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J. E. Colwell, L. W. Esposito, A. R. Lemos, M. T. Mellon (LASP, University of Colorado)
A series of low-velocity impact experiments into simulated planetary regolith has been performed in microgravity conditions on the space shuttle and the NASA KC-135 and in terrestrial conditions in the laboratory. All impacts are in vacuum. Target materials include JSC-1 lunar regolith simulant, JSC-Mars-1, and quartz sand, and impactors are spheres between 0.6 and 2 cm in diameter in a variety of densities. Impact velocities between 1 and 300 cm/s have been studied with a range of impact angles. Ejecta velocities are measured through direct imaging in microgravity experiments and a combination of imaging and ejecta collectors in ground-based experiments. Coefficients of restitution are measured from direct imaging in microgravity experiments, and total ejecta mass is measured with collectors in ground-based experiments.
The amount of ejecta produced and the coefficient of restitution are strongly dependent on target properties such as size distribution, grain shape, and relative density. Impacts into low density quartz sand in microgravity resulted in net accretion for impact speeds below about 15 cm/s with 2-cm quartz sphere impactors. In ground-based experiments the ejecta mass is seen not to depend only on the kinetic energy of the impactor. Ejecta velocity distributions follow simple power-laws similar to those for hypervelocity impacts, but with a different power-law exponent.
Data have been returned from 9 space shuttle experiments, and more than 30 each of ground-based and airplane-based impacts in a variety of parameters. We will report on these data including coefficients of restitution, and functional forms for ejecta mass and velocity, and their implications for the conditions necessary for planetesimal formation in the early protoplanetary nebula.
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Bulletin of the American Astronomical Society, 34, #3< br> © 2002. The American Astronomical Soceity.