HYPERVELOCITY IMPACT IN LOW EARTH ORBIT: FINDING SUBTLE IMPACTOR SIGNATURES ON THE HUBBLE SPACE TELESCOPE

Abstract

HYPERVELOCITY IMPACT IN LOW EARTH ORBIT: FINDING SUBTLE IMPACTOR SIGNATURES ON THE HUBBLE SPACE TELESCOPE A T Kearsley 1,2,5, J L Colaux 3, D K Ross 4, P J Wozniakiewicz 2,5, L Gerlach 6, P Anz-Meador 4, J-C Liou 7, T Griffin 8, B Reed 8, J Opiela 4, V V Palitsin 3, G W Grime 3, R P Webb 3, C Jeynes 3, J Spratt 2, M J Cole 5, M C Price 5 and M J Burchell 5. 1 Dunholme, Raven Hall Road, Ravenscar, YO13 0NA, UK (kearsleys@runbox.com); 2 Natural History Museum (NHM), Cromwell Road, London, UK. 3 Ion Beam Centre, University of Surrey, Guildford, UK. 4 ESCG-Jacobs, NASA-JSC, Houston, TX, USA. 5 School of Physical Sciences, University of Kent, Canterbury, Kent, UK. 6 European Space Agency (ESA, retired), Noordwijk, The Netherlands. 7 NASA Johnson Space Center, Houston, TX, USA. 8 NASA Goddard Space Flight Center (GSFC), Greenbelt, Maryland, USA. ABSTRACT Introduction Return of large surface area components from the Hubble Space Telescope (HST) during shuttle orbiter service missions has allowed inspection of large numbers of hyper-velocity impact features from long exposure in low Earth orbit (LEO). Particular attention has been paid to the origin of the impacting particles, whether artificial Orbital Debris (OD) or natural Micrometeoroid (MM). Extensive studies have been made of solar cells (Graham et al., 2001; Kearsley et al 2005, Moussi et al., 2005) and recently, the painted metal surface of the Wide Field and Planetary Camera 2 WFPC2 radiator shield (Anz-Meador et al., 2013; Colaux et al., 2014; Kearsley et al., 2014a; Ross et al., 2014). Both of these materials from HST have layers of complex chemical composition, into which particle fragments and melt may infiltrate during impact. Experimental light gas gun (LGG) impacts (e.g. Price et al., 2014) have shown that impactor remains may be dispersed and dilute, often as a very thin and patchy coating within an irregular impact-generated pit. In previous studies, the low concentration of particle residue, the rugged topography of impact features, and especially the complex multi-element composition of the impacted surface were considered significant barriers to recognition of extraneous impactor-derived components. Analysis was both difficult and time consuming (e.g. Graham et al., 2001), and a substantial proportion of impactors (25-65%) could not be identified. Recent advances in energy dispersive X-ray microanalysis (EDX) now permit routine identification of impactor origins using scanning electron microscope (SEM); particle induced X-ray emission (PIXE) and micro-X-ray fluorescence (µ-XRF) instruments (Kearsley et al., 2012, 2014b). Here we demonstrate how these techniques have allowed impactor composition to be isolated, and the particle source determined for the great majority of WFPC2 samples studied. Methods To analyse impact melt on the zinc orthotitanate (ZOT) and aluminium alloy (Al-6061) of the WFPC2 radiator shield we used the Oxford Instruments INCA SEM-EDX spectrum pro-cessing software to separate peak and background X-ray counts for specified X-ray emission lines. From tables of likely OD and MM signature elements (e.g. Kearsley et al., 2005), and knowledge of the pristine WFPC paint and alloy compositions, we extracted data for the fol-lowing elements: Mg, Al, Si, S, K, Ca, Ti, Cr, Mn, Fe, Ni, Cu and Zn. Two types of graphical plot were developed, to highlight extraneous element signatures in small impacts on the ZOT paint (Fig. 1), and larger craters into the Al-alloy (Fig. 2). The impactor origin was then clas-sified by reference to a suite of decision trees (Kearsley et al., 2012). A Bruker X-Flash 6050 EDX detector was also used to obtain signal from the interior of deeper craters. PIXE maps and spectra were acquired in the Ion Beam Centre, University of Surrey (Colaux et al., 2014). Results Figure 1. WFPC2 impact feature 339: a) SEM backscattered electron (BE) image; b) SEM depth model; c) SEM-EDX maps show high Mg concentration in the impact melt lining the impact feature d) plots of SEM-EDX X-ray counts for Mg and Fe show much higher levels in impact melt (red) than in clean ZOT paint (blue), and a similar level to impact residue from LGG impacts of olivine grains (open black squares). Excess Mg and Fe contents in frothy impact melt show impactor was a micrometeoroid. Figure 2). WFPC2 impact feature 462: a) SEM BE image; b) SEM depth profile; c and d) PIXE EDX maps show Fe and Ni across crater pit and surrounding metal, some iron-rich in-clusions in the Al alloy, but Ni only enriched in pit; e) PIXE EDX spectra show high Fe and Ni on crater floor, similar to micrometeoroid metal composition; f) plot of Mg/Al versus Cr/Fe X-ray counts in SEM-EDX spectra from the crater edge (red) show enrichment of Mg and Fe over alloy composition (black, grey, yellow and green), indicating a mafic silicate mi-crometeoroid component has also been added from the impacted micrometeoroid. Summary and conclusions Together, SEM-EDX and PIXE-EDX maps, spectra and X-ray count plots showed 166 MM residues and 2 OD residues in this survey of 188 impact features on WFPC2, ~ 90% of those examined, considerable enhancement of impactor recognition over an earlier study of HST impacts (~75% identified as MM or OD in origin, Kearsley et al., 2005). Acknowledgements ESA contract 40001105713/12/NL/GE awarded to NHM and the University of Surrey; Bruker for expertise in use of the X-Flash detector and loan of the M4 Tornado µ-XRF. References quoted Anz-Meador P. et al. (2013) Proc. 6th European Conf. Space Debris, ESA SP 723: s1b_anzme.pdf, CD-ROM. Colaux J. L. et al. (2014) LPSC 45 Abstract #1727. Graham, G.A. et al. (2001) Proc. 3rd European Conf. Space Debris, ESA SP 473:197–203. Kearsley A.T. et al., (2005) Adv. Space Res. 35:1254–1262. Kearsley A. T. et al. (2012) Technical Note 1 for ESA contract 40001105713/12/NL/GE. Kearsley A. T. et al. (2014a) LPSC 45 abstract #1722. Kearsley A.T. et al. (2014b) LPSC 45 abstract #1733. Moussi A. et al. (2005) Adv. Space Res. 35:1243–1253. Price M. C. et al. (2014) LPSC 45 abstract #1466. Ross D. K. et al. (2014) LPSC 45 abstract #1514.