MMRTG | Multi-Mission Radioisotope Thermoelectric Generator | Facty Universe

For every space exploration program, there is a need of power source to supply electricity and heat for many instruments and systems in the spacecraft. For a successful, reliable and long lasting power supply, NASA scientists have created a power system known as RPS i.e. Radioisotope Power System which can produce electricity as well as heat for up to 15 years continuously.

Model of MMRTG including GPHS, Credit: Department of Energy / NASA

There are several types of RPS have been tested by NASA scientists till date for a safe, reliable and long lasting power supply and currently NASA is using MMRTG technology i.e. Multi-Mission Radioisotope Thermoelectric Generator. It was flexibly designed for working and operating within the atmosphere of a planet such as Mars as well as in vacuum such as in various spacecrafts. Rovers such as Curiosity and Perseverance equipped with MMRTG technology and spacecraft like Cassini, New Horizon, Voyager 1 and  Voyager 2 are equipped with RPS and still providing power and sending data back to Earth even if they have reached outside of the Solar System. RPS system doesn't use power from Sun thus it can be and is being used for Deep Space Missions.

How does an MMRTG works?

MMRTG convert heat energy into electrical power, it consist of a heat source isotope (usually Plutonium-238) and a set of thermocouples. It works on the principle of Seebeck Effect that says that if two dissimilar, conductive material connected in a close circuit an the two junction points are being kept at different temperature (one is hot and another is cold) then it produces a potential difference between two junction and this device is known as thermoelectric couple or thermocouple.

It uses Plutonium isotope decay heat energy at one junction point of thermocouple and another junction kept in space or in atmosphere of having relatively low temperature, this temperature difference causes the production of electricity. MMRTG can produce approximately 110 watts of power at the launch and can run up to or more than 17 years, at the end of it's life it can maintain the power output of more 72 watts.

How much does an RTG cost?

It cost $109 million to manufacture an MMRTG as the Plutonium core itself being so costly around $5600 per kilogram and we are using around 5 kg of Plutonium in a single MMRTG unit, its safety and design is also complicated hence required a high amount of cost to cover the Plutonium core with various layers of Iridium metal cladding, graphite sleeves etc. to prevent the leak of radiation that can harm anyone.

Design and Safety of MMRTG

GPHS illustration
Credit: Department of Energy / NASA

MMRTG has dimensions of 25 inches (64 centimeters) in diameter and 26 inches (66 cm) tall and has 49 kilograms of weight that consist of 4.8 kilograms of Plutonium dioxide as fuel. While RPG/RPS never been a cause of either any type of spacecraft accident or death of an astronaut, it has been observed to perform well as according to it's design. Many layers of different safety features have been used to prevent the leak and release of isotope residue such as ceramic form of Plutonium dioxide, graphite sleeves, iridium metal cladding and a carbon fiber material that forms an aero shell (a kind of box) of eight GPHS (General Purpose Heat Source). GPHS generally protects MMRTG mechanism from accidental scenarios and impacts while reentry.

Technical Specifications

Credit: NASA / Department of Energy