The objective of ODC was to capture and return analyzable residues of the man-made and natural particulate environment in low-Earth orbit for a detailed assessment of its composition and potential origins. The JSC scientific team has recently published the results of their findings from ODC in Optical Analysis of Impact Features in Aerogel from the Orbital Debris Collection Experiment on the MIR Station.
ODC exposed highly porous, low-density (0.02 g/cm3) SiO2 aerogel as the basic collector medium. Based on laboratory impact simulations by a number of groups, this material is ideally suited to gently decelerate and capture hypervelocity particles, as demonstrated by unmolten remnants of silicate and aluminum particles fired at velocities as high as 7 km/s.
This capability offers a significant improvement over traditional, comparatively dense collector media, including those employed on the Long Duration Exposure Facility (LDEF). The latter resulted in pervasive melting, if not complete vaporization of many impactors, leaving little or no residue for analysis. The expectation was that ODC would return a larger number and wider diversity of particles than all previous collection efforts in low-Earth orbit.
ODC exposed two identical trays, Tray 1 nominally pointing into the ram direction, Tray 2 in the opposite direction. The macroscopic survey of all impact features > 3 mm revealed that Tray 1 was dominated by low-velocity waste impacts, ~78%, in comparison to 25% on Tray 2. A high track abundance on Tray 2 was affected by discrete clusters of tracks, all of the same orientation (azimuth and inclination), suggesting that they may have resulted from a swarm of secondary projectiles from a local, primary impact.
Harvesting and compositional analysis of individual particles is tedious; therefore, significant effort went into the development of suitable techniques, minimizing the inadvertent loss of particles typically 10 microns or smaller. The compositional analyses, using a scanning electron microscope with energy dispersive spectrometers, concentrated on a survey-type inventory of diverse particle types and associated impact features.
Among the man-made particles detected were metallic aluminum, stainless steel, soldering compounds, and paint flakes. The swarm event was apparently due to some natural impactor, containing Fe, Mg, and Ca, which must have fragmented on impact with a neighboring structure on Mir.
In summary, the optical analysis of the Mir collectors has been completed, as has the survey-type assessment of man-made or natural classes of particles. Although ODC observations suggest that the utility of aerogel for the capture of hypervelocity particles may be velocity limited, its performance is vastly superior to traditional, non-porous media. Hundreds of impactor residues were returned by ODC. Future ODC efforts will concentrate on the compositional analysis of a statistically significant fraction of these particles and an improved assessment of their origins.