Soft capture of earth-orbiting hypervelocity particles with aerogel

The Orbital Debris Collection (ODC) experiment exposed ∼ 0.6 m2 of SiO2- based aerogel (0.02 g/cm3) for 18 months on the US Docking Module of the Mir space station. Post-flight inspection revealed hundreds of millimeter-sized hypervelocity features, ranging from deep penetration tracks to rather shallow pits. Intermediate morphologies suggest that deep tracks and shallow pits are the end members of a morphological continuum that seems controlled by impact velocity. Some 163 particles were recovered from the termini of deep tracks. Scanning (SEM) and Transmission Electron Microscopy (TEM) suggest that shock and thermal alterations are minimal and that individual phases and their textures are well preserved. SEM investigations combined with Energy Dispersive X-Ray Spectroscopy (EDS) reveal a wide variety of natural and man-made particle types, akin to those from other space-exposed surfaces. Very shallow aerogel depressions contain human-waste products. The transparent pits, ∼ 20% of all hypervelocity features, contain no residues detectable with SEM-EDS methods, suggesting that the utility of aerogel to soft capture hypervelocity impactors is velocity-limited. This velocity threshold is estimated at ∼ 18–20 km/s for the ODC aerogel.