Radioisotope Power Systems For Extended Lunar Science Investigation
The Moon’s surface environment offers a significant challenge for most space systems and, particularly, for power technologies. Equatorial diurnal temperatures range from about 400K at noon to 100K during the night. Craters and permanently shadowed regions can experience a constant 40K and below. The length of the lunar day is about 708 hours with 354 hours of constant sunlight and 354 hours of darkness. Polar regions can range from total darkness in deep polar craters and at higher elevations, potentially nearly continuous sunlit conditions are possible. However local topographical features such as outcrops or more distant mountains and ridges would obscure the Sun even in these extended sunlit areas and thus create intermittent shadowed periods depending on Sun angle and the obscuring topographical features. Thus, the power system technology that a mission selects becomes a critical trade. Surviving the long lunar night poses a significant challenge for photovoltaic arrays and energy storage systems. Providing the energy required for both nominal operations and electric heaters to maintain keep-alive temperatures for electronics and other vital systems during the lunar night reduces the amount of payload dedicated to science and exploration investigations. However, radioisotope power systems (RPS) produce power by converting the heat produced by natural isotopic decay into electricity. The plutonium-238 (Pu-238) fuel source offers a high energy density along with an 88-year half-life. NASA has flown multiple versions of the Radioisotope Thermoelectric Generator (RTG) technology using Pu-238 for decades to distant planets and the planetary surfaces of the Moon and Mars. While the extreme range in lunar temperatures and long duration diurnal periods pose an extreme environment for conventional power system technology, RPS has proven their capability to operate for years in such conditions as proven by the Apollo mission Systems for Nuclear Auxiliary Power (SNAP) SNAP-27 RTG and more recently the Multi-Mission Radioisotope Thermoelectric Generator (MMRTG) currently powering the Curiosity rover. An even greater challenge is a lunar polar mission to provide ground truth of specific locations, concentrations, depth, etc. of potential water ice and other volatile deposits as indicated from orbiting instruments. RPS technology is uniquely capable to provide the required electrical and thermal power for the multi-year exploration timelines likely required to fully perform such a mission. It is anticipated that significant amounts of heat will be required for rover mechanical subsystems and electronics. The constant heat produced from isotope decay can supply this heat thus reducing the electric power demands of electric heaters. Many processes for the extraction of volatiles from the regolith requires heat, therefore the waste heat available from the RPS reduces the electric power requirement. Advanced RPS systems using higher efficiency dynamic conversion technology can produce about four times the electric power of the conventional RTG from the equivalent quantity of radioisotope fuel. Several advanced technology concepts continue to be in development and their features will be discussed as well as current and future RTG concepts.