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Mir Orbital Debris Collector Data Analyzed

[From Vol. 4 No. 2 issue of Orbital Debris Quarterly News, April 1999.]

Mir Orbital Debris Collector Data Analyzed
By F. Horz
http://sn-callisto.jsc.nasa.gov/newsletter/v4i2/v4i2.html

The Mir Environmental Effects Package (MEEP) was deployed on the Mir space
station by STS-76 and retrieved (right) by STS-86 after an 18-month exposure
in orbit. The payload, managed by NASA Langley Research Center, included the
Orbital Debris Collector (ODC) that was designed and built at JSC. 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.

[Image caption:
http://sn-callisto.jsc.nasa.gov/newsletter/v4i2/v4i2.html#news1]
                  
Detailed analysis of a man-made particle. Note the cone-in-cone structure
and the misalignment of the last few cones with the overall track axis. The
recovered particle exhibits a highly irregular, if not jagged and sharp-edged
surface, suggesting that very little rounding, much less melting, occurred
during the capture process. The dominance of Ti and Zn identifies this particle
as paint (pigments), with with other elements being part of the organic binder.

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