Capture of hypervelocity particles in aerogel: in ground laboratory and low earth orbit

\ PERGAMON

Planetary and Space Science 36 "0888# 078Ð193

Capture of hypervelocity particles in aerogel] in ground laboratory and low earth orbit M[ J[ Burchella\\ R[ Thomsona\ H[ Yanoa\b a

Unit for Space Sciences and Astrophysics\ Physics Department\ University of Kent at Canterbury\ Canterbury\ Kent CT1 6NR\ U[K[ Planetary Science Division\ The Institute of Space and Astronautical Science\ 2!0!0 Yoshinodai\ Sa`amihara\ Kana`awa 118\ Japan

b

Received 0 July 0886^ revised 3 June 0887^ accepted 4 June 0887

Abstract We have investigated in the laboratory the capture in aerogel "density 81[429[4 kg m−2# of small particles travelling at "4[029[1# km s−0[ The particles used were soda glass spheres and irregularly shaped olivine and iron particles\ with mean diameters in the range 64Ð244 microns[ We have measured the impact site for each particle\ characterised by the mean diameter of the entrance hole in the aerogel\ the minimum and maximum radii of the damaged region in the surface of the aerogel around the entrance hole\ the length of the track in the aerogel caused by passage of the particle into the aerogel|s interior\ and the diameter of the captured particle "if seen# found near the end of the track[ For each type of particle we establish relationships between the observed parameters and the pre!impact particle size[ We _nd that the processes resulting in the observed surface features and the capture of the particles in the interior of the aerogel are di}erent[ We also _nd that the particle shape "spherical:irregular# does not unduly in~uence penetration depths in the aerogel[ We have studied the e}ects of non!normal incidence on the observed impact features and _nd that the angle of incidence can be reconstructed to within 21>[ We compare the laboratory obtained data with that measured for four particles captured in a sample of aerogel ~own in a Low Earth Orbit on board the EuReCa spacecraft[ The density of one of the particles is predicted to be "06652235# km m−2[ Using the ability to reconstruct impact direction the probable nature of the particles is shown to be micrometeoroids with retrograde trajectory[ Þ 0888 Elsevier Science Ltd[ All rights reserved[

In the Solar System there is a ~ux of small particles[ The origin of these particles is varied\ and the ~ux is dependent on both location in the Solar System and epoch at which measurements are made[ The particles are of great interest for a variety of reasons[ For example\ they can retain chemical compositions which re~ect the nature of their source[ We can imagine a whole set of possible origins for these particles including comets\ asteroids\ beta!meteoroids\ interstellar dust\ _ne ejecta from surfaces of bodies impacted by other objects\ man! made orbital debris and so on[ It would clearly be of great value to determine the relative contribution of each source to the total ~ux[ This can best be done via infor! mation in both chemical composition and orbits of the particles[ Capturing the particles intact with a knowledge of the impact direction would seem the most desirable option[

The simplest capture method is to use the Earth|s atmo! sphere to slow the particles\ and then harvest them from the upper atmosphere[ This has been done for many years\ but\ although successful\ it is subject to selection biases and has no directional information[ It would there! fore be better to collect the particles in space[ However\ their speed relative to an observer launched from the Earth is typically measured in tens of kilometres per second[ Thus any attempt to trap the particles in space for close study involves a collision at hypervelocities[0 Such a hypervelocity impact in a dense\ thick "relative to particle size# target results in at least partial vaporisation of the projectile and formation of a crater in the target[ Whilst some properties of the particle can be inferred from the crater size and shape\ it is di.cult to precisely determine particle density\ let alone its composition[ Fur! ther\ crater shape is relatively insensitive to impact direc! tion until large angle incidence\ e[g[\ impacts at greater than typically 59> from the normal are required to sig!

 Corresponding author[ Tel[] 9933 0116 653999^ fax] 9933 0116 651505^ e!mail] M[J[BurchellÝukc[ac[uk

0 i[e[\ where the impact speed exceeds the speed of the compression wave in both target and projectile[

0[ Introduction] why use capture cells<

9921Ð9522:88:, ! see front matter Þ 0887 Elsevier Science Ltd[ All rights reserved[ PII] S 9 9 2 1 Ð 9 5 2 2 " 8 7 # 9 9 9 7 4 Ð 2

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ni_cantly alter crater shape in metals "Gault and Wede! kind\ 0867^ Christiansen et al[\ 0882#[ The use of high purity metal target surfaces "e[g[\ Horz et al[\ 0881a# can provide some compositional infor! mation via study of impact residues[ Hypervelocity impact ionisation can also provide elemental infor! mation[ For example\ time of ~ight spectroscopy of the ion species in impact produced plasma can provide infor! mation on composition "e[g[ Kissel and Krueger\ 0876^ Ratcli} and Allahdadi\ 0885#[ However\ these methods are indirect because of the destruction of the impactor[ An alternative is to use a low density target material[ This causes the energy of the particle to be dissipated as it penetrates into the target\ permitting capture[ The relatively intact particle can then be retrieved and studied\ yielding chemical composition[ Directional information on the impact would ideally be available from the track produced in the capture medium[ The ideal material for such a capture cell is one e}ec! tively continuous on a macroscopic level\ but with plen! tiful microscopic voids\ thin cell walls and an open cell structure which permits use in a vacuum[ Two such materials immediately suggest themselves as suitable\ organic polymer foams and silica aerogels[1 The use of such materials in hypervelocity capture cells deployed in Low Earth Orbit "LEO# has been carried out for some time "see below#[ When deciding whether to use an organic polymer foam "e[g[\ polystyrene# or silica aerogel as the capture medium\ consideration has to be taken of the particular properties of both[ Polymer foams are cheaper\ can be more easily made in greater volumes\ and are less sensitive to handling problems "i[e[\ are less brittle and friable#[ However\ in LEO the impact ~ux means that after a few months there will be several impacts of particles of greater than 0 mm diameter on a surface area of just a few hundred cm1\ "e[g[\ Gardner et al[\ 0885#[ This\ combined with required depths for capture in aer! ogel of only a few cm "see later#\ means that large volumes are not required and so manufacturing di.culties and costs are not an issue\ removing one of the attractions of foams[ The density of aerogel can be controlled during manufacture\ and densities of 0Ð499 kg m−2 can be obtained as required "changing the rate of energy dis! sipation during capture#[ Finally\ unlike foams\ aerogels are fairly transparent\ making location of trapped par! ticles and measurement of impact features relatively easy[ This combination of properties makes aerogel well suited as a capture medium for micrometeoroids in space[ 0[0[ History of aerogel capture cells Aerogels have been studied and used in capture cells for several years[ A basic introductory review is given by 1 Aerogels are dried silica gels in which are trapped fairly uniformly distributed voids[ A description of the history of aerogels and their structure and manufacture\ is given in Fricke "0877#[

