Lunar South Pole Illumination: Review, Reassessment, and Power System Implications

[Abstract] This paper reviews past analyses and research related to lunar south pole illumination and presents results of independent illumination analyses using an analytical tool and a radar digital elevation model. The analysis tool enables assessment at most locations near the lunar poles for any time and any year. Average illumination fraction, energy storage duration, solar/horizon terrain elevation profiles and illumination fraction profiles are presented for various highly illuminated sites which have been identified for manned or unmanned operations. The format of the data can be used by power system designers to develop mass optimized solar and energy storage systems. Data are presented for the worse case lunar day (a critical power planning bottleneck) as well as three lunar days during lunar south pole winter. The main site under consideration by present lunar mission planners (on the Crater Shackleton rim) is shown to have, for the worse case lunar day, a 0.71 average illumination fraction and 73-117 hours required for energy storage (depending on power system type). The illumination at this site for each lunar day during a year varies dramatically, with as many as seven lunar days with negligible shadowing (i.e. maximal illumination/very little energy storage required). The maximum duration shadowing period for this site is primarily due to distant high terrain in the Malapert Mountain region (from 84 to 86 S, -10 to +45 E). Two potential sites with higher average illumination fraction and lower energy storage hours than the Shackleton site are shown to possibly have erroneously high site heights. In addition, a site at the Malapert Mountain peak counter-intuitively had a much lower average illumination fraction and much higher energy storage hour range, due primarily to nearby mountainous high terrain. This paper shows that by increasing the Shackleton site height by 100 m using a tower reduces the number of energy storage hours by 15-21%, although whether this is a mass optimized solution for a power system awaits further analysis. Completely eliminating energy storage through the use of practical tower heights does not appear feasible due to the nature of the shadowing terrain. Linking the Shackleton site with one ~10 km away was shown to improve the average illumination fraction from 0.71 to 0.84 and reduce the energy storage hours from 117 to 68 hours. Again, this may not be a mass optimum power system solution due to either heavy power beaming equipment or power cables (compared with simply increasing the energy storage size at the site). Linking other sites and including towers at both sites are shown to not completely eliminate the need for energy storage.