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Macroscopic Inspection of the
Orbital Debris Collection Experiment
Friedrich Hörz, Glen Cress, Mike Zolensky
Earth Sciences and Solar System Exploration Division
SN, NASA Johnson Space Center
Houston, Texas 77058
Thomas. H. See, Ronald P. Bernhard,
and Jack L. Warren
C23, 2400 NASA Road 1
Houston, Texas 77058-3711
Detailed analysis of hypervelocity impact features on returned materials from the Solar Maximum mission (e.g., Warren et al., 1989) and the Long Duration Exposure Facility (LDEF; see Levine, 1993), revealed the feasibility to differentiate, on compositional grounds, among man-made and natural impactors. Indeed, a variety of particle subclasses could be established, such as paint flakes, metallic aluminum, aluminum-oxide (and others) in the case of man-made debris, or natural objects of bulk-chondritic composition, as well as monomineralic silicates and sulfates (e.g., Mandeville et al., 1991, Amari et al., 1992, Hörz et al., 1993). Of particular interest was the discovery of aluminum-rich particles on all LDEF surfaces, including the trailing edge (Bernhard et al., 1996) where encounters with man-made objects was not expected. These findings suggest prolific particle sources in highly elliptic orbits, generally consistent with transfer vehicles to geosynchronous orbits and associated effluents (Al2O3) from solid-fuel rocket motors (e.g., Kessler, 1992). Although LDEF exposed 86 instrument trays, each ~1 m2, these observations are confined to a single tray. This specific tray exposed pure gold substrates; most other LDEF surfaces suitable to investigate the hypervelocity particle environment in low-Earth orbit (LEO) consisted of aluminum. While it is possible to analyze for aluminum in the gold substrate, this cannot be done with collectors that are themselves composed of aluminum.
The need to expose non-aluminum surfaces in LEO appears obvious to improve on our understanding of aluminum-rich debris, the reason why the Orbital Debris Capture Experiment (ODC) was included as one of the four experiments composing the Mir Environmental Effects Package (MEEP). The ODC instrument employed a standard MEEP container that was hinged such that the two surfaces, Trays 1 and 2, respectively, could be exposed in opposing directions. For ODC, Tray 1 pointed in the general forward-facing direction, while Tray 2 faced in the rearward direction relative to the velocity vector of the Mir Space Station. Reconstruction of the specific exposure geometry relative to Mir is in progress.
The ODC experiment employed aerogel, made from pure SiO2, as the basic collector medium. This material has physical properties, foremost its very low bulk-density, (e.g., Fricke, 1988 or Hrubesch and Poco, 1990) that permit relatively gentle deceleration of high-speed particles (e.g., Barrett et al., 1992, Anderson and Ahrens, 1993; Tsou, 1995). Particles residing at the terminus of long, carrot-shaped penetration tracks were recovered in an unmelted state in laboratory impact tests at velocities as high as 7 km/s (Barrett et al., 1992, Mendez, 1994, Tsou, 1995, Burchell and Thomson, 1996, Hörz et al., 1997). Following a modest impact-test program (Hörz et al., 1997) we selected aerogel of 0.02 g/cm3 density for ODC, which seemed a reasonable compromise between the integrity of a captured particle and total stopping power of the collector. Obviously, higher-density aerogels cause higher shock-stresses and temperatures upon impact as well as more ablation. Lower-density aerogels would have to be extremely thick relative to our design goal of terminating a particle of 50 mm diameter (at normal incidence and at 7 km/s laboratory velocity). Aerogels <0.01 g/cm3 are not only difficult and costly to fabricate, but excessively cumbersome to handle and process.
The aerogel used in ODC was purchased from the Jet Propulsion Laboratory and came in monolithic "tiles" approximately 10 x 10 cm on the side and ~10-12 mm thick. Each ODC tray accommodated 36 such tiles that were press-fit into an aluminum frame. This frame was backed by a solid aluminum plate and had anodized (red) 0.5 mm thick aluminum hold-down grid on the front. Individual members of the hold-down grid were 2 mm wider than the frame members, which allowed for a 1 mm overlap along the entire circumference of an aerogel tile. This hold-down grid was installed last and it essentially sandwiched the frame and its collectors between the (opaque) backup plate and the (transparent) hold-down grid. Although frictional forces alone were expected to secure the lightweight aerogel in place during all flight operations, this hold-down grid provided an additional positive mechanism to secure each tile.
