Ado. Spcc~c ~ Vol.5, No.2, pp.879O, 1985 0273—1177/85 $0.00 .50Printed in Great Britain. All rights rt~scrvd. Copyright I C~SPAR

SPACE SHUTTLE MICROABRASIONFOIL EXPERIMENT (MFE):IMPLICATIONS FOR ALUMINIUM

OXIDE SPHERE CONTAMINATION

OF NEAR-EARTH SPACE

W. C. Carey. D. G. Dixon andJ. A. M. McDonnell

SpaceSciencesLaboratory, tJni~er.cii~of Kent. Canu’rlurv,

Kent. CT2 7NT. U.K.

ABSTRACT

In the Microabrasion Foil Experiment (MFE) flown onboard Shuttle flight STS-3 as part ofthe OSS—1 scientific payload, four hypervelocity perforation events were recorded by thecapture—cell array. Previous investigations /1/ of hypervelocity craters on Skylab IVwindows have suggested the presence in near—Earth orbit of a significant population ofman—made debris in the form of aluminium oxide spheres from rocket thruster firings. Cos.ccdust investigators on the first LDEF mission (launched in April 1~8L4) have expressedconcern over the possibility of ‘Contamination’ by these A1

203 particles. For Al203particles in near—Earth orbit the expected impact velocities are of the order of 7-10kms

1. At a velocity of 10 kms1, the three near—marginal MFE events are attributed toparticles —21jm in diameter and the largest perforation )23om diameter) to a particle i—’ urnin diameter. Morphological evidence from the three near-margInal events clearly indicatesa low density (1—3 g.cm3) for the impacting projectile which is rot er.tirely consister.twith a density of p~3.97 g.cm3 for aluminium oxide. Themical analysis shows siliconenhancement near all crater rims and also calcium for the larger crater. The flux ratededuced for an incident particle mass >1.8x10”12g is 2.~x10”5 rn’~’2.s1 corrected for Ear:shielding. The interplanetary flux at this mass has been placed at ~.Ox10~ m2.s1, an:hence if the )‘~FE craters were to be considered caused by aluminiu~ oxide, our ‘natural’particle flux would be a factor of -~4 below the inter:lanetary fig,ire. Considering theadditional effects of gravitational enhancement near the Earth, the data suggests thatA1

203 contamination is not a serious threat to the co~ection and atalysis of at least :~earnaller craters in 5~jm foil.

INTRODUCTION

The Microabrasion Foil Experiment (MFE) was flown as part of the scientific payload on tneSTS—3 Space Shuttle mission in March 1982. The MFE capture cell array comprised of adouble layer foil structure /3/ witn the top layer of 5~m—thick alurtiniurn foil held —irr~above a Kaptor sheet by a gold-coated brass spacer gric, in a 1m

2 rectangular array wit~. aneffective detection area of 0.~m2. MFE was exposed over 8 days in a circular Earth orb:tof 2~41 km altitude and 380 inclination, recording a total of one arge (23iim diameter) srithree near—marginal perforation events (5.1, 6.5 and 7.5 ~m in diarreter, respectively~.

COMPARISONOF FLUX DATAa — _____________________________—-

LUNAR-. MORPISON~1INNER

MFE SHJVTLE CAPTURE CELL

•~SKYLAB IV (MAX)1984 ~“ CLANTUN (~AL

gtassy.._ TIC DONNELL1q76

/1SKYLAB IV(PROBABLE)

~lJ S.

l.~ MASS ~*~)

Fig. 1. Comparison of data sources relevant to flux definition in the near-Earth, and :neMFE data. An upper limit at m-1.8x1012g shows the divergence whIch would exist If the ~IFEdata were attributed to Al

203 space contaminants.

g7

88 W.C. Carey, D.C. Dixon and J.A.M. HcDonnell

Evaluation of the minor particulate component (<lOOum diameter) of the near-Earth dustflux, and in particular, the problem of distinction between man—made debris and naturalsources is considered by examining the MFE data in the context of several data sources;namely (1) lunar microcrater records, (ii) earth orbiting satellite data, (iii) deep spacedata, and (iv) recovered capture cell data and spacecraft surfaces.

