SUMMARY REPORT - space-env.esa.intspace-env.esa.int/R_and_D/eureca/Sum_Rpt.pdf · Contract No. 11887/96/NL Meteoroid & debris Flux & Ejecta Modelling SR CONTENTS 1 OVERALL OBJECTIVES

Contract No. 11887/96/NL Meteoroid & debris Flux & Ejecta Modelling SR

METEOROID and DEBRISFLUX and EJECTA MODELS

ESA CONTRACT NO. 11887/96/NL/JG

SUMMARY REPORT

Contributors:

Unispace Kent (UK)J.A.M. McDonnell A.D. Griffiths J.C. Zarnecki

D.J.CatlingS.F.Green

M.A.FowlerN. McBride

E.A.Taylor

HTS (Switzerland)C. Lemcke

T.Schilliger

MMS (Filton, UK)L. Berthoud

M.K. HerbertJ.D. Hobbs

R. Turner

ONERA (Toulouse,France)

J.C. MandevilleM. Rival

SAS(Belgium)R.Aked

W.Carey

Century Dynamics (UK)L AbellG.E. FairlieN. Robertson

Open University/OxfordBrookes (UK)M.M. GradyG.A. GrahamA.T. KearsleyI.P. Wright

TUBS (Germany)J. BendischD. Rex

TUM (Germany)E. Schneider

ConsultantsE. Grün K. Paul

UNISPACE KENT December 1998


ESA STUDY CONTRACT REPORT

ESA CONTRACTNo.11887/96/NL/JG(SC)

SUBJECTMeteoroids & Debris Flux and Ejecta

Modelling

CONTRACTORUnispace Kent

ESACR( )No

STAR CODE No. of volumes 1This is Volume No. 1

CONTRACTOR’SREF:

ABSTRACT:

This document is the Summary Report of the Meteoroid and Debris Flux and Ejecta ModellingContract. Full reporting of the work packages is presented in Technical Notes 1 through 9.Results from each work package are also presented in chapters 1-9 of the Final Report togetherwith overall conclusions in chapter 10.

The work described in this report was done under ESA contract.Responsibility for the contents resides in the author or organisation that prepared it.

Names of Authors: Edited by J.A.M.McDonnell and A.D.GriffithsContributors: M.K.Herbert, L.Berthoud, J.C.Zarnecki, W.C.Carey, J-C.Mandeville, C.LemckeG.A.Graham, G. Fairlie, D.J.Catling

NAME OF ESA STUDY MANAGERG.Drolshagen

DIV: EMDIRECTORATE: TOS

ESA BUDGET HEADING:N3201/061.512/96.WO7 + W10


CONTENTS

1 OVERALL OBJECTIVES AND STUDY PLAN 11.1 Overview 1 1.2 Data Sources 41.3 Data Processing 5 1.4 Flux Data Processing and Fitting 7 1.5 Analysis and Results of Spatial Distributions 11 1.6 Modelling 13

2 IN DEPTH ASSESSMENT OF CURRENT STATUS 16 2.1 Residue Analysis 16 2.2 Impact Calibration for Solar Arrays 20 2.3 New Studies Defined 21

2.3.1 Hypervelocity Impact Penetration 21 2.3.2 HST SA Photo CD Scanning 25 2.3.3 Hydrocode Simulation of Solar Array Impact 25 2.3.4 Impact Ejecta Studies 25 2.3.5 Impact Residue Analysis 25

3 NEW RESULTS 26 3.1 Impact Penetration: SA Damage Equations 26 3.2 Solar Array Photo CD Scanning 30 3.3 Hydrocode Simulations of Solar Array Impact 30 3.4 Ejecta Studies 31 3.5 Impact Residue Analysis 33 3.6 Solar Array Flux Data 34 3.7 Modelling and ESABASE Enhancements 35

4 SYNTHESIS OF IMPACT ENVIRONMENT: CAPABILITIES IN SUMMARY 384.1 Flux Anisotropies in Time and Direction 38

4.1.1 Earth Apex Bias 384.1.2 Alpha and Beta Particulates 384.1.3 Meteoroid Streams 384.1.4 Space Debris 384.1.5 Secondary Impact Environment 38

5 ACHIEVEMENTS OVERVIEW 405.1 Impact Data from Optical Surveys 405.2 Analysis of Impact Test Data 405.3 Impact Simulatlions 415.4 Analysis of Residue Studies 415.5 Ejecta Modelling 415.6 Simulation 425.7 Data Interpretation 425.8 Further Studies: Residue Analysis, Photo CD Scanning and Enhanced Simulations Using

Hydrocodes 435.8.1 Residue Analysis 435.8.2 HST photo CD Analysis 435.8.3 Hydrocodes 43

5.9 Enhanced Simulation and Synthesis 44

6 TABULAR DATA 44

7 REFERENCES 51

Contract No. 11887/96/NL Meteoroid & Debris Flux & Ejecta Modelling SR

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METEOROIDSEURECA

Solar ArraySPACE DEBRIS

HUBBLE Solar Array

Analytical Engine 1

WP1, WP2, WP4

LDEF + MD SIG

Explorers 16 & 23

Pegasus Series

Solar Max

Impact Processes WP3, WP5, WP8

Calibration WP2, WP3

Analytical Engine 2

WP7

SYNTHESIS & OUTPUT

PRODUCT (WP10) REPORTS

Advanced Modelling

WP9

Simulation ESABASE MASTER

WP6

Figure 1 Elements of the space input to the Database. Analytical Engine 1 represents the scrutinyand QA of impact data sources; Analytical Engine 2 the processing of such flux data with fluxsensitive calibrations leading to the comparison of such data in common parameter space.Comparison with modelling leads to update of model parameters.

impact craters on glass. For the enhanced modelling, Work Package 9, where we generatepotential improvements in the modelling, we have therefore the best available space impact dataand impact parameters; the advanced modelling supports, in interactive fashion, the critical scrutinyof data taking place in Work Package 7. It is necessarily multifarious and this is, perhaps, a virtue.As input to the advanced modelling, we take advantage of the different and complex exposures ofthe data set; only with these differences of exposure can we decode the source complexities fromboth meteoroids and space debris. Aspects of the question ‘Debris or Meteoroids?’ are addressedin Work Package 5 and Work Package 8 of the Technical Notes. We see here though that, infuture, we must always address both these aspects of the environment.

A vital case study in the comparisons of differing data sources, which led to verydifferent predictions of model flux parameters, was that of LDEF and EURECA (and HST). Thegeocentric attitude stabilisation of LDEF and the heliocentric attitude stabilisation of EURECA ledto opportunity for model development in both meteoroids and space debris and a separation of the


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two components from flux considerations rather than form chemical residue studies. These twoareas will be shown now to be in good accord on the question.

The output product of the study is:• better understanding of the debris and meteoroid environment with emphasis on the

LEO satellite arena;• coherent penetration equations for materials (especially solar cell arrays and aluminium)

and equivalence in terms of a common reference material;• flux distributions in terms of impactor diameters and information on other properties

such as density, shape and flux anisotropy;• an enhanced tool for study of the impact environment (ESABASE 2.01)• identification of undefined aspects of the environment and study areas ahead.

W P 1

S u rv e ys

W P 4

R e sidu e A naly se s

W P 2

C alibrat ion s

W P 3

O th e r Fac ilitie s/

Te st s

W P 5

Seco n dary E je c ta

W P 6

M o del P re d ictio n s ESA BA SE

M ast er

W P 7

D ata In te r- p ret a tion

W P 8

Test s O th e r

Fac ilit ie s

Fac ilit ie s EM I M u n ich

M P I -H TU M

H y droc ode s ( C D)

W P 10

Fin a l R ep o rt

W P 9

En h an ce d M o de lling

to W P 2 C o n su ltan ts: K . P au l E . Grü n

N . M c Bride A .D . G rif fith s

D . Gardn e r

Figure 2. Key planning strategy and synergisms for the study plan with authorship of majorreporting.


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The modelling enhancements follow the results of studies from a number of contractors andconsultants within this contract. The Executive Summary reviews the contract in thematic orderand makes no attempt to accredit particular workers to particular achievements.

1.2 DATA SOURCES

Long Duration Exposure Facility (LDEF)The majority of dust detection experiments on LDEF involved passive detection. In addition,

large areas of the spacecraft not specifically prepared for impact experiments were exposed to thedust environment. These surfaces and experiments differed either in target material or targetthickness. In order to obtain a uniform data set, only impacts in Aluminium surfaces were used.This procedure means that potentially large areas of exposed surface such as thermal covers for theUltra-High energy Cosmic Ray Experiment (UHCRE) have not been used; the aim is, however, togive uniformity and reliability of data. The scaling factors for conversion of crater dimensions tothose in Aluminium are poorly defined for most materials and the crater structure in composites ormulti-layered materials is significantly different.

LDEF data sources used in this analysis were:• UKC Micro-Abrasion Package (MAP) (McDonnell et al, 1984). Aluminium foil perforation

fluxes on East, West, North, South and Space faces. The top foil surfaces ranged inthickness from 1.5 to 30µm.

• Craters on Map foil surfaces. Extending flux measurements below 1µm.• Space Debris Impact Experiment (SDIE) (Humes, 1984). Craters in Aluminium in the

diameter range a few to ~350µm on all peripheral faces. Since the resolution of quoteddiameters was ~ 5µm, the cumulative fluxes were determined for crater diameters at thelower end of each diameter "bin" before conversion to Fmax. Data were obtained from theM&D SIG database and Humes (1991) for Space face.

• Data for spacecraft structural surfaces is available from the Micrometeoroid and SpaceDebris Special Investigator Group (M&D SIG) database (See et al, 1991a,b; Zolensky et al,1993). It includes:

• Craters on Intercostals and Longerons,• Craters on clamps,• Craters on Al surfaces of experiment S0010 (Exposure of Spacecraft coatings – i.e. not a

dedicated meteoroid and debris experiment)• Craters on experiment A0187-1 (Chemistry of Micrometeoroids Experiment CME) (Hörz

et al, 1991),• Craters on clamps measured with UKC SEM,• Space face data from Love et al. (1995). European Retrievable Carrier (EURECA) All Post Flight Analysis (PFA) impact data from the EURECA spacecraft (UnispaceKent et al. 1994) were made available. The different spacecraft surfaces surveyed for impactdamage include:• Spacecraft solar cell arrays and rear surface kapton thermal blankets• Beta cloth Multi-Layer thermal Insulation (MLI)• The two painted aluminium signs (or logo) and two scuff plates• The Timeband Capture Cell Experiment (TiCCE) which consisted of aluminium foils of

thickness 2.5, 3.2 and 9.2 µm. Its pointing direction in Heliocentric space proved valuableto the modelling.


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Hubble Space Telescope Solar Arrays (HST) The HST data set consists of PFA impact data from the single retrieved solar cell array;unlike the EURECA rigid panel design the HST array consisted of solar cells mounted on a ~0.2mm thick flexible blanket allowing the largest particles to punch straight through the array. Thiskind of paired front (sun facing) and rear face damage features is not seen on the ~23 mm thickEURECA arrays. No “through perforations” had been detected on EURECA although one wassuspected but subsequently discounted.

Historic Spacecraft DatabaseWe incorporate a subset of reliable well referenced spacecraft impact flux measurements

recorded in the time period 1962-1984 (McDonnell & Baron 1993, McDonnell et al 1997 andreferences therein). Figures for launch date, operational altitude and detector thickness (convertedto a common reference material) are given in table 1. The detected flux has been corrected to takeaccount for differing values of gravitational enhancement and earth shielding for each spacecraft.The influence of orbital debris is considered to be negligible during this time frame giving us ourmost reliable measurement of the natural meteoroid flux in low Earth orbit. These reference fluxdistribution is compared to modelling predictions.

