The radiator shield on the Wide Field and Planetary Camera 2 (WFPC2) was subject to optical inspection following return from the Hubble Space Telescope (HST) in 2009. The survey revealed over 600 impact features of > 300 micrometers diameter, from exposure in space for 16 years. Subsequently, an international collaborative programme of analysis was organized to determine the origin of hypervelocity particles responsible for the damage. Here we describe examples of the numerous smaller micrometeoroid (MM) impact features (< 700 micrometers diameter) which excavated zinc orthotitanate (ZOT) paint from the radiator surface, but did not incorporate material from underlying Al alloy; larger impacts are described by [3]. We discuss recognition and interpretation of impactor remains, and MM compositions found on WFPC2

Anz-Meador, P.

Burchell, M. J.

Colaux, J.

Cole, M. J.

Gerlach, L.

Griffin, T.

Grime, G.

Jeynes, C.

Kearsley, A. T.

Liou, J.C.

Opiela, J.

Palitsin, V.

Price, M. C.

Ross, D. K.

Webb, R.

Wozniakiewicz, P. J.

The Wide Field and Planetary Camera 2 (WFPC2) was returned from the Hubble Space Telescope (HST) by shuttle mission STS-125 in 2009. In space for 16 years, the surface accumulated hundreds of impact features on the zinc orthotitanate paint, some penetrating through into underlying metal. Larger impacts were seen in photographs taken from within the shuttle orbiter during service missions, with spallation of paint in areas reaching 1.6 cm across, exposing alloy beneath. Here we describe larger impact shapes, the analysis of impactor composition, and the micrometeoroid (MM) types responsible

Webb, R. P.

Liou, J. C.

Grime, G. W.

Colaux, J. L.

Griffin, G. T.

NASA Technical Reports Server

MICROMETEOROID IMPACTS ON THE HUBBLE SPACE TELESCOPE WIDE FIELD AND 
PLANETARY CAMERA 2: LARGER PARTICLES. A. T. Kearsley1, G. W. Grime2, R. P. Webb2, C. Jeynes2, V. 
V. Palitsin2, J. L. Colaux2, D. K. Ross3,4, P. Anz-Meador3,4 , J.C. Liou4, J. Opiela3,4, G. T. Griffin5, L. Gerlach6, P. J. 
Wozniakiewicz1,7, M. C. Price7, M. J. Burchell7 and M. J. Cole7 1Natural History Museum (NHM), London, SW7 
5BD, UK (antk@nhm.ac.uk) 2Ion Beam Centre, University of Surrey, Guildford, 3Jacobs Technology, Houston, TX, 
USA 4NASA-JSC, Houston, TX, USA, 5NASA-GSFC, USA, 5consultant to European Space Agency, ESA-ESTEC, 
Noordwijk, The Netherlands 6School of Physical Science, University of Kent, Canterbury, CT2 7NH, UK. 
 
 
Introduction: The Wide Field and Planetary Cam-
era 2 (WFPC2) was returned from the Hubble Space 
Telescope (HST) by shuttle mission STS-125 in 2009. 
In space for 16 years, the surface accumulated hun-
dreds of impact features [1] on the zinc orthotitanate 
paint, some penetrating through into underlying metal. 
Larger impacts were seen in photographs taken from 
within the shuttle orbiter during service missions [2], 
with spallation of paint in areas reaching 1.6 cm across, 
exposing alloy beneath. Here we describe larger impact 
shapes, the analysis of impactor composition, and the 
micrometeoroid  (MM) types responsible. 
 
Fig. 1. The WFPC2 radiator shield wrapped in Llu-
malloy sheeting at the Johnson Space Center (NASA-
JSC), locations of the large craters indicated by arrows. 
Methods:  Samples were cut using the tech-
nique of [3], and examined in a Zeiss EVO 15 LS 
scanning electron microscope at NHM. Digital eleva-
tion models (DEM) were created from stereo pairs, to 
reveal the shape and size of features (e.g. Fig. 2b). A 
silicon drift energy dispersive X-ray detector (EDX) 
was used to collect point X-ray spectra and maps. Al-
though data were collected under quantitative analysis 
conditions and matched to analysis standards, due to 
the complex surface topography and porosity, matrix 
correction was not attempted. Instead, spectral decon-
volution was performed, using Oxford Instruments 
INCA software to measure X-ray counts in characteris-
tic element peaks and background (sigma), to establish 
detection above a statistically defined background  (3 x 
sigma). After characterisation of both the paint and 
underlying alloy, incorporation of impactor remains 
was recognised in two types of analytical plot [4], vali-
dated by experiments in the light gas gun at the Univer-
sity of Kent [5]. 
Results: 63 impact features > 700µm across, each with 
paint spallation > 300 µm, showed combinations of 
five main components (e.g. Fig. 3): a) an exposed sur-
face of Al alloy, often with adhering fragments of 
paint; b) a bowl-shaped pit or field of compound pits, 
penetrating into the alloy; c) frothy impact melt, de-
rived mainly from paint (Fig. 3); d) droplets/coatings 
of alloy-dominated metal melt; and occasionally e) 
retained fragments of the impacting particle (Fig 4). 
 
