Numerical simulation of grazing impacts from micron sized particles on the XMM-newton mirrors

Abstract

This paper presents the results of numerical simulations of micron-sized particles hypervelocity impacts on the XMM telescope mirror surfaces at grazing angles. The scope of the study was to demonstrate the possibility that very high energy peaks detected by the Charge Coupled Devices (CCDs) placed at the end of the mirror surfaces were caused by micrometeoroid impacts. First simulated were tests performed with the dust accelerator of Max Planck Institut fur Kernphysik (Heidelberg, Germany) at impact velocities around 5 km/s and impact angles between 1 and 5 degrees from the mirror surface plane. The results showed the projectile leaving the target plastically deformed but not fragmented with an exit angle much smaller than the entry one, thus confirming the experimental evidence that some material travelling after the impact close to the target surface reached the location of CCDs. Additional simulations with iron projectiles were performed at increasing impact angles up to 45° from the mirror surface plane. They showed that the amount of material leaving after impact nearly parallel to the mirror surface progressively diminishes due to the onset of projectile fragmentation and scattering, but some material is still found even for high impact angles. This effect is probably due to the high density of the gold coating layer of the XMM mirror, as it was not observed in analogous simulations with a softer aluminium coating. In order to better approximate the conditions of a real natural meteoroid impact, simulations at I -degree incidence and velocities up to 30 km/s were performed with aluminium projectiles, whose density is closer to the expected values for meteoroids in space. The results showed that the velocity increase induces more projectile erosion phenomena in the contact zone with the target, further reducing the exit angle. Above 20 km/s the projectile exit path is practically parallel to the mirror surface, thus supporting the hypothesis that a micrometeoroid impacting at grazing angle on the mirror surface can find its way up to the XMM CCDs focal plane.