The Interstellar Probe mission was initially proposed to travel 1000 astronomical units (AUs) in 50 years, after launching in the 2030s. In a later mission concept review, this was reduced to 400 AUs. The spacecraft uses Next Generation Radioisotope Thermoelectric Generators (RTGs), which are similar to General Purpose Heat Source (GPHS)-RTGs, to provide electrical power to the onboard scientific and communications equipment. The highest possible specific power (power per unit mass) of the RTG is desired. The current plan is for these RTGs to use the latest design of the GPHS module, called the Step 2 module. It would have a specific power of 4.7 Welectric/kg [Beginning of Life (BOL) 276 Welectric for 16 modules]. This briefing presents justification for a proposal to instead use the original design Step 0 module, which is smaller and lighter than the Step 2 module, in an RTG case of the same length. In doing so, a higher specific power for the RTG of 5.3 Welectric/kg [BOL 300 Welectric for 18 modules] can be achieved. The Step 0 GPHS module has been qualified for survivability to accidental impacts, and orbital decay and shallow angle reentries. It is important to note that this is the GPHS-RTG design flown on the Cassini mission.
