A 5.2 mm crater in Al-metal represents the largest found on LDEF. We have examined this crater by field emission scanning electron microscopy (FESEM), energy dispersive spectroscopy (EDS) and time-of-flight/secondary ion mass spectroscopy (TOF-SIMS) in order to determine if there is any evidence of impactor residue. Droplet and dome-shaped columns, along with flow features, are evidence of melting. EDS from the crater cavity and rim show Mg, C, O and variable amounts of Si, in addition to Al. No evidence for a chondritic impactor was found, and it hypothesized that the crater may be the result of impact with space debris

Bunch, T. E.

Paque, Julie M.

Zolensky, Michael

English

NASA Technical Reports Server

N95- 23838
IMAGE AND COMPOSITIONAL CHARACTERISTICS
LDEF "BIG GUY" IMPACT CRATER
T, E, Bunch
NASA Ames Research Center
Moffett Field, CA 94035-1000
Phone: 415/604-5909, Fax: 415/604-6779
OF THE
") _,/J
Julie M. Paque
SETI Institute
2035 Landings Drive
Mountain View, CA 94043
Michael Zolensky
NASA Johnson Space Center
Houston, TX 77058
ABSTRACT
A 5.2 mm crater in Al-metal represents the largest found on LDEF. We have examined this crater by
field emission scanning electron microscopy (FESEM), energy dispersive spectroscopy (EDS) and time-
of-flight/secondary ion mass spectroscopy (TOF-SIMS) in order to determine if there is any evidence of
impactor residue. Droplet and dome-shaped columns, along with flow features, are evidence of melting.
EDS from the crater cavity and rim show Mg, C, O and variable amounts of Si, in addition to A1. No
evidence for a chondritic impactor was found, and it is hypothesized that the crater may be the result of
impact with space debris.
PRECEDING
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https://ntrs.nasa.gov/search.jsp?R=19950017418 2020-06-16T08:30:51+00:00Z
INTRODUCTION
The largest crater on LDEF measures 5.2 mm in diameter and has a depth to diameter ratio of 0.5. The
"Big Guy" crater was located on tray H03; experiment M0001, J. Adams, Principal Investigator.
Although the experiment faced out into space, this particular impact occurred on an A1 metal Z-frame,
which projected in the ram direction (see photographs in ref. 1, p. 410-411). In addition, the impactor
penetrated eight layers of lexan before encountering the A1 surface. Our objective in examining this crater
was to determine the nature and origin of the impactor. The crater was examined carefully for impactor
residue or other evidence of the impactor's origin.
METHODOLOGY
The crater was first observed by a stereo microscope, then by field emission scanning electron
microscopy (FESEM). Samples were then analyzed for impactor residue by energy dispersive
spectroscopy (EDS) equipped with a light element detector, followed by cursory examination by time-of-
flight/secondary ion mass spectroscopy (TOF-SIMS; see ref. 2 for instrumental technique).
The Hitachi FESEM is equipped with a light element detector, allowing for the analysis of boron,
carbon, nitrogen, and oxygen by energy dispersive spectroscopy (EDS). However, this analysis can be
complicated by contamination, which can be very significant if steps are not taken to minimize the effects.
The contamination can arise from several sources including sample handling, preparation and contaminants
in the electron column. Organic contaminants on the surface will result in polymerization around the
electron beam if it is focussed, due to the heat generated in a small area and the electrostatic attraction of the
beam. This effect is minimized both by careful handling and by rastering the beam over a several square
micron area during analyses.
RESULTS
An FESEM image of the crater is shown in Fig. 1. A small amount of melt is visible in the crater
bottom with more melt on the upper wall near the rim. In general, the melt liner appears to be quite thin.
No intact projectile or dark patches on the rim were found. The only prominent melt-forms are droplet to
dome-shaped columns (see Fig. 2). These objects on the rim appear to be darkened on their top surfaces
when viewed through the stereo microscope, although this could also be an artifact due to illumination
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under the microscope. Figure 3a shows an example of the unusual domed objects, with the horizontal
layering effect. Figure 3b is an example of a melt form that appears to have collapsed before it completely
froze.
Figure 1. FESEM imagery mosaic of the Big Guy crater.
477
FESEM-EDSanalysesof thecratercavity and
rim showthepresenceof Si, Mg, C, and O. The
amounts of Si, in particular, vary considerably,
depending on the location of the analysis. The top
of droplets and dome-shaped columns have the
highest amounts of Si, melt liners intermediate
