MORPHOLOGY OF METEOROID AND DEBRIS IMPACT CRATERS FORMED IN SOFT METAL TARGETS ON THE LDEF SATELLITE

We have measured the depths, average diameters and circularity indices of over 600 micrometeoroid and space debris impact craters formed in surfaces exposed to space aboard the Long Duration Exposure Facility s atellite. The target surfaces had a variety of orientations and physic al properties. The average depth-diameter ratio of craters formed in a luminum targets by nearly normal impacts is between 0.56 and 0.60, hig her than the canonical and widely accepted value of 0.50 which corresp onds to a hemispherical shape. The depth-diameter ratio does not chang e significantly with target Brinell hardness values between 40 and 90, or with average impact velocity above 5 km s(-1). The depth-diameter ratio is found to vary as roughly the one-tenth power of target densit y. Less than 10% of the craters examined had major-to-minor axis ratio s higher than 1.5, consistent with the production of shallow, elongate d craters exclusively by grazing impacts. The variation in depth-diame ter ratio for circular craters most likely results from variation in p rojectile shapes.