The problem of quantitative assessment is considered from the aspect of particle flux

counts, and where available, from chemical evidence or impact crater characteristics.The flux data from several source classes significant in this area is displayed in Figure

1. Early Earth satellite data and deep space data Is considered to be immune from thealuminium oxide sphere contamination arising from the recent growth in the traffic ofnear—Earth space. Numerous data from Class (ii) is summarised in the flux model curve(dotted line) of Figure 1. Data from lunar microcrater records is derived from Morrisonand Zinner /3/. Considering various sources of data and the dynamical processes ofevolution in the zodiacal complex, a recent flux distribution for the interplanetary regionat 1 AU has been derived /14/. Special attention if focussed on how two sources of data inClass (iv) compare with these ‘background’ data sets, namely:

A. Apollo window data /11,15/and B. Shuttle capture cell (MFE) data /2/.

Sources A and B (employing glass and aluminium detectors respectively) have to be‘normalised’; a relationship between crater diameter D~and particle diameter d~for glasstargets /6 / of

— c v2”3 rn1”3 cm (1)

where log10c —3.960, v is in kms

1 and m is in grammes.

A degree of flexibility in the interpretation of the Apollo data exists, since not allcraters can be attributed to hypervelocity impacts and extensive micro— damage of thewindows was possible. The geometrical exposure conditions in the Skylab complex also ledto a high probability of secondary hypervelocity ejects particles. The larger pitlesscraters are thought unlikely to be hypervelocity; the smaller glassy lined craters,however, are not only hypervelocity but 6 of the 7 examined showed evidence of aluminiumand are hence probably due to aluminium oxide spheres though some could be secondaryejecta. The S—i 149 dust—collection experiment on Skylab /7/ recorded a total of 78 impactcraters ranging in diameter from 1.9i,im to l35ijm on highly—polished metal surfaces. Distinctvariation in crater morphology was observed, but attempts to correlate morphology withcrater parameters such as depth—to—diameter ratios, etc. were unsuccessful, and it wassuggested that crater morphology may be related to parameters such as the strength andstructure of the incident particle. Chemical (EDS) analysis /7/ showed aluminium to be thepredominant element detected in crater residues, and although some of the impacts may beattributable to secondary impacts from the Skylab spacecraft, most of the aluminium-containing craters occurred on surfaces that were out of the spacecraft field of view (i.e.on surfaces shielded from secondary impact).

The effect of discounting the larger Skylab craters as cosmic or orbiting debris issignificant for the whole data set as a cumulative flux plot (Figure 1). Minor uncertain-ties in area—time product and shielding still exist in published data for the Apollo datain addition to the possibility of secondary impact ejecta, but a fair degree of convergenceof the data sources can be obtained in terms of both flux and size distribution.

RESULTS FROM MFE

For capture cell arrays, the deduction of Impacting particle parameters requires extensivedecoding of the morphological characteristics of the perforation event, together withchemical analysis of both the impact site on the top foil and any associated impact debriswithin the capture cell (I.e. on the near Kapton surface in the case of MFE). The detailsof the analysis procedure used for MFE are presented elsewhere /8/. Table 1 gives asummary of the probable impacting particle diameter responsible for each perforation eventfor an assumed particle density and velocity. From the MFE chemical analysis data, we haveas yet inconclusive chemistry, though electron microprobe and ion microprobe data will bepresented /9/. However, evidence of silicon enhancement near all four crater rims is seen,and also calcium enhancement for the larger crater.

Crater Dixon (D) shows evidence in the symmetrical front—near lip formation (Figure 2a) ofan impact velocity of at least 10 kms1 and is well above marginal; craters Carey (C).McDonnell CM) and Rook (R) are near—marginal (Figures 2b, c and d). The detailedmorphology of the rear of craters C, N and 8 imitates the impact of an iron projectile ontOa gold target, and by this comparison, impacts due to high density particles are precluded.Therefore particle densities considerably lower than iron are suggested for these threeperforations, though the circular symmetry and depth—to—diameter ratios preclude very lowdensity projectiles. A density of p-1—3, typical of silicates, fluffy Brownlee-typeparticles or aluminium oxide spheres would then be appropriate.

Microabras ion Foil Experiment 89

TABLE 1 Deduced Diameters and Masses of Incident Particles

Density (g.cnr3) 7.8 1.0

Velocity(kins—1) 10 15 20 10 15 20

Crater diain(Mm) 9.2 7.1 5.6 4.5 3.3 2.9

Dixcn (23~,un) mass(g) 3.2xl0~ 1.5x10”9 7.2x10° 4.8xlO~~ l.9x10’’ l.

