Apollo 11 (Early Apollo Surface Experiment Package-EASEP) has used Two RHUs for its Lunar surface mission. In fact, the Apollo 11 package EASEP was solar posered, but was heated by two 15W RHUs.

For missions of Apollo 12 through 17, One SNAP-27 RTGs was used for the Lunar surface mission.

Viking 1 lander has used Two SNAP-19 RTGs for its Mars surface mission. The RTG has operated for 6 years until Lander was shut down.

Viking 2 lander has also used Two SNAP-19 RTGs for its Mars surface mission. The RTG has operated for 4 years until the relay link was lost.

Galileo has used Two GPHS-RTGs-103 RHUs on its orbiter, 17 RHUs on its atmospheric probe. The spacecraft has performed Venus and Earth flybys, Jupiter orbit, probe to Jupiter's atmosphere. Its de-orbited has occurred in September, 2003.

Ulysses use One GPHS-RTGTwo for its mission as Jupiter flybys and Solar polar observations. Its RTGs still operating.

Cassini-Huygens has used Three GPHS-RTGs, such as 82 RHUs on the orbiter cassini, and 35 RHUs on the Huygens probe. Each RHU produces about 1 watt of thermal power. Its mission performed was Venus, Earth and Jupiter flybys, Saturn orbit, Huygens lander to Titan. The mission was terminated in September 2017.

New Horizons has used One GPHS-RTGs for its Pluto & Kuiper Belt flybys. Its RTGs still operating.

Mars Science Laboratory Rover curiosity has used One MMRTG for its mission on the Mars' surface.

Mars Pathfinder Sejourner Rover has used Three RHUs for its Mars' surface mission.

Mars exploration Rover Spirit & Opportunity has used Eight RHUs each for their Mars surface missions.

Voyager 1 & 2

At this moment, Voyager 1 and 2 are traveling away from the Sun, probing the outer edges of our solar system and analyzing the interaction of the solar wind and the interstellar medium nearly four decades after launch. The two Voyager spacecrafts have contributed to our understanding of the giant planets of our solar system as well as the limits of the Sun’s influence, but it is easy to forget that both Voyagers ended their primary mission phases soon after their encounters with Saturn, which for Voyager 2 occurred in summer 1981. More than 30 additional years of scientific discovery by the Voyagers have resulted from repeated extensions of the mission. LEARN MORE about THIS FANTASTIC VOYAGE


Credit: NASA /Jet Propulsion Laboratory

From a source of heat comes power to explore!

A Radioisotope Thermoelectric Generator, or RTG, are a family of radioisotope power system, or RPS, that use direct thermoelectric conversion to generate electric power to spacecraft. It do that by converting heat generated by the natural radioactive decay of its fuel source, plutonium dioxide 238, into electricity using devices called thermocouples. RTGs have no moving parts.
The latest Radioisotope power systems, or RPS, to be qualified for flight, the Multi-Mission Radioisotope Thermoelectric Generator (MMRTG) is powering the Mars Science Laboratory rover, Curiosity, which landed on Mars in August 2012. Designed to be used in the vacuum of space as well as within the atmosphere of Mars, the MMRTG continues to perform as designed, providing both power and heat for the rover.

Power Evolution

Since the early 1960s, RTGs have been long-lived and highly reliable power sources for more than two-dozen Earth-orbiting and planetary missions. RTG flight system designs progressed from the Systems for Nuclear Auxiliary Power (SNAP) series, to the Multi-hundred Watt Radioisotope Thermoelectric Generators (MHW-RTG), to RTGs based on the GPHS.
The SNAP series of RTGs included a number of different configurations. SNAP-19 use PbTe/TAGS [lead telluride/tellurides of antimony, germanium, and silver] as the thermoelectric material in the converters, producing 40 We per unit. SNAP-19 RTGs powered the Viking 1 and 2 Mars landers, and the Pioneer 10 and 11 outer-planet flyby missions.
Because it use silicon-germanium (Si-Ge) thermoelectrics, the MHW-RTG had a much higher power output than the SNAP series (~160 We) and has real success with the Voyager 1 and Voyager 2 missions, launched in 1977 and still operating in 2015.
After the MHW-RTG, the Department Of Energy (DOE) developed the GPHS module to provide a standard, modular plutonium dioxide-based heat source. This GPHS module has been the basis of the RTGs developed since then. The GPHS-RTG was the first of these standardized designs, using 18 GPHS modules and Si-Ge thermoelectrics to generate nearly 300 We. The GPHS-RTG has been used very successfully on four planetary missions: Galileo, Ulysses, Cassini and New Horizons.
The GPHS module provides steady heat for a radioisotope power system. Image credit: NASA
Then, we remember that, RPS provide electricity and heat that enable spacecraft to undertake scientific missions to environments beyond the capabilities of solar power, chemical batteries and fuel cells. Sometimes is referred to a type of "nuclear battery."
RPS are flexible because, like Cassini's spacecraft, run its systems directly off of its RPS and, others like the Mars Science Laboratory rover, use the RPS to charge batteries and run its systems and instruments off of stored battery power. In either case, the RPS is attached directly to a spacecraft like a power cord being plugged in.
These technologies are capable of producing electricity and heat for decades under the harsh conditions of deep space without refueling. All of these power systems, flown on more than two dozen NASA missions since the 1960s, have functioned for longer than they were originally designed.
The RPS used to power NASA spacecrafts are supplied by the U.S. Department of Energy (DOE). NASA and DOE continue to collaborate on maintaining and developing several types of RPS.