Tsou 0884[ The _rst demonstration of capture in a low density medium was by Werle et al[ "0870#\ who _red stainless steel spheres "0[1 and 1 mm diameter at 9[4Ð7[9 km s−0# into porous alumina[ They measured penetration lengths\ and showed that simple scaling with velocity was not applicable because a maximum penetration depth was found at 1Ð2 km s−0[ Several groups subsequently used low density foams _nding similar results "Tsou\ 0884#[ Since then silica aerogel has also been studied as a capture medium[ Tsou et al[ "0877# made the _rst report of successful capture in the aerogel "glass spheres of 09Ð 099 mm were _red at 4 km s−0 into aerogel of density 049 kg m−2#[ Tsou et al[ "0878# demonstrated that 0[5 mm diameter aluminium spheres\ _red at 4 km s−0\ resulted in capture of between 54 and 64) of the pre!impact particle mass in aerogel of density 49 kg m−2[ To obtain higher impact velocities\ Tsou et al[ "0889# used electrostatic acceleration to _re micron sized iron particles at up to 03[4 km s−0 into silica aerogel of 49 and 59Ð69 kg m−2[ At the highest velocity they observed no captured particles in the impact sites[ At a lower velocity "1Ð01 km s−0# they reported a captured particle of 1[4 mm diameter at the end of a 067 mm track[ They also used a plasma drag accelerator to _re glass beads "09Ð79 mm diameter# at between 1Ð5 km s−0 into silica aerogel "49 kg m−2#[ Carrot shaped tracks with particles trapped at their ends were observed in these latter impacts[ These early papers concentrated on establishing the general techniques of capture in aerogel and few detailed measurements were given[ The _rst comprehensive measurements of impact sites in silica aerogel capture cells were made by Barrett et al[ "0881# "a preliminary report of the work was given in Zolensky et al[ "0889##[ They reported on various projectile types "olivine\ enstat! ite\ pyrrhotite and calcite# of irregular shape "size 094Ð 014 mm#\ impacting silica aerogels "density 19\ 39\ 59 and 019 kg m−2# at 4[0Ð6[1 km s−0[ They observed carrot shaped tracks in the aerogel[ For each projectile type they studied the maximum track length as a function of aerogel density and impact velocity[ They concluded that there was a rough negative correlation of maximum track length with increasing aerogel density\ but found no vari! ation of maximum track length with velocity "4Ð6 km s−0# in aerogel of density 019 kg m−2[ They also showed that there was an inverse correlation with aerogel density of the scaled parameter "track length#:"pre!impact pro! jectile diameter#\ and a positive correlation of this scaled parameter with the ratio of projectile:target density[ However\ they observed a scatter in their data when plotting these scaled parameters\ and concluded that den! sity e}ects were not the only determinants of track length[ When reviewing laboratory experiments on capture cells\ Horz et al[ "0881b# _tted track length "T# normalised to pre!impact projectile diameter "Dp# vs the ratio of projectile and target densities "rP:rT#[ They used the data

M[J[ Burchell et al[ : Planetary and Space Science 36 "0888# 078Ð193

of Barret et al[ "0881# and Christiansen and Ortega "0889# "concerning capture in fused silica tiles# and found T:DP  1[960"rP:rT#9[759 "r  9[81#

"0#

"r is the regression coe.cient for the _t#[ The data used was for impacts between 4Ð6[4 km s−0\ with no correction for velocity[ The correlation coe.cient is good\ indicating that there was some underlying similarity between cap! ture in silica aerogels and other forms of silica[ Horz et al[\ 0881b also summarised available reports on track curvature[ They assigned the gently curving carrot shaped tracks observed in aerogel by Barrett et al[ "0881# as being due to irregularities in the projectile shape a}ecting the capture process[ No details as to the mag! nitude of this curvature is given in either paper\ but Horz et al[ "0881b# claimed this meant that examination of tracks in aerogel would not yield the pre!impact direction of the particles[ This conclusion echoes a slightly weaker such statement in Zolensky et al[ "0889#\ where curvature of the carrot tracks was also noted\ indicating di.culties in reconstructing directions[ However\ none of these pap! ers gives details as to the magnitude of the curvatures observed[ A separate study\ Bunch et al[ "0880#\ focused on the thermal and mechanical alteration of particles during impact processes[ They made three light gas gun shots "1[1\ 3[6 and 4[5 km s−0# into aerogel of density 099 kg m−2[ They _red olivine "fosterite#\ glass\ pyroxene and pyrite particles "typical grain size 199 mm#[ The average penetration length was 09Ð39 times pre!impact particle diameter[ The typical captured particle mass fraction was 1[6) at the lower velocity\ rising to 5[3) at the highest velocity[ This small value for the captured mass fraction would seem to establish that aerogel is not a good medium for capturing particles[ This con~icts with the earlier results of Tsou et al[ "0878^ see above#\ who reported a recovered mass fraction of between 59Ð64)[ This latter result is in wider circulation than that of Bunch et al[ "0880#\ and is taken as a typical indicator of recoverable mass fraction[ However\ given this discrepancy\ possible variation of the captured mass fraction with projectile size and type should be considered an open question[ Finally\ since the purpose of capture cells is to operate in space\ we consider reports speci_c to space exposed aerogels[ Maag and Linder "0881# gave results for capture cells ~own on three space shuttle ~ights "STS 30!B\ 30!D and 50!B#[ Post!~ight\ tracks were observed in the aerogel "density 24 kg m−2# but no detailed measurements were given[ Tsou et al[ "0882# reported on particles captured in aerogel during the shuttle ~ight STS!36[ The aerogel had a density of 19 kg m−2 and an exposed surface area of 9[054 m1[ The cells were exposed for 069 h and four trapped particles were found which appeared to have undergone hypervelocity impacts "a rate of 2[3 m−1

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day−0\ however the exposure direction was dependent on STS attitude and thus the rate is not easily compared to other experiments#[ The four impacts were associated with carrot shaped tracks and particles of sizes 09Ð39 mm were observed at their ends[ Finally\ Brownlee et al[ "0883# gave preliminary results for aerogel capture cells ~own in LEO on the spacecraft EuReCa[ The aerogel was in four trays\ each with a surface area of 9[90 m1[ They were exposed pointing spacewards "at an angle mid! way between the direction to the Sun and the Earth|s direction of orbital motion# for 00 months in 0881Ð0882[ Brownlee et al[ "0883# report on the aerogels in three of the trays[ They found 01 impact sites "09 long tracks and 1 surface craters#\ an impact rate of 0[12 m−1 day−0[ The longest track was over 1 mm long[ Entry holes in the aerogel of 099 mm and captured particles of order 09 mm diameter were found[ The previous work involving capture in aerogel clearly leaves many questions unanswered[ In this paper we address some of these issues via laboratory experiments of impacts on aerogel[ We particularly investigate in a systematic fashion variation of track length with pro! jectile size and type\ dependence of track length on pro! jectile shape\ the magnitude of the captured mass fraction and the relationship between angle of impact and track direction[ We also present detailed data on 3 of the impacts which occurred in LEO on aerogel ~own on fourth tray on the satellite EuReCa[ Note that we make no chemical analysis of the captured material^ this is left for a later paper[ 1[ Experimental details We have used a two stage light gas gun to _re small particles at "4[029[1# km s−0 into silica aerogel of density "81[429[4# kg m−2[ The aerogel density was chosen to be in the range of that covered by previous laboratory experiments\ and is identical to that ~own in capture cell experiments on the Russian space station Mir as part of EuroMir |84 "on the European Space Exposed Facility\ a European Space Agency program^ see Shrine et al[\ 0886#[ The projectiles used were spherical soda glass beads "size 64Ð264 mm\ density "1387205# kg m−2 and spherical to within 1)2#\ irregular olivine grains "size 64Ð 264 mm\ density "2195219# kg m−2 and aspect ratio of up to 1 ] 0# and irregularly shaped iron particles "size 64Ð 264 mm\ density "6799249# kg m−2 and aspect ratio of up to 1 ] 0#[ The particles were _red in size ranges which were sub!sets of the total size range\ so that e}ects due to particle size could be investigated[ An investigation of e}ects due to particle shape at similar particle density was possible between the spherical soda glass and irregular 2 i[e[\ repeated measurements of diameters of many particles produced values which clustered within 1)[

