LONG TERM MICROPARTICLE IMPACT FLUXES ON LDEF DETERMINED FROM OPTICAL
SURVEY OF INTERPLANETARY DUST EXPERIMENT (IDE) SENSORS

C. G. Simon1, J. P. Oliver2, W. J. Cooke, K. I. Downey2

Institute for Space Science & Technology, Gainesville FL 32609

P. C. Kassel

NASA Langley Research Center, Hampton VA 23665

Phone: (804) 864-4621 Fax: (804) 864-7607

1Current Address: Air Consulting and Engineering, Inc.

2106 NW 67 Place, S-4, Gainesville FL 32653

Phone: (904) 335-1889 Fax: (904) 335-1891

2Also University of Florida, Gainesville FL 32611

SUMMARY

Many of the IDE metal-oxide-silicon (MOS) capacitor-discharge impact
sensors remained active during the entire Long Duration Exposure
Facility (LDEF) mission. An optical survey of impact sites on the active
surfaces of these sensors has been extended to include all sensors from
the low-flux sides of LDEF (i.e. the west or trailing side, the earth
end, and the space end) and 5-7 active sensors from each of LDEF\'s
high-flux sides (i.e. the east or leading side, the south side, and the
north side). This survey was facilitated by the presence of a relatively
large (\>50 µm diameter) optical signature associated with each impact
site on the active sensor surfaces. Of the \~4700 impacts in the optical
survey data set, 84% were from particles in the 0.5 to 3 µm size range.
An estimate of the total number of hypervelocity impacts on LDEF from
particles \>0.5 µm diameter yields a value of \~7 x 106. Impact feature
dimensions for several dozen large craters on MOS sensors and germanium
witness plates are also presented.

Impact fluxes calculated from the IDE survey data closely matched
surveys of similar size impacts (\_3 µm diameter craters in Al, or
marginal penetrations of a 2.4 µm thick Al foil) by other LDEF
investigators. Since the first year IDE data were electronically
recorded, the flux data could be divided into three long term time
periods: the first year, the entire 5.8 year mission, and the
intervening 4.8 years (by difference).

The IDE data show that there was an order of magnitude decrease in the
long term microparticle impact flux on the trailing side of LDEF, from
1.01 to 0.098 x10-4m-2s-1, from the first year in orbit compared to
years 2-6. The long term flux on the leading edge showed an increase
from 8.6 to 11.2

x10-4m-2s-1 over this same time period. (Short term flux increases up to
10,000 times the background rate were recorded on the leading side
during LDEF\'s first year in orbit.) The overall east/west ratio was 44,
but during LDEF\'s first year in orbit the ratio was 8.5, and during
years 2-6 the ratio was 114.

Long term microparticle impact fluxes on the space end decreased from
1.12 to 0.55 x10-4m-2s-1 from the first year in orbit compared to years
2-6. The earth end showed the opposite trend with an increase from 0.16
to 0.38 x10-4m-2s-1. Fluxes on rows 6 and 12 decreased from 6.1 to 3. 4
and 6.7 to 3.7

x10-4m-2s-1, respectively, over the same time periods. This resulted in
space/earth microparticle impact flux ratios of 7.1 during the first
year and 1.5 during years 2-6, while the south/north, space/north and
space/south ratios remained constant at 1.1, 0.16 and 0.17,
respectively, during the entire mission.

This information indicates the possible identification of long term
changes in discrete microparticle orbital debris component contributions
to the total impact flux experienced by LDEF. A dramatic decrease in the
debris population capable of striking the trailing side was detected
that could possibly be attributed to the hiatus of western launch
activity experienced from 1986-1989. A significant increase in the
debris population that preferentially struck the leading side was also
observed and could possibly be attributed to a single breakup event that
occurred in September of 1986. A substantial increase in the
microparticle debris population that struck the earth end of LDEF, but
not the space end, was also detected and could possibly be the result of
a single breakup event at low altitude.

These results point to the importance of including discrete orbital
debris component contribution changes in flux models in order to achieve
accurate predictions of the microparticle environment that a particular
spacecraft will experience in earth orbit. The only reliable, verified
empirical measurements of these changes are reported in this paper.
Further time-resolved in-situ measurements of these debris populations
are needed to accurately asses model predictions and mitigation
practices.

INTRODUCTION

The MOS capacitor discharge impact sensors of the Interplanetary Dust
Experiment provided a unique opportunity for measuring the short term
and long term time-resolved flux of small microparticles that impacted
LDEF during its 5.8 year long mission. We have reported these data in
several papers (Refs. 1-7) that address different aspects of the
microparticle environment in low Earth orbit (LEO). In this paper we
report further results of an optical survey of impacts on IDE sensors
that remained active during the entire LDEF mission.

