The LCROSS Impact Cratering Experiment

The large Earth-Departure-Upper Stage (the “EDUS”) and the LCROSS Shepherding Spacecraft (SSc) will both slam into the permanently shadowed regions near the lunar south pole on October 9, 2009. The goal of this mission is to excavate possible ice buried below the surface, thereby providing a measure of potential reservoirs of water for future human exploration. Impact experiments at the NASA Ames Vertical Gun Range (AVGR) have contributed to the mission design and planning. These experiments have included predictions for target selection (Schultz, 2006), a re-assessment of excavation at early times (Hermalyn and Schultz, 2009), and excavation depths (this study). Such predictions are critical for designing instrument sensitivity/selection for the SSc and earth-based telescopic observing campaigns. Because the EDUS has an effective low density (with concentrations at two ends), we have explored the effects of impactor density and configuration (hollow, solid) on the early-stage cratering process, including excavation depths. Most ejecta scaling studies use loose quartz or flint-shot sand in order to track late-stage excavation scaling. This approach does not work well at earlier stages, which comprise a greater fraction of growth at larger scales (see Hermalyn and Schultz, 2009; Hermalyn and Schultz, this volume). Experiments using solid and hollow aluminum spheres impacted a variety of target types (fine and coarse sand, fine pumice, and JSC-1a) in order to assess their effect on this earlier stage of crater growth. Tracers were placed at different depths allowed tracking of excavation. Results have direct implications to the LCROSS experiment and observations (after appropriate scaling). First, the effective low-density impactor significantly reduces excavation depths to a projectile diameter or less, even in sand. This becomes more important for regolith-like targets since the hollow projectile collapses and target compression prevents deep penetration. Second, a high-angle ejecta plume emerges from the center. This is not due to the simultaneous impacts by the two heavy mass ends of the EDUS (described by Korykansky et al., 2009) but a result of projectile implosion and cavitation during the penetration stage. Third, projectile fragments survive and are scattered within and around the resulting crater. Fourth, the impact flash and thermal decay observed by the trailing SSc can be related to the luminous efficiency and can be used to assess initial coupling process, including the presence of near-surface volatiles (Ernst and Schultz, 2008). The goal of this effort is to derive the depth of buried volatiles based on evolution of the impact flash, sequence of ejecta emergence into sunlight, and evidence of a high-angle plume.