Multi-mission radioisotope thermoelectric generator

Did You Know?

• Solid-state thermoelectric couples convert heat from plutonium-238 decay to electricity via the Seebeck effect.
• Unlike photovoltaic solar arrays, RTGs are not dependent upon solar energy, enabling deep space missions.
• The MMRTG design concept was based on the SNAP-19 thermoelectric converter, which powered Pioneer 10, Pioneer 11, and Viking 1 and 2 landers.

Background

Space exploration missions require safe, reliable, long-lived power systems for electricity and heat. Radioisotope thermoelectric generators (RTGs) serve as nuclear batteries converting heat into electricity. Radioisotope power has been used on eight Earth orbiting missions, eight outer planet missions, and Apollo missions after Apollo 11. Outer Solar System missions include Pioneer 10 and 11, Voyager 1 and 2, Ulysses, Galileo, Cassini, and New Horizons. Voyager RTGs have operated since 1977. In total, 26 missions and 45 RTGs have been launched by the United States.

Design and specifications

The MMRTG is powered by eight Pu-238 dioxide general-purpose heat source (GPHS) modules from the US Department of Energy, initially generating about 2 kW thermal power. It incorporates PbTe/TAGS thermoelectric couples, where TAGS denotes a material of tellurium, silver, germanium, and antimony. Designed to produce 125 W electrical at start of mission, falling to about 100 W after 14 years, the MMRTG has a mass of 45 kg, with 4.8 kg of plutonium dioxide fuel, providing about 2.8 W/kg electrical at beginning of life. It can operate in vacuum and planetary atmospheres, with design goals including high safety, optimized power over a minimum 14-year lifetime, and minimized weight.

Usage in space missions

Curiosity, the MSL rover that landed in Gale Crater on August 6, 2012, uses one MMRTG for heat and electricity. On February 20, 2015, a NASA official reported enough plutonium for three more MMRTGs like Curiosity's. One was used by Mars 2020 and its Perseverance rover; the other two remain unassigned, possibly available by late 2021. An MMRTG launched on July 30, 2020, aboard Mars 2020, supplies Perseverance with heat and power. That MMRTG, designated F-2, was built by Teledyne Energy Systems and Aerojet Rocketdyne under DOE contract, with a lifespan up to 17 years. The upcoming NASA Dragonfly mission to Titan will use one of the two MMRTGs recently contracted to the Aerojet Rocketdyne/Teledyne team, charging lithium-ion batteries to power a quad helicopter. The proposed PROMISE rover mission to the moon would use an MMRTG to survive the lunar night.