RTG Power

Radioisotope thermoelectric generators, abbreviated RTGs, have operated in space for a total combined time of more than 300 years. Not a single thermocouple has ever ceased producing power.

The rover is upside down in this straight from the back view. The image is centered on a cylinder with fins, the spacecraft's Radioisotope Thermoelectric Generator.

Radioisotope Thermoelectric Generators

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 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.

before , A sweeping landscape gives way to hills on the colorized horizon in this artistic Mars composite image.
after , A labeled view of a Martian landscape.

Before and After

Curiosity's 'Postcard' of 'Marker Band Valley'

June 13, 2023

Multi-Mission Radioisotope Thermoelectric Generator (MMRTG)

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.

This 3D animation shows the main components of the latest generation of nuclear generators used to provide power for some NASA missions that explore the solar system.

Previous Generations of RTGs

Systems for Nuclear Auxiliary Power-19B (SNAP-19B) RTG

Power source for Nimbus III meteorological satellite.

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


Power source for the Viking 1 and 2 Mars landers and the Pioneer 10 and 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 five 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.


Power source for the Apollo Lunar Surface Experiment (ALSEP) packages carried to the Moon by Apollo crews. Astronauts on Apollo missions 12, 14, 15, 16 and 17 deployed RTG-powered ALSEPs.

  • 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 in 1977.

Multi-Hundred Watt RTG (MHW-RTG)

Power source for Voyager 1 and 2 spacecraft

  • Output 158 Watts electric at beginning of mission.
  • The RTGs still operating over 45 years later on the edge of interstellar space.

General Purpose Heat Source RTG (GPHS-RTG)

Power source for the Galileo, Ulysses, Cassini, and 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 GPHS RTG.
  • New Horizons RTG is still operating and enabled an extended mission deeper into the Kuiper Belt beyond Pluto.
  • Galileo operated for nearly 14 years, while Ulysses and Cassini functioned for nearly 19 years in space.