Two different sensitivity MOS sensors made up the IDE experiment. The
sensitivity of the detectors is proportional to the thickness of a
thermally grown layer of Si02 on top of a 250 µm thick, 51 mm diameter
Boron-doped Si wafer substrate. The higher sensitivity detectors (0.4 µm
thick dielectric, sensitive to \~0.2 µm and larger hypervelocity
particle impacts) drained their batteries during the longer-than-planned
LDEF mission. However, the low sensitivity IDE detectors (1.0 µm thick
dielectric, sensitive to \~0.5 µm and larger hypervelocity particle
impacts) remained powered during the entire mission, except for those
incapacitated by large hypervelocity impacts.

The first year, time-resolved IDE data are re-presented in this paper in
three separate categories: (1) Multiple Orbit Events (MOES), (2) Spikes,
and (3) Background. These are described briefly with the tabulated
results. More detailed descriptions of these categories, along with
examples, can be found in Refs. 6 and 7.

Details of the optical survey procedures were previously published (Ref.
5). Briefly, the IDE sensors have the unique characteristic (among LDEF
surfaces) of producing a clearly visible 50 µm diameter \"discharge
zone\" around microparticle impact sites. This zone is formed by the
evaporation of the 0.1 µm thick surface layer of aluminum (the top
electrode) caused by heat from the electrical discharge of the MOS
capacitor when struck by a particle with sufficient energy to breakdown
the silicon dioxide insulator and \"trigger\" the sensor. The electrical
discharge spark also creates a central crater with a diameter of \~10
µm. Particles \>3-4 µm in size are large enough to form a spall-zone
larger than the 10 µm \"spark\" crater, but impacts from smaller
particles that triggered the sensors always resulted in a 10 µm central
crater. These characteristics made it possible to quickly scan large
areas of even low impact flux surfaces on LDEF (trailing side, space and
earth ends) using optical microscopy. Pre- and post-flight photographs
of each sensor provided a record of the few discharges produced during
manufacturing and pre-flight testing. Careful correlation yielded
accurate counts of impact induced discharges.

Another method of determining small particle impact fluxes on large
areas of LDEF was counting penetrations through thin foils. The Multiple
Abrasion Package (MAP) experiment consisted of large areas of thin
aluminum and brass foils dedicated to this purpose (Ref. 8). The MAP
foils were mounted adjacent to the IDE sensor arrays on 5 of the 6
orthogonal sides of LDEF (earth end excluded). After retrieval, the
foils were back lighted and all penetrations counted. After additional
optical and electron microscopic examination and correction for
interferences (secondary impacts and non-impact induced pinholes) data
from the MAP foils and from witness plates/foils of the FRECOPA
experiment (Ref. 9) were plotted in smoothed flux curves (Ref. 10, 11).
Although the MAP foils did not provide any level of time resolution,
they did provide an excellent independent record of the microparticle
impact flux for the entire 5.8 year LDEF mission which closely matched
the IDE results for the same time period.

RESULTS AND DISCUSSION

Impact crater morphologies

Crater size data are presented for those interested in the material
response of the MOS structure and single crystal germanium (Ge) to
hypervelocity impacts. Complete optical survey results for the MOS
sensors are appended to the end of this report (Table 1A). Tabulated
crater size classification and measurement data are incomplete since
these parameters were not part of the initial goals of the survey, but
were added after the survey was underway. Results of an optical survey
of two 250 µm thick, 25 mm diameter single-crystal Ge witness plates
mounted on LDEF tray B-12 (north side) were previously reported (Ref.
12) and are presented here in graphical form. The passive Ge targets
were also part of IDE.

Dimensions were recorded for four morphological characteristics of
hypervelocity impact craters into the crystalline materials: central
crater/shatter zone, inner spall zone, outer spall zone, and fracture
zone. Figures 1 and 2 show plots of the measured central
crater/shatter-zone maximum dimensions versus the maximum dimensions of
the associated outer spalls and fracture zones of craters in the MOS and
Ge surfaces, respectively. Note the convergence of the MOS data to a
spall zone size equal to the central crater size at 10 µm in Fig. 1 (not
predicted by the simple linear fit shown). This is consistent with the
domination of the crater formation mechanism by the electrical discharge
energy for small impactors (0.5-3 µm) into active MOS sensors. Above
this size, the impactor energy dominated the crater formation resulting
in \"classic\" crater/spall morphologies.

Figure 2 shows a linear response for fracture zone maximum dimensions
versus central crater size, even for large features in the silicon MOS
sensors. (Fracture zones were not measured for the Ge witness plate
impacts.) The MOS sensors were bonded to aluminum frames with \~4 mils
of silicon RTV. Apparently, shock waves from large impactors were
reflected through the silicon adhesive with enough efficiency to cause
fractures with the same relative dimensions as seen with smaller
impactors. Figure 2 also shows that fractures that extend beyond the
outer spall zones are not significant for craters smaller than \~150 µm,
but are 6-7 times the maximum central crater dimension in larger
impacts.

Figure 3 is a plot of central crater size versus spall zone maximum
dimensions for impacts into the Ge witness plates. The largest central
crater size observed was 188 µm. The near zero intercept in this plot
shows the more \"normal\" response to small impactors of the passive
crystalline Ge substrates compared to the \"active\" silicon MOS
substrates. Observed spall zone maximum dimensions in Ge are \~6x the
central crater size, while in Si they are \~3x the central crater size.
This is consistent with the relative crystal lattice energies of Ge and
Si.

