A Fortuitous Impact Experiment at Mars

Introduction: The Mars Science Laboratory (MSL) reached Mars on August 6, 2012. Shortly before crossing the atmospheric interface – which marks the beginning of the entry, descent, and landing (EDL) phase of the mission – the spacecraft ejected two pieces of tungsten ballast, each weighing 75 kg. Collectively, these ballast are called the Cruise Balance Mass Device (CBMD). Unlike the rovers, the CBMD did not benefit from a sky-crane enabled soft touchdown with the surface, and they proceeded on an intersect course with the Red Planet. Unlike impacts that formed the innumerable other craters on Mars, for which we do not know the speed, angle, and material properties of the impactor, we can estimate these things for the CBMD. Because an impact crater shape is due to both impactor and target properties, researchers typically have to make assumptions about either the impactor or target to infer properties about the other. For this case, however, we do know the impactor properties, enabling us to infer accurate target properties by careful examination of the crater and its ejecta. This also enables us to help calibrate the impact models for oblique impact angles, and provide a known data point at impact energies beyond those available in lab-based impact experiments. Unexpectedly, however, MRO images revealed not just two new craters, but four fresh craters between 4-5 m diameter, and a strewn field of numerous smaller craters, see image below. We assume that two of the larger craters resulted from the CBMD, and the remaining large craters and numerous smaller ones resulted from a disrupted cruise stage, since no other candidate new craters have been found over a 50-km wide region. Taking into account differences in the separation procedure for the Cruise Stage, and for CMDB and their dramatically different aerodynamic behavior during the atmospheric passage, the coincidence of impact sites is astonishing. Here we briefly describe the CBMD, the craters formed, and initial interpretation of the Martian surface at the impact sites. CBMD and Impact Conditions: The CBMD are 75 kg, roughly wedge-shaped objects. They were ejected from the aeroshell with ≥ 1.5 m/s speed, about 5 min prior to entry. To estimate impact location, we ran simulations (using Satellite Tool Kit, or STK) of the CBMD trajectory from the point of release to impact. For this initial analysis, we utilized simplifications and considered a few cases, including various combinations of drag, ejection speed and ejection orientation of the CBMD relative to MSL. Depending on whether the mean ejection angle of the CBMD was ±z (up or down), or ±y (left or right) to the direction of motion, for a 1.8 m/s ejection speed, the pair of simulated impacts can be as close together as ~160 m or as far apart as ~ 1 km, similar but slightly more constrained than the predicted separations found in [1]. Table 1 lists the distances between each of the four “large” craters relative to one another. Based on the STK simulations, crater A is too distant from the others to be one of the CBMD impacts, leaving craters B, C, and D as candidate craters caused by the CBMD. Impact Conditions. Although our rough, initial simulations cannot exactly match the observed impact sites, some of the cases are within a few km, a small enough difference that the impact speed and angle of our simulations should approximate the actual impact conditions. The best match case impacts with at least