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The multi-mission radioisotope thermoelectric generator (MMRTG) is a type of radioisotope thermoelectric generator developed for NASA space missions such as the Mars Science Laboratory (MSL), under the jurisdiction of the United States Department of Energy's Office of Space and Defense Power Systems within the Office of Nuclear Energy. The MMRTG was developed by an industry team of Aerojet Rocketdyne and Teledyne Energy Systems.
Space exploration missions require safe, reliable, long-lived power systems to provide electricity and heat to spacecraft and their science instruments. A uniquely capable source of power is the radioisotope thermoelectric generator (RTG) – essentially a nuclear battery that reliably converts heat into electricity. Radioisotope power has been used on eight Earth orbiting missions, eight missions travelling to each of the outer planets as well as each of Apollo missions following 11 to Earth's moon. Some of the outer Solar System missions are the Pioneer, Voyager, Ulysses, Galileo, Cassini and New Horizons missions. The RTGs on Voyager 1 and Voyager 2 have been operating since 1977. Similarly, Radioisotope Heat Units (RHUs) were used to provide heat to critical components on Apollo 11 as well as the first two generations of Mars rovers. In total, over the last four decades, 26 missions and 45 RTGs have been launched by the United States.
RTGs convert the heat from the natural decay of a radioisotope into electricity. The MMRTG's heat source is plutonium-238 dioxide. Solid-state thermoelectric couples convert the heat to electricity. Unlike solar arrays, the RTGs are not dependent upon the Sun, so they can be used for deep space missions.
In June 2003, the Department of Energy (DOE) awarded the MMRTG contract to a team led by Aerojet Rocketdyne. Aerojet Rocketdyne and Teledyne Energy Systems collaborated on an MMRTG design concept based on a previous thermoelectric converter design, SNAP-19, developed by Teledyne for previous space exploration missions. SNAP-19s powered Pioneer 10 and Pioneer 11 missions as well as the Viking 1 and Viking 2 landers.
The MMRTG is powered by 8 Pu-238 dioxide general-purpose heat source (GPHS) modules, provided by the Department of Energy. Initially, these eight GPHS modules generate about 2 kW thermal power.
The MMRTG design incorporates PbTe/TAGS thermoelectric couples (from Teledyne Energy Systems), where the TAGS material is a material incorporating Tellurium (Te), Silver (Ag), Germanium (Ge) and Antimony (Sb). The MMRTG is designed to produce 125 W electrical power at the start of mission, falling to about 100 W after 14 years. With a mass of 45 kg the MMRTG provides about 2.8 W/kg of electrical power at beginning of life.
The MMRTG design is capable of operating both in the vacuum of space and in planetary atmospheres, such as on the surface of Mars. Design goals for the MMRTG included ensuring a high degree of safety, optimizing power levels over a minimum lifetime of 14 years, and minimizing weight.
Curiosity, the MSL rover that was successfully landed in Gale Crater on August 6, 2012, uses one MMRTG to supply heat and electricity for its components and science instruments. Reliable power from the MMRTG will allow it to operate for several years.
In February 20, 2015, a NASA official reported that there is enough plutonium available to NASA to fuel three more MMRTG like the one used by the Curiosity rover. One is already committed to the Mars 2020 rover. The other two have not been assigned to any specific mission or program, and could be available by late 2021.
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