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olivine particles[ Also\ an investigation of e}ects due to particle density was possible by comparing results for iron with those for olivine[ The e}ect of non!normal incidence on the aerogel of soda glass spheres "64Ð89 mm diameter# was then investigated\ with angles of incidence from the normal of 9\ 01\ 19\ 29\ 34\ 59\ 57 and 67>[ The two stage light gas gun at the University of Kent _res a sabot which is discarded in ~ight "and impacts a stop plate#[ The sabot accelerates a load which continues in ~ight after the sabot is discarded[ The load passes through two light screens "which are focused onto pho! tocells# and hits the target[ Velocity measurement is via the timing information from the two light screens\ com! bined with signals from piezo!electric transducers moun! ted on the stop plate and the target[ The transducers produce signals due to the shock waves in the stop plate and target\ and a correction to the timing is applied for passage of the signals through the plates[ The velocity of each shot is measured to 0) accuracy[ The load used is chosen individually for each shot[ It is usually a small volume of the desired particles\ plus a 0 mm stainless steel ball bearing[ The ball bearing is used to tap down the load\ keep it in place during handling and mounting in the gun\ ensure stability of the sabot during its launch and provide accurate timing infor! mation "hence velocity# for the shot[ This techniques has been used to _re similar soda glass beads at thin foils "Baron\ 0885#[ The resulting holes in the foils are circular and\ for any shot\ of uniform size similar to projectile size\ indicating that this launch technique does not break up the projectiles during launch[ For the shots at non! normal incidence the ball bearing was not used "increas! ing the risk of a mishap during _ring and slightly reducing accuracy of velocity measurement#[ The scatter within a single shot on observed parameters like track length is the same in the shots with and without the ball bearing\ again indicating that its presence is not a}ecting the data! taking[ The three types of projectile used were soda glass\ olivine and iron[ The soda glass was a normal industrial product\ and was sub!divided into _ne size ranges by sieving[ The ranges were 64Ð89 mm\ 037Ð041 mm\ 069Ð 064 mm\ 109Ð144 mm\ 149Ð299 mm and 299Ð244 mm[ The olivine was in powdered form\ and was sieved into size ranges 64Ð89 mm\ 014Ð049 mm\ 109Ð144 mm\ 149Ð299 mm and 299Ð244 mm[ The iron was also in powdered form\ and was similarly sieved into size ranges 64Ð89 mm\ 049Ð 109 mm\ 149Ð299 mm\ 299Ð244 mm and 279Ð314 mm[ The aerogel was supplied by Henning Aeroglass of Sweden\ in sheets of approximately 19×19 cm surface and 2 cm depth[ The large sheets were cut in our lab! oratory into individual targets\ each of 4×4 cm1 surface area[ The aerogel was cut by scoring the surface with a sharp cutting knife\ and then placing a support under the scored line and applying downward pressure to the edges of the aerogel slab[ The aerogel then fractured cleanly\

usually following the scored line[ This method is similar to the tradition method of splitting tiles used in kitchens[ It was quick\ highly e}ective and did not damage the aerogel surfaces or interior[ The aerogel was stored pre! shot in desiccating jars with drying agents[ This was to prevent moisture from contaminating the aerogel[ As well as the aerogel density the stopping of particles will depend on the average cell wall thickness\ or if the density is known\ the average cell size[ To determine the cell size in the aerogel\ the method of magnetic resonance relaxation was used "Strange et al[\ 0882^ 0886#[ A sample of aerogel was soaked in cyclohexane and place in a strong magnetic _eld[ The material was cooled and the temperature at which freezing of the liquid occurred was measured by looking for the associated change in mag! netic resonance properties[ This freezing point is associ! ated with cell size[ The bulk of the cells were between 39 and 79 nm size\ with a peak in the distribution at 48 nm[ The aerogel was mounted in the light gas gun slightly below the central axis[ This prevents the 0 mm ball bear! ing striking the aerogel[ The load of smaller particles spreads slightly in ~ight "covering a 1 cm diameter area at the target#\ permitting some of them to strike the aerogel[ After a shot\ the signals from the light curtains and piezo!electric transducers were used to measure the vel! ocity for the shot[ The aerogel was then removed from the target chamber and examined under optical stereo microscopes[ Magni_cation of up to ×099 were su.cient to observe in detail the impact features[ The aerogel was mounted under the microscopes on xÐy stages with pos! ition measuring gauges accurate to 22 mm[ By tracking across the surface of the aerogel\ impact features could be found and measured[ Captured particles could be observed by focusing below the aerogel surface[ By turn! ing the samples on one side the tracks inside the aerogel could be observed and measurements made of their length and angle to the surface[ By attaching cameras to the microscopes\ images were taken and preserved for future study[

2[ Results 2[0[ Normal incidence impacts The three types of projectile used "soda glass\ olivine and iron# were _red separately each shot using a load drawn from just one of the size ranges available[ The average velocity was "4[029[1# km s−0\ with no obvious variation of velocity with projectile type[ Successful shots were obtained in all the soda glass and olivine size ranges[ However\ for iron at the larger size ranges the impacts broke the aerogel targets[ Therefore data are presented for iron only in the size ranges 64Ð89 mm\ 049Ð109 mm and 149Ð299 mm[

