Pergamon Adv. Space Res. Vol. 23, No. 1, pp. 95-100, 1999 0 1999 COSPAR. Published by Elsevier Science Ltd. All rights reserved Printed in Great Britain 0273-I 177/99 $20.00 + 0.00 PII: SO273-1177(98)00235-X HYPERVELOCITY IMPACTS IN LOW EARTH ORBIT: COSMIC DUST VERSUS SPACE DEBRIS G. A. Graham ‘, A. T. Kearsley 2, M. M. Grady 3, I. P. Wright ‘, A. D. Griffths 4 and J. A. M. McDonnell 4 ’ PSRI, The Open University, Milton Keynes MK7 6AA, U.K 2 Geology, Oxford Brook& University, Oxford OX3 OBP, U.K 3 Mineralogy Department, The Natural History Museum, London SW 7 5BD. U.K 4 USA, The University of Kent at Canterbury, Canterbury CT2 7NR, U.K ABSTRACT The understanding of the micron-sized populations of natural micrometeoroids and artificial space debris in low Earth orbit has benefited considerably from the post-flight investigations of retrieved surfaces from spacecraft, such as the Long Duration Exposure Facility. The returned solar array from the Hubble Space Telescope has added to this repository and has offered a further opportunity to document these particles. 25 individual solar cells were specially selected on the basis that they contained impact craters (diameter 100-1000 pm) which had the most potential to retain impactor residue chemistry. The solar cells were subject to a detailed investigation using analytical scanning electron microscopy which identified 29 impact craters, the analysis of which identified 3 residues as artificial in origin, 6 unclassified and 20 as natural in origin. The limited number of unclassified residues identified indicates that the methods of analysis employed in this investigation are a significant step forward for such studies and, if employed on a greater number of samples, will improve the calculations of the time-integrated flux rates for micrometeoroids and space debris in the low Earth orbit environment. Notwithstanding the small sample set examined, the observed chemical classification of the impact residues in terms of micrometeoroid to space debris (in the particle size range 8-80 pm) corresponds well to the flux model that predicts the dominance of natural particles. 01999 COSPAR.Publishedby Elsevier Science Ltd. NOMENCLATURE Diameters and crater Classes as defined by Herbert and McDonnell (1997) D,, = conchoidal cracking diameter (pm) D, = Particle diameter (pm) INTRODUCTION The characterisation of low Earth orbit (LEO) micron-particle populations has benefited extensively from the post-flight investigations of retrieved surfaces from spacecraft, i.e. the Long Duration Exposure Facility (LDEF) (e.g. Zolensky et al., 1994) and the European Retrievable Carrier (EuReCa) (e.g. McDonnell et al., 1995). The micron-particle populations can essentially be sub-divided into two categories: 1) natural micrometeoroids (particles originating from comets, asteroids and possibly secondary ejecta from planetary impacts) (Brownlee, 1994), and 2) artificial space debris (fragments generated from spacecraft collisions, by-products of normal spacecraft operations etc). The returned solar array from the Hubble Space Telescope (HST) has offered a further opportunity to document the abundance of these particles, as prior to its retrieval the array was in LEO for 3.62 years at an orbital altitude of 6OOkm, in essentially a sun-facing direction. The array was subject to a detailed post-flight investigation led by ESA, with micrometeoroid and debris studies focused upon the hypervelocity 95