Microparticle Impact Fluxes

Before discussing the IDE optical survey results, it is important to
review a summary of the IDE time-resolved data for the first year of
LDEF\'s orbital lifetime. These data are presented in Table 1 by
detector sensitivity, location on LDEF, and impactor category (MOES,
Spikes or Background). See Refs. 4 and 6 for more discussions of
impactor classifications. \"Spike\" events were identified by visual
inspection of the data file. During these events the IDE recorded tens
to hundreds of impacts on the space and/or north and/or leading (east)
sides of LDEF within a few minutes. These events were dominated by very
small particles as evidenced by the large relative proportion of impacts
from this category of particle on the high sensitivity IDE sensors
compared to their low relative abundance on the low sensitivity IDE
sensors.

MOES events were identified both by visual inspection of the data file
and by the use of an extraction algorithm. The algorithm searched the
IDE data in groups of three arrays at a time (space/north/east,
space/south/east, space/south/west, space/north/west, earth/north/east,
earth/south/east, earth/south/west, earth/north/west) for multiple
impacts within a narrow window (34°, or ± 4.4 minutes). If two out of
three adjacent orbits recorded impacts in the window, the impacts were
identified as a MOES. While this method necessarily extracted some
random data, in practice the MOES were very strong events which
dominated the data set and limited the number of identifiable \"random\"
impacts.

Table 1. Summary of time-resolved IDE microparticle impact data for
LDEF\'s first year in orbit. \"High\" sensitivity detectors were
triggered by hypervelocity particles \>0.2 µm in size. \"Low\"
sensitivity detectors were triggered by hypervelocity particles \>0.5 µm
in size. Results are listed in order of increasing flux.

LDEF Detector Impact Flux (x 10-4m-2s-1) % of Total Flux

Location Sensitivity Total MOES Spikes Background MOES Spikes Background

G-10 high 0.162 0.041 0.0 0.121 25 0 75

(earth) low 0.157 0.037 0.0 0.120 24 0 76

C-3 high 1.59 0.721 0.0 0.869 45 0 55

(west, trailing) low 1.01 0.368 0.0 0.642 36 0 64

H-11 high 1.85 0.882 0.360 0.608 48 19 33

(space) low 1.12 0.436 0.043 0.641 39 4 57

B-12 high 9.21 5.00 2.51 1.70 54 27 19

(north) low 6.14 3.30 0.364 2.48 54 6 40

D-6 high 12.7 10.5 0.017 2.18 83 0.1 17

(south) low 6.70 5.19 0.0 1.41 77 0 23

C-9 high 17.1 12.1 1.82 3.18 70 11 19

(east, leading) low 8.61 5.59 0.182 2.84 65 2 33

Totals high 68 11 21

low 64 2.5 34

The IDE sensor optical survey results are summarized in Table 2 by LDEF
tray location in order of increasing total-mission flux. The size range
(0.5 - 3 µm) of impactors labelled \"small\" in the table is based on
the minimum central crater diameter of 10 µm for impact induced
discharges on the MOS sensors. As mentioned above, the crater formation
is dominated by the electrical discharge energy for small particles.
This masks all information about the impactor\'s size for particles
smaller than \~3 µm.

The relative abundance of \"small\" impactors can be used to define the
population of a set of particles that are significantly affected by
non-gravitational forces. It is these particles that undergo rapid
deviation from their parent body orbits, even when leaving with very
small velocity differences (\_v). Thus, the orbits of microparticles
shed from large bodies in circular orbits quickly increase in
eccentricity and have very limited lifetimes. These particles can remain
under the influence of the parent body for some time. For example, if
they reside in shaded wake regions they will be shielded from solar
pressure and aerodynamic drag effects. As a result, enhanced
shedding/breakup events will release three classes of microparticles
based on their leaving characteristics: (1) large \_v, (2) small \_v,
and (3) \"dribblers\".

Shedding rate enhancements have a variety of causes, including
differential thermal expansion /contraction induced vibrations and
frictional wear, reaction wheel motion, solar panel motion, thruster
firings, surface charge polarity shifts, plasma discharges, and
hypervelocity impacts. Major sources of microparticles released during
shedding include fuel residues, residual dust/dirt, and erosion products
such as paint particles and thermal blanket flakes. The actual and
potential particle release-rates from these sources will change
drastically during an orbiting spacecraft\'s lifetime. These rate
changes depend on a spacecraft\'s materials of construction and its
mission operational profile, including the space environment that it
experiences during its lifetime. For example, LDEF results showed that
impacts into old painted surfaces released more secondary debris
(spalled paint) than did impacts into \"new\" paint, (this was due to
the loss of organic binder in the paint as a result of exposure to solar
ultraviolet \[UV\] radiation and atomic oxygen \[AO\]), and metal-coated
hydrocarbon based foils such as Al-coated Mylar erode quickly when
exposed to AO and UV, releasing bits of residual Al thin film. (See Ref.
13 for discussions of these and other examples.)

