General Purpose Heat Source

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Exploded view of a stack of General Purpose Heat Source modules.

The General Purpose Heat Source is a U.S. DOE-designed radioactive heat source for Radioisotope Thermoelectric Generators (RTG) or Stirling radioisotope generators (SRG). It is meant for space applications and is packaged as a stackable module.


GPHSs are fueled with plutonium-238 dioxide. Each module has a temperature of over 600 degrees Celsius[citation needed] and delivers 250 watts at the time of manufacture. They measure 9.948 cm wide x 9.32 cm deep x 5.82 cm high and weigh no more than 1.44 kg each.


GPHSs are designed with safety in mind and employ iridium-clad plutonium-238 dioxide pellets. The generated alpha particles are blocked by the cladding, thus no further radiation shielding is necessary. The pellets are encased within nested layers of carbon-based material and placed within an aeroshell housing to comprise the complete module.

The modules can withstand extreme conditions including a launch-pad explosion or a high-speed reentry. Overheating and impact tests were performed on several sample modules.[1]


GPHSs of this, or very similar, design were used in the GPHS-RTGs of Cassini-Huygens, New Horizons, the Galileo probe, and the Ulysses probe. They are used in the Multi-Mission Radioisotope Thermoelectric Generator, as used by Mars Science Laboratory. They are also used in the Advanced Stirling Radioisotope Generator.

Stages of assembly[edit]


  1. ^ Los Alamos report "General-purpose heat source safety verification test series: SVT-11 through SVT-13". 1986-05-01, OSTI 5664400. doi:10.2172/5664400
  • [1] Department of Energy (DOE) Office of Nuclear Energy GPHS article
  • [2] "Radioisotope Thermoelectric Generators (RTGs)" NASA Galileo Information

External links[edit]