Launch Mean Equivalent Detected CorrectedSPACECRAFT Date Altitude Thickness of Flux Flux

[km] 2024-T3 Al [µm] [/m2/s] [/m2/s]Explorer 16 16 Dec 965 45.82 3.86E-06 2.80E-06

1962 86.9 1.99E-06 1.44E-06Explorer 23 06 Nov 722 45 4.10E-06 3.04E-06

1964 91.8 2.40E-06 1.78E-06Ariel II 27 Mar 824 11.76 0 0

1964 14.7 5.26E-05 3.87E-05Pegasus 2-3 25 May 1965 627 198.94 2.42E-07 1.83E-07

30 July 1965 531 397.88 5.64E-08 4.27E-08Solar Max 14 Feb 1980 560 225.0 7.85E-07 5.99E-07MFE 22 Mar 1982 241 5 2.60E-05 2.20E-05

Table 1. Reduced data from key exposures of penetration experiments over the first 30 years inspace; valuable for examining the possible time dependence of the Meteoroid population and themore recent advent of Space Debris. The study relates these to recent space environmental models(Figure 8) We note especially the following experiments/exposed surfaces: Explorer 16 & 23 “beercans”: beryllium-copper or stainless steel pressure cells punctured by the impactor. Ariel II:detection of sunlight through penetrations in thin (12-30 µm) aluminium foil. Pegasus 1, 2 and 3:capacitive discharge on penetration of 37, 200 or 400 µm equivalent thickness of aluminium. SolarMaximum Mission: penetration of retrieved 125 µm thick aluminium thermal control louvers.STS-3/Microabrasion Foil Experiment (MFE) They can be readily related - with calibration - toLDEF and the NASA 90 & 96 models. 1.3 DATA PROCESSING For LDEF, which we give as an example of the precision afforded to the space data, theflux generation process took place in three stages, producing three sets of Excel workbooks): Each of the 14 faces are processed as a separate set of these 3 files. The last two sets of filescontain or modify data from the previous (parent) file and contain a directory path reference to


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their parent file in the header row of each work sheet to allow data traceability. The majority of thedata was acquired from the Meteoroid and Debris Special Investigation Group (M&DSIG)database at NASA Johnson Space Centre. The database can be accessed at the following URL:

http://www-curator.jsc.nasa.gov/curator/seh/ldef/db1.htm The database is now directly searchable via web browser using the URL (or by following thelink from the first URL):

http://sn-charon.jsc.nasa.gov/dbsearch/ldef/searchmenu.htm The raw data files available for each face were combined to form a “*contained the following columns for all impact features:

• spacecraft face,• feature ID,• exposed area,• measurement type (eye, M&DSIG, optical microscope, SEM),• measurement quality number/reliability factor (1 = lowest, 3 = highest),• measured ballistic limit in Aluminium, Fmax [µm] (where available),• measured geometric mean crater lip crest diameters [µm] (standard M&DSIG database

measurement),• measured crater depth, Tc [µm] (where available),• calculated Fmax [µm],• used Dc – calculated or measured (if available) crater diameter at original target surface level,• number of impacts (usually 1) that the entry refers to,• material (Aluminium, etc.),• surface – origin column from M&DSIG surfaces file),• date that data was added/changed,• reference to the file that the data was imported from (file 1, etc.: directory path is defined in

first row of the worksheet)• comment (carried over from reference file). The M&DSIG features data file forms the majority of the new file (contributing feature ID,geometric mean diameters [µm] and comment columns) which is split into 3 leaves (or pages) inthe spreadsheet depending on the impacted surface type – i.e.:• Experiment (the impact occurred on an aluminium experimental surface)• Peripheral (the impact occurred on an aluminium clamp, intercostal, longeron or other

spacecraft structural surface on the periphery of the experiment trays)• Other Material (the impact occurred on a non aluminium surface) For experiments the data was sorted into blocks of data on the basis of principle investigatorand/or surface area. The peripheral data was similarly split into clamp, intercostal and longeron.The M&DSIG data contains so called “exceptions records” which consisted of a sub-set of thesame craters measured at a different NASA site. These are considered redundant and thereforewere discarded from the data set. Surface areas were taken directly from the M&DSIG surfaces file unless more reliable datawas available in published papers. Examination of the initial size distributions for the intercostalsshowed that for Dc � 250 µm data was only available in the M&DSIG database for 4 of the


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available 6 intercostals on each face. Therefore two intercostal size distributions were calculatedwith the smaller distribution using 2/3 of the total intercostal area. The measured crater lip rim diameters, Dr were converted to crater diameter at the originallevel of the target surface, Dc using the expression

Dc = 0.75 Dr due to Humes 1992. The conversion from Dc to Fmax (McDonnell et al 1993) proceeds as follows:

Fmax = (Fmax /Tc)*(Tc/Dc)*Dc

where: (Fmax /Tc) = 1.5 and (Tc/Dc) = 0.58 1.4 FLUX DATA PROCESSING AND FITTING The experiment, peripheral and (where appropriate) Microabrasion Abrasion Package (MAP)data sheets form each face’s “.org” file were transferred to a corresponding “.fit” file so that a fluxplot could be produced to allow the fitting of a “best fit” flux. During this process the measurementtype and quality factor columns were removed and a cumulative impact count column added.Additionally column headers for flux and Poisson error (along with copies of the Dc and Fmax

columns) were added to assist the flux plotting (see below). The Cumulative Flux was calculated, after appropriate binning of the data, in theconventional manner namely as the ratio of cumulative impact count to the time area product foreach impact in a data block. The LDEF exposure time (= 1.82×108 seconds) is a global variable inthe header rows of each work sheet. The flux error was calculated using Poisson statistics as theflux divided by the square root of the cumulative number of impacts. This error estimate could beunreliable at small sizes (high fluxes) where “roll–off” may occur (due to some small crater beingmissed). The calculated flux (with Poisson errors) for each data block was then plotted against Dc andagainst Fmax on two Excel charts. Also plotted on each chart were lines for the “Best Fit” and upperand lower estimated error limits. The fitting of the “Best fit” and error lines was performed bydragging points in each line to the appropriate position using the mouse (which in Excel 5automatically updates the worksheet data values on which the line is based).

HST and EURECA Solar Arrays

The solar arrays from HST and EURECA were recovered after prolonged exposure todust and other space debris. Being the focus of this contract some pertinent details areappropriate:

EURECA . In August 1992, EURECA was deployed into a 425x424km altitude orbit from whichit manoeuvred itself into the 508x502km operational orbit. The purpose of EURECA was toprovide an unmanned retrievable platform for performing microgravity experiments. Like HST, thestructure of EURECA consists of a main spacecraft body flanked by two wings containing the solarcells. In June 1993, after 326 days exposed to the low earth orbit meteoroid and debrisenvironment, EURECA was recovered by Space Shuttle Endeavour (STS-57). Both of the solararray wings were returned intact for post flight analysis although only data from the -X wing hasbeen analysed for this report. The -X wing of EURECA is again made up of five sections, denotedas F1-F5, F1 being closest to the main spacecraft body.


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HST. The mission was launched in April 1990 into a 614km low earth orbit with the primaryobjective of performing astronomic observations not possible on the Earth’s surface. The structureof HST is composed of a main body telescope, which is powered by a solar array comprising twodouble roll out wings each with two flexible blankets. Due to a problem with one of the “booms used to unfold and retract the wings, the +V2 wing was jettisoned in orbit. The -V2 wing,however, was successfully removed by the astronauts of the space shuttle Endeavour (STS-61).This task was performed as part of the first service mission after 1320 days exposure to open space.The returned -V2 wing of HST is comprised of two blankets, designated as “upper” and “lower”,each containing five Solar Panel Assemblies (SPA’s). For identification purposes these been labelled A-E and AA-EE for the upper and lower blankets respectively. The SPA’s aredivided into three solar-cell strings, each containing 106 solar cells in series. Data Description. Upon retrieval, the solar arrays from both HST and EURECA were scanned forimpact features. During scanning, arbitrary origins were identified to act as the basis for positionco-ordinates of relevant features. From this data, impact craters were classified according to theirposition on the array with respect to the origin. Hence the raw data received for this analysis wasin the form of a crater identification label with subsequent x and y position co-ordinates

EURECA Flux versus Conchoidal Fracture Diameter Fit

1E-10

1E-9

1E-8

1E-7

1E-6

1E-5

1E-4

0.01 0.1 1 10

Conchoidal Fracture Diameter [mm]

Cumulative Flux [m -2s -1]

Best FitUpper LimitLower Limitf1 swathf2-10 swathESTEC SEM

Figure 5. Flux fitting results (thick line) and the data for EURECA’s Solar array for theConchoidal crater feature (diameter Dco) in the cover glass. The face is accessible to bothmeteoroids and Space Debris throughout part of its Earth orbit, as is HST (Figure 6)


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HST Flux versus Conchoidal Diameter Fit

1E-10

1E-9

1E-8

1E-7

1E-6

1E-5

1E-4

1E-3

0.001 0.01 0.1 1 10

Conchoidal Diameter [mm]


Best Fit

Upper Limit

Lower Limit

Survey

Catch All

Berthoud SEM

Berthoud Optical

Figure 6. Flux fitting results (thick line) and the data for Hubble Space Telescope Solar array forthe Conchoidal crater feature (diameter Dco) in the cover glass.

Figure 7. Flux data from historic space penetration or impact measurements, reanalysed and scaledto the equivalent thickness of aluminium 6061 T6. It is compared to the meteoroid fmax fluxcalculated from ESABASE using the Grün distribution for the LDEF 6 point average at 500 km(comparable to a random tumbling plate). Compatibility with a constant meteoroid flux over longperiods is demonstrated.


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For EURECA, the wing was only divided up twice from the total area. Firstly into fivepanels (F1 - F5); these were then split into halves along the y axis. As with HST, the total area andthe sections F1 - F5 showed a random distribution of impacts. However, unlike HST, the nearestneighbour statistic did not show such a variation of values when the panels were divided into halvesdue to a larger number of craters in each section. This result is unexpected when the times ofexposure of the two arrays are compared. HST was subject to almost four times the length ofexposure than EURECA. The differences between the array areas must also be taken into accountif an accurate comparison is to be made.

Clustering test for HSTBecause the nearest neighbour approach might not recognise non-random spatial

distributions from a number of clustered sources, a test was performed on the HST SA to establishits sensitivity on a quantitative basis.Results show that 40% of the impacts would have to be concentrated entirely in only 20% of theSPA area for significant detection. Clearly this would be seen in the variance spatial studies referredto below and so this nearest neighbour approach is not very sensitive to a single cluster. By thesame token, perhaps the actual existence of secondary crater groupings need only be small to showup in the variance studies.

EURECAThe standard deviations results from EURECA showed that only 60% of the data points

lie within ± 1 standard deviation. A possible reason for this discrepancy is the low number of binsthe data is divided into. This is made more probable by the fact that when the data is further splitinto ten bins, the condition previously described are all adhered to. Hence this again suggests anormal or Gaussian distribution of the data.

Results in contextIn addition to spatial techniques to study clustering, analysis of the significance of

variations is made using Poisson Statistics. We examine for the surfaces, the actual countcompared to the mean expected; the mean was calculated from the total on each row and on eachcolumn of the rectangular arrays in all cases. The approach establishes the variance of each cell andidentifies those cells in bands corresponding to deviations of ± 1 sigma, 2 sigma etc. for theconchoidal impact crater features. The method has the advantage of simplicity and readyunderstanding; it is a first approach prior to either cluster analysis or to establishing trends. Smalltrends are not highlighted in the variance tests and yet can, of course, be extracted by fittingthrough the mean values of cells or rows. Strictly speaking, the variance should be studied relativeto such trends, but because the trend is weak, the loss of accuracy is trivial. For the EURECA SAsurfaces we have shown the trends as a function of distance for the spacecraft which is attributed tospacecraft shielding of meteoroids and secondary ejecta from impacts on the spacecraft body. Thedistribution is consistent with a Poisson (random) distribution based on the mean for any panel. ForHST the fit through the columns as a function of distance from the SA spreader arm and as afunction of distance from the spacecraft for both SPA sets is puzzling. Certainly the data is not“well distributed” and, in fact, very little conforms to the normal expectation of a Poissondistribution in terms of the number of sub-area divisions being within +/- 1, 2 sigma counts etc.

Reasons for this distribution are not clear and yet, because they are not understood, itwould be premature to jump to the conclusion that the flux was in some significant way “grouped”.The distribution could be biased by small differences in the array thickness, crater visibility etc. Or,there may be a quite small real non-randomness due to secondaries on top of a random background.

