Giant craters on Mathilde

Mathilde possesses at least five giant craters, where crater diameters are comparable to the mean radius of the body. Initially, it seems remarkable that this asteroid survived such an onslaught of impacts without disrupting. We consider implications of giant cratering on Mathilde, including effects of oblique impacts. Experimental databases and theory indicate that oblique impacts are (1) less likely to disrupt a target, (2) produce less ejecta relative to a normal impact, and (3) most often do not generate elongated craters. Moreover, oblique impacts reduce crater sizes and ejecta volumes particularly in the case of giant craters, where projectile fragments can miss the target. The morphology of giant craters on Mathilde and the lack of obvious ejecta are consistent with formation by oblique impacts,which could help explain why Mathilde survived. The composition and low density of Mathilde probably indicate a porous and volatile-rich body. Target porosity attenuates shock waves and enhances survival without catastrophic disruption. It is not surprising that several giant craters can be formed on a body without disrupting it. For a realistic projectile population, the probability of making a giant crater on Mathilde is 2.1 to 2.6 times greater than the probability of disruption. The probability of Mathilde surviving the formation of five giant craters is approximately 1 in 5 to 1 in 7. (C) 1999 Academic Press.