Laboratory investigation of oblique hypervelocity impacts with relevance to in situ meteoroid and space debris detectors

Inferring the properties of the near-Earth meteoroid and space debris environments as sampled by in situ detectors and retrieved spacecraft surfaces requires understanding of the hypervelocity impact process. To infer the nature of an impact after the event, we need to establish relationships between the relevant impact parameters and the resulting impact features. This task is performed in the laboratory by controlled hypervelocity impact experiments using various acceleration techniques. A facet of understanding impact processes in space is the investigation of behaviour under impact from an oblique angle. Despite the fact that impacts normal to the target surface are the exception under real conditions, impact angle studies are often given low priority in investigation of a material's response to hypervelocity impact, with other parameters such as projectile size, velocity and density being initially studied at normal incidence. The author has identified, in his analysis of space-flown surfaces and previous applications of empirical relationships to space data, two areas where oblique impact studies are lacking leading to an uncertain interpretation of the near-Earth environment. The first of these areas is oblique penetration of thin aluminium targets as observed on capture cell detectors such as the EURECA-TICCE experiment. An experimental programme was performed using the University of Kent's light-gas gun to fire steel and aluminium ball bearings through aluminium plates, covering more angles and target thicknesses than similar previous studies. It is found that the method by which an empirical equation derived from normal impact studies has been modified for application to space data using an assumed angle dependence does not predict the laboratory data well. An alternative method of applying the same laboratory-derived equation is presented that more closely reproduces the oblique experimental data. This new method is shown to give a significantly different estimation of the size distribution of meteoroids and debris when applied to the EURECA-TICCE penetration record. The second area is oblique impacts on solar cells as observed on the EURECA and HST solar arrays. A second experimental programme using the light-gas gun was performed firing 50 f.lm soda-lime glass beads at solar cell samples over a range of impact angles from 0-75° from normal. It is found that, of impact crater features previously used as a guide to impact angle, only the pit circularity is primarily related to impact angle. It is also found that the conchoidal diameter, previously believed to have a power law dependence on impact angle, is insensitive to impact angle for angles less than 45° from normal and decreases in size linearly with the cosine of the impact angle for angles greater than 45° from normal. This experimental programme was extended to determine the survivability of the glass beads to launch in the light-gas gun and it was found that although it is likely that soda-lime glass beads reach the target intact, other commonly used small projectiles such as meteorite-analogue mineral powders do not.