Table 2. Results of optical survey of IDE sensors that remained active
during the entire LDEF mission (April, 1984-January, 1989). Scanned area
= 19.6 cm2 per sensor. Total exposure time = 2106 days. Sensors
responded to iron particles \>0.5 µm at velocities \>2km/s in ground
tests.

\# of \# sensors \# \"active\" average \# of % of small impact

LDEF sensors with craters sensors (n) impacts std. dev. (0.5-3 µm) flux

tray scanned \>1mm used for per \"active\" (n-1) impactors x10-4m-2s-1

flux calcs. sensor

C-3 (west) 32 4 (13%) 23 8.82 7.18 (81%) 92 0.247

G-10 (earth) 32 0 (0%) 31 12.2 6.01 (49%) 99.7 0.342

H-11(space) 22 5 (23%) 14 23.1 6.02 (26%) 77 0.647

B-12 (north) 8 5 (63%) 7 137 13.8 (10%) 85 3.84

D-6 (south) 10 3 (30%) 6 149 16.0 (11%) 78 4.17

C-9 (ram) 9 8 (89%) 5 384 30.2 (8%) 85 10.8

A summary of the long-term flux rates measured by IDE sensors and MAP
foils is presented in Table 3. The MAP data are from smoothed curves
generated using penetration counts (corrected for secondary impacts)
from several different foil thicknesses at each location. Marginal foil
penetration occurs at the point where the crater diameter is \~1.1 times
the foil thickness for small impacts (Ref. 10). These data indicate that
particles that will penetrate a 2.4 µm thick Al foil, or make an \~3 µm
diameter crater in Al, will also trigger the low sensitivity IDE
sensors. This result is consistent with ground-based calibration tests
(Refs. 14, 15). The narrow range (2.34 to 2.72 µm) of equivalent foil
penetration thicknesses for MAP fluxes that match IDE fluxes is
particularly gratifying. These results were arrived at independently by
two different groups of investigators.

The MAP data show a shift to thicker foils for the same flux value on
the east side of LDEF compared to the IDE data. This indicates that the
velocity enhancement effect on threshold sensitivity of the MOS sensors
is not as strong as the enhanced penetration effect of Al foil due to
higher velocities.

Table 3. Long-term LDEF impact flux rates measured by IDE sensors and
MAP foils. Sensors responded to iron particles \>0.5 µm at velocities
\>2km/s in ground tests. Scanned area =19.6 cm2 per sensor. Total
exposure time =2106 days. MAP data is from smoothed cumulative flux
curves for marginal foil penetrations corrected for secondary impacts
(Refs. 10, 11).

LDEF days days days MAP smoothed equivalent MAP Al

tray 1-346 347-2106 1-2106 fluxes for days 1-2106 foil thickness (µm)

C-3 (west) 1.01 0.098 0.247 0.247 2.36

G-10 (earth) 0.157 0.379 0.342 - -

H-11(space) 1.12 0.553 0.647 0.647 2.36

B-12 (north 6.14 3.39 3.84 3.84 2.42

D-6 (south) 6.70 3.67 4.17 4.17 2.34

C-9 (east) 8.61 11.2 10.8 10.8 2.72

Flux ratios calculated from IDE and MAP results are presented in Table
4. These data illustrate the limitations of using \"typical\"
microparticle impact flux ratios for environment effects predictions.
The data also confirm the higher microparticle impact flux on the south
side of LDEF compared to the north side throughout the entire mission,
despite the fact that the north side was pointed 8° into the ram.

The changes in flux ratios can be used to identify possible discrete
microparticle orbital debris component contributions to the total impact
flux experienced by LDEF. Our observations show that the fluxes on the
north, south and space sides decreased at about the same relative rate
(by \~50%), leaving the north/space and south/space flux ratios
relatively constant at \~6. These ratios should be 0.78 for meteoroid
impacts alone (due to the partial earth shielding of the north and south
sides). Thus, it is apparent from the overall flux ratios and from the
first year, time-resolved IDE data that the microparticle impact flux on
the north and south sides was dominated by orbital debris. Further, the
population of debris in orbits capable of striking these surfaces
apparently decreased during years 2-6. During the same time period the
microparticle impact flux increased by 30% on the ram (east) surface of
LDEF.

Kessler has reported on orbital debris component contributions to the
observed impact flux on various sides of LDEF (Ref. 16). He showed that
particles in highly inclined orbits with apogees near LDEF\'s would have
impacted the leading (east) side of LDEF more frequently than the north
or south sides. A specific example in (16) for a near circular orbit
inclined 100° with an apogee of \~500 km shows a 3:1 ratio of impactors
preferentially striking the leading side versus the north or south
sides. An increase in the debris particle population in a highly
inclined orbit such as this could produce the observed long term flux
increase on the east side of LDEF while allowing for the observed long
term flux decrease on the north and south sides. We note that an
anti-satellite test took place in September of 1985 (vehicle designation
P-78) at an altitude of \~500 km and resulted in a drastic increase in
the debris population in orbits inclined \~100° (Ref. 17).

