Multi-Mission Radioisotope Thermoelectric Generator
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.
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.
Design and specifications
The MMRTG is powered by 8 Pu-238 dioxide GPHS modules, provided by the Department of Energy. Initially, these 8 GPHS modules generate about 2 kW thermal power.
The MMRTG design incorporates PbTe/TAGS thermoelectric couples (from Teledyne Energy Systems). 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
Radioisotope power has been used on 8 Earth orbiting missions, 8 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, Ulyssess, Galileo, Cassini and Pluto New Horizons missions. The RTGs on Voyager 1 and 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 in the United States.
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 at least one Mars year (687 Earth days).
On Nov. 20, 2013, NASA reported suspending operations on the Mars Curiosity rover in order to diagnose an electrical problem first observed on Nov. 17. Apparently, an internal short in the rover's power source, the MMRTG, caused an unusual and intermittent decrease in a voltage indicator on the rover, though power output was unaffected. On Nov. 23, 2013, the short had cleared, and Curiosity resumed full science operations, with no apparent loss of capability.
- This article incorporates public domain material from the United States Department of Energy document "Space Radioisotope Power Systems Multi-Mission Radioisotope Thermoelectric Generator".[dead link]
- SNAP-19: Pioneer F & G, Final Report, Teledyne Isotopes, 1973
- Bechtel, Ryan. "Radioisotope Missions". US Department of Energy.
- Webster, Guy (November 20, 2013). "Rover Team Working to Diagnose Electrical Issue". NASA. Retrieved November 21, 2013.
- Staff (November 25, 2013). "Curiosity Resumes Science After Analysis of Voltage Issue". NASA. Retrieved November 25, 2013.
|Wikimedia Commons has media related to Radioisotope thermoelectric generators.|
- NASA Radioisotope Power Systems website – RTG page
- Idaho National Laboratory MMRTG page with photo-based "virtual tour"