M[J[ Burchell et al[ : Planetary and Space Science 36 "0888# 078Ð193

Where successful shots were obtained the aerogel was scanned as described above[ A side!view of a typical track is shown in Fig[ 0[ As reported by others "e[g[\ Tsou\ 0878^ Bunch et al[\ 0880^ Barrett et al[\ 0881#\ most tracks are carrot shaped\ tapering from the entrance hole to almost a point at their end "see Fig[ 0#[ Ninety _ve percent of carrot tracks have an observable trapped particle near their end[ However\ not all carrot tracks end in a single point[ Approximately 04 of the carrot tracks split in the second half of their length\ giving a second track which proceeds close to the _rst[ These bifurcated carrot tracks are in other respects similar to single carrot tracks\ both the major and minor track ends may contain captured particles[ It is clear therefore that in some cases the pro! jectile splits during capture and the two parts start to move apart[ Another type of impact seen is referred to as {star! bursts|[ These impacts are characterised by only pen! etrating typically 03 of the length of carrot tracks in the same shot[ The impact site on the surface is indis! tinguishable from that associated with a carrot track\ but upon penetrating the surface the projectile splits into 4 or more separate short tracks[ The projectile thus appears to have arrived as a single entity\ but then immediately breaks apart into a multitude of smaller parts[ The possi! bility that the method of acceleration has adversely stressed the projectiles has to be considered[ This could then lead to break up on impact[ Similar {starbursts| are seen in the shots without the ball bearing\ indicating that the presence of this in the sabot was not responsible for the subsequent splitting of the projectile[ However\ as already stated\ there are extreme accelerations in the launch process of a light gas gun\ which may be causing problems with some of the glass spheres[ Alternatively\ the method of manufacture of the glass beads could be the origin of ~aws in the beads[ The soda glass beads were produced from a spray of molten glass which then set as spheres\ the most spherical of which were selected by sorting[ It is possible that stresses:~aws were intro! duced into some the beads during this manufacture pro! cess or subsequent handling[ These possibilities cannot be excluded^ however\ only a small fraction of the data is a}ected by the {starburst| phenomena[

082

Fig[ 1[ Surface features for a typical impact[

In Fig[ 1 we show a typical impact site in the surface plane[ There is a central hole surrounded by a region of fracturing[ This region may be slightly depressed relative to the larger surrounding surface of aerogel[ For each site we measure the typical width of the central hole and the minimum and maximum radii of the fracture zone[ We do not quote an average for the radius of the fracture zone\ because the di}erence between minimum and maximum is on average quite large "a factor of 1 or 2#[ Although the central hole shown in Fig[ 1 is circular\ this is not always the case[ For the spherical soda glass beads we _nd that the entrance holes have ratios of the minor! :major axis which range from 9[5Ð0[9[ This can be con! trasted to the holes caused by similar glass beads impacting thin _lms at velocities equal to those here "Baron\ 0885#\ where the ratio of minor:major axes is very tightly distributed near 0[9\ implying that the soda glass was still spherical when it impacted the foils[ This indicates that the aerogel entrance hole is not a par! ticularly sensitive indicator of projectile shape[ In Fig[ 2 we show a top view of an impact in aerogel which occurred whilst the aerogel was in LEO on board EuReCa[ The image is a montage of three layers taken by the stereo microscope at di}erent depths in the aerogel[ The entrance hole is in the top left of the _gure\ as one then looks towards the bottom right one sees the track at an increasing depth inside the aerogel[ Bright regions indicate damaged areas in the aerogel which re~ect rela! tively more light[ At approximately half the total track length the projectile split into two "the region is marked

Fig[ 0[ Side view of a standard carrot shaped track[

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Fig[ 2[ Surface view of a split carrot track seen in space exposed aerogel "track 2 in Table 1#[

by a bright area\ the structure in the aerogel at this point re~ecting a large amount of light#[ The two sub!particles then proceed towards the bottom right of the _gure and the trapped particles at the ends of the sub!tracks are the relatively solid features[ The splitting of the particle into two is a feature also observed in the laboratory data[ However\ the lack of a fracture zone around the entrance hole is di}erent to the laboratory work[ It is not clear if this is due to the lower density of the space exposed aerogel "5125 compared to 8129[4 kg m−2# or is an e}ect of a possibly greater impact velocity[ More details of the impacts in the space exposed aerogel are given in a later section of this paper[ Using the laboratory data we examined in detail 50 normal incidence impact sites "13 soda glass\ 13 olivine and 02 iron impacts#[ Four starbursts are seen in soda glass impacts[ No clear preference for starbursts occur! ring related to projectile size was noted\ although the small statistics involved may obscure this[ If the fraction of starbursts was independent of projectile type\ we expect four for olivine and two for iron[ However\ we only observed one olivine and one iron starburst[ Apply! ing Poisson statistics for the olivine result\ the probability of observing one when four are expected is 9[96[ Although low\ this is not su.cient to de_nitively rule that soda glass and olivine are behaving di}erently[ The earlier comments concerning the possibility that starbursts arise due to either stressing of the projectile during launch or handling apply[ However\ as stated\ given the low rate of starbursts in the following analysis we simply exclude the starburst impacts[ In Fig[ 3 we show the impact hole diameter and maximum and minimum surface damage radii vs original projectile size for the tracks "carrot and split!carrot#[3 If the impacts are close together\ the surface features can 3 The data shown in all _gures have the associated uncertainty shown by error bars[ When these are not visible\ their magnitude is less than the size of the symbol used to show the datum[

merge\ and in such cases are excluded from the data[ It is clear that the data show quite a large degree of scatter[ The velocity change from shot to shot was minimal "the mean spread in velocities was 3)#\ and the size range of the original projectile samples is shown[ The observed scatter is larger than is plausible from either e}ect[ Given the scatter on the data a _t to determine the underlying behaviour is di.cult[ Also that can be done is to _t to the data a function whose mean at any point is the mean of the observed data[ The quality of the _t "as given by the regression coe.cient r# will be poor\ and the _t curve is really no more than a trend line permitting a par! ametrization of the data[ In each case the simplest such function is a straight line "no improvement was found with higher order polynomials#[ The results of such _ts are shown on Fig[ 3 "and subsequent _gures#[ As the data shown has a large scatter\ the mean predicted values from the _t curves shown must be supplemented with an estimate of the spread of the data[ Assuming the data has a Gaussian spread the standard deviation "s# of the residuals is found for each parameter\ and given in Table 0 for soda glass "the other data sets are similar#[ Thus if the experiments were repeated one would expect any new data to follow the _t lines within the range of s given[ The _ts do not in general extrapolate properly to the origin\ indicating either that the true function to describe the data has been hidden by the large s values\ or a di}erent behaviour takes over at small projectile sizes[ It is impossible to distinguish between these possibilities[ In general the surface impact features are similar in size for the soda glass and olivine and\ within a factor of 1\ the data from the iron projectile impacts is also similar[4 It seems therefore\ that for compact projectiles at equal velocity these impact features are mostly determined by projectile size\ not composition[ The length of the carrot tracks "note that split carrot tracks are excluded from the analysis of track length and captured particle size# is shown in Fig[ 4[ Again a scatter is seen on the data which is not readily explained by variations in velocity between shots\ or the spread in projectile size inside a single shot "see Table 0 for s#[ For iron\ no _t is sensible as the scatter in track length is larger than the change in projectile size used[ Instead we simply average track length\ and _nd "for iron# a mean track length of "0069923999# mm[ Typically we see that at 71[4 mm projectile diameter\ the soda glass and olivine projectiles penetrate 79Ð89 times their original diameter[ This is similar to the earlier work of Barrett et al[\ 0881[ However\ the iron data penetrates some 039 times pro! jectile size "as evaluated at 71[4 mm#[ The captured projectile diameter "for carrot tracks# is shown vs pre!impact diameter in Fig[ 5[ The capture diameter is de_ned as the mean of the minimum and 4 No _t is shown to the iron data as no meaningful trend line was found[

