Hypervelocity impacts in low Earth orbit: Cosmic dust versus space debris

The understanding of the micron-sized populations of natural micrometeoroids and artificial space debris in low Earth orbit has benefited considerably from the post-flight investigations of retrieved surfaces from spacecraft, such as the Long Duration Exposure Facility. The returned solar array from the Hubble Space Telescope has added to this repository and has offered a further opportunity to document these particles. 25 individual solar cells were specially selected on the basis that they contained impact craters (diameter 100-1000 mu m) which had the most potential to retain impactor residue chemistry. The solar cells were subject to a detailed investigation using analytical scanning electron microscopy which identified 29 impact craters, the analysis of which identified 3 residues as artificial in origin, 6 unclassified and 20 as natural in origin. The limited number of unclassified residues identified indicates that the methods of analysis employed in this investigation are a significant step forward for such studies and, if employed on a greater number of samples, will improve the calculations of the time-integrated flux rates for micrometeoroids and space debris in the low Earth orbit environment. Notwithstanding the small sample set examined, the observed chemical classification of the impact residues in terms of micrometeoroid to space debris (in the particle size range 8-80 mu m) corresponds well to the flux model that predicts the dominance of natural particles.