POSTFLIGHT INSTRUMENT PROCESSING
The entire MEEP container housing ODC was received at the Johnson Space Center on October 10, 1997 and subsequently transferred to the Facility for the Optical Inspection of Large Surfaces (FOILS) laboratory where it was opened and disassembled. The exteriors of the MEEP container appeared relatively clean and pristine, with modest, honey colored staining in some areas, akin to many LDEF surfaces, yet much less pronounced. A total of nine hypervelocity craters, all <500 Ám in diameter, accumulated on the exposed container surfaces. Following this inspection of the exterior surfaces, the container was opened as shown in Figure 1. All aerogel tiles were firmly in place and exceptionally pristine, attesting to the space worthiness of this very delicate material and to highly successful EVA operations during deployment and retrieval by the STS 76 and STS 86 crews, respectively. A number of large impact features and penetration tracks were readily apparent. Subsequent disassembly of the trays and harvesting of individual tiles occurred in a class 1000 laminar flow bench. Care was exercised to systematically record each tile's original orientation with regard to an instrument reference frame that can be related to the MEEP's orientation relative to MIR. Following their removal, each of the highly transparent tiles was photographically documented (i.e., mug shots) using special illumination techniques to highlight tracks >1 mm in size. An example of this photography is presented in Figure 2, illustrating a particularly interesting tile containing a large number of impact features. In summary, all operational aspects of the ODC experiment, including harvesting of the exposed tiles, must be considered nominal, if not outstanding.
Using the mug-shot photographs and other products for recording purposes, all tiles were systematically scanned with the naked eye, aided by using a hand lens and flexible glass-fiber optics to provide variable illumination geometry. A combination of back and side lighting was determined to be the best for highlighting impact features of various sizes and geometry. This first-order inspection aimed at identifying and recording the occurrence of all tracks >1 mm in length, yet tracks of millimeter dimensions are easily overlooked. It became obvious and necessary to distinguish three major classes of features:
These are the classical, carrot-shaped penetration tracks that one produces experimentally with silicate or metal projectiles in aerogels over a wide range of densities and velocities. Some typical examples are illustrated in Figures 3a - 3c. Experimental tracks in 0.02 g/cm3 aerogel at 6 km/s have a depth or length (L) that is 300 to 500 times the projectile diameter (Dp; at 50 Ám Dp scales, Hörz et al., 1997) and an aspect ratio (largest diameter of carrot/total length) of <0.1. Such classical carrot morphologies are present in the aerogel, but generally only at very small sizes (<5 mm); the population of the smallest tracks recorded (i.e., ~1-2 mm long) during this initial survey are without exception of the traditional, carrot-shaped morphology. On the other hand, larger tracks (i.e., typically >5 mm) tend to deviate from this carrot shape, and are commonly straight-walled, cylindrical cavities with a fairly blunt terminus and a large aspect ratio (0.2-0.5), giving them a stubby appearance (see Figure 3c). The largest carrot track is ~14 mm long (oblique impact angle) and barely penetrated the entire aerogel collector. Generally, some projectile residue is visible at the terminus of most tracks, yet not all, including some of the large tracks. Nevertheless there should be a rich harvest of diverse particles as also shown in Figure 3.
These features resemble impact craters in dense target media to the first order, but they do differ in substantial ways, thus the term "pit". The typical pit (see Figure 4) is a fairly centro-symmetric depression with an aspect ratio >0.5 and frequently close to 1, sometimes even >1. It has neither a raised rim nor a prominent spall zone at the target surface, unlike hypervelocity craters in either ductile or brittle materials, respectively. It also has a very irregular surface relief at the bottom and in the walls with the roughness and irregularities being of much larger amplitude - for any given crater size - than the typical hypervelocity crater in other solids. The rim area of the pits is more irregular than typical hypervelocity craters in metals (e.g., Bernhard et al., 1995) and exhibit many small-scale promontories and reentrant areas. Substantial radial fracture systems may surround the entrance hole, yet no spall zones are observed. The typical pit wall and bottom have a crackled appearance caused by conchoidal fracturing at very small scales, producing a thin layer of highly reflective material on account of multiple-light reflections. This renders macroscopic observations regarding the presence and distribution of (highly transparent) "melts" essentially impractical. The general impression is that one looks at mechanically fractured, freshly exposed walls with little evidence of melting. Detailed SEM studies will have to confirm this impression. The deeper pits may be viewed as a hemispherical depression connected to cylindrical walls of variable length; the stubby tracks mentioned above may be transitional features to pits. Exceptionally short, bulbous shapes with aspect ratios >1 are present as well; such features have entrance holes at the target surface smaller than the damage zone at some depth.