It is an area which would benefit from further study; which needs to be studied andresolved in terms of crater dimension also, noting the identification and overall influence ofsecondary cratering on EURECA. In the case of EURECA the secondaries are identified only at


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the smaller dimensions (800 microns); this would need to arise on HST at somewhat largerdimensions to be seen in the survey data but could result from the larger angle subtended by thetelescope body and the longer exposure of 4 years. The area-time product determines the largestintercepted particle and the largest secondary crater. The wide field camera on HST is well placedto act as a host surface for generating secondary impactors and is relatively efficient (compared tothe low efficiency of thermal Blanket). Residue studies of the chemistry in the HST microcratersnow provide very evidence to the existence and origin of such matter. Work reported in TechnicalNote 4, led by ONERA in this contract, addresses the modelling of secondary cratering onEURECA and HST; in Technical Note 8 we present residue studies.

For LDEF, and the targets thicker than the MAP foils, we have a less homogenoussource; certainly the trays where intercostal survey was conducted show variances exceeding the 1sigma expectancy but, significantly, only for the smaller craters. We are, unavoidably, faced withroll-off in the flux distributions analysis; the apparently high count of cells exceeding the normalvariance is best viewed in the context of flux distributions where both size and pointing directioncan be considered. Examining firstly whether such variances could arise from observational effectsor bias, it is noted that the intercostal counts are made by different members of a team of PostFlight analysts; several problems arise:

• different observers have differing visual acuity.• different lighting conditions may pertain• different degrees of spacecraft contamination (masking) exists• differing surface heterogeneity exists.

Because of the high slope of the flux distribution, counts down to a particular crater sizeread a value strongly dependent on the smallest crater reliably counted by an observer. Roll-off canbe seen to reduce, typically by a factor of 2, the crater count at some 2 to 3 times greater than thesmallest reported crater. Occasionally, a single crater 10 times less than the “roll-off figure” will bediscovered - perhaps under “perfect viewing conditions or a small polished section of the metal -and duly reported. The effect on the flux distribution is dramatic!

SummaryThe results of the test performed to date show a distribution of primary impact craters

not inconsistent with random distribution except (i) where there are specific instances of small butnumerous secondaries from line of sight impacts on other surfaces and (ii) a trend on the EURECASA assemblies clearly associated with secondaries from impacts on the spacecraft body(ESA/ERNO Signs). The spatial distribution on HST Solar Arrays is not fully understood, but theexposure relative to the spacecraft body (shielding and potential secondary ejecta generation)cannot be examined in full.

1.6 MODELLING

IntroductionIn order to model the impact exposure of a spacecraft to the meteoroid environment, one

must consider the mass distribution of the meteoroids (or rather the flux distribution), their velocitydistribution, the effect of the Earth (gravitational enhancement, and shielding), the exposuregeometry of the spacecraft and the effect of its motion, and the hypervelocity impact response ofthe spacecraft surfaces. These elements have been combined into a meteoroid model applied toLDEF and EURECA at Canterbury; it mimics the advanced ESABASE modelling at HST


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performed in close co-operation; the exchanges on anomalies were valuable in developing adequateQA. Elements are briefly described:

Meteoroid mass distributionThe ‘Grün flux’ (Grün et al. 1985) is generally used to model the natural sporadic

meteoroid influx to Earth. The flux values are given as cumulative mean values (i.e. time averagednumber of particles, m-2 s-1, of individual masses >= M) for a spinning flat plate detector at 1 A.U.,outside the gravitational influence of the Earth (but moving in an Earth-like orbit). This Grün fluxwas derived from a variety of sources (meteor, lunar crater, and spacecraft data) and isotropy ofthe meteoroid environment was not assumed per se, but in defining the flux as that to a spinning flatplate, the model is essentially ‘isotropic’.A parametric form of the cumulative flux, F(M) (M in g), at 1 A.U. is written (from Grün et al.1985) in the form of a polynomial in log space as a function only of particle mass, at a velocity of20 kms-1 and particle density ρ = 2.5 gcm-3.

For modelling purposes (see below) it is useful to use a differential distribution i.e. theflux of meteoroids with mass within a given mass interval dM, where dM is taken in equallogarithmic steps; this was performed.

Meteoroid velocity distributionThe derivation of the Grün flux was performed at an assumed meteoroid mean speed of

20 kms-1 but when using the Grün flux in a detailed model, we incorporated the entire meteoroidvelocity distribution. This was especially important when considering detectors or impact damagewhich is velocity dependent. The velocity distribution used here originates from the Harvard RadioMeteor Project (HRMP) data where ~ 20000 meteor observations were taken (Southworth andSekanina 1973; Sekanina and Southworth 1975) so offering a statistically reliable data set. Taylor(1995a) reappraised the data using an improved analysis of ionisation probability and massdistribution index. Taylor also identified a numerical error in the original code used to reduce thedata which resulted in a significant under-estimation of numbers of fast meteors (particularly 50 to70 kms-1 meteors where the under estimation is by a factor of ~ 100).

The Long Duration Exposure Facility (LDEF) provides the single largest source ofimpact data from Earth orbit. LDEF, which was recovered by the Shuttle after 5.75 years exposureto the LEO particulate environment (at altitude ~ 470 km), was a 12-sided cylindrical satellite in agravity gradient stabilized orbit, such that it maintained an essentially fixed orientation with respectto Earth. The satellite was aligned with the ends pointing in ‘space’ and ‘Earth’ directions, and theside faces maintaining the same geometry relative to the orbital velocity vector. This is important,because the exposure of the space face with respect to heliocentric co-ordinates, was effectivelyspinning (with a period of ~ 90 minutes) resulting in an essentially isotropic exposure tointerplanetary space. Thus, the application of an isotropic meteoroid model is particularly valid forthe space face, and to a lesser extent, the spacecraft as a whole. The space face offers an excellentmeteoroid detector since for geometric reasons, little orbital debris can impact it. As isotropy canbe assumed for LDEF, one does not need to consider in detail the actual orbital history because oneinstantaneous flux contribution is the same as another. One need only consider the orientation withrespect to Earth, and the relative motion of the spacecraft.

Model ResultsBy applying the model in the way described above, predictions of the flux as a function

of ballistic limit Fmax was obtained for each face of LDEF. Model predictions for all the faces arecompared to the actual data. The space face (accessible only to meteoroids) is taken as the test ofthe model.


15

LDEF Space face: ESABASE and Measured Best Fit Comparison

1E-10

1E-9

1E-8

1E-7

1E-6

1E-5

1E-4

1E-3

1 10 100 1000 10000

Ballistic Limit in Aluminium [µm]


NASA 96 & HRMP

NASA 96 & Grün

Best Fit

Figure 8. Test of the impact exposure model for meteoroids applied to LDEF’s Space face(inaccessible to Space Debris). The data show excellent correspondence to flux distributioncalculated from the Grün mass distribution but using a revised HRMP velocity distribution.Application of the model to other faces revealed additional contributions from Space Debris.

The East to West ratio yields values consistent with the data at medium and largedimensions (over 50 microns penetration, fmax). The Earth face is very low compared to all otherfaces, due to considerable Earth shielding; we see this as anomalous compared to the actual fluxdespite the attention given to it in the modelling!

Factors that may be relevant in understanding this anomaly in LDEF’s database are:

• The flux is highly dependent on the value of Earth shielding used which is dependent onmeteoroid velocity and the effective atmospheric height.

• The impacts which do reach the plate are of grazing incidence and could yield crater dimensionsgreatly exceeding those at normal incidence. Certainly those visible by eye on the (painted) blacksurface resemble “streaks” rather than “stars”. The semi-major axis can be greatly enhanced bythe down stream ricochet effect.

Historic Spacecraft ExposuresHistoric spacecraft: The most reliable data from spacecraft flown prior to LDEF was

summarised in Table 1 and plotted in Figure 7. This figure compares the data (expected to be forthe most part debris free at this epoch) with ESABASE predictions for the Grün sporadic modelusing the Erickson velocity distribution. The ESABASE predictions are generally in goodagreement with the flight data (< a factor of 2) but randomly distributed above or below. Howeverthe ESABASE predictions are for a random tumbling plate in the modelled spacecraft’s orbit andwill not therefore reflect the peculiarities of detector orientation (with respect to the velocityvector) from spacecraft to spacecraft. Only the Solar Maximum Mission data (one of the mostrecent data sets) stands out a clear exception to the general agreement being up to two orders ofmagnitude above the meteoroid model at the smallest sizes. Laurance & Brownlee (1986)attributed this high flux to microdebris. However McDonnell and Sullivan (1992) subsequentlyshowed that the choice of the penetration equation used to relate crater or perforation diameter toimpactor diameter (in this case Pailer and Grün 1980) has a major effect on the value of Fmax finally


16

derived for a given flux level. Using the same penetration equation “twice” to convert damagediameter to ballistic limit (via particle diameter) they produced a much smaller flux excess for solarmax.

We illustrate, to judge the options available for characterising debris, the effect of twodifferent models of the space debris population, namely NASA 90 and NASA 96. Figure 9 plotsthe space debris compared to the Meteoroid (Grün) flux; this modelling of the different debrispopulations is a key element in Work Packages 6 and 9 of this contract.

1.E-12

1.E-10

1.E-08

1.E-06

1.E-04

1.E-02

1.E+00

1.E-01 1.E+00 1.E+01 1.E+02 1.E+03 1.E+04


FailureFlux [m -2s -1]

Grün

NASA 90

NASA 96

Figure 9. Space debris fluxes plotted from NASA 90 and NASA 96 compared to the GrünMeteoroid flux for a random exposed surface. Both models show that meteoroids exceed spacedebris for ballistic limits of 100 to 1000 microns. NASA 90, a previous baseline for space debrischaracterisation, was replaced by NASA 96 and improvements to the Grün distribution used in theenhanced modelling calculations and simulations performed with, but not developed within, thecontract.

2 IN DEPTH ASSESSMENT OF CURRENT STATUS.

2.1 RESIDUE ANALYSIS

A severe limitation of the analysis of residues, needed to be understood to address themeteoroid-debris partitioning of the total impact environment, is the suitability of host surfaces forpost flight analysis. The Solar Arrays - potentially extremely valuable for their large area-timeproduct are very diverse in terms of their physical, chemical and elemental composition. Thedifficulties experienced, and the resulting status, is summarised in the following conclusionsestablished within the contract; we look first at the pre-contract situation regarding the HSTanalyses (and further ahead we look at the EURECA analyses).

A total of 9 impact reports were reviewed, which indicated:


18

Figure 10. SEM image of HST SA section (above) and (below) elemental mapping showing thechemical signatures. Some 20 different elements are present within the array at manufacture, andunder impact may be incorporated within the impact melt.

3. On normalising the chemical analysis to a “per hole” basis, the following percentages werefound:


19

Type OUNatural 8%Debris 34%

Unknown 58%

Table 3 Summary classification of meteoroid and debris discrimination prior to the contract.

4. The elements present in the MLI materials significantly complicated the unambiguousidentification of residue material.

Comparison of EURECA/HST Data with LDEF/Solar Max/Mir DataAn interesting aspect which has become evident through the review of the chemical data

sets is that dedicated collector surfaces do not appear to have such an advantage as one wouldexpect over non-dedicated surfaces.1. In general, precisely the same “difficulties” regarding chemical characterisation were

encountered during the HST analyses as were during the LDEF, Solar Max and Miranalyses. Even in instances where dedicated collection surfaces were used, e.g. Aucollector surfaces on LDEF, problems in identifying crater residues remained, and inparticular - 50% of the impacts studied did not have an unambiguously determined chemicalsignature - this ratio remains very consistent throughout all studies to date, in that itrepresents the “best” figure available. Even where identifications are made however, theclassification is usually given rather tentatively.

2. The relative proportion of natural, debris and unknown impact types determined from theHST data compares very favourably with LDEF and Mir data, i.e.:

IMPACT TYPE HST DATA MIR DATA LDEF DATANatural 18 - 27% 23% 17 - 23%Debris 11 - 40% 9% 4 - 11%

Unknown 42 - 62% 68% ~ 50%

Table 4 Comparison of impact types in various data sets; the unknowns critically reduced theopportunity to discriminate between meteoroids and space debris prior to the new study.

Only HST data has been included here as there were no high level of chemical data setsavailable from EURECA. We see that the high percentage of unknowns restricted the refinementof the meteoroid to debris ratio, hence stimulating the drive for new techniques (Section 3.5).

The results of a second Solar Array residue study (performed by the Open University, NaturalHistory Museum and Oxford Brookes University) on HST samples will now be assessed; thetechnique employed represents a significant step forward in the analysis of impactor residuematerial.