Fig. 2. WFPC2 sample 121: a) optical image and b) 
DEM, showing: 1 paint external surface; 1” partially 
exfoliated paint; 2 exposed surface of aluminium alloy; 
3 compound crater pits in the surface of alloy.  
https://ntrs.nasa.gov/search.jsp?R=20140003986 2019-08-29T14:22:47+00:00Z
Fig. 3. WFPC2 sample 121: a) BEI of frothy impact 
melt outside compound pits; b) plot of melt composi-
tion in Mg/Al versus Cr/Fe X-ray counts compared to 
ordinary and carbonaceous chondrite Cr-rich spinels. 
In most features, EDX of frothy impact melt (Fig. 
3a) showed enrichment of elements indicating the MM 
composition (eg Cr-rich spinel, Fig. 3b). The common-
est signature was increased Mg and Fe, and occasion-
ally S, Ca and Ni. Impactor fragments were found in X-
ray maps of three large craters, revealing Mg, Si and 
O-bearing compositions: one non-stoichiometric; one 
probably pyroxene; and one a shocked Mg-rich olivine 
(Fig. 4). If no clear signature was seen in EDX data, 
the impact was submitted for ion beam analysis [6]. 
  
Conclusions: The WFPC2 radiator surface pre-
served a signature of impacts by MM across a wide 
size range, from the smaller particles of [2] to large Mg 
silicate, sulfide and oxide grains. Impact features pene-
trating to the alloy layer occasionally contain fragments 
of the impactor, fused to the surface within melt (Fig. 
4). More frequently, the nature and composition of the 
impactor could only be determined by long duration 
(200 s) EDX spectra of impact melt, in which subtle 
traces of material other than the paint or alloy could be 
distinguished and interpreted. 
 
Fig. 4. WFPC2-424: a) optical image of largest impact 
on radiator; b) EDX maps show location of: c) Mg-, 
Fe-silicate fragment, probably mainly shocked olivine. 
Acknowledgements: UK participation in this col-
laborative research program was funded by ESA con-
tract 4000105713_12_NL_GE. 
References: [1] Opiela, J.N. et al. (2012) 
NASA/TP-2012-217359 [2] ODQN 13.3, NASA, July 
2009 [3] Anz-Meador P. et al. (2013) Proc. 6th Euro-
pean Conf. Space Debris, ESA SP723: s1b_anzme.pdf, 
CD-ROM [4] Kearsley A. T. et al. MAPS submitted [5] 
Burchell M.J et al. (1999) Meas. Sci. Tech., 10, 41-50. 
[6] Grime G. et al. (2014) LPSC 45 Abstract # XXXX. 


Micrometeoroid Impacts on the Hubble Space Telescope Wide Field and Planetary Camera 2: Larger Particles

MICROMETEOROID IMPACTS ON THE HUBBLE SPACE TELESCOPE WIDE FIELD AND 
PLANETARY CAMERA 2: SMALLER PARTICLE IMPACTS. D. K. Ross
1,2
, P. Anz-Meador
1,2
 , J.C. Liou
2
, J. 
Opiela
1,2
, A. T. Kearsley
3
, G. Grime
4
, R. Webb
4
, C. Jeynes
4
, V. Palitsin
4
, J. Colaux
4
, T. Griffin
5
, L. Gerlach
6
, P. J. 
Wozniakiewicz
3,7
, M. C. Price
7
, M. J. Burchell
7
, and M. J. Cole
7
 
1
Jacobs Technology, Houston Tx 
(daniel.ross@nasa.gov) 
2
NASA-JSC, Houston, TX, USA 
3
Science Facilities, Natural History Museum (NHM), Lon-
don, SW7 5BD, UK 
4
Ion Beam Centre, University of Surrey, Guildford, UK 
5
NASA-GSFC, USA, 
6
consultant to 
European Space Agency, ESA-ESTEC, Noordwijk, The Netherlands 
7
School of Physical Science, University of 
Kent, Canterbury, CT2 7NH, UK. 
 
