Effect of Initial Conditions on Impact Flash Decay

Introduction: Light flashes due to thermal heating and vaporization are produced during hypervelocity impact events. The shape of such a flash's light curve may be used to determine certain initial conditions of the impact. The measured flash peak intensity depends on the impact velocity and angle and varies with target and projectile type, as shown in previous studies [1,2]. Here, we examine the dependence of the flash decay curve on initial conditions for macroscopic impacts. Experiments: Laboratory experiments performed at the NASA Ames Vertical Gun Range (AVGR) allow the use of particulate (as well as solid) targets that more closely simulate the surfaces of planetary bodies. The experiments were performed in near-vacuum conditions (< 0.5 Torr) in a chamber large enough to allow the free expansion of the impact plume and ejecta without interference from chamber walls [3]. Previous photometric studies used only solid target materials and the analysis of the evolution of the light curves was limited to shorter times due to interactions with the walls [4]. A photodiode system with a rise time of 40 ns and a spectral range of 350 – 1100 nm recorded the impact flash signatures from above and from the side. Particulate (pumice dust, sand, sieved perlite, and powdered dolomite) and solid (solid pumice blocks, and frozen water-saturated perlite with a coherent crust) targets were impacted by Pyrex and copper spheres (0.318 – 0.635 cm, 0.299 g and 0.318 cm, 0.150 g respectively) with velocities between 4.05 – 6.14 km/s. Impact angle ranged from 30 – 90 degrees (from the horizontal). Under these conditions, Pyrex fails completely on impact. Analysis: A sample flash curve for a Pyrex impact into pumice is shown in Figure 1. Two components can be seen, which may represent two superimposed curves: an intensity peak lasting from 50 – 100 ms, and a long-lasting decaying blackbody signal. This first component represents a light curve signature of the projectile depending on impactor velocity and angle [1], as well as size, density, and composition. The thermal plume produced by the impact emits the long-lasting signal, and is related to the target. Trendlines fit to the decay curves show a power-law dependence on time where I ~ t-a. Sample decay curves and their decay exponents (a) are shown in Figure 2. Results: The value of a is constant for a given set of target, projectile, and viewing conditions but changes with varying …