The next generation radioisotope power system
Advanced Stirling Radioisotope Generators (ASRGs)
are a new type of RPS that is being developed by NASA and the Department of Energy. Like an RTG
, the ASRG converts heat energy to electricity, but it has moving parts. Inside the device, a moving piston is driven by the heat of the nuclear fuel source. The piston moves a magnet back and forth through a coil of wire more than 100 times per second to generate electrical current in the wire. To prevent physical wear, the piston is suspended in a helium gas bearing, meaning it does not actually touch the inside of the mechanism.
An ASRG features a power output of more than 130 Watts electric (or We) using two general purpose heat source (GPHS) modules - slightly more power than the more conventional MMRTG, which uses eight GPHS modules. With an operational efficiency goal of over 25 percent, the ASRG would be about four times more efficient than the more traditional MMRTG.
Like the MMRTG, the ASRG is designed to be modular, meaning that more than one unit could be used in different combinations depending on a mission's scientific and operational needs.
NASA is currently studying the first potential in-space use of the ASRG. No flight opportunity has yet been selected.
Why does NASA need an ASRG?
The energy conversion process used by an ASRG allows it to use about one quarter of the plutonium-238 used in previous radioisotope systems to produce a similar amount of power. This greater efficiency helps extend the limited U.S. supply of this special material. Like the MMRTG, the ASRG is designed to work in the atmosphere of planets like Mars, as well as in the vacuum of deep space. Both the ASRG and MMRTG could be used alone or in different multiples, depending on a proposed mission's scientific and operational needs. For example, the ASRG will be lighter than an MMRTG, while the MMRTG can supply more heat for mission systems that may benefit from it.
About the size of a carry-on suitcase, the ASRG converts its total input heat of 500 thermal watts into about 130 electrical watts available to power spacecraft systems and instruments. The remaining 75 percent of the input heat could be used for keeping those systems and instruments at their proper operating temperatures. Unused heat is emitted through a rectangular outer housing on the ASRG featuring large radiator fins, and any excess electricity is shed through a set of electrical resistors called a shunt.
Who is building the ASRG?
The U.S. Department of Energy is building the ASRG for NASA, with key contributions from NASA's Glenn Research Center, Cleveland, OH, Lockheed Martin, Valley Forge, PA, and Sunpower Inc., Athens, OH.
Fact sheet: ASRG - Advanced Stirling Radioisotope Generator (PDF, 830 KB)