The general hiatus in western launch activities caused by the loss of
several vehicles in 1986 had to result in a lower replenishment rate for
the population of microparticles in 7° and 28° inclined orbits. This
could account for the general lower activity on the north and south
sides of LDEF. Since Kessler also showed in (16) that orbital debris
particles in elliptical orbits with inclinations of \~5-40°could strike
the west side of LDEF (with a peak contribution at LDEF\'s orbit of
28.5°), this population must also have been drastically reduced during
the 1986-1990 time period. The reduction of microparticles released by
western launch activities could also account for this observation.

A further observation of the time-resolved IDE data showed that \~40% of
the impacts recorded by the low sensitivity detectors occurred over a
relatively narrow section of LDEF\'s orbit (± 30° for the whole mission)
centered at \~20° west with respect to the LDEF-Sun angle. (Most of
these impacts were identified by the extraction algorithm as belonging
to an extended MOES event. See Table 1.) These events were first thought
to be a ß-meteoroid signature, but the space end sensors showed no sign
of this signature. In addition, the flux on the west side dropped by an
order of magnitude from the first year to years 2-6. We also noted that
the west panel average impact rate during the first year was relatively
constant. Thus, we are left with the conclusion that this
sun-synchronous component contribution must have decreased drastically
during years 2-6 and that it could be attributed to a special case of an
orbital debris MOES.

A substantial increase in the microparticle debris population that
struck the earth end of LDEF (all small particles) but not the space end
could possibly be the result of a single breakup event at low altitude.
Such an event occurred in September of 1986 when a Delta upper stage in
a 23° orbit was struck by a smaller sensor package (the Delta\'s
payload) in a 39° orbit (Ref. 17). The intercept took place at 220 km
altitude and should have produced a large outward component of
microparticle debris.

Table 4. Long-term LDEF impact flux ratios measured by IDE sensors and
MAP foils. These data are for impactors that would create 3 µm diameter
or larger craters in an aluminum surface, or penetrate a 2.5 µm thick Al
foil.

LDEF days days days MAP smoothed data

tray ratio 1-346 347-2106 1-2106 for days 1-2106

H-11/G-10 (space/earth) 7.13 1.46 1.89 -

H-11/C-3 (space/west) 1.11 5.64 2.62 2.87

H-11/D-6 (space/south) 0.167 0.151 0.155 0.146

H-11/B-12 (space/north) 0.182 0.163 0.168 0.166

D-6/B-12 (south/north) 1.09 1.08 1.09 1.13

C-9/C-3 (east/west) 8.52 114 43.6 58.8

C-9/D-6 (east/south) 1.29 3.05 2.58 3.00

C-9/B-12 (east/north) 1.40 3.30 2.80 3.39

CONCLUSIONS

There are several important conclusions that can be drawn from this
study. First, the temporal anisotropy of the microparticle environment
experienced by LDEF extended throughout its orbital lifetime. This has
serious implications on environment predictions that rely on isotropic
long term flux and flux ratio assumptions, and points to the importance
of including discrete orbital debris component contribution changes in
these models. Further time-resolved in-situ measurements of these debris
populations are needed to accurately asses model predictions and
mitigation practices.

Second, the IDE experiment proved that most encounters with manmade
particles occur with an orbital beat frequency that indicates repeated
intersections with streams of orbiting particles shed from larger
orbiting objects (Refs. 2-4, 6, 7). These particles are typically very
small, submicron to a few microns, and account for \~2/3 of the IDE
impacts during the first year. There is no reason to assume that this
situation changed during years 2-6. This is also consistent with the 84%
of small (0.5 to 3 µm) particle impacts on IDE sensors observed in the
optical survey.

At least two other investigators have detected similar multiple-orbit
encounters on different missions since the IDE data were first reported
(Refs. 18, 19). It is apparent that each spacecraft\'s local orbital
environment will be subject to a different microparticle impact regime
based on interactions with the manmade particle environment of the day.
Because the particles are small, they are not expected to stay in orbit
for long due to perturbation by non-gravitational forces. Thus we are
left with the conclusion that for multiple orbit intersections that
lasted days to weeks, there must have been a continuous emission source
for these particles, such as a satellite shedding paint particles and/or
other materials.

All of this leads to the conclusion that changes in the manmade
microparticle population that interacted with LDEF were the major
reasons for the observed changes in long term microparticle impact
fluxes. Limited chemical analysis of impact sites on IDE sensors
supports this conclusion (Ref. 20). Approximately 1/3 of the sites
examined on the space and west side sensors were from manmade
microparticles, mostly paint. This is a significantly higher proportion
than expected, but is statistically consistent with the flux data.

