Impact Cratering From Ldefs 5.75-Year Exposure - Decoding Of The Interplanetary And Earth-Orbital Populations

The IDEF multiple-foil microabrasion experiment (MAP) was exposed in five pointing directions stabilized relative to the orbit motion vector. Penetration records from this exposure provide an excellent opportunity for the decoding of the possible contribution from Earth orbital (bound) components and hyperbolic (unbound) particulates of extraterrestrial origin. Complemented by other experimental data at larger dimensions on LDEF, a preliminary flux distribution is derived for the nominal east- (ram), west-(trailing), and space-pointing detector surfaces. Modeling of the orbital dynamics for these two classes of population, and their collisional probabilities with IDEFs near-circular orbit, demonstrates a high anisotropy in the flux rate for the different detector locations. Bound and unbound particulates are also seen to have quite different signatures regarding anisotropy. According to the modeling, the west- and space pointing flux distributions must dominantly represent the unbound extraterrestrial populations. The different impact velocities on these two faces also permits, with computer modeling, the derivation of the average geocentric particle impact velocity, an extension of the same modeling permits, further, a transformation to predict the flux distribution of the same unbound particulates on the east face. Hence we can identify any excess flux observed on the east face as a possible component in Earth orbit. The east-to-west flux ratio for the MAP data is 34 +/- 7 for the penetration of aluminum at 5 mum and 7.3 +/- 1.7 at 30 mum; the space-to-west ratio is 4.9 +/- 1.0 at 5 mum (Niblett, 1991). These data demonstrate, using the modelling developed and that of Zook (1991), that LDEF impacts on all detectors are dominated by unbound and hence extraterrestrial particulates above particulate masses of 6.4 x 10(-10) g mass. However, for small particulates an orbital component is clearly identified on the east and side (N,S) faces that exceeds the interplanetary flux distribution by a factor of around 4 on the east face. The source is not yet identified, but the possible role of space debris and aerocaptured natural interplanetary dust is discussed. The data are compared to craters reported on the Solar Maximum Mission (SMM) louvres (Laurence and Brownlee, 1986). The SMM data are consistent with IDEF MAP data as a crater distribution, but very significant revisions to the interpretation of the SMM data are identified. The application of a new dimensionally scaled penetration formula, incorporating hypervelocity impact calibration to 16 km s-1 velocity, shows that the interplanetary component measured on the LDEF foils is consistent with interplanetary sources measured at 1 AU heliocentric distance.