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Fig[ 3[ Impact hole diameter\ maximum fracture zone radius and minimum fracture zone radius\ all vs projectile size[ "Soda glass\ olivine and iron as indicated#[ For the iron data no _ts are made\ the lines shown are those _tting the soda glass "solid# and olivine "dashed# data[

Table 0 Standard deviation of the distribution of each parameter about best _t prediction for all normal incidence soda glass impacts

diameter

Entry Hole damage radius

Standard deviation ")# 13

Minimum surface damage radius

Maximum surface damage radius

Track length diameter

Captured particle

28

21

14

15

maximum diameters of the captured projectile[ The associated error represents the full range of these values\ and thus varies from particle to particle[ We can see that for soda glass\ at 71[4 mm original projectile diameter\ the mean captured particle diameter is 54 mm\ i[e[\ approximately 79) of the original diameter is captured\ which corresponds to a captured volume of 49)[ For

olivine at 71[4 mm original projectile diameter\ the mean captured particle diameter is 56 mm\ i[e[\ again 79) of the original diameter\ with a captured volume of some 49)[ Thus olivine and soda glass are similar[ However\ a smaller fraction of the iron projectiles is captured intact\ with almost all the iron data in Fig[ 5c being below the curves for soda glass and olivine[

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Fig[ 5[ Captured particle diameter vs projectile diameter[ "a# Soda glass "b# Olivine "c# Iron[ Also shown in "c# are _t curves for soda glass "solid# and olivine "dashed#[ Fig[ 4[ Track length vs projectile size[ "a# Soda glass "b# Olivine "c# Iron[ Fits shown in "c# are those for soda glass "solid# and olivine "dashed#[

This di}erence between soda glass and olivine on one hand and the iron on the other is interesting[ At equal velocity and size\ the clearly denser projectile "iron# is penetrating 49) further on average and losing slightly more of its volume in the process[ However\ the entrance hole and surface features are very similar for all three materials[ 2[1[ Oblique incidence impacts The data for normal incidence yielded tracks which seemed aligned with the impact direction[ This was true for both the spherical soda glass and the irregular olivine and iron[ To test if this permits reconstruction of the impact direction shots were carried out using aerogel blocks with surfaces inclined to the impact direction[ This oblique incidence work used soda glass particles of 64Ð 89 mm diameter[ The average impact velocity was as before 4[029[1 km s−0[ The angles of incidence from the normal were 9\ 01\ 19\ 29\ 34\ 59\ 57 and 67> "accurate to 20>#[ In Fig[ 6 we show typical surface features at various angles "all views are made normal to the aerogel surface#[ The entrance holes and fracture region clearly evolve with angle[ The most regular of these features is the entrance hole[ As the angle from the normal increases\ these

develop a de_nite elliptical shape[ We measure hole length "in the direction of the impact velocity vector pro! jected onto the surface# and width "transverse to the length# and use the ratio width:length as the circularity[ This ratio is 0 for a circular hole\ and decreases as the hole becomes more elliptical[5 We show the hole length\ diameter and circularity for each angel of incidence in Fig[ 7[ It can be seen that the hole length increases with angle of incidence\ but hole diameter stays unchanged[ The circularity thus decreases with angle of incidence[ The best _t to the circularity is shown in Fig[ 7\ but it should be noted that there is a large spread of the data at any angle of incidence[ This in turn means that the impact angle reconstructed from the circularity is uncer! tain to 204>[ The track length was measured at each angle of inci! dence[ Based upon a total of 27 tracks "4 or 5 at each angle except 59> where only two impacts were obtained#\ no variation of track length with angle was found[ The track lengths are therefore averaged\ with mean length 5019 mm and s  0649 mm "18)#[ This is well within 0s 5 For normal impacts we _nd the largest and smallest diameters of the entrance hole[ Since which corresponds to width and length is unde_ned\ we choose the length to be the measured diameter most closely aligned to the direction of the vertical when the target was _red upon[ Thus the circularity for normal incidence can be greater than 0[

M[J[ Burchell et al[ : Planetary and Space Science 36 "0888# 078Ð193

086

Fig[ 6[ Surface features for inclined incidence impacts "64Ð89 mm soda glass#[ Direction of impact is always from the bottom[ Angle of incidence is shown in degrees in top of each image[ The vertical shadow at the top of some images is cast by the sub!surface track which has been illuminated from below[

of the result for normal incidence impacts in Fig[ 4 and both data sets have similar scatter of the data "s of 14) in normal incidence impacts#[ Given that the two data sets

were taken with "normal incidence# and without "oblique incidence# a stainless steel ball bearing accompanying the load in the sabot\ this suggests that the ball bearing has

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M[J[ Burchell et al[ : Planetary and Space Science 36 "0888# 078Ð193

Fig[ 8[ Track inclination vs actual impact angle "64Ð89 mm soda glass#[

Fig[ 7[ Entrance hole size and shape vs angle of impact "for soda glass 64Ð89 mm diameter#[

not in~uenced the results in the normal incidence data set[ Similarly the size of the captured projectile was measured\ and found to be una}ected by angle of inci! dence[ The average captured particle diameter was 62 mm with a s of 02 mm[ Thus the captured diameter is 77) of the pre!impact diameter\ i[e[\ 57) of the original volume was captured intact[ The direction of the track relative to the aerogel surface was also measured[ This was not taken by observing the position of the captured particle relative to the surface and entrance hole[ We _nd that in the _nal few percent of track length the particle deviates from its previous trajectory[ Therefore the point where this deviation occurred was located and the angle to the surface mea! sured relative to the entrance hole in the aerogel surface[ This is shown as a function of angle of incidence in Fig[ 8[ The _t to the data shows that the mean reconstructed angle is indistinguishable from the impact angle[ The s of the distribution of the measured angles about the impact angle is 21>[ The results thus show that the aerogel can be used to reconstruct impact direction with good accuracy[ 3[ Discussion When looking at the results\ the most striking feature on almost all observed parameters "track length\ hole

diameter\ etc[# is the large scatter on the data[ Much e}ort has been directed to understanding this[ The data was obtained by two di}erent launch methods "with and with! out a ball bearing present in the sabot# and no gross change was observed in the mean or standard deviation of the track length etc[ Also the projectiles used had di}erent manufacturing techniques "the soda glass was formed direct from molten glass\ whereas the olivine and iron projectiles were formed by techniques involving cru! shing#[ Thus internal stresses and fractures in the pro! jectiles may well have di}ered\ yet the degree of scatter was similar in all cases[ Further\ Baron "0885# uses the same soda glass beads and launch technique with thin foils as the target[ The measurements for perforations in the foils had well de_ned properties[ The inference is thus that it is the aerogel which is causing the spread in the data[ If the large scatter in the data is indeed related to the aerogel it should have been apparent in earlier work[ Zolensky et al[ "0889# reported what they described as an extreme variability of track length inside single experi! ments "a factor of 1#[ A more detailed report "Barrett et al[\ 0881# observed two types of penetration in aerogel^ those which give long tracks\ and those which give short ones\ with the two distributions clearly distinct[ The short tracks were attributed to impacts by projectiles which fragmented during the launch process[ For the long tracks "i[e[\ un!fragmented projectiles# they gave the vari! ation in track length as being typically 20 mm for tracks of length 4Ð5 mm[ This is a spread of 19)\ not too dissimilar to that observed here[ They attributed this to possible variations in projectile size[ However\ some of the work in this paper here used projectiles of very restric! ted size range\ yet still observed large scatter in the data[ Horz et al[ "0881b# suggested that previous experiments

