Radioisotope Power Systems (RPS) have been a reliable and durable source of electricity for NASA solar system exploration missions from the dusty surface of Mars to the frigid realm of the outer planets.
An RPS consists of two basic elements: a hot fuel source provides heat to a power convertor, which transforms this infrared energy into electrical power.
The joint RPS Program conducted by NASA and Department of Energy (DOE) currently manages two power units intended for use in space. The flight-qualified Multi Mission Radioisotope Thermoelectric Generator (MMRTG) uses the temperature difference between its hot nuclear fuel source and the cooler outside environment to generate electricity. This is done using metallic junctions called thermocouples, which have no moving parts; similar technology (minus the fuel source) is used on consumer products such as hot water heaters and natural gas ovens. RPS similar to the MMRTG have flown safely and successfully on a wide variety of solar system exploration missions for more than four decades.
A second RPS unit now in-development by NASA and DOE is called the Advanced Stirling Radioisotope Generator (ASRG). The ASRG uses a Stirling engine cycle to generate an alternating current of electricity from a moving piston. This piston is driven by the expansion and contraction of an internal gas heated by a similar nuclear fuel source as used the MMRTG. However, the ASRG can produce electricity about four times more efficiently than the more traditional MMRTG.
This greater efficiency allows an ASRG to use only two General Purpose Heat Source (GPHS) modules, compared to the eight modules in an MMRTG, to produce a similar amount of power (130 watts for an ASRG versus 110 watts for an MMRTG). The ASRG is undergoing rigorous qualification testing to be ready for its first flight as soon as 2016.
Advanced Technology for Future RPS
Beyond the ASRG, new technology research and development supported by the RPS Program aims to further increase the efficiency, operational flexibility and scalability of these power sources to provide a wider variety of options for future science missions.
Such research outcomes would significantly enhance our ability to explore some of the most extreme environments imaginable beyond Earth, and should enable more effective use of the limited domestic supply of plutonium-238, the fuel used in all U.S. RPS-powered missions to date.
The RPS Program is currently investing in three energy conversion research areas, with varied goals and objectives, depending on the technology category: thermophotovoltaics, advanced Stirling systems, and improved materials and convertors.
Advanced Thermoelectric Materials and Converters
The RPS program is supporting research and testing of a variety of new advanced materials designed toward the goal of doubling the electrical conversion efficiency and specific power of future radioisotope thermoelectric generators (RTGs), using about half of the plutonium-238 required for the current MMRTG.
The objective of this effort is to develop new materials that are capable of doubling the thermoelectric efficiency of the best-performing materials used today by using complex hybrids of different compounds assembled thanks to the precise techniques available through nanotechnology. Eventually, a factor of four times improvement in efficiency may be possible.
One example of such materials are advanced blends of silicon germanium and lead telluride, which have demonstrated their ability to meet technical and operational goals, such as reliable reproducibility. Once proven, such materials would be integrated in an Advanced Thermoelectric Couple (ATEC) toward the goal of developing thermoelectric couples capable of providing 10 percent energy conversion efficiency and 6-8 Watts per kilogram of specific power for a 17-year lifetime.
Summary of Progress
Overall, the current RPS Program technology research investments have been reviewed and confirmed by external experts to have clear, realistic development objectives, with measureable evidence of progress within a reasonable development timeline to support the integration of these technologies into flight systems for potential missions by NASA and its partner DOE in a timely way.
New proposals will be solicited approximately every four years to keep the program on the cutting edge of RPS-related technology using a mix of directed and competitively selected projects, with the next major round targeted for 2014.