| | Radioisotope Thermoelectric Generator  | 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 The current RTG model is theMulti-Mission Radioisotope Thermoelectric Generator, orMMRTG, is based on the type of RTG flown previously on the twoViking landers and thePioneer 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 theMars Science Laboratory rover, Curiosity, which is scheduled land on Mars in August 2012. 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 eightGeneral Purpose Heat Source (GPHS) modules to produce about 110 Watts of electrical power in total.
Previous generations of RTG | | SNAP-19B RTG
Power source forNimbus III meteorological satellite. SNAP stands for "Systems for Nuclear Auxilliary Power."
Features:
- 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
| | SNAP-19 RTG
Power source forViking 1 & 2 Mars landers,Pioneer 10 & 11 spacecraft
Features:
- 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.
| | SNAP-27 RTG
Power source forApollo Lunar Surface Experiment Package (ALSEP). Deployed on Apollo missions 12, 14, 15, 16 and 17
Features:
- 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 forVoyager 1 & 2 spacecraft
Features:
- 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 forGalileo,Cassini,Ulysses &New Horizons spacecraft
Features:
- Output 292 Watts electric at beginning of mission
- A total of 18general 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.
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