M[J[ Burchell et al[ : Planetary and Space Science 36 "0888# 078Ð193

have used irregular grains as projectiles\ and the resulting lack of control of the cross!section during impact may have introduced scatter into the data[ Here we have used spherical soda glass beads and irregularly shaped grains of olivine\ with the scatter in the data being present in both data sets[ Having considered all these arguments it seems that the scatter in the data is most likely due to the aerogel "or more accurately the stopping mechanism in the aerogel# rather than the projectile[ Even allowing for the scatter in the data some deduct! ions can be made[ For normal incidence impacts some trends are apparent[ All three projectile types behave similarly in terms of the impact features seen on the surface of the aerogel[ The sizes of the entrance holes and fracture zones seem mostly determined by projectile mean diameter\ not by projectile shape or density[ However\ di}erences are apparent in the observed track length and the fraction of the projectile captured intact[ At a given projectile size\ the soda glass and olivine has shorter tracks than iron does and a larger fraction of the original particle is trapped[ It is tempting to associate this with the di}erent energy of the impact "at a _xed velocity#[ However\ it should be remembered that without a vali! dated model of the capture process which may depend on other factors such as projectile melting temperature etc[\ this cannot be con_rmed[ What can be reasonably inferred however\ is that the surface and interior features result from di}erent processes[ It might thus be possible that the resulting observable features have di}erent sensitivity to the impact conditions[ However\ the spread of the observed data about its mean value shows that this is not the case "see Table 0 for the standard deviations of the distribution of surface hole size\ surface damage maximum and mini! mum radius\ track length and captured particle size#[ The two most precise indicators of pre!impact projectile size at normal incidence are entrance hole size "s  13)# and track length "s  14)#[ Thus despite the possible di}erence in the processes involved\ the precision of reconstruction of pre!impact size is the same[ This how! ever does not require that the processes involved are the same\ only that they cause similar magnitude ~uc! tuations[ The study of non!normal incidence yields a de_nite result^ namely that the impact angle can be reconstructed to within 21> from the track in the aerogel[ This con! tradicts the belief of previous authors\ e[g[\ Zolensky et al[ "0889# and Horz et al[ "0881b#[ Their belief was based upon the observation of gentle curves in the carrot tracks for normal incidence impacts[ In our work\ the relation between impact angle and track angle became readily apparent when the impact angle was actually varied[ We also carefully allowed for curvature of the tracks by mea! suring only the _rst portion of a track to obtain the trajectory[ For example\ when looking at the sample of tracks impacting at normal incidence\ there are four

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tracks which towards the end of the track start to show a signi_cant track curvature[ If the angle of incidence is measured from the end of the track to the entry hole\ the di}erence from the normal is\ in the worst case\ 09>[ However\ with the same track\ if only the _rst half of the track is used to _nd the angle of incidence\ it deviates from the normal by only 9[7>[ With this approach\ the angular reconstruction shown in Fig[ 8 is obtained[ This marks aerogel out as a good material for reconstruction of impact direction for compact projectiles at the sizes and velocities used here[ To determine compatibility of the data presented here with that of other authors\ we consider several parameters[ Horz et al[ "0881b# obtained a _t to various data sets in terms of normalised track length "track leng! th:projectile diameter# and the ratio of projectile:target density "Eqn 0 above#[ This normalised track length for the impacts presented here is shown in Fig[ 09a\ along with the prediction of Eqn 0[ There is rough agreement between our data and the prediction of Eqn 0\ indicating that the data here is compatible with previous results[ However\ a large spread is seen in this normalised track length at any ratio of the projectile and target densities\ with a similar magnitude spread present in the data used in Horz et al[ "0881b#[

Fig[ 09[ "a# Track length:projectile diameter and "b# track leng! th:captured particle diameter vs ratio of projectile:aerogel densities[

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To test the idea that the stopping mechanism is intro! ducing a large scatter into the data\ we de_ne a new normalised track length[ This time the track length is divided by the captured particle diameter DC "as distinct from the pre!impact diameter#[ This new normalised track length is shown against the ratio of projectile:target densities in Fig[ 09b[ With the exception of a handful of data points\ the scatter has been reduced[ If the two extreme points are removed from the soda glass data and the one extreme point from the olivine data the s is 16) in Fig[ 09a and 11) in 09b[ This indicates that part of the mechanism causing some of the scatter a}ects simi! larly both track length and captured size fraction[ Again ignoring the very divergent points\ we _t the data in Fig[ 00b and obtain] T:DC  39[3¦0[51"rP:rT# "r  9[55#

"1#

Since T:Dc is measurable and rT is known\ it should

Fig[ 00[ Captured volume fraction vs projectile diameter[ "a# Soda glass "b# Olivine "c# Iron[ Solid lines are predictions for the respective data sets from the _ts in Fig[ 5[ In "c# the soda glass "dotted# and olivine "dashed# are shown for comparison[

thus be possible to determine rP for any projectile[ However\ the scatter in the data "s  11)# means that the resulting determination of projectile density will not be particularly precise[ When considering the captured particle\ it is more usual to look at the captured volume fraction rather than diameter[ In Fig[ 00 the captured volume fraction is therefore shown[ The data used are the same as those in Fig[ 5 "the volume is found from the mean diameter assuming a spherical particle\ and the error bars represent the range given by the maximum and minimum measured diameters# and the lines shown are obtained from the _ts to that data[ There is of course again a large scatter on the data\ but it appears that the captured fraction falls with increasing pre!impact size[ Also it can be seen that iron has on average a smaller fraction captured than do soda glass or olivine at similar pre!impact size[ As discussed earlier\ Bunch et al[ "0880# _red fosteritic oli! vine and glass "199 mm diameter# into aerogel of den! sity ¼ 099 kg m−2 "similar to that used here#[ At 4[5 km s−0\ they found a typical captured volume fraction of 5[3)[ Whilst a single olivine or glass particle can have such a low capture fraction in the data here\ such a value cannot be described as typical[ Indeed the curves shown in Fig[ 01 suggest a capture fraction of 14) and 19) respectively\ for olivine and glass[ Apart from the stat! istics involved\ there seems no clear reason for the dis! crepancy in captured volume between the two data!sets[ Since the track length in the aerogel is particle depen! dent "iron penetrates further than soda glass and olivine#\ we can ask if it is dependent on impact energy or on some property of the particle "e[g[\ melting point#[ Theoretical models of capture\ e[g[\ Anderson and Ahrens "0883#\ have unfortunately focused on polymer foam capture

