The multi-mission radioisotope thermoelectric generator (MMRTG) is a type of

radioisotope thermoelectric generator A radioisotope thermoelectric generator (RTG, RITEG), or radioisotope power system (RPS), is a type of nuclear battery that uses an array of thermocouples to convert the Decay heat, heat released by the decay of a suitable radioactive material i ...

(RTG) developed for

NASA The National Aeronautics and Space Administration (NASA ) is an independent agencies of the United States government, independent agency of the federal government of the United States, US federal government responsible for the United States ...

space missions such as the

Mars Science Laboratory Mars Science Laboratory (MSL) is a robotic spacecraft, robotic space probe mission to Mars launched by NASA on November 26, 2011, which successfully landed ''Curiosity (rover), Curiosity'', a Mars rover, in Gale (crater), Gale Crater on Augus ...

(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 techni ...

. The MMRTG was developed by an industry team of

Aerojet Rocketdyne Aerojet Rocketdyne is a subsidiary of American Arms industry, defense company L3Harris that manufactures rocket, Hypersonic flight, hypersonic, and electric propulsive systems for space, defense, civil and commercial applications. Aerojet traces ...

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 Apollo is one of the Twelve Olympians, Olympian deities in Ancient Greek religion, ancient Greek and Ancient Roman religion, Roman religion and Greek mythology, Greek and Roman mythology. Apollo has been recognized as a god of archery, mu ...

missions after

Apollo 11 Apollo 11 was a spaceflight conducted from July 16 to 24, 1969, by the United States and launched by NASA. It marked the first time that humans Moon landing, landed on the Moon. Commander Neil Armstrong and Lunar Module pilot Buzz Aldrin l ...

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

Galileo Galileo di Vincenzo Bonaiuti de' Galilei (15 February 1564 – 8 January 1642), commonly referred to as Galileo Galilei ( , , ) or mononymously as Galileo, was an Italian astronomer, physicist and engineer, sometimes described as a poly ...

'', '' Cassini'' and ''

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

'' missions. The RTGs on ''

Voyager 1 ''Voyager 1'' is a space probe launched by NASA on September 5, 1977, as part of the Voyager program to study the outer Solar System and the interstellar medium, interstellar space beyond the Sun's heliosphere. It was launched 16 days afte ...

'' and ''

Voyager 2 ''Voyager 2'' is a space probe launched by NASA on August 20, 1977, as a part of the Voyager program. It was launched on a trajectory towards the gas giants (Jupiter and Saturn) and enabled further encounters with the ice giants (Uranus and ...

'' 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 thermoelectric couples convert the

heat In thermodynamics, heat is energy in transfer between a thermodynamic system and its surroundings by such mechanisms as thermal conduction, electromagnetic radiation, and friction, which are microscopic in nature, involving sub-atomic, ato ...

produced by the natural decay of the

radioisotope A radionuclide (radioactive nuclide, radioisotope or radioactive isotope) is a nuclide that has excess numbers of either neutrons or protons, giving it excess nuclear energy, and making it unstable. This excess energy can be used in one of three ...

plutonium-238 Plutonium-238 ( or Pu-238) is a 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 suitable for usage ...

electricity Electricity is the set of physical phenomena associated with the presence and motion of matter possessing an electric charge. Electricity is related to magnetism, both being part of the phenomenon of electromagnetism, as described by Maxwel ...

.SNAP-19: Pioneer F & G, Final Report
, Teledyne Isotopes, 1973 The physical conversion principle is based on the

Seebeck effect 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 ...

, obeying one of the

Onsager reciprocal relations In thermodynamics, the Onsager reciprocal relations express the equality of certain ratios between flows and forces in thermodynamic systems out of equilibrium, but where a notion of local equilibrium exists. "Reciprocal relations" occur betw ...

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 spatial gradient is a vector quantity with Dimensional analysis, ...

generates an

electron The electron (, or in nuclear reactions) is a subatomic particle with a negative one elementary charge, elementary electric charge. It is a fundamental particle that comprises the ordinary matter that makes up the universe, along with up qua ...

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 ...

solar arrays, RTGs are not dependent upon

solar energy Solar energy is the radiant energy from the Sun's sunlight, light and heat, which can be harnessed using a range of technologies such as solar electricity, solar thermal energy (including solar water heating) and solar architecture. It is a ...

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

Pioneer 10 ''Pioneer 10'' (originally designated Pioneer F) is a NASA space probe launched in 1972 that completed the first mission to the planet Jupiter. ''Pioneer 10'' became the first of five artificial objects to achieve the escape velocity needed ...

'' and ''

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

'' missions as well as the ''

Viking 1 ''Viking 1'' was the first of two spacecraft, along with '' Viking 2'', each consisting of an orbiter and a lander, sent to Mars as part of NASA's Viking program. The lander touched down on Mars on July 20, 1976, the first successful Mars lan ...

'' 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 ''V ...

'' landers.

Design and specifications

The MMRTG is powered by eight Pu-238 dioxide general-purpose heat source (GPHS) modules, provided by the

US Department of Energy US or Us most often refers to: * Us (pronoun), ''Us'' (pronoun), the objective case of the English first-person plural pronoun ''we'' * US, an abbreviation for the United States US, U.S., Us, us, or u.s. may also refer to: Arts and entertainme ...

(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 Tellurium is a chemical element; it has symbol Te and atomic number 52. It is a brittle, mildly toxic, rare, silver-white metalloid. Tellurium is chemically related to selenium and sulfur, all three of which are chalcogens. It is occasionally fou ...

(Te),

silver Silver is a chemical element; it has Symbol (chemistry), symbol Ag () and atomic number 47. A soft, whitish-gray, lustrous transition metal, it exhibits the highest electrical conductivity, thermal conductivity, and reflectivity of any metal. ...

(Ag),

germanium Germanium is a chemical element; it has Symbol (chemistry), symbol Ge and atomic number 32. It is lustrous, hard-brittle, grayish-white and similar in appearance to silicon. It is a metalloid or a nonmetal in the carbon group that is chemically ...

(Ge) and

antimony Antimony is a chemical element; it has chemical symbol, symbol Sb () and atomic number 51. A lustrous grey metal or metalloid, it is found in nature mainly as the sulfide mineral stibnite (). Antimony compounds have been known since ancient t ...

(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. The MMRTG has a length of 66.83 cm (26.31 in), and without the fins it has a diameter of 26.59cm (10.47 in), while with the fins it has a diameter of 64.24cm (25.29). The fins themself have a length of 18.83cm (7.41 in)

Usage in space missions

Curiosity Curiosity (from Latin , from "careful, diligent, curious", akin to "care") is a quality related to inquisitive thinking, such as exploration, investigation, and learning, evident in humans and other animals. Curiosity helps Developmental psyc ...

'', the MSL rover that was successfully landed in

Gale Crater Gale is a crater, and probable dry lake, at in the northwestern part of the Aeolis quadrangle on Mars. It is in diameter and estimated to be about 3.5–3.8 billion years old. The crater was named after Walter Frederick Gale, an amateur a ...

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 was used by

Mars 2020 Mars 2020 is a NASA mission that includes the rover ''Perseverance (rover), Perseverance'', the now-retired small robotic helicopter ''Ingenuity (helicopter), Ingenuity'', and associated delivery systems, as part of the Mars Exploration Progra ...

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

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

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

'' 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 produce and deploy, 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 2012 in science

Background

Function

History

Design and specifications

Usage in space missions

Cost

See also

References

External links