3xlo~~~

Crater diam(Mm) 1.5 1.2 0.89 2.3 1.8 1.5

Rook (7.5Mm) mass(g) l.4XlO~~ 7.1x1O’

2

2.gxlO~i2 6.4x10’

2 3.Oxl0’~ i.8xio~2

Crater diam(pin) 1.33 0.98 0.77 2.2 1.6 1.4

Carey (6.5Mm) mass(g) 9.7X10—’2 3.9x10’’2 1.9x10’~2 5.6xlO’~2 2.1xl0~2 l.4xl0~2

Crater

McDonnell diain(pm) 1.2 0.83 0.66 2.1 1.5 1.2

(5.lMm) mass(g) 7.O*1O12 24x1O~2 1.2x10’2 48<10_12 ~•~~~—12 9.ox1o~3

From the point of view of flux comparisons, crater Dixon (23pm diameter) is in excellentagreement with the Skylab data, and from their chemical analysis, they would consider 6/7of these craters to be derived from aluminium projectiles. Chemical evidence from craterDixon may in future decide this issue; chemical evidence Is abundant on the Kapton secondsurface where a debris area of -200pm diameter is seen, though interpretation is compli-cated by the composition of the Kapton. For the smaller near—marginal craters, attributionto this source would place an upper limit on the flux of ‘cosmic’ particles considerablylower than other data. Morphological details of the rear (exit) of the perforation cratersare shown in Figure 3.

a) Crater Dixon, 23Mm diam. b) Crater Carey, 6.5pni diam.

c) Crater McDonnell, 5.lIim diam. d) Crater Rook, 7.Spm diam.

Fig. 2. Exit side of the four hypervelocity impact craters recovered from the MFE SpaceShuttle experiment. Crater Dixon at 23um perforation diameter shows symmetrical rear andfront lip formation; the three perforations near the marginal limit exhibit distinctspallation features seen in impacts of particles of low density.

90 W.C. Carey, D.C. Dixon and J.A.M. McDonnell

a b

c d

Fig. 3. Schematic crater profiles for the craters shown in Figure 2. (a) crater Dixon(23pm diam.), (b) crater Carey (6.5pm diam.), (c) crater McDonnell (5.lpm diam.) and (c)crater Rook (7.Spm diam.)

CONCLUSIONS

In summary, there is strong morphological evidence to Indicate the three near-marginalevents were caused by low density particles (1—3 g.cm

3); and if all four events areattributed to man—madedebris, an upper limit to the ‘true’ cosmic dust flux significantlylower than other data (arrowed point in Figure 1) would be implied.

Clearly, the choice of aluminium as a surface for a capture—cell is disadvantageous fromthe point of view of distinguishing between Man—madealuminium debris and natural cosmicdust. Future capture—cells (on LDEF and Space Shuttle) will deploy non—aluminium foils andcollection surfaces, though the sensitivity will not be as high as for 5pm—thick aluminiumfoil. Demonstration of the technique offers the opportunity for return of convincing datato the laboratory in the future for the discrimination and quantitive assessment of thenatural and man—madenear-Earth particulate population.

REFERENCES

1. U.S. Clanton, H. Zook and P.A. Schultz, Hypervelocity impacts on Skylab IV/Apollowindows, Proc. Lunar Planet. Sd. Conf. 11th, 1980, p. 2261.

2. J.A.M. McDonnell, W.C. Carey and D.C. Dixon, Cosmic dust collection by the capture celltechnique on the Space Shuttle, Nature, Vol. 309, No. 5965, p. 237 (19814).

3. D.A. Morrison and E. Zinner, 120514 and 76215: new measurements of interplanetary dustand solar flare fluxes, Proc. Lunar Planet. Sci. Conf. 8th, 1977, p. 8111.

11. C. Grun, H.A. Zook, H. Fechtig and R.H. Giese, Collisional balance of the meteoriticcomplex, submitted to Icarus.

5. B.C. Cour-Palais, Results of the examination of the Skylab/Apollo windows formicrometeoroid impacts, Proc. Lunar Planet. Sci. Conf. 10th, 1979, p. 1665.

6. J.—C. Mandeville and J. Vedder, Mlcrocraters formed in glass by low density projec-tiles, Earth Planet. Sd. Lett., 11, p. 297 (1971).

7. D.S. Hallgren and C.L. Hemenway, Analysis of Impact craters from the S-1149 Skylabexperiment, in: Interplanetary Dust and Zodical Light. eds. H. Elsasser and H. Fechtig,Springer—Verlag, Heidelberg 1975, p. 270.

8. W.C. Carey, J.A.M. McDonnell and D.G. Dixon, An empirical penetration equation for thinmetallic films used in capture cell techniques, to be published in Proc. of IAUColloquium No. 85, Marseille, France, June 19814.

9. J.A.M. McDonnell, E. Zinner, W.C. Carey and D.C. Dixon, Space Shuttle OSS-1 micro-abrasion foil experiment (MFE): chemical analysis of Impact sites, to be published inProc. of IAU Colloquium No. 85, Marseille, France, June 19811.