A total of 29 solar cells were investigated in this study, all of which were extracted from SPAB. The activities carried out consisted of:1. An optical survey to locate impact features;2. SEM examination for imaging and chemical analysis:

- Backscattered electron imaging (BEI) to locate candidate residue material- X-ray mapping in 20 characteristic energy intervals (at low and high resolution)


21

for all the data Fig 14. The next ‘clump’ is from plasma drag accelerator data which can launchparticles of up to 80 micron up to about 10 kms-1. The final group of data is light gas gun datawhere, typically, mm-sized particles are launched at velocities from 3-8 kms-1.

The addressing of the problem of how to build an impact damage equation for alldimensions is illustrated by reference to figure 11. A section through the array shows a mediumsize penetration where the depth of penetration is comparable to the thickness of the solar cellcover glass (150 and 180 microns for EURECA and HST respectively). For smaller particles theimpact material is the cover glass and the medium semi-infinite; the ratio between damagedimension and particle dimension is fairly constant and affected only by a small dimensional scalingfactor. At the penetration distance shown, the ratio will vary quite strongly as critical barriers arereached regarding fracture, spallation, impedance changes and shock wave reflections. For verymuch larger particles, the target is thin; at infinite sizes the damage approaches the particle diameterwith only a small marginal growth around the ‘shadow’ of the particle as it projected through thecell. These points in the approach were folded into the priorities established for the impact testprogramme and the analysis of data to generate the (single function) damage equations.

For small particles, fit for Dco was found to be:

coD = 0.52 p0 .49ρ p

1.21d −0.10V 0. 27cos Θ

We review the impact test data and analyses leading to the damage equations presented in section3.1

Database consolidation of EURECA and HSTThe data from Post Flight Analysis has been well documented in numerous reports and

databases. We use the ESA image data from EURECA and HST comprising mostly Hirox imagesfrom some 500 microns dimension and above in terms of the conchoidal diameter. Other data isless numerous but, on the contrary side, extremely valuable in extending the range of definition ofthe impactor flux.

An important task in Work Package 2 of this contract was to review the PFA data and toascribe reliability factors; corrections if appropriate. Detailed measurement of the Hirox images,portrayed in figure 12 led to a dataset used as the input to the damage equations. 2.3 NEW STUDIES DEFINED 2.3.1 Hypervelocity Impact Penetration Damage equations for the quantitative understanding and accurate decoding of impacts forboth Space Debris and for Meteoroids was found to be adequately represented by the McDonnellSullivan ballistic limit equation for solid particles of varying density on metallic targets. Aluminiumwas taken as the reference material for different materials used for space exposures following anearlier review of space data by Baron (1996). The relationship between craters on a thick targets and the penetration of foils was wellunderstood from actual space impact craters; for larger penetrations of thin foils the GardnerCollier and McDonnell equation was available and consistent with the McDonnell Sullivanequation.


22

Figure 11. Schematic (sectional view) of an impactor penetration on a Solar Array; the dimensionand velocity yield a depth of penetration comparable to the cover cell thickness.


23

Figure 12. Hirox microscope images of the categories of solar array impacts . The categoriescorrespond to HST impacts for the front (cover glass) side but for impactors from the top (left) orfrom the back side (right). Features shown in the vivid colouration correspond to depth damagethresholds (see figure 11) which can be used to distinguish between the direction of impact(between top and back) and therefore the appropriate choice of damage equation.

We show (Figure 13) sectional views of the EURECA and HST solar array. For HST weneed to understand impact damage relationship for both sides; for EURECA, only one rearpenetration had been suspected but subsequently found not to be a through penetration.


24

Sca le: 1m m = 10 µ m

Solar Array Cell Cross-Section

Ka pto n H ( 25 µm )

FE P (1 2.5 µm ) ( to sca le)

ta ken fro m a pr es en ta tio n by L o tha r Ge rla ch E ST EC (XP G)

( to sc ale)

DP 4 6 97 1 ( 5- 10 µm ) ( to sca le)

t ake n fro m BA e ST SA re port (J u ly 1 9 94 )

~175µm Array Blanket

HSTSol ar Arr ay Fron t

S olar A rr ay R e ar

40µm

150µm

250µm

70-80µm

Silv er M es h (5 0 µm )

CM X cover g las s

D C 93500 A dh esi ve

B SFR So lar C ell

RTV S691Glas s fibre fill ed with DC 93500 (35 µm)

Glas s fibre fill ed with DC 93500 (35 µm)

Aluminium Honeycomb (not to scale )

22000µm

A l um in ium /K apto n Term al B la nk e t

120µm

50µm

Ca r bo n F ibre Re inf or ced Pla st ic

Solar A r ray R e ar

30µm

150µm

180µm

70µm

130µm Thermal B lanket

RTV S691

G lass fib re fi lle d w ith XB 30 52 (55 µ m)

B SF R so lar cel l

DC9 35 0 0

CM X cover slid e

Ca rb o n Fi bre R ein fo rc ed P la s t ic

120µm

Sol ar Ar ray Fron t

EURECA

o r = 26 .8 ×× 50 .1m m

= 1342.68 m m2 (type B)

Ratio to tal cell thick ness to cover slid e th icknes s = 2 .40

C over g las s = 20 .8 ×× 59.4mm

= 1235 .52 mm 2 (typ e A)

EURECAR atio to tal cell thi cknes s to cover

slid e th icknes s = 2 .93

C over glas s = 21.1 ×× 40 .5m m

= 854.55 m m2

HST

Figure 13. Sectional view of the EURECA and HST solar array. For HST we have impactpenetration from both sides; for EURECA, only one rear penetration through to the front of thesolar array had been suspected but was subsequently discounted.


25

The impact programme focused, therefore, on understanding Solar Array penetration;because HST and EURECA arrays were essentially similar, and because of the availability of HSTcells, most impacts were performed on HST cells. The planned programme centred on:

• Glass projectiles of varying size, namely 30 microns to several millimetres at constantvelocity

• Glass particles of varying velocity• Particles of different density

Impacts were to be performed on both sides of the SA cells and on Aluminium as a referencematerial. 2.3.2 HST SA Photo CD Scanning Although the database comprised a total of some one to two thousand useful impacts on theSA structures, for EURECA all had impacted from the Sun pointing side; for HST, although thePFA photography had logged impacts both sides in a survey, attention had naturally focused inquantitative terms on the front Sun pointing side; thus the anti-Sun side had not been analysed fullyand the flux of the smaller particles was unknown. For larger impactors causing front surfacedamage, although the impact direction could be resolved, we were unable to determine theimpactor size without the calibrations called for in Section 2.3.1. Thus, after a feasibility study, a scanning of the rear FFA Photo CD image library wasinitiated. 2.3.3 Hydrocode Simulation of Solar Array Impact Because of the central role of the SA penetrations in decoding space impactenvironment, it was deemed important to model the penetration of at least a simplifiedrepresentation of a solar array. Hope was offered in extending hydrocode modelling to representbrittle fracture - the mechanism leading to conchoidal fracture which is the key impact damageparameter used for decoding flux data. A study was performed therefore using equations of state and other properties of sodalime glass, shown to be comparable to the CMX cover glass. Two phases were defined so as todemonstrate feasibility prior to full commitment. 2.3.4 Impact Ejecta Studies Instances on LDEF foils (MAP experiment) and on EURECA (TICCE) had shownìrashesî of secondary craters associated with a larger primary crater. On MIR flux data hadpersistently shown fluxes higher at small dimensions than expected form interplanetary meteoroidsor orbital debris. The secondary ejecta Work Package 5 was therefore vital to understanding thelocal impact environment and its integration and testing within ESABASE; additional effort wasplaced therefore on quantifying the geometry of the known instances of secondary impacts fromreal meteoroids in space and on the study in Hypervelocity accelerator facilities of secondarycratering by analysing witness plates. 2.3.5 Impact Residue Analysis Following the critical review of residue analysis, and the (demonstrated) hope of muchimproved discrimination of impactor origin in the complex SA host surfaces, HST SA cells weremade available for study; the plan was a mix of space impact studies and laboratory impacts. Thelaboratory programme included preparing blind test samples to see if the discrimination was reliable- using stainless steel and laboratory materials in the first instance. Later, different ìmeteoroidîsamples (analogues) were prepared and accelerated in the Light Gas Gun to test the discriminationbetween different types of meteoroid.


26

3 NEW RESULTS

3.1 IMPACT PENETRATION: SA DAMAGE EQUATIONS

In the reports, we analysed the dataset in semi-infinite and in finite datasets. The finite datasetwas found to be lacking in data, so to investigate where exactly those gaps lie, a series of plots ofthe damage diameter Dco against particle size (dp), density, incidence angle and velocity was made.To view the new results in perspective figure 14 illustrates the data assembled within the contractprior to the new shots programme. A very wide range of velocity and projectile size would seemto be available but when analysed for specific target types and size-velocity the coverage is oftenvery weak. We found that there was a lack of shots over 7 kms-1 for both HST and EURECA. Noparticles with densities over 3 g/cm3 were used for the HST, EURECA and finite glass shots; noneof the HST targets have been impacted with particle diameters dp> 100 microns, but withEURECA and finite glass targets there was a fairly wide range of particles.

0

5

10

15

20

25

30

0.1 1 10 100 1000 10000

Particle diameter [µm]

Particle velocity [km/s]

Figure 14. Impact data showing particle and velocity; good coverage exists in general over theimpactor parameter space, but this is for many different target types and did not provide goodcoverage for medium and large impactors on SA cells.

As an example of the data analysis and derivation of new damage equations, we show all datarelevant to the HST SA Front top penetrations, existing and new, in the region of 5 kms-1 (Figure15). This includes two sources of Front Top impact test data, namely:• Work Package 2 database of pre-existing shots• Test results from Work Package 3 Results from impact tests (conducted previous to this contract) cover a wide range ofbrittle materials; including lunar rock, different types of semi infinite glass targets and solar cellsamples.

Only the cell data from HST, EURECA, Olympus and ECS were used (despite slightdifferences in the cover slips and solar cell thickness used and the ways the cells were mountedonto different backing structures to form each array). New test results include front top impacts(Technical Note 3) were conducted at 12 different particle sizes between 36 µm and 2 mm, withsoda lime glass (SLG) projectiles (density = 2.45 gcm-3) at ≅ 5 kms-1 on both HST and EURECAsolar cells. Incidence angles of 0, 30, 45, 60 and 75° were explored


27

Solar Cell Partical to Spall Zone Diameter Calibrationfor Front Top Impacts at 5 kms -1

0.01

0.1

1

10

100

0.001 0.01 0.1 1 10

Particle Diameter [mm]

Conchoidal FractureDiameter [mm]

HST (Baron/Herbert WP3)

HST (Dirr/Shine)

HST (Catling WP3)

EURECA (Baron/Herbert WP3)

EURECA (Paul/Schäfer)

Olympus (Schneider)

ECS (Shrine/Kuczera)

Taylor 1998 (eq 4)

Asymptotic Dco

Normal Impact Calibration

HST Extra Calibration

V = 5 kms -1

Figure 15. Impact data available from previous studies (crosses/open symbols) and new dataacquired throughout the study (solid symbols). The transition from a damage equation for a semi-infinite target (brown) through to a thin target asymptote (blue) threads a path through the data(red). It is scaled (Figure 16) to other velocities.

Front Top impacts (see Technical Note 3) were conducted at 12 different particle sizesbetween 36 µm and 2 mm, with soda lime glass (SLG) projectiles (ρ = 2.45 gcm-3) at ≅ 5 kms-1 onboth HST and EURECA solar cells. Incidence angles of 0, 30, 45, 60 and 75° were explored inWork Package 3; however the 0° was selected for use in the initial formulation of the impactcalibration curve (below) after consideration of the opposing functional dependencies of angulardependence. The (geometric) mean conchoidal diameter is found to be almost independent ofincidence angle. Where velocities were not 5 kms-1 a correction was made on figure 14 to derive

an effective dp at 5.00 kms-1. Function fitting for the damage equations.

The approach taken is to firstly to agree on equations for the limiting asymptoticbehaviour, specifically for thick or for thin targets. For thick targets we have the Taylor equationalready referred to.

For thin targets we follow the approach of formulae based on thin film hypervelocityimpact formulae e.g. Carey, McDonnell, Dixon, (Carey et al. 1985) or Gardner, McDonnell,Collier, (Gardner et al. 1997). In the intermediate region we use the vital new calibration data.