The General Purpose Heat Source module, or GPHS

GPHS is the essential building block for the radioisotope generators used by NASA. These modules contain and protect the plutonium-238 (or Pu-238) fuel that gives off heat for producing electricity. The fuel is fabricated into ceramic pellets of plutonium-238 dioxide (238PuO2) and encapsulated in a protective casing of iridium, forming a fueled clad. Fueled clads are encased within nested layers of carbon-based material and placed within an aeroshell housing to comprise the complete GPHS module.
Each GPHS is a block about four by four by two inches in size, weighing approximately 3.5 pounds (1.5 kilograms). They are nominally designed to produce thermal power at 250 watts at the beginning of a mission, and can be used individually or stacked together.
Modules have been subjected to extreme testing conditions that significantly exceeded the intensity of a wide range of potential accidents. Such tests have included simulating multiple re-entries for a single module through Earth's atmosphere, exposure to high temperature rocket propellant fires, and impacts onto solid ground.
The enhanced GPHS modules used in the latest generation of radioisotope power systems incorporate additional rugged, safety-tested features that build upon those used in earlier generations. For example, additional material (20 percent greater in thickness) has been added to the graphite aeroshell and to the two largest faces of the block-like module. These modifications provide even more protection to help to contain the fuel in a wide range of accident conditions, further reducing the potential for release of plutonium-238 that might result.

Radioisotope Heaters

A Radioisotope Heater Unit, or RHU, employs a small, pencil eraser-sized pellet of plutonium dioxide to generate heat for spacecraft structures, systems, and instruments, enabling their successful operation throughout a mission. Some missions employ just a few RHUs for extra heat, while others have dozens. NASA has also studied the potential for using the same small fuel pellet in an RHU to power a compact system that could provide a few dozen milliwatts of electrical power.

Radioisotope power systems & heaters by mission

Credit: https://solarsystem.nasa.gov/rps/rhu.cfm

Why Spacecrafts' Missions are extended? 

Today, nearly four decades after their launch in 1977, spacecraft Voyager 1 and 2 are traveling away from the Sun, probing the outer edges of our solar system and analyzing the interaction of the solar wind and the interstellar medium.
By their odyssey, they do an incredible contribution to our understanding of the giant planets of our solar system as well as the limits of the Sun’s influence.
As we know, both Voyagers ended their primary mission phases soon after their encounters with Saturn which, for Voyager 2, occurred in summer 1981. The 30 additional years missions allowed to Voyagers 1 and 2, given us a lot of scientific discoveries.
Who make the decision for additional years?

Benefits of Some Mission Extensions

During 2016, there were approximately 14 active Planetary Science Division missions. Follow some examples of Major Science Results Made Possible by Extended Missions in Planetary Sciences.


Global subsurface oceans were discovered in Titan (Lorenz et al., 2008; Iess et al., 2012) and in Enceladus (Thomas et al., 2016).

The erupting jets and curtains of water on Enceladus



Pioneer 10 has used Four SNAP-19 RTGs & 12 RHUs for its Outer planet flyby at Jupiter. Its RTGs has operated more than 31 years.

Pioneer 11 has also used Four SNAP-19 RTGs & 12 RHUs for its Outer planet flybys at Jupiter & Saturn. Its RTGs has operated for more than 22 years.

Voyager 1 use Three MHW-RTGs & 9 RHUs for its Outer planet flybys at Jupiter, Saturn, and far away in interstellar space. Its RTGs still operating!

Voyager 2 use Three MHW-RTGs & 9 RHUs for its Outer planet flybys at Jupiter, Saturn, Uranus, Neptune, and its interstellar space voyage. Also, its RTGs still operating!


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.
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 30 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 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.
NASA missions enabled by radioisotope heater units

. Cassini-Huygens Story 
. The Huygens Probe Mission
. End of Mission
. Some of the most promises futures missions' Scenarios on Moon's Titan, like New Spacecraft and/or Balloons and/or Submarine in Titan's Lakes...