Imaging of Planetary Surfaces

There have been significant improvements in recent years in high-resolution imaging of planetary surfaces beyond Earth, improving both the utility and beauty of the images. Three examples are described here: the narrow-angle camera (NAC) of the Imaging Science Subsystem (ISS) of Cassini at Saturn [1], the High Resolution Imaging Science Experiment (HiRISE) on Mars Reconnaissance Orbiter [2], and the NACs of the Lunar Reconnaissance Orbiter Camera (LROC) on LRO [3]. Each of these cameras was designed according to the constraints and science objectives of each mission. The ISS-NAC, built at the Jet Propulsion Laboratory, is optimized for imaging a diverse array of targets, typically at large ranges, and with high priorities given to multispectral imaging (via two 12-slot filter wheels) and movies with high time resolution. ISS observes in framing mode (1024 × 1024 pixels, 0.35° field of view, 6 μrad/pixel).  In contrast to Cassini, MRO and LRO are in tight circular orbits suitable for pushbroom imaging to optimize spatial resolution. MRO orbits at an altitude near 300 km, about as low as practical for a circular orbit given the Mars atmosphere, whereas LRO orbited near 50 km altitude during its primary science phase and is now close to a 40 x 190 km orbit.   Although the instantaneous fields of view (IFOV) differ by a factor of 10 (1 μrad/pixel for HiRISE; 10 μrad/pixel for LROC), the mapping scales are similar (0.3 m/pixel for HiRISE at 300 km range; 0.5 m/pixel for LROC at 50 km). The FOVs are 1.14° for HiRISE and 2.85° for each of two LROC NACs for a combined FOV of 5.58°. Maximum images sizes are 20,048 × 118,000 pixels for HiRISE and 5,000 × 52,200 pixels for each LROC-NAC. HiRISE, built at Ball Aerospace and Technology Corp., uses time-delay integration (TDI) with up to 128 lines to achieve a high signal:noise ratio, essential for high-quality imaging through the dusty atmosphere of Mars, but which places tight constraints on spacecraft pointing stability.  HiRISE acquires 3-color coverage over 20% of the swath width via additional CCDs with color filters.   All 3 imaging systems have acquired image sequences or movies of dynamic phenomena at Saturn and Mars or changing lighting conditions over the Moon.  [1] Porco, C.C. et al., 2004, Space Science Reviews 115, 363-497 [2] McEwen, A.S. et al., 2007, J. Geophys. Res. 112, E05S02. [3] Robinson, M.S. et al., 2010, Space Science Reviews.