We were able to modify the thin film formulae to better suit the SA situation by noting:1) we have information from the results of large space impactors (though uncertain on

the projectile size) and2) we have calibration data from WP3 showing the approach to the asymptote. The

impact of a 2mm steel ball and 4.5 mm (diameter) cylindrical nylon “solid sabot”through the HST cell is especially valuable in this region.


28

The test shots confirmed the extension of the behaviour of conchoidal fracture in themechanism of smaller impacts, showing that spall platelets initiated near the lowest penetration andhave a fairly constant angle of qc = 1/6 radians (~10°) aiming towards the free surface. Thus a spallinitiation at the bottom of the cover glass (Tc = 150µm) means a value of Dco = 2f/qc and for f =150µm the Dco = 12 x 150 = 1800µm = 1.8mm. A different value of Tc pertains to the penetrationof the silicon (thickness 250µm). This will in fact spall back through the silicon to initiate a second(cover cell) spall. It is found that the perforation incremental hole diameter (Di = Dco - DH ) islargely independent of size for large impactors and the asymptotic approach to Dco = dp is thusexpressed by an equation Dco = DH + Di. We have looked in the real HST data at the Dco - DH

distribution and found that a function of this nature is warranted. In fact the fit yields a value of:

Dco = 1.080 DH + 1.335 (mm)

This functional form and the asymptote - intrinsic to the actual data set is thus used to fitthe large impactor calibration data. It is seen to be velocity independent, in contrast to the thicktarget data where crater size increases with e.g. velocity2/3. Values of the fit of the type found inthe space data are chosen to give a best fit through the calibration data though the form precludesanything more than good judgement. The transition between the two calibration sets (f1 and f2) isgiven by a spline of the form:

Foverall = f1.f2/(f2+f2)

Special note was taken of the potential effect of angle in the operations of the impactdamage equations for the solar arrays. The data refer to normal incidence both for the acquiredshots at some 5-6 km s-1 and the projection of these to some 10 km s-1 for space debris and to 21.4km s-1 (the average meteoroid velocity).

Penetration of metallic particles (ballistic limit = fmax) shows a dependence upon thecosine of the instance angle measured from the target normal but for surface damage (conchoidalfracture). It is not the penetration depth (fmax) which is of importance at shallower angles; all ofthe energy is deposited into the cover cell for modest penetration depths and the lateral shockwave expansion (rather than the penetration depth) determines the extent of the conchoidal zone.Although we have characterisation for the cosinusoidal variation of fmax with impact angle, we haveno functional relationships to invoke for the equivalent variation for Dco. But from energyconsiderations, and the limited amount of data available to date, there is no evidence to suggestthat Dco decreases with impact angle and hence the effective isotropic damage equation is expectedto be almost identical to the normal damage equation.

The transition to velocities applicable to the test solar cells is now made. Taking, for aLEO random tumbling plate, the model of McBride (see Technical Note 9) we find a mean resolvedimpact speed of 21.4 kms-1 - very close indeed to the Grün distribution assumptions of 20 kms-1.The beauty of the asymptotic approach at large impactor sizes, and the spline approach, is nowapparent, since we can scale the impact formula for thick targets by the velocity exponent deducedfrom laboratory impacts which extend to very high velocities according to the relationship:

Dco = AdxpV

y

But, in noting that the Dco values of the calibration shots are “in resonance” with the solar cellstructure (which is constant) we must be careful not to scale Dco from a constant dp for increasingvelocities, but more to scale down the size of the causative impactor. To a first approximation theasymptotic approach and calibration shots of 2mm and 4.5 mm projectiles remains velocityindependent; certainly a fair assumption though not explicitly correct, since DH - dp will have a


29

finite, though small change from 5 kms-1 to 21.4 kms-1. We fit the same type of function withamendment to obtain the best fit through the “21.4 kms-1” data set, shown in figure 16.

Conversion of Front Top Calibration to MeteoroidWeighted Mean Impact Velocity of 21.4 kms -1

0.01

0.1

1

10

100

0.001 0.01 0.1 1 10

Particle Diameter [mm]

Dia

met

er [

mm

]

HST Extra CalibrationHST Extra CalibrationVel. Scaled BB & SSHST (Baron/Herbert WP3)HST (Dirr/Shine)HST (Catling)EURECA (Baron/HerbertWP3)EURECA(Paul/Schäfer)Olympus (Schneider)ECS(Shrine/Kuczera)Taylor 1997 (eq 4-9)Asymptotic DcoNormal Calibration

V = 21.4 kms -1

Figure 16 Calibration data scaled to the average (Grün) meteoroid velocity of 21.4 kms-1 and thedamage equation fitting for meteoroids. A similar damage equation for space debris at 10 kms-1

was generated and tested.

In a parallel manner to the deviation of the damage equations at 21.4 km s-1, a damageequation was developed for space debris, namely with a density of 4.0 gcm-3 and velocity 10.0 kms-1.

For back impacts which damage the front surface of the HST cells we make the premisethat only calibration data in that specific scenario is valid but with one exception - namely that theasymptotic approach to infinite particle size will be independent of impact direction. The failureobserved on the cover cells is a different one (D outer spall = DOS) in this case but at largedimensions where Dos approaches Dco .

For the rear surface impact features we have no conchoidal spall, and Dco is replaced byDL the crater lip diameter (also referred to as Dsc where a scorched zone exists around the centralhole); at large sizes we have a hole diameter DH which is the same as a unit observed on the frontof the cover cells. The relationship between DL and dp is examined in a manner analogous to thefront cells where Dco–DH is examined. We find DL/dp tends to ~35 at small dimensions and thatDL–Dp tends to a value of ~3 at the largest sizes. The material properties of the woven glass fibreblanket mean that the same particle (for dp <= 0.1 mm) will produce a smaller damage feature Dsc,on the rear of the array, than on the front side solar cell cover glass (Dco, Dos). Hence these rear sidefeatures are harder to find at a given size resulting in an earlier roll-off for the rear top and rearback curves) at Dsc ≈ 1 mm. However between 1 and 3–5 mm both data sets are well approximatedby the Grün curve.

Due to the similar structure of the solar cells for both HST and EURECA and the similarresults from the existing EURECA front side test shots. The HST calibration curve was used toconvert the EURECA flux data from impact feature diameter to particle diameter. The HST andEURECA data agrees within the 1 sigma errors and follow the same general shape. This would be


31

Figure 17. Hydrocode result of the simulation of impact by a sphere of 53 micron diameter at4.31 kms-1 on a complex target comprising glass cover cell, adhesive layer and finite thickness glassat rear to mimic the silicon solar cell. Conchoidal and spall features can be identified.

3.4 EJECTA STUDIES

Total ejected massAmong different equations, that of Gault (1973) was found to be the most reliable, being

based on a wide range of experimental conditions; we have:

Me = 10-10.061 (ρp/ρt)1/2 Ec

1.133 (cos i)2 (cgs units)

or Me = 7.41 10-6 (ρp/ρt) 1/2 Ec

1.133 (cos i)2 (SI units)

This relation accounts for incidence effects in the case of oblique impacts, as alsoprojectile and target material density. According to Gault, it can be used for 10mm to 10m craterson brittle targets.

On ductile targets, both theoretical works and experimental observations agree on thefact that the amount of ejected matter is less important than on a brittle target in the same impactconditions. This can be explained on the one hand by the absence of spall, and, on the other hand,by a lower number of cone fragments due to plastic deformation (compression, lip formation) thatprevent material disruption.

The total ejected mass is partitioned in the three ejection processes :

Me = Mjet + Mcone + Mspalls for a brittle target,Me = Mjet + Mcone for a ductile target.

JettingJetting is the ejection of matter from projectile / target interface while the projectile is

penetrating into the target. This matter is ejected in a jet. Most of the projectile is ejected in the jet,except for low energy impacts or very grazing impact incidence. We found the mass ejected in thejet (Mjet) its duration, physical state, size distribution, velocity and directionality. Experimental andtheoretical approaches proved that the total jet mass was small and comparable to the projectilemass but increasing for oblique impacts.

Significant is the cone feature; fragments are generated during crater formation by tensileand compressive rupture of target near the crater (comminution according to the Grady-Kipp

(l) T=1.320µs


32

theory). These fragments are ejected at roughly the same elevation angle, that gives thecharacteristic conical shape of the debris cloud (cone around the perpendicular to target axis atimpact site). This average elevation angle is equal to p/2-qmax where qmax is the half-aperture at conevertex. Because only a very low fraction of the mass is ejected in the jet, we consider that the totalejected mass is partitioned between cone fragments and spall. Studies have shown that this partitiondepends on impact velocity and crater size. For small craters (e.g. micron dimension), there is nospall and cone fragments represent 100% of the total ejected mass. For large craters, conefragments represent only 40% and spall 60%. For increasing impact energy, the comminuted zoneis more important and the relative mass of cone fragments increases.

The smallest fragment size depends on target mechanical characteristics (micro-cracks,grain size); the usual ‘cut-off value’ is 0.1mm for a brittle target. We have no indication whetherthis value is the same for a ductile target.

The largest fragment size on a brittle target, its size is close to projectile size. On aductile target, its size decreases (about 1/5 of projectile size).Regarding the critical size distribution we found that the total number of fragments is very high,because of the very numerous dust particles (less than 1mm). The number of fragments decreaseswith increasing fragment size; the size distribution is given by a power-law function :

n(d) dd proportional to da dd

where n(d) dd is the number of fragments of size (mean diameter) between d and d+dd, and a is thepower-law slope (coefficient close to -3.5 for a brittle target, and -2.6 for a ductile target). Thusthere is a greater number of small fragments on a brittle target. This can be explained by the tensilestrength of ductile material, that prevents micro-cracks growth and diminishes materialfragmentation. The coefficient depends slightly on impact velocity : for increasing impact kineticenergy, there are more activated micro-cracks and consequently more smaller fragments; as a resultthe coefficient a decreases. This power-law distribution over-estimates the number of small ejectaby comparison to experimental measurements. The lower validity limit is estimated to 30mm.

The ejection velocity of cone fragments is not homogeneous. It is firstly dependent onejecta size: the smallest particles are the fastest. Maximal ejection velocities are close to impactvelocity. Mean ejection velocity is estimated between to be 0.5 and 1 km/s. Ejecta generated on abrittle target seem to be slightly fastest than those created on a ductile target. For oblique impacts,velocities of fragments ejected in the impact azimuthal direction are relatively faster. We alsoestablished the spall mass (the difference between total ejected mass and cone fragments mass).Spall results from tensile rupture of target material near free surfaces, around primary impact site orat target limits. They are generated on brittle materials : glass, rocks and hard metals. Onheterogeneous multi-layers targets (SA solar cells) the spall mass from the conchoidal generation isrelatively higher.

The spall velocity is low: 10 to 100 m/s. There is a weak dependence on spall size (1/dvariation) and on the impact velocity.

Ejecta generated under grazing impacts and ricochet phenomenonEffects related to a "reasonable" impact incidence (elevation higher than 30°, or

incidence lower than 60°) have been discussed already: ellipticity of primary crater, cone flattening,increase of number and velocity of cone fragments in the impact azimuth direction, and decrease inthe opposite direction (backstream). These modifications do not induce major changes compared tothe normal impact case. For more grazing angles (less than 30° elevation), new phenomena can beobserved. The primary crater becomes very elongated and the pit bottom is no longer smooth buthas an irregular profile, giving evidence of high shearing of target material. The projectile is notcompletely melted during impact : upper part remains in a solid state and is not completelydisrupted by the shock wave. For extreme grazing incidences (less than 10° elevation), shearing


33

effects split up the projectile following a plane parallel to target surface: this is called thedecapitation phenomenon. The remaining upper part of the projectile then impacts the target justbeyond the first crater: in this case there is a pair of craters.

Results of the critical analysis of the ejecta phenomenon and new experiments performedto ‘fill in gaps’ and the application of instances of secondary ejecta in real space exposures led to aversatile and realistic model and coding within ESABASE 2.01 release. We show one example ofits application to the EURECA SA. With a metallic host surface close to the scanned SA surfacesand no other nearby surfaces capable of generating significant secondary ejecta (i.e. MLI blanket)the configuration was an ideal test of the new ejecta codes. In Figure 18 we show the elements ofthis; the secondary cratering on the nearby parts of the array was found to be quantitatively inalmost agreement with the high sensitivity scanning of the smaller (600 micron ‘f1’ crater set) in thePFA data.

