Solar System Exploration: Science & Technology: Power & Propulsion: View Feature

Enabling Exploration: Small Radioisotope Power Systems

Editor's Note : This is an excerpt from the summary of 'Enabling Exploration with Small Radioisotope Power Systems,' a report prepared by the Jet Propulsion Laboratory, California Institute of Technology and the Department of Energy through an agreement with NASA.




	Cover image of the report Enabling Exploration with Small Radioisotope Power Systems

Radioisotope Power Systems (RPSs) generate electrical power by converting the heat released from the nuclear decay of radioactive isotopes into electricity. First used in space by the United States in 1961, these devices have consistently demonstrated unique capabilities over other types of space power systems for applications up to a kilowatt, and studies have indicated that these benefits may extend to at least applications requiring up to several kilowatts [1]. A key advantage is their ability to operate continuously, independent of orientation to and distance from the Sun. Radioisotope systems are also long-lived, rugged, compact, highly reliable, and relatively insensitive to radiation and other environmental effects. As such, they are ideally suited for missions involving autonomous operations in the extreme environments of space and planetary surfaces.

Twenty-eight U.S. space missions that have safely flown radioisotope energy sources since 1961. Four of these missions flew radioisotope heater units only, whereas twenty-one successfully used RPSs to produce power for scientific instruments and spacecraft operations. Some of the most notable RPS flights are the Apollo lunar missions, the Viking Mars landers, and the Pioneer, Voyager , Ulysses , Galileo and Cassini outer planetary probes. The different RPS units that the U.S. has developed and flown over the years have all provided electrical power levels ranging from ten to several hundred watts. The current system, the General Purpose Heat Source (GPHS)-Radioisotope Thermoelectric Generator (RTG), typically generates 285 watts of electrical power (We) at the beginning of its life (BOL). Two new units currently under development, the Multi-Mission RTG (MMRTG) and Stirling Radioisotope Generator (SRG), will each provide at least 110 We at BOL.

The increased use of smaller spacecraft over the last decade, in combination with studies of potential science applications, has suggested the need for RPSs yielding much lower power levels. Radioisotope generators lend themselves to small, long-lived power applications mainly because the rate of heat production per unit mass of fuel is independent of the size of the system. In fact, development of generators with power levels as low as tens of milliwatts appears to be quite feasible using existing plutonium-238 (Pu-238) heat sources and thermoelectric energy conversion technology. Such RPS power supplies have the potential to extend the capability of small science payloads and instruments, and to enable applications such as:

Long-lived meteorological/seismological stations broadly distributed across planetary surfaces
Small landers at extreme latitudes or in regions of low solar flux
Surface and atmosphere-based mobility systems
Subsurface probes, including impactors and autonomous boring devices
Deep space micro-spacecraft and sub-satellites

Such units could also find application in future human exploration missions involving use of monitoring stations and autonomous devices, similar to the ALSEP units deployed on the Moon during the Apollo program.

Although flight-qualified RPS units in this size and power range do not presently exist, their potential to support a broad range of exploration tasks has led NASA and the Department of Energy (DOE) to consider the development of small-RPS units such that they might be available for missions by the early part of next decade. Starting in 2003, NASA's Office of Space Science and DOE convened a series of studies and technical interchange meetings to review the potential applications, associated requirements, and methodology for pursuing development of small-RPS.

The results of initial surveys and studies point to many scientifically valuable mission concepts and applications that could benefit from or be enabled by small, reliable, long-lived RPSs. An important assumption in all the efforts to date has been the use of existing plutonium fuel capsule designs, since testing and evaluating new fuel configurations would likely impose excessive costs on any future development. This led to the categorization of small-RPS according to the power level groupings shown in Figure. 1-1. The upper end of small-RPS power capability (10 to 20 watts (We)) was best achieved by designing systems around single General Purpose Heat Source (GPHS) modules, which are the thermal building blocks used in current RTGs. The lower end of power output (40 to 100 milliwatts (mWe)) could be met with systems based on one to several Radioisotope Heater Unit (RHU) fuel capsules. The mid-range (100 mWe to several watts) could be accommodated by a number of options, including heat source assemblies composed of multiple RHU fuel capsules and fractional GPHS units based on one or two GPHS fuel capsules. Thermoelectrics were considered to be the most viable technology for thermal-to-electric power conversion, although Stirling cycles could offer unique advantages at the lower and middle ranges of power.