The Technology of Discovery

3US Space Flights Enabled by RTGs

Young H. Lee and Brian K. Bairstow

Jet Propulsion Laboratory/California Institute of Technology, Pasadena, California

For more than 60 years, the US has produced a string of successful space flights using RTGs with no mission losses caused by an RTG failure. Those flights, their missions, and the means of how RTGs enabled the missions are presented.

3.1 SNAP‐3B Missions (1961)

3.1.1 Transit 4A and Transit 4B

The US Navy sponsored the Transit satellite program developed by the US Defense Advanced Research Projects Agency (DARPA) and the Johns Hopkins Applied Physics Laboratory (APL). The satellites’s signals were used for navigation and hydrographic and geodetic surveying. These missions furthered our understanding of ionospheric refraction of radio waves and of the Earth’s shape and gravitational field. The Transit spacecraft were precursors to the Global Positioning System (GPS) [1,2], and the Transit 4A and 4B missions marked the first successful use of RTGs in space.

Solar power and batteries had powered previous Transit spacecraft. However, the use of RPS technology had been under consideration from the start of the program because of concerns that the hermetic seals on the rechargeable spacecraft batteries would not last for the five‐year mission requirement. The Transit program worked with the Atomic Energy Commission (AEC) to introduce the SNAP‐3 RTG as an auxiliary power source. The first SNAP‐3 RTG used a polonium heat source. ...