Figure 18. Application of the ejecta model to the EURECA SA. A metallic host surface close tothe scanned SA surfaces (the ESA/Erno sign) generates secondary ejecta an ideal test of the newejecta codes. The primaries and the sign were known and measured. Secondary cratering on thenearby parts of the array was found to be quantitatively in almost agreement with the highsensitivity scanning of the smaller (600 micron “f1”) craters in the PFA data.

Although on EURECA and on LDEF - both with ‘open geometry’ - secondaries are aminor effect, the successful test of the ejecta codes validates their use in applications wheresecondaries are important, such as the local environment of MIR or Space Station Freedom.

3.5 IMPACT RESIDUE ANALYSIS Table 6 presents the data from the Open University and Oxford Brookes residue studies. Only 6 ofthe 29 sites are not identified. Perhaps the most exciting aspect of the conclusions of the contract -not apparent until the differing work packages had been drawn together - was the very closeagreement between the two different sources of data on the “Meteoroids versus Debris” match.From the modelling of flux data only (ignoring chemistry) and, now, from residue analyses(examining only chemistry) we get surprising agreement. The data below show the percentage of


34

meteoroids (as 70%) in the sample of 29. This pertains to a limiting size of over 100 microns but ifthe size distribution is considered the percentage of debris increases to between 40 and 80% withunknowns 40% and meteoroids between 20% to 60%. Clearly it supports the dominance ofmeteoroids in the mid range but also confirms the increasing debris ratio at larger sizes.

Sample Elemental Morphology Dco ClassificationNo. Identification ums160 Sn, Cu circular 1364 Debriss161 Fe, Ni, S circular 457 Meteoroids161 Mg, Fe, Si circular 423 Meteoroids165 Mg, Fe, Ni, S, Si oblique 447 Meteoroids165 Ca, Na, Cl, Al, C & O circular 176 Meteoroids166 No Analysis circular 638 Spalled Melt - Unknowns167 Mg, Fe, Si & Fe, Ni, S circular 1028 Meteoroids176 No Analysis circular 1037 Spalled Melt - Unknowns178 Mg, Fe, Si circular 527 Meteoroids179 No Analysis oblique 605 Spalled Melt - Unknowns180 Fe, Ni circular 501 Meteoroids180 Fe, Ni &S circular 99 Meteoroids181 Mg, minor Fe, Si circular 596 Meteoroids182 Mg, Fe, Ca, Al, C, O, Si oblique 538 Meteoroids183 No Analysis circular 1246 No Residue - Unknowns204 Fe, S, minor Ni circular 416 Meteoroids278 Mg, Fe, Ni, S circular 434 Meteoroids278a Fe, trace Ni, S circular 91 Meteoroids162 Mg, S, Fe, Ca, K circular 950 Meteoroids163 no data oblique 650 Unclassifieds164 Mg, S, Fe, Ni oblique 830 Meteoroids164 Mg circular 385 Meteoroids169 no data circular 1010 Unclassifieds170 (not SA) Fe, S, Ni circular 515 Meteoroids170 Fe, Cr, Ni circular 1030 Debriss175 Mg, Fe, Ni, S circular 250 Meteoroids177 Fe, Ni circular 530 Meteoroids275 Ti, C, N, O circular 153 DebrisS276 Fe, S circular 500 Meteoroid

No 20 6 3% 70 20 10

Table 6. Results of Chemical analyses on the HST Solar Array cells; 20 out of 29 are identified asmeteoroids with only 6 unidentified. The fraction of debris increases with size, however, in theregion of 1 mm damage dimension.

3.6 SOLAR ARRAY FLUX DATA With improved data on the solar arrays; with improved understanding of impacts anddecoding; with new damage equations consistent with those for ductile targets we are able topresent different estimates of the flux of impactors for the EURECA and HST mission and - earlier- for LDEF. We present here the conchoidal flux distributions determined for the SA surfaces. Examining the significance of the data in figure 19 both curves agree well with the Grünmeteoroid model except at particle sizes below 5 µm. Despite petering out at dp ~ 2 µm, theEURECA data also supports the conclusion from the HST data above of the existence of a distinctmicro particle population (probably space debris) in LEO.)


35

Comparison of Calibrated EURECA and HST Front TopS.A. Data and the Grün Meteoroid Model

1E-11

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1E-4 1E-3 1E-2 1E-1 1E+0

F max or Particle Diameter, d p [mm]

CumulativeFlux [m -2s -1]

Grün Fmax in Al.Grün dpEURECA Calibrated FT dpHST Calibrated FT dp

Figure 19. Particle diameters decoded from the EURECA and HST front top impacts (solardirection) these are compared to those modelled (for a randomly tumbling plate) by a basic Gründistribution using a constant 20 kms-1 and meteoroid density of 2.5 gcm-3. The equivalent damagein aluminium is shown (Fmax). For more specific modelling of the Grün distribution applied to theHST front surfaces, enhanced ESABASE results are shown in Figure 25. 3.7 MODELLING AND ESABASE ENHANCEMENTS

Analyses performed in the contract show the improvements in prediction accuracyprovided with the enhanced environment models; these are implemented in the latestESABASE/DEBRIS software release. They have been applied to EURECA to HST to LDEF andto the historic spacecraft data. The main enhancements are:1. The inclusion of an ejecta model2. The implementation of the NASA 96 model for space debris risk prediction3. The advanced directional meteoroid models for apex enhancement and α/β-separation4. The HRMP meteoroid velocity distribution model5. The Jenniskens meteoroid stream model

Apart from testifying the theoretical basis of the models with simulation results whichcompare well to the analysis of the flight specimen (Technical Note of WP 7) simulations haveoffered the opportunity to gain experience in the numerical behaviour of the models and to deriveguidelines in the usage of the models.

The analyses with the special damage equations derived for the HST solar arrayshighlighted the utility of the user subroutine option in the ESABASE/DEBRIS analysis tool; resultsare shown in section 4. More important from the prediction point of view is the proof of howsensitive predictions are to the damage equations used. For standard targets (e.g. aluminium),reliable damage equations are available. However for complex structures, such as the HST solararrays, the derivation of reliable impact equations was a demanding task, exemplified by thecomplex formulation of the equations used. We note salient points from the study area.

The Taylor HRMP velocity distribution model was seen to provide an accurateprediction of the flux enhancement due to Earth gravity (based on true meteoroid and escapevelocities), coupled with a detailed velocity vs. mass distribution of the meteoroids. Results of theLDEF space face show that the predictions with this model are higher than those obtained with the


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NASA 90 velocity distribution (which is coupled to the Grün formula of gravity enhancement).This corresponds to the observation of the LDEF space face impacts. It is thus recommended touse the HRMP velocity distribution for meteoroid analysis.

For spacecraft with fixed orientation to the sun, the use of the Apex enhancementproduces a more detailed flux prediction of the spacecraft, as exemplified by the EURECA TiCCEresults. For Earth oriented or randomly oriented spacecraft, the Apex option results are notnoticeably different from the sporadic model results. The dramatic effect is shown in modelling ofthe apex enhancement on the EURECA TICCE flux distribution of craters and perforations (Figure20). The pointing direction - between Earth apex and sun-pointing leads to a considerableenhancement of the flux. This ‘fitting’ of the isotropic and directional fluxes was used to determinethe extent of apex bias required; a figure if 65% directional and 35% isotropic was established.

EURECA TiCCE

1.00E-12

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0.1 1 10 100 1000 10000

Ballistic Limit [micron]

Fai

lure

Flu

x [/

s/m

^2]

Alfa

Beta

Alfa+Beta

Grün_HRMP

Apex_HRMP

Figure 20. Effect of the modelling of the apex enhancement on the EURECA TICCE crater fluxdistribution. The pointing direction - between apex and sun-pointing leads to a considerableenhancement of the flux and was used to determine the extent of apex bias required (65%directional and 35% isotropic).

The alpha/beta-separation introduces the possibility to account for the directional effect ofthe beta-particles which are ejected from the sun. However, as for the ejecta model, the effect ofthese particles is only visible for small meteoroid masses, and consequently low ballistic limits; thealpha/beta-separation model only provides additional information for micro-impacts. Figure 21shows this separation; incorporated as a tool, its performance has yet to be ‘exercised’ on highsensitivity data.

Finally, the Jenniskens model provides the possibility to include the flux and directionaleffects due to meteoroid streams. The direction, epoch as well as the velocity amplitude of thestreams are accounted for. This model is best suited for shorter mission times or analysis periods.For longer missions, the directional effect is smeared out and the flux prediction looses in accuracy,as highlighted in the Final Report. It is not recommended to use the Jenniskens model for mission(analysis) times exceeding one month.


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38

4 SYNTHESIS OF IMPACT ENVIRONMENT: CAPABILITIES IN

SUMMARY.

4.1 FLUX ANISOTROPIES IN TIME AND DIRECTION

4.1.1 Earth Apex BiasWe have established a directionality of the Meteoroid Flux, consistent with the Grün

distribution in terms of mass but now capable of calculating the impact (damage) distributions forheliocentric pointing directions. The previous isotropic model led to damage fluxes a factor of 4below this. For geocentric stabilised missions, this apex bias is masked and, thus, its importancewas not appreciated.

4.1.2 Alpha and Beta ParticulatesWe have tested an enhanced ESABASE a capability of resolving the Meteoroid Flux into

an apex (alpha) and a solar (beta) flux, consistent with the Grün distribution in terms of mass. It isnow capable of calculating the directional incidence of the very small particles for missions whereheliocentric pointing directions are experienced - e.g. solar arrays or in experiments with timeresolved capability.

4.1.3 Meteoroid StreamsBased on the Jenniskens model, ESABASE has been demonstrated in its meteoroid

stream mode to calculate the impact damage for LDEF at three different masses. It shows theability to detect - or to react to - particular streams.

4.1.4 Secondary Impact EnvironmentFor open geometry, secondary impacts are not significant regarding the local environment, but inthe vicinity of larger structures, we now have the knowledge for understanding, and a tool forpredicting, the local impact environment. We can understand and model therefore, the higherfluxes detected close to other structures. Further, and even for open structures such as LDEF orEURECA, we can calculate the emission of secondary particles which contribute to thehypervelocity impact environment of other spacecraft in near Earth space.

4.1.5 Space Debris and MeteoroidsConsiderable testing of NASA 90, NASA 96 and to lesser extent Master has been performed onmissions where the impact damage distributions are well established. Improvements in NASA 96are noted and quantitative discrepancies established in the case of LDEF to fold into updates. Wesee in Figure 23 potentially higher space debris fluxes compared to NASA 96. NASA 90 predictedno flux on the West.

The East (ram direction) face of LDEF showed a very different picture of the meteoroiddebris scene. The micro debris enhancement is very significant; being mid way between NASA 90and NASA 96


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Comparison of Meteoroid and Debris Modelson the LDEF West Face

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0.1 1 10 100 1000 10000



ESABASE NASA96

ESABASE Grün/HRMP

Derived Debris

McBride-Apex Enhanced

N.B. NASA 90 predicts zero fluxat all sizes on the West face.

Figure 23. Space debris component isolated from the LDEF West facing surfaces, the Grünmeteoroid flux and modelling from NASA 96. The debris in this exposure configuration becomesmore significant at larger sizes and exceeds the meteoroid flux at values of Fmax in excess of 200µm.

Comparison of Meteoroid and Debris Modelson the LDEF East Face

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ESABASE NASA96

ESABASE NASA 90

ESABASE Grün/HRMP

Derived Debris

McBride-Apex Enhanced

Figure 24. Space debris component isolated from the LDEF East facing (Ram) surfaces and theGrün meteoroid flux. The debris becomes more significant at smaller sizes and exceeds themeteoroid flux at some 20 microns and below in terms of the ballistic limit Fmax.


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5 ACHIEVEMENTS OVERVIEW

5.1 IMPACT DATA FROM OPTICAL SURVEYS

• The post flight analysis (PFA) database has been assessed and associated measurements, whichwere used to derive flux distributions, have been examined.

