THE NEAR-EARTH SPACE IMPACT ENVIRONMENT - AN LDEF OVERVIEW

Impact data from craters on finite targets and thin foil perforations are used to establish impact distributions for key pointing directions on the returned spacecraft LDEF over sizes ranging from sub-micron to millimetres. Using thin and thick target data corresponding to the same average impactor fluxes, equivalences are established. From such merged distributions and from variations in flux with pointing direction, average flux distributions at constant crater size are established for LDEF's principal pointing directions and, incorporating LDEFs offset angle, further models for the flux distribution relative to LDEFs orbit vector. Though natural meteoroids dominate at medium dimensions, orbital particulates and space debris are evident at micron and millimetre crater dimensions. For the natural particulates a mean velocity ranging between 16.7 and 22kms-1 (depending on particle size) and a density of 1.0+/-0.3gcm-1 is indicated. For the microdebris, crater morphology shows that a higher density is required. Asymmetries in the North and South fluxes point to a non-uniform meteoroid population; this natural component on LDEF is shown to agree very well with that calculated for the interplanetary meteoroid flux. The model fluxes are available (electronic and hard copy) through the Meteoroid and Debris Special Investigation Group Database.