A third important result is a practical lesson learned in measuring
microparticle impact fluxes on surfaces that have been exposed to a very
low impact-flux environment, namely that inspections of small areas can
lead to radically different flux values due to the statistical
uncertainty of the small data set. The IDE sensors on the Earth end and
trailing side of LDEF averaged \~10 impacts per sensor, but the range of
values for the number of impacts found on individual 20 cm2 sensors was
1 to 26. We recommend that several hundred cm2 be carefully examined
when determining flux rates based on impact feature densities in the
range of one impact per cm2.

A third conclusion resulting from this study was the utility of careful
pre-flight photographic documentation of flight hardware that will
eventually be returned for close examination. The pre- and post-flight
photographic archive of IDE sensors allowed the long term impact count
to be determined with a high degree of accuracy, and identified many
pre-flight contaminants that were still in place after retrieval. New
contaminants were also easily identified. This situation provides an
excellent opportunity for the study of contamination and its effects.
All of these photographs are electronically archived with the rest of
the IDE data (available on CD), and physically archived along with the
IDE hardware at NASA/LaRC.

ACKNOWLEDGEMENTS

This study was supported by NASA grant NAG-1218. The authors gratefully
acknowledge the helpful discussions and reviews of this work provided by
Don Kessler and Don Humes.

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10\. D.H. Niblett, S. Mullen, M.J. Neish and J.A.M. McDonnell,
\"Comparison of Flux Data Deduced from Observed Impacts on LDEF with
Predictions from Meteoroid and Debris Models\", First European
Conference on Space Debris, April 5-7, Darmstadt, Germany, ESA SD-01,
pp. 165-170 (1993).

11\. J.A.M. McDonnell, et al., \"The Near Earth Space Impact
Environment - An LDEF Overview\", Proceedings of the IAF/COSPAR World
Congress, W. Flury, ed., Pergamon, (1993).

12\. C.G. Simon, J.L. Hunter, D.P. Griffis and J.J. Wortman, \"Ion
microprobe elemental analysis of impact features on Interplanetary Dust
Experiment sensor surfaces\", LDEF-69 Months in Space: First Post
Retrieval Symposium, NASA CP 3134, pp. 529-548 (1992).

13\. A.F. Whitaker and J. Gregory, eds., \"LDEF Materials Results for
Spacecraft Applications\", NASA CP 3257 (1994).

14\. P.C. Kassel, \"Characteristics of Capacitor-Type Micrometeoroid
Flux Detectors When Impacted With Simulated Micrometeoroids\", NASA TN
D-7359 (1973).

15\. J.J. Wortman and P.C. Kassel, \"Metal-Oxide-Silicon Capacitor
Detectors for Measuring Micrometeoroid and Space Debris Flux\"
(submitted to Journal of Spacecraft and Rockets, 1994).

16\. D. Kessler, \"Origin of Orbital Debris Impacts on LDEF\'s Trailing
Surfaces\", LDEF-69 Months in Space: Second Post Retrieval Symposium,
NASA CP 3194, pp. 585-593 (1993).

17\. D.J. Nauer, \"History of On-orbit satellite Fragmentations\", 7th
ed., Teledyne-Brown Technical Report CS93-LKD-018, July (1993).

18\. C.R. Maag, W.G. Tanner, T.J. Stevenson, J. Borg, J.-P. Bibring,
W.M. Alexander and A.J. Maag, \"Intact capture of hypervelocity impact
particles and ejecta\", First European Conference on Space Debris, April
5-7, Darmstadt, Germany, ESA SD-01, pp. 125-130 (1993).

19\. J. Edelman, ed., \"Summary: Second International Space Forum in
Toronto\", Space Flight Environment International Engineering
Newsletter, Vol. 5, No. 2, p. 10 (May-June, 1994).

20\. C.G. Simon, J.L. Hunter, D.P. Griffis, V. Misra, D.A. Ricks, J.J.
Wortman, and D.E. Brownlee, \"Elemental analyses of hypervelocity
microparticle impact sites on Interplanetary Dust Experiment sensor
surfaces.\" LDEF-69 Months in Space: Second Post Retrieval Symposium,
NASA CP-3194, pp. 677-692 (1993).

APPENDIX

Table 1A. Optical survey results for IDE low sensitivity MOS sensors
listed by LDEF location in order of increasing impact flux. Sensor
dielectric thickness =1.0 µm; active surface area =19.6 cm2/sensor;
total exposure time =2106 days. In ground tests, these sensors responded
to iron particles \> 0.5 µm diameter at velocities \>2 km/s (Refs. 14,
15). LDEF data showed that the sensors responded to any impactor that
would produce craters in aluminum \>3 µm diameter, or penetrate a 2.4 µm
thick Al foil. Dimensions for craters are listed in microns (±5) as
central-crater-dia/outer-spall-max-dimension/fracture-zone-max-dimension.
A question mark (?) indicates that a measurement was not made; a dash
(-) indicates that the feature was not present or not discernable.
Sensors that gave all indications of having been active during the
entire LDEF mission were used to calculate impact fluxes are marked with
an asterisk (\*).