• A detailed description of impact crater morphology on Solar Arrays (SA) has been developedand a classification of crater types relevant to impactors on HST (front and rear) and onEURECA (front). The taxonomy, developed to high standards, provides opportunity forfurther studies on the damage mechanisms and improved decoding of the front to rear fluxratios of HST and of secondary ejecta.

• The PFA measurement techniques were found to be very satisfactory; a recommendation forusing Photo CD as the baseline medium for future analyses is made.

• High variability was found in PFA measurements of the crater pit; the conchoidal diameter wasfound to have been measured consistently by different observers

• Sample sets measured by different observers lead to a correlation not significantly different than1.00 as a best fit over the whole range for Dco; errors were established as ± 10% typically forany one crater, which is insignificant when some 500 data points may well be included in theflux distributions.

• A consolidated data set of 1300 craters was confirmed (Table 7 gives the scope of the database); this includes data from the EURECA signs and scuff plates.

Spacecraft Survey onto Front onto Back Totals

HST Catch All 137 N.A. 137

Survey 417 270 687

Subtotal HST 554 270 824

EURECA f1 swath 169 10 179

f2-10 swath 535 132 667

Subtotal EURECA 704 142 846

Totals Both 1258 412 1670

Table 7. Total impact crater set available for analysis from EURECA and HST sources.

5.2 ANALYSIS OF IMPACT TEST DATA

• A database was assembled for all significant hypervelocity impact measurements on (glassy)brittle targets. The data, extending from 1 to 30 kilometres per second and sub-micron tocentimetre scale, was scrutinised for reliability and bias.

• The data was examined and analysed in multi-parameter space to derive damage equations forthick targets; a new equation was generated providing calibration for small impactors in termsof the conchoidal damage Dco.

• Requirements for impact testing were identified (potentially for work packages 3 and 8)especially in the range of marginal penetration of the cover glass and silicon. Results of impacttests were obtained and data used to compare to the damage equations for thick targets.

• Study of the solar array damage flux distribution - input to the damage equations of workpackage 2 in order to yield the impactor dimensions - were endorsed with acceptable QualityAssurance. A composite solar array conchoidal distribution for high and low sensitivities wereestablished for EURECA (front surface) and for HST (front and rear surface).


41

• Features in the distribution were related to damage thresholds in the solar arrays, later to beexamined in the hypervelocity impact testing of work package 3 and in the analysis within workpackage 2.

• The conchoidal damage feature was confirmed as the appropriate one for conversion within thedamage equation set.

• Impact tests were performed at very large impactor sizes, where the thickness of the cells ismuch smaller than the impactor; this extended calibration to beyond the largest space impactorson HST in its 4 year exposure.

• A composite damage equation was derived for small and large meteoroid particles (velocity 21kms-1, density 2.5 gcm-3) and for space debris (velocity 10 kms-1 , density 4.0 gcm-3)

• The damage equations were applied in simulation to meteoroid impacts according to the Grünsize distribution and the new velocity distribution. The results were compatible, in the regionwhere meteoroids dominate, with observed distribution in terms of size distribution (slopes).The modelling also reproduced the effect of the ballistic limit (roll off) in the flux curves.

5.3 IMPACT SIMULATIONS

• A programme of hypervelocity impacts was mounted, in response to the recommendations ofWork Package 3, at the University of Kent and at the Ernst Mach Institute, Freiburg.

• Some 150 hypervelocity impact sites were generated at small and large sizes.• The hypervelocity impact sites were measured by the same PFA analysis techniques as for space

impacts and data presented to Work Package 2 for developing impact damage equations

5.4 ANALYSIS OF RESIDUE STUDIES

• Assembly of all data pertinent to post flight residue analysis on solar arrays and on otherrelevant missions such as LDEF was performed.

• Critical analysis of all studies was performed and potentially conflicting results assessed.• Quality Assurance of existing data and bounds for the dust and debris population ratios were

established; the single largest factor identified in debris identification was the proportion of“unknowns”.

• Improved techniques for the discrimination of meteoroid dust and debris were identified.Analysis of impact sites on HST was recommended and subsequently performed at anotherinstitution leading to a very significant reduction in the “unknowns”.

• Meteoroids were confirmed, from residue analyses, as the dominant impact erosion mechanismin the mid size range; debris was shown to be dominant at very small (micron) dimensions andat large dimensions (sub millimetre).

5.5 EJECTA MODELLING

• Literature and documentation from experiments identifying impact ejecta for metallic andbrittle targets has been collated and reviewed.

• Examination of the processes of ejecta generation has been performed to enable criticalassessment and parameterisation of the experimental data.

• An ejecta model has been generated for both the high speed jet component and also for thespall fragments covering ejecta size, velocity and angle(s).

• Hypervelocity impact tests were performed to test the new ejecta model.


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• These model equations were compared to experimental data.• Application of the model to space impact configurations where secondary cratering on LDEF,

EURECA and HST was performed and very satisfactory performance demonstrated.• The model was delivered to Work package six and nine to be implemented within ESA’s

software.• Secondary damage was demonstrated to be a significant factor at small crater sizes (below 200

micron) in regions where line of sight to other parts of the spacecraft exist.• The local micro-debris environment was established as a significant experimental design factor

for large structures such as MIR and Space Station. This resulted from large exposed areassuch as Solar Arrays.

5.6 SIMULATION

• Modelling tools for the flux of space debris and meteoroid impactors were applied to data in theESA database and data from spacecraft identified in Work Package 7.

• The new modelling tools were applied to space impact data and from historic spacecraftexposures through to recent exposures such a LDEF, EURECA and HST.

5.7 DATA INTERPRETATION

• Data was assembled on all significant space exposures relevant to impact cratering orpenetration; additional, but limited, data from plasma detection was included.

• The data was scrutinised with regard to accuracy and reliable calibration, observational effects,and statistical significance.

• For historic spacecraft measurements prior to the age of Space Debris, LDEF, EURECA andHST a flux data base was established in terms of a single parameter - the ballistic limit ofpenetration in 6066 T6 aluminium.

• It was found that the Grün isotropic flux and previous velocity distributions gave fair but notfull agreement with the LDEF meteoroid component.

• The space face of LDEF was found to be exclusively meteoroid derived and used to calibrate amodel.

• Comparison of flux impact data on LDEF’s different pointing directions and between LDEF andEURECA showed that the isotropic Grün flux was not justified for spacecraft with a bias in theheliocentric reference frame - e.g. sun pointing solar arrays (such as EURECA or HST) orexperiments such as Ticce.

• A model with a “heliocentric” (Earth apex) enhancement of 65%, and 35% isotropic, wasestablished as the best fit the critical EURECA flux anisotropies..

• The new model was applied to LDEF, EURECA and HST discriminating between meteoroidsand space debris on the basis of flux only.

• A separation of meteoroids and debris was performed yielding two distinct debris componentsin the LEO environment.

• A very significant forward (ram direction) enhancement for the micro debris, with orbitalinclinations and eccentricities of moderate value

• A component of large dimensions: space “milli-debris” with higher eccentricity and inclination,which impacts on the trailing face of earlier orbiting spacecraft.

• Meteoroids were found to be dominant at mid-sizes - supporting the chemical residue analysisof work package 8.


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• Space debris fluxes impacting LDEF and EURECA were derived, enabling comparison (inWP9) with NASA 90 and NASA 96/ORDEM.

• Space debris was found to have different signatures on the leading and trailing faces of LDEF;two distributions were identified, requiring different orbital populations.

5.8 FURTHER STUDIES: RESIDUE ANALYSIS, PHOTO CD SCANNING ANDENHANCED SIMULATIONS USING HYDROCODES

5.8.1 RESIDUE ANALYSES

• Techniques were established with potential to discriminate, with better reliability, between spacedebris and meteoroids.

• Residue studies were performed on HST solar cells which had identifiable space impacts.• Samples of meteoroid material – minerals and ground meteorites were prepared and impacted

on HST solar cells in the Kent Light Gas Gun.• Reliability of the residue analysis technique was established by blind tests of different impactors

generated by the Light Gas Gun.• Residue studies resulted in a reliable detection of 23 out of 29 impactors; space debris was

found to be identified in only 3 sites demonstrating meteoroids, at 20 out of 29, to be dominant.• The size distribution of “meteoroids to debris” was found to support the flux modelling showing

that meteoroids of medium dimensions (fmax = 50 to 500 microns) thus supporting thediscrimination found in analysis of the space flux data on HST.

• Discrimination was found to be possible between different types of meteoroid.

5.8.2 HST PHOTO CD ANALYSIS

• Post flight data from HST was assessed; the rear surface of HST was identified and scanned athigh resolution using photo CDs.

• Corrections to earlier post flight data on the rear of HST identified; a new rear incident flux wasgenerated, also extending the previous PFA sensitivity by a factor of 2 in crater dimension.

• A photo-mosaic of a complete HST SPA rear unit was established with resolution down to 200microns crater size.

5.8.3 HYDROCODES

• Hydrocode studies were examined which could be applied to brittle targets such as Solar Arrayelements

• Hydrocode software was developed using Johnson Holquist polynomial EOS with a principalstrength model, and crack softening.

• Laboratory test data from a HVI impact on a brittle target was selected for calibration.• Fracture mechanisms were identified, precursors of the conchoidal fracture phenomenon on SA

structures.• A layered target model was established to study the marginal penetration thresholds of a multi-

layer target; this comprised cover glass, an adhesive layer and a second brittle “substrate”target.

• The penetration of this complex target was modelled successfully in the hydrocode study.


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5.9 ENHANCED SIMULAION AND SYNTHESIS.

Comparison of Grün Meteoroid Model with the Calibrated HST Solar Array Fluxes

1E-14

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1E-4 1E-3 1E-2 1E-1 1E+0 1E+1Particle Diameter, dp [mm]

Cumulative

Flux [m-2s-1]

Calibrated FT

Calibrated RT

Calibrated RB

Calibrated FB

ESABASE Front Grün/HRMP

ESABASE NASA96 Front

ESABASE Front Total

Figure 25: Comparison of particle flux deduced for the front impacts on the solar arrays with thatmodelled by the enhanced ESABASE. Both curves agree well with the Grün meteoroid modelusing the HRMP revised velocity distribution. The enhancement at larger dimensions, namely abovethe Grün meteoroids flux, may point to a significant increase in space debris at millimetredimensions.

• Enhanced impact flux modelling software was prepared and tested.• A meteoroid stream model was incorporated including appropriate temporal and directional

characterisation.• An option for anisotropic meteoroid flux distributions within the Grün distribution was

incorporated, comprising• Earth apex of motion flux enhancement• α and β meteoroid separation• a new velocity distribution based on re-analysed Harvard Radio Meteoroid data• The enhanced software modelling tools were applied to space impact configurations.• Secondary ejecta modelling was performed and compared to actual space exposures.