Interestingly, these pits contain no obvious projectile material in the form of discrete particles or some discoloration of the crater walls/bottoms, nor do they possess parasitic tracks reflecting discrete projectile fragments; such fragment tracks have been observed experimentally utilizing fluffy, low-strength impactors (Hörz et al., 1997).
A most outstanding characteristic of these features is their seeming size dependency: most features >2 mm in diameter observed on the entire ODC collector are pits; carrot-shaped tracks of that size are rare. The largest pit is ~9 mm across (see Figures 2 and 4a) and barely penetrated the 11 mm thick aerogel host, leaving some discoloration (projectile residue?) on the aluminum substrate.
The origin of these pits is poorly understood. They do not have an experimental analogue at impact velocities of 3-7 km/s. Intuitively, their general appearance suggests a low-density impactor at exceptionally low-encounter velocity, akin to the structures produced by Hörz et al. (1997) using fluffy, compressed powders for projectiles. However these experimental structures display numerous parasitic fragment-tracks that emanate from the main, bulbous cavity, features that are absent on ODC. The other alternative for the ODC pits would involve impact velocities beyond current laboratory capabilities, which may sufficiently melt and vaporize the impactor. In this view, the observed cavity would be generated by an expanding gas-cloud that is centered at some depth; a substantially linear source of expansion seems compatible with the stubby track variety which is transitional from tracks to pits. This latter scenario would point to a velocity-dependent limit of aerogel as a suitable collector medium for essentially unmelted residues of hypervelocity particles. We favor this interpretation on intuitive grounds. The detailed analysis phase of ODC will hopefully clarify which of these contrasting views is correct, if either.
These features represent extremely shallow depressions with aspect ratios >>1, generally 2-3. Typically, they are of irregular and distinctly non-circular outline as illustrated in Figure 5, which portrays one of the largest specimen observed. Invariably, these depressions are associated with either discrete flakes of a white material (on occasion assuming yellowish hues), or they are lined with such material. In many cases, only part of the flakes penetrated the aerogel and they seem to stick to the surface. It is such flakes that provide especially strong evidence for low-encounter velocities; the latter cannot be quantified at present, but it certainly was < 3 km/s and most likely a few tens of meters per second. Some of the larger flake depressions are associated with concentric fracture zones at depth, unlike the above pit craters. We suspect that these flakes may represent human waste products that co-orbit with the MIR station.
The results of our optical survey are summarized in Table 1 and illustrated in Figure 6. Obviously, the frequency of flake encounters is much higher on Tray 1 compared to Tray 2. The relative frequencies of pit type events are similar on both surfaces, and genuine, slender carrot-shaped penetration tracks are about a factor of two higher on Tray 2 compared to Tray1. Until the detailed, cumulative exposure geometry of these surfaces relative to the MIR station and its velocity vector is understood, some of these statistics are difficult to interpret. Suffice to say that there is variation in particle types as a function of pointing direction on MIR.
|Table 1. Absolute and relative frequency of genuine tracks, pit-type features and flakes >1 mm that were captures by ODC aerogel with a cumulative surface area of ~0.35 m2 per tray. The differences between Tray 1 and Tray 2 are given by the Tray 1/Tray 2 ratio.|
|Tray /Tray 2||0.37||0.80||4.46|| |
In addition to the above features, distinct clusters of very small, but predominantly carrot-shaped tracks occur on numerous tiles of Tray 2. A typical cluster, from tile 2F01, is illustrated in Figure 7. All of these tracks exhibit a remarkably constant azimuthal orientation and relatively shallow angles of incidence, ~30░ to 40░ from the tiles' horizontal surface. This constant orientation persists over many individual tiles and suggests these tracks were caused by a unique event consisting of a single swarm of very small impactors. The largest such tracks (see Figure 7) measure a few millimeters in length, and is accompanied by a myriad of much smaller tracks, including tracks not visible to the naked eye as evidenced by preliminary microscopic examination. None of these swarm-tracks are included in Table 1 as they represent effectively a single collisional event.
