGs provided constant energy, there were times when the heat needed to be vented to prevent over heating, as much as there was a need to maintain the temperature for system functions.