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Rear-Back Debs Mets Front-Back Debs Metsdp 10 km/s 21.4 km/s dp 10 km/s 21.4 km/s[mm] [mm] [mm] [mm] [mm] [mm]

1.00E-04 0.00E+00 0.00E+00 1.00E-04 0.00E+00 0.00E+002.00E-04 0.00E+00 0.00E+00 3.00E-04 0.00E+00 0.00E+005.01E-04 0.00E+00 0.00E+00 5.00E-04 0.00E+00 0.00E+001.00E-03 0.00E+00 0.00E+00 1.00E-03 0.00E+00 0.00E+002.00E-03 0.00E+00 0.00E+00 3.00E-03 0.00E+00 0.00E+005.01E-03 0.00E+00 0.00E+00 5.00E-03 0.00E+00 0.00E+001.00E-02 0.00E+00 0.00E+00 1.00E-02 0.00E+00 0.00E+002.00E-02 0.00E+00 0.00E+00 3.00E-02 0.00E+00 0.00E+005.01E-02 0.00E+00 0.00E+00 5.00E-02 0.00E+00 0.00E+001.00E-01 2.01E-01 7.47E-01 1.00E-01 0.00E+00 3.71E+001.06E-01 3.33E-01 8.92E-01 1.26E-01 3.67E+00 3.87E+001.12E-01 4.54E-01 1.03E+00 1.42E-01 3.70E+00 3.97E+001.18E-01 5.64E-01 1.15E+00 1.58E-01 3.74E+00 4.05E+001.26E-01 6.94E-01 1.30E+00 1.79E-01 3.78E+00 4.16E+001.58E-01 1.10E+00 1.80E+00 2.00E-01 3.83E+00 4.26E+002.00E-01 1.44E+00 2.25E+00 2.51E-01 3.97E+00 4.49E+002.51E-01 1.73E+00 2.68E+00 3.16E-01 4.16E+00 4.77E+003.16E-01 2.02E+00 3.12E+00 3.98E-01 4.44E+00 5.12E+003.98E-01 2.34E+00 3.64E+00 5.01E-01 4.82E+00 5.57E+005.01E-01 2.73E+00 4.25E+00 6.31E-01 5.34E+00 6.16E+006.31E-01 3.19E+00 4.96E+00 7.94E-01 6.00E+00 6.90E+007.94E-01 3.72E+00 5.77E+00 1.00E+00 6.81E+00 7.79E+001.00E+00 4.32E+00 6.67E+00 3.00E+00 1.24E+01 1.40E+011.26E+00 4.99E+00 7.62E+00 5.00E+00 1.58E+01 1.72E+011.58E+00 5.71E+00 8.61E+00 1.00E+01 2.22E+01 2.37E+012.00E+00 6.50E+00 9.64E+002.51E+00 7.37E+00 1.07E+013.16E+00 8.33E+00 1.19E+013.98E+00 9.43E+00 1.31E+015.01E+00 1.07E+01 1.45E+016.31E+00 1.23E+01 1.61E+017.94E+00 1.41E+01 1.80E+011.00E+01 1.64E+01 2.04E+01

Table 9: Solar array damage characteristics from calibrations for particle diameter (dp) and impactdamage diameter (Dl) seen on the front and rear surface and caused by impacts onto the opposite(back) surface (meteoroid density = 2.5 gcm-3 and impact velocity = 21.4 kms-1 or debris density =4 gcm-3 and impact velocity = 10 kms-1).

The solar array impact flux in terms of particle diameter is shown interpreted as either allmeteoroids or all debris in tables 10 – 14. The choice of meteoroid or debris parameters duringconversion to particle diameter should be considered in the light of the meteoroid to debris ratiosinferred within the contract. Alternative interpretation, using the space debris damage curvesbelow may be considered.


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Debris Cumulative Meteoroid Cumulativedp [mm] Flux [/m2/s] dp [mm] Flux [/m2/s]

2.33E-03 2.30E-05 2.93E-03 2.30E-052.71E-03 2.02E-05 3.41E-03 2.02E-053.16E-03 1.67E-05 3.98E-03 1.67E-053.98E-03 1.42E-05 5.01E-03 1.42E-054.64E-03 1.10E-05 5.84E-03 1.10E-055.41E-03 7.69E-06 7.36E-03 7.69E-066.81E-03 5.38E-06 8.58E-03 5.38E-068.58E-03 3.84E-06 1.08E-02 3.84E-061.00E-02 3.00E-06 1.26E-02 3.00E-061.17E-02 2.37E-06 1.58E-02 2.37E-061.47E-02 1.88E-06 1.85E-02 1.88E-061.85E-02 1.46E-06 2.33E-02 1.46E-062.15E-02 1.29E-06 2.93E-02 1.29E-062.71E-02 1.09E-06 3.41E-02 1.09E-063.41E-02 9.59E-07 4.30E-02 9.59E-073.98E-02 6.43E-07 5.41E-02 6.43E-075.01E-02 3.01E-07 6.31E-02 3.01E-076.31E-02 1.78E-07 7.94E-02 1.78E-077.94E-02 1.25E-07 9.26E-02 1.25E-079.26E-02 1.05E-07 1.16E-01 1.05E-071.16E-01 4.74E-08 1.36E-01 4.74E-081.47E-01 1.62E-08 1.71E-01 1.62E-081.71E-01 3.28E-09 2.15E-01 3.28E-092.15E-01 2.19E-09 2.51E-01 2.19E-092.93E-01 1.41E-09 3.41E-01 1.41E-093.98E-01 1.00E-09 4.30E-01 1.00E-095.41E-01 5.62E-10 6.31E-01 5.62E-10

Table 10: EURECA solar array front side flux in terms of particle diameter using the solar arraycalibration curve for the HST front top impacts (assuming meteoroid density = 2.5 gcm-3 andimpact velocity = 21.4 kms-1 or debris density = 4 gcm-3 and impact velocity = 10 kms-1).


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3.69E-04 1.78E-04 4.64E-04 1.78E-044.64E-04 1.62E-04 5.84E-04 1.62E-045.41E-04 1.48E-04 6.81E-04 1.48E-046.31E-04 1.38E-04 7.94E-04 1.38E-047.94E-04 1.29E-04 1.00E-03 1.29E-049.26E-04 1.12E-04 1.17E-03 1.12E-041.08E-03 1.00E-04 1.36E-03 1.00E-041.36E-03 8.13E-05 1.71E-03 8.13E-051.58E-03 6.61E-05 2.00E-03 6.61E-051.85E-03 5.25E-05 2.33E-03 5.25E-052.33E-03 3.89E-05 2.93E-03 3.89E-052.71E-03 2.69E-05 3.41E-03 2.69E-053.16E-03 1.66E-05 3.98E-03 1.66E-053.98E-03 1.20E-05 5.01E-03 1.20E-054.64E-03 9.12E-06 5.84E-03 9.12E-065.41E-03 7.76E-06 7.36E-03 7.76E-066.81E-03 6.46E-06 8.58E-03 6.46E-068.58E-03 5.01E-06 1.08E-02 5.01E-061.00E-02 3.80E-06 1.26E-02 3.80E-061.17E-02 3.00E-06 1.58E-02 3.00E-061.47E-02 2.09E-06 1.85E-02 2.09E-061.85E-02 1.35E-06 2.33E-02 1.35E-062.15E-02 9.33E-07 2.93E-02 9.33E-072.71E-02 6.92E-07 3.41E-02 6.92E-073.41E-02 5.75E-07 4.30E-02 5.75E-074.30E-02 3.47E-07 5.41E-02 3.47E-075.01E-02 1.66E-07 6.31E-02 1.66E-076.31E-02 1.20E-07 7.94E-02 1.20E-077.94E-02 1.00E-07 9.26E-02 1.00E-079.26E-02 7.76E-08 1.16E-01 7.76E-081.16E-01 3.09E-08 1.36E-01 3.09E-081.36E-01 1.38E-08 1.71E-01 1.38E-081.71E-01 7.94E-09 2.00E-01 7.94E-092.15E-01 3.47E-09 2.51E-01 3.47E-092.93E-01 7.08E-10 3.41E-01 7.08E-103.98E-01 2.75E-10 4.30E-01 2.75E-10

Table 11: HST solar array front top flux in terms of particle diameter (assuming meteoroid density= 2.5 gcm-3 and impact velocity = 21.4 kms-1 or debris density = 4 gcm-3 and impact velocity = 10kms-1).


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3.69E-02 2.80E-07 4.54E-02 2.80E-073.98E-02 2.78E-07 5.01E-02 2.79E-074.30E-02 2.76E-07 5.41E-02 2.76E-074.64E-02 2.73E-07 5.84E-02 2.70E-075.01E-02 2.68E-07 6.31E-02 2.68E-075.41E-02 2.61E-07 6.81E-02 2.53E-075.84E-02 2.46E-07 7.36E-02 2.46E-076.31E-02 2.35E-07 7.94E-02 2.35E-076.81E-02 2.16E-07 8.58E-02 2.16E-077.36E-02 1.91E-07 9.26E-02 1.91E-077.94E-02 1.57E-07 1.00E-01 1.57E-078.58E-02 1.33E-07 1.08E-01 1.33E-079.26E-02 1.06E-07 1.17E-01 1.09E-071.00E-01 8.59E-08 1.26E-01 9.28E-081.08E-01 6.15E-08 1.36E-01 6.20E-081.17E-01 4.34E-08 1.47E-01 5.25E-081.26E-01 3.07E-08 1.58E-01 3.79E-081.36E-01 2.30E-08 1.71E-01 2.99E-081.47E-01 1.61E-08 1.85E-01 2.30E-081.58E-01 1.13E-08 2.00E-01 1.45E-081.71E-01 8.90E-09 2.15E-01 1.08E-081.85E-01 6.77E-09 2.33E-01 8.90E-092.00E-01 5.98E-09 2.51E-01 8.10E-092.15E-01 4.65E-09 2.93E-01 5.71E-092.33E-01 3.59E-09 3.41E-01 3.32E-092.51E-01 2.26E-09 4.30E-01 9.31E-102.71E-01 1.73E-09 5.01E-01 8.78E-102.93E-01 9.31E-10 8.97E-01 3.71E-103.16E-01 9.22E-10 1.58E+00 2.08E-103.41E-01 8.78E-103.69E-01 8.25E-103.98E-01 7.88E-104.30E-01 6.78E-106.31E-01 4.99E-107.36E-01 3.71E-101.38E+00 2.08E-10

Table 12: HST solar array rear top flux in terms of particle diameter (assuming meteoroid density =2.5 gcm-3 and impact velocity = 21.4 kms-1 or debris density = 4 gcm-3 and impact velocity = 10kms-1).


50


8.88E-02 5.19E-08 1.08E-01 5.19E-089.57E-02 5.09E-08 1.17E-01 5.17E-081.00E-01 4.96E-08 1.29E-01 5.06E-081.08E-01 4.80E-08 1.36E-01 4.88E-081.17E-01 4.42E-08 1.47E-01 4.61E-081.26E-01 3.63E-08 1.58E-01 4.21E-081.36E-01 2.67E-08 1.71E-01 3.23E-081.47E-01 2.03E-08 1.85E-01 2.35E-081.58E-01 1.48E-08 2.00E-01 1.69E-081.71E-01 1.16E-08 2.15E-01 1.40E-081.85E-01 8.92E-09 2.33E-01 1.02E-082.00E-01 5.99E-09 2.51E-01 6.53E-092.15E-01 3.87E-09 2.71E-01 3.87E-092.33E-01 3.60E-09 2.93E-01 3.60E-092.51E-01 3.34E-09 3.16E-01 3.07E-092.71E-01 2.81E-09 3.41E-01 2.28E-092.93E-01 2.01E-09 3.69E-01 1.21E-093.16E-01 1.48E-09 3.98E-01 9.48E-103.41E-01 9.48E-10 5.01E-01 6.82E-104.30E-01 6.82E-10 7.54E-01 4.16E-106.99E-01 4.16E-10 7.94E-01 2.08E-107.54E-01 2.08E-10

Table 13: HST solar array rear back flux in terms of particle diameter (assuming meteoroid density= 2.5 gcm-3 and impact velocity = 21.4 kms-1 or debris density = 4 gcm-3 and impact velocity = 10kms-1).


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8.58E-02 3.25E-08 1.00E-01 4.36E-089.26E-02 3.00E-08 1.08E-01 4.24E-081.00E-01 2.78E-08 1.17E-01 4.02E-081.08E-01 2.48E-08 1.26E-01 3.72E-081.17E-01 2.18E-08 1.36E-01 3.47E-081.26E-01 1.97E-08 1.47E-01 3.30E-081.36E-01 1.63E-08 1.58E-01 3.04E-081.47E-01 1.28E-08 1.71E-01 2.82E-081.58E-01 1.24E-08 1.85E-01 2.48E-081.71E-01 8.13E-09 2.00E-01 2.14E-081.85E-01 5.14E-09 2.15E-01 1.93E-082.00E-01 3.42E-09 2.33E-01 1.50E-082.15E-01 2.14E-09 2.51E-01 1.28E-082.33E-01 1.71E-09 2.71E-01 8.56E-092.51E-01 1.28E-09 2.93E-01 5.14E-092.71E-01 8.56E-10 3.16E-01 2.14E-093.16E-01 4.28E-10 3.41E-01 1.71E-09

3.69E-01 1.28E-093.98E-01 8.56E-104.30E-01 4.28E-10

Table 14: HST solar array front back flux in terms of particle diameter (assuming meteoroid density= 2.5 gcm-3 and impact velocity = 21.4 kms-1 or debris density = 4 gcm-3 and impact velocity = 10kms-1). 7. REFERENCES

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Documents

SUMMARY REPORT - space-env.esa.intspace-env.esa.int/R_and_D/eureca/Sum_Rpt.pdf · Contract No. 11887/96/NL Meteoroid & debris Flux & Ejecta Modelling SR CONTENTS 1 OVERALL OBJECTIVES