Fig[ 01[ "a# Hole diameter:captured particle diameter and "b# track length:hole diameter\ both vs ratio of projectile:aerogel densities[

M[J[ Burchell et al[ : Planetary and Space Science 36 "0888# 078Ð193

cells[ They suggest that drag and ablation are the main mechanism for energy loss in polymer foam capture cells\ but state that this may have to be modi_ed for aerogel to increase the e}ect of viscous drag "e[g[\ a molten aerogel shell has been observed by some authors around captured particles in aerogel#[ If the capture process depends on properties of the capture medium\ and not the particle\ then track length may indeed be an indicator of impact energy[ By contrast we might expect the projectile mass loss during capture to re~ect projectile type[ This is plaus! ible as the projectile is subject to heating and some abra! sion[ Yet the reduced scatter on the data in Fig[ 09b compared to 09a suggests that the processes which deter! mine track length and capture size are somewhat corre! lated[ Again however\ without a suitable model for the capture process in silica aerogels or data from other vel! ocities\ these points cannot be fully investigated[ Whilst we have considered the e}ects of varying pro! jectile size and type\ and\ by limited comparison to other data the e}ect of varying aerogel density\ we have not considered the in~uence of another major variable\ namely projectile velocity[ This is critical\ as the impact speeds of natural particles in LEO will typically be 19 km s−0[ This is further complicated because it is generally believed that for example track length in porous media has a maximum at about 2 km s−0 "Barrett et al[\ 0881#[ As the capture cells will not directly measure the impact speed\ the obvious solution would be to _re calibration projectiles at a full range of speeds in the laboratory[ However\ we are unable to achieve this[ There is a need therefore to either determine which observed features or ratios thereof are independent of impact speed\ or to understand the impact process su.ciently well enough to be able to extrapolate the results presented here to other velocities[ Since the impact speed is not measured directly in a capture cell\ the former is clearly the more desirable[ Since no detailed model exists of the capture process we cannot know which impact features possess the same velocity dependence[ We cannot therefore con_dently form velocity independent ratios of measurements[ We have already looked at track length:captured size "see above#[ We also select two other ratios for study] a\ entrance hole diameter:captured particle diameter\ and b\ track length:entrance hole diameter[ For the data at normal incidence\ the values of these ratios are shown in Fig[ 01\ "a# for a and "b# for b\ both shown vs the projectile:target density ratio[ In general\ given the large scatter on the data\ and the relative lack of variation with the ratio of projectile and target densities\ the value of using these ratios seems limited[ In summary\ then\ the process of capture does not clearly emerge from the data[ General trends are detect! able\ but since most measurements are subject to large scatter\ these trends may not aid in analysis of impacts obtained under unknown conditions[ The exception is

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the impact direction[ This stands out as being measurable to a fair precision[ In the rest of this paper we examine impacts in space exposed aerogel "i[e[\ unknown impact conditions# and analyse them in terms of what was learnt from the laboratory studies[ 3[0[ Observations of particles captured in low earth orbit The European Retrievable Carrier spacecraft "EuReCa# was deployed from the space shuttle in August 0881 into a circular orbit at 491 km altitude and 17[4> inclination[ It was constructed by the European Space Agency as a retrievable platform for microgravity and space science experiments[ After 215 days in orbit it was retrieved and returned to Earth[ Throughout its orbital lifetime it was oriented "Fig[ 02# so as to have one direc! tion "x axis# on the platform pointing at the northern hemisphere\ a second "y# axis pointing in the direction of the Earth|s orbital velocity vector "referred to henceforth as the Earth|s apex direction# and the third "z# axis at the Sun[ The z!axis was very stable throughout the ~ight\ with slight variation in x and y axis directions[ One of the experiments on board the satellite was the Timeband Capture Cell Experiment "TICCE#\ a device to monitor the small particle ~ux[ It was located on a plane at 34> to the Sun pointing direction and the Earth|s apex face and had an unobstructed view of space[ Amongst other packages it contained four aerogel samples[ Each sample had a surface area of 099×099 mm1 and depth 5 mm[ The aerogel had a density of 5125 kg m−2 "see Hrubesh and Poco "0889# for more details#[ Three of the samples were analysed at the University of Washington "Seattle\ U[S[A[# and a preliminary report

Fig[ 02[ Orientation in space of EuReCa spacecraft[ Aerogels are shown as shaded area on main body[

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has been published in Brownlee et al[\ 0883[ The fourth sample was analysed separately at the University of Kent "Canterbury\ U[K[# and detailed observations made of the impact features "Yano\ 0884#[ It is the impacts on this fourth sample that are considered below[ Four carrot tracks were observed in this sample of aerogel[ With a surface area of 099 cm1\ the impact ~uence is similar to that observed in the rest of the EuReCa aerogel "09 tracks in 299 cm1#[ They were measured using a Leica TCS 3D confocal stereo laser microscope[ The target was illuminated with an argon laser and the track! ing of the aerogel sample as it moved in the _eld of view was accurate to approximately 0 mm[ Track depths were found by focusing down through the aerogel and a cor! rection is made for the refractive index "n# of the aerogel[ This was found to be n  "0[90729[994#\ in good agree! ment with the predicted value of 0[904 from the formula of Cantin et al[ "0863# who predicted n  0[9¦ 9[14×density "density in g:cm2#[ Details of the measured tracks are given in Table 1[ In the surface plane of the aerogel each impact caused a hole\ but no associated fracture region[ The holes were not circular\ and the minimum and maximum diameters were found "and hence circularities\ which were in agreement with the observed track directions relative to the aerogel surface#[ Two tracts split into two "i[e[\ bifurcated carrot tracks#^ the two branches are referred to as a and b in Table 1\ with details of the common entrance hole also given[ In both cases\ the split occurred at about half the total track length[ It is one of these tracks that is shown in Fig[ 2[ This splitting is similar to that seen in the laboratory data "where 0 in 4 tracks split#[ A captured particle was seen in both unsplit carrot tracks\ and in both branches in each of the bifurcated carrot tracks[ When attempting a detailed analysis\ it should be remembered that the aerogel density on TICCE is di}er! ent to that used in the laboratory work reported here[ Further\ the impact velocities are unknown as are the

