Revisiting Ldef: High Resolution Elemental and Isotopic Characterization of Hypervelocity

Introduction: The Long Duration Exposure Facility (LDEF) satellite was flown in Low Earth Orbit (LEO) for a duration of 69 months from 1984 through 1990 [1]. Its main mission was to study the effects of the space environment on a variety of exposed surfaces, but it also carried experiments dedicated to the capture of impacting particles, which included not only cosmic dust, but also a significant fraction of manmade orbital debris. The LDEF satellite was gravitygradient stabilized in orbit [1], resulting in different surfaces facing the same directions (e.g., leading and trailing edge, space and earth ends) during the entire time in space. This configuration made it possible to study separate regimes of particle bombardment on different sides of the satellite, with, e.g., fewer impacts by man-made debris on the trailing edge [2]. Samples from LDEF were extensively studied during the early 1990s with a wide array of analytical tools available at the time [e.g., 3 5]. The recent return of the Stardust spacecraft with hypervelocity impacts of cometary dust in Al foil targets has renewed interest in the detailed characterization of crater debris and possible inferences about the original projectile material [6]. Here we revisit impact features from the LDEF satellite using state-of-the-art analytical techniques with two major objectives in mind: (1) Prepare for the future analysis of interstellar impacts from the second collector on the Stardust spacecraft [7, 8], where the impact parameters (speed and angle) are less well defined than on the cometary side and may resemble more the conditions found in LDEF impacts. (2) Investigate whether elemental and isotopic characterization of LDEF impact features at a sub-micrometer scale can provide additional information about the nature of the LEO environment. Experimental: We performed high resolution elemental and isotopic imaging measurements of particle impact features in Au and Al targets which were originally mounted in space-facing and trailing locations on the LDEF satellite [2, 9 11]. Measurements were performed by SEM-EDX, NanoSIMS and Auger spectroscopy. The latter two techniques are highly surface sensitive and offer spatial resolutions of 100 nm and 20 nm, respectively. The advantage of Auger spectroscopy is that it is non-destructive (which is important for the ISPE [7, 8]), while the NanoSIMS allows isotopic measurements combined with depth profiling. For the isotopic measurements we acquired secondary ion images of C, C, O, O, and O . The analytical conditions were similar to those used in studies of cometary craters [12] and should allow the detection of C and O isotopic signatures of presolar grain at high spatial resolution, if such particles are present among the impact debris [12, 13]. Results and Discussion: Elemental measurements of all analyzed LDEF craters (6 in Au, 2 in Al) indicates the presence of projectile residues on the crater floors, the inner walls and on the rims. High concentrations of Fe, Mg, and Si (and the absence of other elements commonly found in man-made debris in the LEO) indicate that these craters were caused by the impact of natural dust particles. Detailed images of the debris show that this material typically is highly heterogeneous on a sub-micrometer scale and that it is finely intermixed with the target materials. This observation is both consistent with earlier observations in such LDEF craters [9, 10] and is comparable to what has been observed on the cometary Stardust collector [6].