General Purpose Heat Source
The General Purpose Heat Source is a radioactive heat source designed for Radioisotope Thermoelectric Generator
Radioisotope thermoelectric generator
A radioisotope thermoelectric generator is an electrical generator that obtains its power from radioactive decay. In such a device, the heat released by the decay of a suitable radioactive material is converted into electricity by the Seebeck effect using an array of thermocouples.RTGs can be...

s (RTG) or Stirling radioisotope generator
Stirling Radioisotope Generator
The Stirling radioisotope generator is based on a Stirling engine powered by a large radioisotope heater unit. The hot end of the Stirling converter reaches high temperature and heated helium drives the piston, heat being rejected at the cold end of the engine. A generator or alternator converts...

s (SRG). It is meant for space applications and is packaged as a stackable module.


GPHSs are fueled with plutonium-238
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. Each module has a temperature of over 600 degrees Celsius 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
Iridium is the chemical element with atomic number 77, and is represented by the symbol Ir. A very hard, brittle, silvery-white transition metal of the platinum family, iridium is the second-densest element and is the most corrosion-resistant metal, even at temperatures as high as 2000 °C...

-clad plutonium-238 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.


GPHSs of this, or very similar, design were used in the GPHS-RTG
GPHS-RTG or General Purpose Heat Source Radioisotope Thermoelectric Generator. These are a specific American design of Radioisotope Thermoelectric Generator used on space missions GPHS-RTG or General Purpose Heat Source Radioisotope Thermoelectric Generator. These are a specific American design of...

s of Cassini-Huygens
Cassini–Huygens is a joint NASA/ESA/ASI spacecraft mission studying the planet Saturn and its many natural satellites since 2004. Launched in 1997 after nearly two decades of gestation, it includes a Saturn orbiter and an atmospheric probe/lander for the moon Titan, although it has also returned...

, New Horizons
New Horizons
New Horizons is a NASA robotic spacecraft mission currently en route to the dwarf planet Pluto. It is expected to be the first spacecraft to fly by and study Pluto and its moons, Charon, Nix, Hydra and S/2011 P 1. Its estimated arrival date at the Pluto-Charon system is July 14th, 2015...

, the Galileo probe, and the Ulysses probe
Ulysses probe
Ulysses is a decommissioned robotic space probe that was designed to study the Sun as a joint venture of NASA and the European Space Agency . The spacecraft was originally named Odysseus, because of its lengthy and indirect trajectory to near Solar distance...


There are used in the Multi-Mission Radioisotope Thermoelectric Generator
Multi-Mission Radioisotope Thermoelectric Generator
Multi-Mission Radioisotope Thermoelectric Generator are a type of Radioisotope Thermoelectric Generator developed for NASA space missions including manned ones such as the Apollo program as well as unmanned ones ranging from Viking 1 through the Mars Science Laboratory...

, as used by Mars Science Laboratory
Mars Science Laboratory
The Mars Science Laboratory is a National Aeronautics and Space Administration mission with the aim to land and operate a rover named Curiosity on the surface of Mars. The MSL was launched November 26, 2011, at 10:02 EST and is scheduled to land on Mars at Gale Crater between August 6 and 20, 2012...


There are also used in the Advanced Stirling Radioisotope Generator
Advanced Stirling Radioisotope Generator
The Advanced Stirling Radioisotope Generator is a radioisotope power system using Stirling power conversion technology currently being developed under joint sponsorship by the United States Department of Energy and NASA for potential future space missions...

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