Measuring hypervelocity impact velocity from micrometeoroid crater geometry

Guided by half-space computer simulations showing hypervelocity impact crater formation for an iron particle impacting an aluminum target and characteristic crater geometry changes with impact velocity over the range 8–40 km s⁻¹, we examined normal surface crater views and cross-sectional views through craters (>0.5 mm diameter) from samples retrieved from the NASA LDEF satellite and examined in the scanning electron microscope (SEM). While geometrical features suggested in the computer simulations were indeed observed for micrometeoroid craters in 6061-T6 aluminum targets and 303 stainless steel targets, there was no consistent estimate for impact velocities in any of the experimental samples, and velocity estimates based on measuring ratios of ejecta width/crater diameter and ejecta height/crater depth as well as ejecta height/crater diameter varied from 8 to 42 km s⁻¹; over the same range simulated. These results point to the need to create reference data from actual hypervelocity impact experiments in the laboratory, and systematic observation of residual crater geometries in the SEM. These experiments also demonstrate the uncertainty in assuming a fixed impact velocity for all impact craters in space materials as well as an apparent futility in attempting to correlate impacting particle velocity with post-mortem characteristics of a given crater.