EXPERIMENTAL STUDIES OF EJECTA KINEMATICS AT THE NASA AMES VERTICAL GUN RANGE

The cratering process can be divided conceptually into three stages: the penetration or compression stage, the excavation stage, and the modificationstage. The excavation stage is a result of the subsurface shock and rarefaction wave propagation through the target material, is responsible for the redistribution of material on a planetary surface during an impact, and has implications for planetary sampling strategies. With the ability to control the initial impact conditions, the laboratory is the optimum setting in which to explore the excavation process and provide ground-truth for numerical models, scaling relationships, and the interpretation of planetary ejecta deposits.

Using the NASA Ames Vertical Gun Range, Peter Schultz was instrumental in developing non-invasive techniques for measuring the excavation process in real time during experimental impacts. Three-Dimensional Particle Imaging and Tracking Velocimetry (3D PIV or PTV) techniques use a laser plane to illuminate ejecta particles moving within the expanding ejecta curtain. Images, taken at high temporal resolution, of these illuminated particles are used to determine the current location and speed of small groups of ejecta particles in flight. These ballistic measurements are then regressed back to the target surface to yield a map of ejection position, speed, and angle over all azimuths around the impact point for both vertical and oblique impacts.

These investigations demonstrated that initial asymmetries due to impact angle persist through the first half of crater growth, even at impact angles of 45 degrees. Further, the center of the subsurface flow-field is not a single, stationary point beneath the target surface even for vertical impacts, in contrast to assumptions made in point-source models. Recent technological developments have permitted high-speed extension of these techniques to allow investigation of ejecta kinematics and the subsurface flow-field at very early times (the first 10% of crater growth). In this presentation, we discuss these novel techniques and their applications to planetary cratering and advances in our understanding of ejecta kinematics.