Our objective is to search for the putative “lunar horizon glow” (LHG) reported during the Apollo-era that was possibly caused by the forward scattering of sunlight by ≈0.1μm-scale dust in the lunar exosphere. More recent searches for LHG, and associated dust population, have only produced upper limits that are significantly lower than the previously inferred abundances. Determining how dust behaves in the lunar environment has important implications for both science and exploration activities on the Moon. Our LHG search was undertaken using one of the star tracker navigation cameras (ST2) on the Lunar Reconnaissance Orbiter (LRO), which is capable of producing images suitable for scientific analysis. The advantage of using ST2, compared with instruments used in recent searches from orbit, is that it offers much better spatial resolution, the ability to probe to lower altitudes, and has a wavelength sensitivity similar to Apollo-era observations. It is also highly complementary to multi-wavelength measurements from other instruments. Absolute photometric calibration of ST2 images is achieved by comparison with concurrent measurements from the SOHO LASCO/C3 coronagraph of coronal and zodiacal light (CZL) observed by both instruments.
Over 30 LHG search operation activities have so far been planned and executed; although some of the early activities were used to learn how ST2 could be re-purposed for science. Many high-quality images were acquired, and in some cases patches of excess brightness were observed just above the limb. These patches were limited in spatial extent to