Introduction The radiator shield on the Wide Field 
and Planetary Camera 2 (WFPC2) was subject to opti-
cal inspection [1] following return from the Hubble 
Space Telescope (HST) in 2009. The survey revealed 
over 600 impact features of > 300 µm diameter, from 
exposure in space for 16 years. Subsequently, an inter-
national collaborative programme of analysis was or-
ganized to determine the origin of hypervelocity parti-
cles responsible for the damage [2]. Here we describe 
examples of the numerous smaller micrometeoroid 
(MM) impact features (< 700 µm diameter) which ex-
cavated zinc orthotitanate (ZOT) paint from the radia-
tor surface, but did not incorporate material from un-
derlying Al alloy; larger impacts are described by [3]. 
We discuss recognition and interpretation of impactor 
remains, and MM compositions found on WFPC2. 
 
Fig. 1. WFPC2-97: Typical smaller micrometeoroid 
impact, Secondary Electron Image (SEI) of a core [2]. 
Methods:  Uncoated core samples were examined 
using backscattered electron imagery (BEI) in a Zeiss 
EVO 15 LS scanning electron microscope (SEM) at 
NHM, with digital elevation models (DEM) created 
from stereo pairs for shape (especially depth) meas-
urement (Fig. 2). Energy dispersive X-ray (EDX) maps 
were created using a silicon drift detector (SDD) and 
INCA SmartMap software, and a Bruker XFlash 5060F 
pole-piece mounted quad detector at Bruker, in Berlin. 
Carbon-coated samples were examined with JEOL 
7600F and 5910LV SEMs at JSC. Point spectra were 
collected from within and outside impact features. Lo-
cations of impact melts were compared to the unaffect-
ed paint surface, to recognise incorporation of impactor 
material. This required detailed characterisation of the 
radiator paint [4]. Due to the irregular surface topogra-
phy and porosity of ZOT paint, matrix correction was 
not attempted. For spectra collected at NHM, deconvo-
lution was performed using the Spectrum Processing 
function in INCA software, giving X-ray counts in el-
ement peaks and background (sigma). These data were 
used to establish whether MM indicator elements were 
detected above statistically defined background (3 x 
sigma), plotted using protocol of [4]. 
 
Fig. 2. WFPC2-339: a) Stereo anaglyph of BEI images; 
b) DEM shows crater is restricted to the ZOT paint 
layer, and does not excavate the underlying Al alloy.  
Results: The smaller impact features contained 
frothy impact melt, mostly containing compoents of the 
https://ntrs.nasa.gov/search.jsp?R=20140005747 2019-08-29T14:05:48+00:00Z
ZOT paint.. Comparison of EDX spectra to clean paint 
composition showed that locally, impact melts had 
enrichment of Mg and Fe (e.g. Fig. 3) without signifi-
cant increase in Al, indicating that these elements were 
not derived from the alloy beneath. These resemble 
silicate MM analogue impacts [4]. A few craters con-
tained Fe and Ni enrichments, indicating probable MM 
metal, or Fe and S (e.g. Fig. 4) like experimental sul-
fide MM analogues of [4]. 
 
Fig. 3. WFPC2-339: a) EDX maps show Mg enriched 
in melt around pit; b) EDX spectrum melt (red) com-
pared to unaltered paint (blue); c) Mg and Fe enrich-
ment in frothy melt (red), above levels in paint (blue). 
Conclusions: The WFPC2 smaller craters give a 
snapshot of the natural dust particle population encoun-
tered by spacecraft during long space exposure in low 
Earth orbit. Despite effects of hypervelocity impact, 
they preserve a record of diverse MM compositions. 
 
Fig. 4. WFPC2-156: a) BEI stereo anaglyph; b) EDX 
maps show Fe and S enriched in melt; b) EDX spec-
trum melt (orange) compared to unaltered paint (blue). 
Acknowledgements: UK research was supported 
by ESA contract 4000105713_12_NL_GE. We thank 
Bruker for use of the XFlash 5060F quad detector. 
 
References : [1] Opiela, J.N. et al. (2012) 
NASA/TP-2012-217359 [2] Anz-Meador P. et al. 
(2013) Proc. 6
th
 European Conf. Space Debris, ESA 
SP723: s1b_anzme.pdf, CD-ROM [3] Kearsley A. T. 
et al. (2014) LPSC 45 submitted [4] Kearsley A. T. et 
al. MAPS submitted. 


Micrometeoroid Impacts on the Hubble Space Telescope Wide Field and Planetary Camera 2: Smaller Particle Impacts

Micrometeoroid Impacts on the Hubble Space Telescope Wide Field and Planetary Camera 2: Smaller Particle Impacts

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