The distribution of impactors within this swarm must have been highly heterogeneous, as evidenced by highly localized concentrations of the resulting tracks. Such tracks typically occur in clusters a few cm across and they exhibit very steep concentration gradients towards the surrounding surfaces on the host tile. Only a few tiles of Tray 2 contain the distinct clusters of swarm particles; some of these tiles are co-located, others are not. Isolated tracks that are part of the swarm may, however, occur on most any tile of Tray 2, and preliminary microscopic analysis also suggests that the swarm event may have affected many tiles. Thus, swarm tracks may occur all over Tray 2, yet distinct clusters are observed on only a few tiles. This highly heterogeneous and lumpy distribution of impactors suggests a relatively local particle source, such as secondary ejecta of some nearby hypervelocity impact that did not have time and space to substantially disperse. Impactor residues are noticeable at the terminus of some of the larger swarm tracks, so we hope to be able to characterize the target or source of this event.
The detailed analysis of ODC includes the systematic microscopic examination of all ODC surfaces followed by Scanning Electron Microscope (SEM) investigation of representative features and the compositional analyses of projectile residues using Energy Dispersive X-Ray Spectrometry (EDS). Examples of such investigations are given below to illustrate the potential of these detailed studies.
Figure 7 is an example of the systematic microscopic scanning contemplated for each tile, and portrays a typical scene obtained via a digital-image acquisition system that will be employed for all features larger than some (small) cutoff size (TBD; <1 mm). The microscope/digital-imaging system is part of a large scanning platform. During the scanning operations, we will record the x/y coordinates of the (center) track's entrance hole, and the x'/y' location of the track terminus/projectile residue, as well as its depth (z) from the local surface, thereby giving a full-vector description of individual tracks (orientation, length, and depth).
Figure 8 is an example of the SEM investigations to come and illustrates an EDS spectrum of one of the white flakes discussed earlier. This material is mainly composed of Na, P, and S, with Si being largely due to the Si-bearing aerogel. This composition is suggestive of human waste, but no positive assignment is possible at this time, because other diagnostic elements for waste-particles previously analyzed contain substantial and diagnostic amounts of K and Cl, which are obviously absent on this single MIR analysis.
The ODC component of the MEEP payload deployed by STS 76 on March 25, 1996 and retrieved by STS 86 on October 1, 1997 performed nominally, if not outstandingly. None of the delicate aerogel collectors was damaged during ground handling, launch, deployment, and retrieval. Every single tile contains some impact feature and projectile residues are clearly visible at the terminus of many penetration tracks. We have completed the macroscopic examination of all surfaces and distinguish between (1) classical carrot-shaped tracks, (2) relatively shallow pits that are poorly understood and have no experimental analogue, and (3) white flakes that are embedded into the aerogel surface. A unique swarm of particles, most likely the fine-grained ejecta of a nearby hypervelocity crater, has affected the rearward facing surfaces of ODC. The macroscopic survey revealed a variety of impact features and we are confident that the detailed analysis phase will reveal a variety of particle types, both natural and man-made. Such detailed analyses will commence shortly.
Acknowledgment: It is a pleasure to recognize the dedication of the STS 76 and STS 86 Crews and of the Langley Research Center MEEP-Team under the leadership of G. Stover; without their efforts, skills and care, ODC would not have been successful. We also thank P. Tsou, JPL, for guidance and advise in handling and mounting the aerogel specimen manufactured in his laboratory. Lastly, substantial discussions with F. Cardenas, M. Cintala, W. Davidson, G. Haynes, D. Kessler, W. Kinard, and D. Potter improved on detailed instrument design and analytical objectives.
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