The multi-mission radioisotope thermoelectric generator (MMRTG) is a type of radioisotope thermoelectric generator (RTG) 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 Office of Nuclear Energy (NE) is an agency of the United States Department of Energy which promotes nuclear power as a resource capable of meeting the energy, environmental, and national security needs of the United States by resolving technic ...

. The MMRTG was developed by an industry team of Aerojet Rocketdyne and Teledyne Energy Systems.

Background

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 to the outer planets, and the Apollo missions after Apollo 11 to the Moon. The outer Solar System missions are the ''Pioneer 10'' and ''11'', ''Voyager 1'' and ''2'', ''

Ulysses Ulysses is one form of the Roman name for Odysseus, a hero in ancient Greek literature. Ulysses may also refer to: People * Ulysses (given name), including a list of people with this name Places in the United States * Ulysses, Kansas * Ulysse ...

'', ''

Galileo Galileo di Vincenzo Bonaiuti de' Galilei (15 February 1564 – 8 January 1642) was an Italian astronomer, physicist and engineer, sometimes described as a polymath. Commonly referred to as Galileo, his name was pronounced (, ). He was ...

'', '' Cassini'' and ''

New Horizons ''New Horizons'' is an Interplanetary spaceflight, interplanetary space probe that was launched as a part of NASA's New Frontiers program. Engineered by the Johns Hopkins University Applied Physics Laboratory (APL) and the Southwest Research ...

'' missions. The RTGs on '' Voyager 1'' and '' Voyager 2'' have been operating since 1977. In total, over the last four decades, 26 missions and 45 RTGs have been launched by the United States.

Function

Solid-state Solid state, or solid matter, is one of the four fundamental states of matter. Solid state may also refer to: Electronics * Solid-state electronics, circuits built of solid materials * Solid state ionics, study of ionic conductors and their use ...

thermoelectric The thermoelectric effect is the direct conversion of temperature differences to electric voltage and vice versa via a thermocouple. A thermoelectric device creates a voltage when there is a different temperature on each side. Conversely, when ...

couples convert the heat produced by the natural decay of the radioisotope plutonium-238 to electricity.SNAP-19: Pioneer F & G, Final Report
, Teledyne Isotopes, 1973 The physical conversion principle is based on the Seebeck effect, obeying one of the Onsager reciprocal relations between flows and gradients in thermodynamic systems. A

temperature gradient A temperature gradient is a physical quantity that describes in which direction and at what rate the temperature changes the most rapidly around a particular location. The temperature gradient is a dimensional quantity expressed in units of degree ...

generates an electron flow in the system. Unlike

photovoltaic Photovoltaics (PV) is the conversion of light into electricity using semiconducting materials that exhibit the photovoltaic effect, a phenomenon studied in physics, photochemistry, and electrochemistry. The photovoltaic effect is commercially us ...

solar arrays, RTGs are not dependent upon the

solar energy Solar energy is radiant light and heat from the Sun that is harnessed using a range of technologies such as solar power to generate electricity, solar thermal energy (including solar water heating), and solar architecture. It is an essenti ...

, so they can be used for deep space missions.

History

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 The Systems Nuclear Auxiliary POWER (SNAP) program was a program of experimental radioisotope thermoelectric generators (RTGs) and space nuclear reactors flown during the 1960s by NASA. Odd-numbered SNAPs: radioisotope thermoelectric generators ...

, developed by Teledyne for previous space exploration missions. SNAP-19s powered '' Pioneer 10'' and ''

Pioneer 11 ''Pioneer 11'' (also known as ''Pioneer G'') is a robotic space probe launched by NASA on April 5, 1973, to study the asteroid belt, the environment around Jupiter and Saturn, solar winds, and cosmic rays. It was the first probe to encounter ...

'' missions as well as the '' Viking 1'' and ''

Viking 2 The ''Viking 2'' mission was part of the American Viking program to Mars, and consisted of an orbiter and a lander essentially identical to that of the ''Viking 1'' mission. ''Viking 2'' was operational on Mars for sols ( days; '). The ''Vik ...

'' landers.

Design and specifications

The MMRTG is powered by eight

Pu-238 Plutonium-238 (238Pu or Pu-238) is a fissile, radioactive isotope of plutonium that has a half-life of 87.7 years. Plutonium-238 is a very powerful alpha emitter; as alpha particles are easily blocked, this makes the plutonium-238 isotope suitab ...

dioxide

general-purpose heat source The general-purpose heat source is a U.S. DOE-designed radioactive heat source for radioisotope thermoelectric generators (RTG) or Stirling radioisotope generators (SRG). It is meant for space applications and is packaged as a stackable module. ...

(GPHS) modules, provided by the US Department of Energy (DOE). Initially, these eight GPHS modules generate about 2 kW thermal power. The MMRTG design incorporates PbTe/TAGS thermoelectric couples (from Teledyne Energy Systems), where TAGS is an acronym designating a material incorporating tellurium (Te), silver (Ag),

germanium Germanium is a chemical element with the symbol Ge and atomic number 32. It is lustrous, hard-brittle, grayish-white and similar in appearance to silicon. It is a metalloid in the carbon group that is chemically similar to its group neighbors s ...

(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.

Usage in space missions

'' 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. (pdf) October 2013 On February 20, 2015, a NASA official reported that there is enough plutonium available to NASA to fuel three more MMRTGs like the one used by the ''Curiosity'' rover. One is already committed to the Mars 2020 and its ''Perseverance'' rover. The other two have not been assigned to any specific mission or program, and could be available by late 2021. A MMRTG was successfully launched into space on July 30, 2020, aboard the Mars 2020 mission, and is now being used to supply the scientific equipment on the Perseverance rover with heat and power. The MMRTG used by this mission is the F-2 built by Teledyne Energy Systems, Inc. and Aerojet Rocketdyne under contract with the US Department of Energy (DOE) with a lifespan of up to 17 years. The upcoming NASA ''

Dragonfly A dragonfly is a flying insect belonging to the infraorder Anisoptera below the order Odonata. About 3,000 extant species of true dragonfly are known. Most are tropical, with fewer species in temperate regions. Loss of wetland habitat threate ...

'' mission to Saturn's moon

Titan Titan most often refers to: * Titan (moon), the largest moon of Saturn * Titans, a race of deities in Greek mythology Titan or Titans may also refer to: Arts and entertainment Fictional entities Fictional locations * Titan in fiction, fictiona ...

will use one of the two MMRTGs for which the Aerojet Rocketdyne/Teledyne Energy Systems team has recently received a contract. The MMRTG will be used to charge a set of lithium ion batteries, and then use this higher-power-density supply to fly a quad helicopter in short hops above the surface of Titan.

Cost

The MMRTG cost an estimated to manufacture and to research and develop. For comparison the production and deployment of the

GPHS-RTG GPHS-RTG or general-purpose heat source — radioisotope thermoelectric generator, is a specific design of the radioisotope thermoelectric generator (RTG) used on US space missions. The GPHS-RTG was used on ''Ulysses'' (1), ''Galileo'' (2), ' ...

was approximately .

References

External links

NASA Radioisotope Power Systems website – RTG page

Idaho National Laboratory MMRTG page with photo-based "virtual tour"

DOE to crank out new plutonium-238 in 2019
{{Mars 2020 Nuclear power in space Mars Science Laboratory Mars 2020 Thermoelectricity Plutonium