C = hypervelocity impact crater with no visual evidence of associated
discharge.

d = impact induced discharge, nominal central crater (\<14 µm diameter).

D = impact induced discharge, central crater/inner spall 14 - 20 µm
diameter.

D = impact induced discharge, central crater/inner spall 20-50 µm
diameter.

D = impact induced discharge, central crater/inner spall \>50 µm
diameter. Dtot = all impact discharges.

Sensor C d D D D Dtot Dtot+C

Number

Tray C-3 (\"west\", or \"trailing\" side)

\*1180 0 7 0 0 60/322/? \*7 7

\*1182 0 9 0 0 0 \*9 9

1183 ?/20/? 9 0 0 0 9 11

?/\~400/?

1191(short) ?/\~5300/45000 12 0 0 0 12 13

\*1192 0 6 0 0 0 \*6 6

\*1208 0 6 0 0 ?/\~100/? \*8 8

?/\~100/?

\*1213 0 7 0 0 0 \*7 7

1224 1 6 0 0 ?/\~500/? 6 7

1237 64/299/506 3 0 0 -/69/106 5 6

368/828/2484

\*1248 0 7 0 0 322/897/2116 \*8 8

\*1249 0 4 0 0 0 \*4 4

\*1268 0 2 0 0 0 \*2 2

\*1271 0 5 0 14/69/- 46/184/- \*7 7

\*1276 0 4 0 0 37/69/207 \*5 5

\*1294 0 4 0 0 0 \*4 4

\*1300 0 24 -/17/- 0 37/140/149 \*26 26

\*1305 0 ? ? ? 0 \*12 12

\*1310 0 26 0 0 0 \*26 26

\*1312 0 ? ? ? 0 \*5 5

\*1320 0 ? ? ? 0 \*6 6

1335(short) 37/322/- 0 0 0 0 1

\*1336 0 ? ? ? ? \*8 8

\*1342 0 2 0 41/248/- 0 \*3 3

\*1356 0 4 0 -/28/- 0 \*5 5

1359 7(secondaries) 18 0 -/30/- 0 19 26

1361 1 9 0 0 0 9 10

\*1365 0 4 0 0 0 \*4 4

\*1382 0 7 -/20/- -/30/- 1 \*10 10

\*1387 0 5 0 0 0 \*5 5

1395(short) 0 11 0 0 0 11 11

\*1401 0 25 0 0 74/322/391 \*26 26

1403 253/1104/2185 0 0 23/106/- 0 2 3

Table 1A (continued)

Sensor C d D D D Dtot Dtot+C

Number

NOTE: On C-3, 4 sensors out of 32 have craters \>1mm. Sensor 1191 was
cracked in half by a large impact. \*n=23, mean = 8.82±7.18. 92% of
impacts were from hypervelocity microparticles \~0.5 to 3 µm diameter.

Tray G-10 (earth facing side)

\*1172 0 21 0 0 0 \*21 21

\*1173 0 1 0 0 0 \*1 1

\*1174 0 1 0 0 0 \*1 1

\*1177 0 5 0 0 0 \*5 5

\*1210 0 10 0 0 0 \*10 10

\*1218 0 6 0 0 0 \*6 6

\*1219 0 16 0 0 0 \*16 16

\*1220 0 12 0 0 0 \*12 12

\*1232 0 14 0 0 0 \*14 14

\*1234 0 15 0 0 0 \*15 15

\*1239 0 12 0 0 0 \*12 12

\*1241 0 7 0 0 0 \*7 7

\*1279 0 6 0 0 0 \*6 6

\*1280 0 14 0 0 0 \*14 14

\*1282 0 9 0 0 0 \*9

\*1284 0 14 0 0 0 \*14\* 14

\*1290 0 9 0 0 0 \*9 9

\*1291 0 13 -/18/- 0 0 \*14 14

\*1297 0 8 0 0 0 \*9 9

\*1304 0 16 0 0 0 \*16 16

\*1322 0 7 0 0 0 \*7 7

1323(short) 0 1 1 0 0 2 2

\*1326 0 2 0 0 0 \*2 2

\*1349 0 19 0 0 0 \*19 19

\*1350 0 14 0 0 0 \*14 14

\*1351 0 20 0 0 0 \*20 20

\*1360 0 19 0 0 0 \*19 19

\*1378 0 19 0 0 0 \*19 19

\*1381 0 23 0 0 0 \*23 23

\*1386 0 10 0 0 0 \*10 10

\*1390 0 16 0 0 0 \*16 16

\*1391 0 19 0 0 0 \*19 19

NOTE: On G-10 sensors there were no large impacts. Sensors 1234, 1291,
1297 and 1381 have Al debris spray droplets from distant impacts. No
impacts into sensor frames were noted. Sensor 1297 had a piece of Fe/Si
rich meteorite residue contained in an Al debris spray droplet
(apparently from an impact into the LDEF walking beam). Sensors 1172 and
1297 have suspected wastewater droplet residues. \*n = 31, mean = 12.2±
6.01. 99.7% of impacts were from particles 0.5 - 3 µm in size.

