CRATERS IN ALUMINUM-1100 BY SODA-LIME GLASS SPHERES AT 1 TO 7 KM S/

To assist in the interpretation of crater populations on space-exposed surfaces from the Solar Maximum Satellite and the Long Duration Expos ure Facility (LDEF), we conducted laboratory simulations of cosmic-dus t impacts into aluminum 1100 targets with similar to 3.2 mm diameter s oda-lime projectiles at velocities (V) between 0.7 to 7 km/s. The resu lting crater diameters (D-C) conform to current cratering equations, w hile crater depths (Pc) are somewhat deeper, yielding typical P-C/D-C ratios of similar to 0.58 at V > 6 km/s. Similar values (P-C/D-C > 0.5 5) are found for craters in aluminum surfaces retrieved from the Long Duration Exposure Facility, which were produced at impact velocities a s high as 20 km/s. The value of P-C/D-C = 0.5 that is being used by ma ny should be abandoned; a P-C/D-C = 0.58 is recommended. Similarly, we demonstrate that mass loss of the target, as determined by pre- and p ost-shot weight measurements; can be almost an order of magnitude less than mass losses that would be inferred from crater-volume measuremen ts. This difference is due to substantial internal deformation of the target which permits large volumes of material to be deformed, and dis placed without physical ejection and dislodgment. In addition, this st udy traces the behavior of the glass impactor. At V < 1.5 km/s a compl exly deformed and sheared projectile resides within the crater cavity. The first projectile melts appear at similar to 2.2 km/s, and mixture s of melt and debris start creeping up the crater walls at > 2.5 km/s. The radially expanding melts develop, by geometric dispersion, into t hin films that tear, leaving a discontinuous melt deposit upon cooling . At V > 5.5 km/s the entire impactor is essentially molten; however, small quantities of unmelted residues still can be found at 7 km/s. In creasingly larger fractions of melt escape the crater between 5 and 7 km/s. Only modestly higher velocities would be needed to have the enti re projectile escape the crater. This appears to be the case for simil ar to 50% of all LDEF craters which lack detectable residues at sensit ivity revels of Scanning Electron Microscopy - Energy Dispersive X-Ray analysis.