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RPS: Radioisotope Power Systems
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Radioisotope Thermoelectric Generator (RTG)

A legacy of exploration

The Mars Science Laboratory rover, Curiosity, took this self portrait, which shows its Radioisotope Thermoelectric Generator (RTG) at center

Radioisotope Thermoelectric Generators, or RTGs, provide electrical power for spacecraft by converting the heat generated by the decay of plutonium-238 (Pu-238) fuel into electricity using devices called thermocouples. Since they have no moving parts that can fail or wear out, RTGs have historically been viewed as a highly reliable power option. Thermocouples have been used in RTGs for a total combined time of over 300 years, and a not a single thermocouple has ever ceased producing power.

Thermocouples are common in everyday items that must monitor or regulate their temperature, such as air conditioners, refrigerators and medical thermometers. The principle of a thermocouple involves two plates, each made of a different metal that conducts electricity. Joining these two plates to form a closed electrical circuit while keeping the two junctions at different temperatures produces an electric current. Each of these pairs of junctions forms an individual thermocouple. In an RTG, the radioisotope fuel heats one of these junctions while the other junction remains unheated and is cooled by the space environment or a planetary atmosphere.

Multi-Mission Radioisotope Thermoelectric Generator

MMRTG pull-apart view
Labeled pull-apart view showing the major components of the MMRTG, or Multi-Mission Radioisotope Thermoelectric Generator. Image credit: NASA

The current RTG model is the Multi-Mission Radioisotope Thermoelectric Generator, or MMRTG. It is based on the type of RTG flown previously on the two Viking landers and the Pioneer 10 and 11 spacecraft (the SNAP-19 RTG). It is designed to be used in either the vacuum of space or within the atmosphere of a planet. The excess heat energy from an MMRTG can be used as a convenient and steady source of warmth to maintain proper operating temperatures for a spacecraft and its instruments in cold environments.

The MMRTG is used to power the Mars Science Laboratory rover, Curiosity, which is currently exploring the Red Planet. This power source was selected for the rover's mission to provide greater flexibility in accessing difficult or remote terrain, to enable effective operation in the dusty Martian environment and throughout its winter season, and for providing heat to maintain effective operating temperatures for its instruments and systems.

The MMRTG produces electricity at an operational efficiency of 6 to 7 percent. Multiple MMRTGs could be combined to provide higher levels of electrical power for missions that require it. Each MMRTG carries 10.6 pounds (4.8 kilograms) of plutonium-238 dioxide as its nuclear fuel, using eight General Purpose Heat Source (GPHS) modules to produce about 110 Watts of electrical power in total.

Fact sheet: MMRTG - Multi-Mission Radioisotope Thermoelectric Generator (PDF, 1.57 MB)

Previous generations of RTG

Power source for Nimbus III meteorological satellite. SNAP stands for "Systems for Nuclear Auxilliary Power."


  • Output 28.2 Watts electric (or We) at beginning of mission
  • NASA's first application of radioisotope power systems
  • Nimbus B-1 launch on 18 May 1968
    • Launch vehicle failure forced destruction by range safety officer
    • Spacecraft and upper stage sank in Santa Barbara Channel
    • RTGs recovered and fuel reused for Nimbus III

Power source for Viking 1 & 2 Mars landers, Pioneer 10 & 11 spacecraft


  • Output 40.3 Watts electric (Pioneer) and 42.6 Watts electric (Viking) at beginning of mission
  • Modified version of SNAP-19B
  • Pioneer 10 & 11 design lifetime was 5 years; spacecraft continued to communicate with Earth for 30 and 22 years respectively.
  • Viking 1 & 2 operational requirement was 90 days; landers operated for six and four years respectively.

Cutaway view of a SNAP-19 RTG used to power NASA's two Viking landers
Cutaway view of a SNAP-19 RTG used to power NASA's two Viking landers

Power source for Apollo Lunar Surface Experiment Package (ALSEP). Deployed on Apollo missions 12, 14, 15, 16 and 17


  • Output 70 Watts electric at beginning of mission
  • Two-year design lifetime. All deployed units operated five to eight years until ALSEP stations were shut down.

Apollo astronaut photo of a SNAP-27 RTG on the Moon
Apollo astronaut photo of a SNAP-27 RTG on the Moon

Multi-Hundred Watt (MHW) RTG
Power source for Voyager 1 & 2 spacecraft


  • Output 158 Watts electric at beginning of mission
  • RTGs still operating over 30 years later at edge of solar system

Cutaway view of a Multi-Hundred Watt RTG
Cutaway view of a Multi-Hundred Watt RTG

General Purpose Heat Source (GPHS) RTG
Power source for Galileo, Cassini, Ulysses & New Horizons spacecraft


  • Output 292 Watts electric at beginning of mission
  • A total of 18 general purpose heat source (GPHS) modules are stacked together to provide the heat source for each GHPS RTG.
  • Cassini and New Horizons RTGs still operating after 14 and 6 years, respectively. Galileo operated for nearly 14 years, while Ulysses functioned for nearly 19 years in space.
Cutaway view of a General Purpose Heat Source RTG
Cutaway view of a General Purpose Heat Source RTG
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Last Updated: 4 Sep 2013