Table 1A (continued)

Sensor C d D D D Dtot Dtot+C

Number

Tray H-11 (space facing side)

\*1193 0 14 0 37x55/138/- 138/290/370 \*18 18

138x184/634/644

101/370/414

\*1194 0 16 0 18x32/69/- 92x110/299/350 \*23 23

23x55/74/74

32/175/359

37/161/-

46/88/-

46/99/299

\*1195 0 10 -/14/- -/23/- 50/253/- \*17 17

-/18/- 37/175/-

-/18/- 14/101/-

1203 83/529/529 14 18/115/- 23/37/- 0 19 20

23/41/-

46/207/-

50/258/276

\*1205 0 24 -/18/- -/23x28/- 92/575/644 \*29 29

32/46/-

41/-/-

1226 (short) 11 (see size 1 9/14/-.(C) 23/55/- (C) 60/207/331 (C) 1 12

listings to right) 12/35/- (C) 23/69/- (C) 92/230/276 (C)

14/41/- (C) 23/138/- (C) 97/313/- (C)

37/92/207 (C) 92x138/276/437 (C)

\*1244 0 15 18/46/- 23/-/- 69/368/- \*23 23

18/55/- 23/69/- 69/552/-

37/175/- 92/552/-

\*1254 0 13 -/18/- -/23/- 92/483/493 \*21 21

14/37/- -/23/-

18/115/- -/32/-

46/166/265

\*1255 0 18 0 0 1 \*19 19

1261 1 15 1 3 2 21 22

1296(short) 0 7 2 0 3 12 12

\*1303 0 27 3 2 1 \*33 33

\*1313 0 21 3 1 1 \*26 26

\*1340 0 28 0 1 6 \*35 35

\*1343 0 12 2 0 1 \*15 15

\*1370 0 16 2 0 0 \*18 18

\*1371 0 18 2 0 1 \*21 21

1372 2 21 3 1 0 25 27

\*1379 0 19 2 1 4 \*26 26

1385 (short) 2 13 2 3 0 18 20

1399 1 21 1 0 0 21 23

1400 2 15 1 0 2 18 20

Table 1A (continued)

Sensor C d D D D Dtot Dtot+C

Number

NOTE: On H-11, 5 sensors out of 22 have craters \>1mm; sensor 1385 has 2
large craters. Sensors 1203, 1226, 1340, 1379, and 1400 have Al debris
sprays from impacts into their frames. Sensors 1195 and 1226 have
suspected wastewater droplet residues. \*n = 14, mean = 23.1±6.0. 77% of
impacts were from particles 0.5 - 3 µm in size.

Tray B-12 (\"north\" side)

\*1175 0 110 28 6 0 \*144 144

\*1202 0 104 11 13 0 \*128 128

1217 5 44 6 2 3 55 60

\*1278 0 107 6 2 5 \*120 120

\*1298 (1) - - - - \*132 133

\*1324 0 145 4 3 3 \*155 155

\*1352 0 114 5 5 2 \*126 126

\*1384 0 121 26 6 - \*153 153

NOTE: On B-12, 5 sensors out of 8 examined have craters\>1mm. Sensor
1298 has 2 large craters.

\*n = 7, mean = 137±13.8. 85% of impacts were from particles 0.5 - 3 µm
in size.

Tray D-6 (\"south \"side

\*1186 0 100 20 10 8 \*138 138

1187(short) 30 ? ? ? 22 52

\*1190 0 122 22 18 0 \*162 162

\*1212 0 116 20 6 2 \*144 144

1225(short) 15 ? ? ? 82 97

1252(short) 14 ? ? ? 37 51

1253 4 64 12 11 9 96 100

\*1263 1 130 21 7 0 \*158 159

\*1311 0 137 23 3 2 \*165 165

\*1314 0 89 29 6 0 \*124 124

NOTE: On D-6, 3 sensors out of 10 examined have craters \>1mm. Sensor
1225 has a 6 mm hexagon crack/spall around a large crater. \*n = 6, mean
= 149±16.0. 78% of impacts were from particles 0.5 - 3 µm in size.

Tray C-9 (\"east\" or \"leading\" side)

\*1176 0 ? ? ? ? \*357 357

1293 10 ? ? ? ? 200 210

1333 11 121 13 11 6 151 162

1334(short) 53 ? ? ? ? 152 205

\*1355 0 321 39 21 7 \*388 388

\*1383 0 339 18 ? 10 \*367 367

\*1396 0 356 46 22 10 \*434 434

\*1406 0 305 47 12 8 \*373 373

NOTE: On C-9, 8 sensors out of 8 examined have craters \>1mm. One sensor
has 3 large craters. Sensor 1355 has a large debris spray from an impact
into its aluminum frame. Sensor 1293 has a suspected wastewater droplet
residue. \*n=5, mean = 384±30.2. 85% of impacts were from particles
0.5 - 3 µm in size.