pre!impact size and density of the particles[ Thus the use of a single measured quantity "e[g[\ entrance hole size# is not sensible[ Instead the ratios of measurements should be considered[ The normalised track length "track leng! th:captured projectile size# can be used with Eqn 1 to predict the density of the impactor[ We do this for the un!bifurcated tracks "0 and 1 in Table 1#[ For track 0 we _nd the particle density to be "06652235# km m−2[ However\ track 1 has a normalised track length smaller than the minimum allowed by Eqn 1 "the normalised track length must exceed 39[6 to apply Eqn 1 and here it is 23[622[5#[ It is not clear if this is an example of a problem arising from the scatter that a}ected the data taken in the laboratory or re~ects the di}erent conditions in space "e[g[\ impact velocity or perhaps a non!compact particle#[ The ratios a "entrance hole diameter:captured particle diameter# and b "track length:entrance hole diameter# are considered next[ For the TICCE aerogel we have the complication that the impacts may be at non!normal incidence[ Whilst track lengths and captured sizes are una}ected by angle of incidence\ the hole size and shape do change[ However\ Fig[ 7 shows that the hole width perpendicular to the entry direction is independent of angle of incidence[ We therefore use for the hole diameter the smallest measured hole diameter for tracks 0 and 1 in Table 1 and _nd the values of a and b[ For track 0\ a"5[220[0# and b"02[729[4#[ For track 1\ a"4[629[5# and b"23[622[4#[ In both cases the ratios are not incompatible with the laboratory data "Fig[ 01#[ However\ this is a further example of where the scatter on the laboratory data precludes any de_nite conclusions being reached[ By contrast the laboratory work showed that the angle of incidence of the particles could be reconstructed from the measured directions of the tracks in the aerogel[ This is obtained for all four tracks "for the bifurcated carrot tracks we estimate the angle using the main track before

Table 1 Measured parameters for four tracks in space exposed aerogel[ Tracks 0 and 1 are carrot tracks^ tracks 2 and 3 are bifurcated carrots with two branches

Track

Max[ hole diameter "mm#

Min[ hole diameter "mm#

0 1 2 2a 2b 3 3a 3b

35[120[9 094[920[9 87[620[9 * * 51[420[9 * *

26[020[9 45[520[9 82[720[9 * * 28[420[9 * *

Circularity

Captured particle diameter "mm#

Track length "m#

Angle of elevation "degrees#

Azimuth angle "degrees#

9[7929[92 9[4229[90 9[8429[90 * * 9[5229[91 * *

4[820[9 09[920[9 * 2[520[9 5[920[9 * 5[320[9 5[320[9

495202 235200 * 148209 132209 * 28228 36726

65 24 36 * * 49 * *

175 1 219 * * 193 * *

M[J[ Burchell et al[ : Planetary and Space Science 36 "0888# 078Ð193

Fig[ 03[ Reconstructed impact direction for space exposed aerogel[ "a# Elevation and "b# Azimuth[ "Impacts 0Ð3 as in Table 1#[

it splits#[ Given that the pointing history of the EuReCa spacecraft is known\ as is the position of TICCE on the satellite\ the impact angles can be used to _nd the direc! tion of the impacts in space[ With respect to the spacecraft centre we give the reconstructed azimuthal and elevation impact angles in Table 1[ The errors on the angles arising from track reconstruction are less than the 1> uncertainty we found earlier "Fig[ 8# for alignment of track with impact direction[ We therefore take this larger value of 1> as the uncertainty in impact direction[ The impact directions are shown diagrammatically in Fig[ 03[ Three impacts are from the Earth|s apex direction "¦y in azi! muth#\ and all of the four impacts come from the Sun direction "¦z direction in elevation#[ Three of the four were also from the ecliptic southern hemisphere "−x in azimuth#[ A chemical analysis of the trapped particles is under way and will be reported separately[ 4[ Conclusions A detailed study of impacts in aerogel in the laboratory has produced a large data set as regards to the behaviour

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of aerogel as a capture medium for small particles[ The mechanisms that govern development of surface features and the deceleration in the interior of the aerogel are shown to be di}erent[ The similarity of all the observed features for soda glass and olivine impacts indicates that the processes are not greatly sensitive to spherical as distinct from non!spherical compact objects[ However\ iron particles do appear to behave di}erently\ although whether this is due to di}erent material properties or a greater energy at equal size and velocity is not established[ What emerges clearly is that all observed features of an impact are subject to large scatter if the impacts are duplicated[ This indicates that the processes leading to both surface features and capture in the interior are not simple parameters of projectile size\ composition and velocity[ Any attempt to model capture in aerogel\ or to use laboratory data to analyze unknown impacts on aerogel must bear this in mind[ Further\ any report of capture in the laboratory should not rely on a single impact to measure any parameter[ Nor can the largest value of any observed parameter "e[g[\ track length# be reliably used as an estimate of the mean expected value under those particular experimental conditions[ It is also seen that the captured mass fraction "under the conditions here# is in the range of 04Ð39)[ Thus use of the phrase {Intact Capture| is possibly misleading[ Since only part of the original bulk material has been captured perhaps {Partial Intact Capture| may be more appropriate[ For non!normal incidence\ the angle made by the track to aerogel surface is found to be a good indicator of impact angle[ This confounds previous beliefs on this matter[ This means that as well as being suitable as a capture medium\ aerogel is also a good medium for recon! structing directional information about impacts[ Four impacts which occurred on an aerogel sample exposed in space have also been studied[ Although the space exposed aerogel was of a di}erent density to that of the laboratory study\ and the impact velocities in space were unknown\ some information was obtained[ Most features of the space impacts were similar to those found in the laboratory "albeit on a di}erent scale#[ The excep! tion was a lack of a damage zone around the impact hole on the space exposed aerogel[ It is not clear if this is due to the higher impact speed\ di}erent aerogel density or smaller incident particles than used in the laboratory[ One of the impacts in space is predicted to have involved a particle with density "06652235# kg m−2[ Using the reconstructed impact direction for the four impacts on the space exposed aerogel\ plus the pointing history of the spacecraft\ it was possible to reconstruct the direction of motion of the impactors in space[ Whilst the statistics of just four impacts is low\ a knowledge of the direction of motion of individual particles in space aids in an understanding of their origin[ Given that all the impacts come from the sun|s direction\ and three

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favour the Earth|s apex direction\ it is suggested that this is indicative of interplanetary dust particles from meteoroids with retrograde trajectories[ Acknowledgements MJB gratefully acknowledges receipt of a grant from the Particle Physics and Astronomy Research Council "PPARC# of the U[K[ Hypervelocity impact facilities used in this work are supported by a PPARC Rolling Grant to the Unit for Space Science and Astrophysics of the University of Kent at Canterbury "U[K#[ MJB thanks the Nu.eld Foundation "U[K[# for equipment support[ HY was supported in this work by an Overseas Research Studentship awarded by the Committee of Vice!Chan! cellors and Principals of the U[K[ We thank J[A[M[ McDonnell\ the PI of TICCE\ for access to the space exposed aerogel[ J[A[M[ McDonnell thanks D[ Brownlee "University of Washington# for supply of the aerogel used in TICCE[ We thank J[B[ Webber of the Physics Dept[\ Univ[ of Kent "U[K[# for measurement of the pore size in the aerogel[ We thank R[ Newsam and F[ McCann of the Biology Department\ Univ[ of Kent "U[K[# for technical assistance with laser microscopy of the TICCE aerogel sample[

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