amounts and unmelted areas on the rim, the lowest
(Fig. 4). The rim analysis also corresponds to an
analysis of a point away from the crater that was
covered with lexan before it was removed for the
crater study. A piece of the frame underside was
polished and analyzed by a Cameca electron
microprobe. This analysis, and an additional
analysis away from the crater in the vicinity of the
EDS analysis, show the average Si content to be
0.34 wt % (range, 0.23 to 0.39) and Mg content is
0.94 (range, 0.83 to 1.02). Thus, the EDS
spectrum of the point away from the crater is taken
to be the background reference composition of
uncratered A1. By comparing analyses of melt
forms, Si shows an increased content of at least 3-
fold or _- 1.0 wt %; Mg shows no significant
change. The presence of C, intrinsic to the crater,
is confirmed by EDS analyses, although some
unknown amount is attributed to contamination
during analysis. TOF-SIMS analyses indicate, in
addition to Si, Mg, C, O, the presence of S, C1 and
Na, which are known contaminants of LDEF
surfaces.
Figure 2. FESEM images. (a) Top side of rim with a bulb-shaped melt form (arrow) with a broken stem.
Lower portion of image looks down into the crater cavity. Note melt liner on the wall surface, but not
the rim. (b) Droplet forms on the lower part of wall. Numbers refer to analyzed spots. (c) Domed
column atop crater rim (sample tilted 45 ° for lighting purposes). Stereo photograph; column diameter
= 0.08 mm.
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Figure 3. FESEMimages. (a) Openarrowpoints to domedobjectwith prominentrings/layers. Solid
arrow indicatesa melt droplet that collapsedinto a "puddle"beforesolidifying. (b) Enlargedview of
collapseddroplet.
DISCUSSION
Analyses of the crater show no chondritic compositional evidence (Si, Mg, A1, Ca, Fe, S_ Ni) for an
extraterrestrial impactor. Silicon and Mg contents are attributed to the intrinsic minor element composition
of the 6061 T6 A1 plate. Silicon is enriched in the melt liner and melt forms relative to A1 away from the
crater. The mechanism for increasing Si content in the A1 is unknown. Either an enrichment of intrinsic Si
occurred during impact melting by some fractionation process or Si was added by an unknown orbital
debris impactor. Carbon content is most likely a contaminant that resulted from vaporization of the
overlying lexan during impact.
Silicon is added to A1 to form a harder, somewhat less ductile alloy; enhanced Si content also
increases the viscosity of molten A1 (ref. 3). The peculiar layered textures of the melt forms may have
resulted from differential cooling rates between the higher viscosity melt surface, which we find to be
enriched in Si, and the underlying portions that may be less viscous. These textures may also have
arisen, in part, from spiral dislocation arrays promoted by non-equilibrium and rapid cooling conditions.
We conclude that compositional heterogeneities together with extremely rapid cooling (the crater forming
event was very fast, on the order of 10 [tsec; ref. 4) probably gave rise to the unusual forms and textures
observed in the Big Guy crater.
It is disappointing to note that the largest crater on LDEF was probably formed by collision with a
piece of space junk. Dale Atldnson (pers. comm., 1993) analyzed the orientation of the crater on LDEF
and concluded from orbital considerations that the impactor debris could have come from a Vandenburg
Air Force Base launch.
479
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Figure 4. EDS analyses of droplet top (a), domed columns (b), and melt liner (c). Horizontal line in
each indicates intrinsic Si content of A1 alloy. The y-axis (counts) uses a logarithmic scale to allow
examination of minor elements in the presence of A1.
480
ACKNOWLEDGEMENTS
We thank Jim Adams for the loan of the Big Guy crater and J. Eflichman for technical assistance.
REFERENCES
1. Adams, J.: Meteoroid and Debris Impact Features Documented on the Long Duration Exposure
Facility: A Preliminary Report. NASA - Johnson Space Center Publ. # 24608, pp 583, 1990.
, Bunch, T. E.; Becker, L.; Bada, J.; Macklin, J.; Radicati di Brozolo, F.; Fleming, R. H and
Erlichman, J.: Hypervelocity impact survivability experiments for carbonaceous impactors. Second
LDEF Sym. NASA CP-3194, 453-477, 1993.
3. Cottrell, A. H.: The Mechanical Properties of Matter. John Wiley and Sons, New York, pp 430,
1964.
4. H6rz, F.; Fechtig, H. and Janicke, J: Morphology and chemistry of projectile residues in small
experimental craters. Proc. 14th LPSC, J. Geophy. Res. 88, B353-363, 1983.
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Image and compositional characteristics of the LDEF Big Guy impact crater

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