Legacy Systems

Radioisotope Thermoelectric Generators (RTGs) and General Purpose Heat Sources (GPHS) have enabled two dozen NASA missions in the last 50 years.


Previous Generations of Radioisotope Thermoelectric Generators (RTGs)

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

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


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.

Multi-Hundred Watt (MHW) RTG

Power source for Voyager 1 & 2 spacecraft.

  • Output 158 Watts electric at beginning of mission
  • RTGs still operating over 40 years later at edge of solar system

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 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.