Implications of Giant Craters on Mathilde

The recent flyby of asteroid [253] Mathilde by the Near Earth Asteroid Rendezvous spacecraft (NEAR) was the first spacecraft encounter with a C-type asteroid. A striking result of the encounter is that there are five giant craters, whose diameters are comparable to the 26.5 km mean radius of Mathilde, on the single observed face of the asteroid. It seems remarkable that Mathilde survived such an onslaught of giant impacts without disrupting. Previous workers have noted that Mathilde's low measured density of 1.3 g/cc, which implies a porous composition, increases its resistance to disruption. We point out that oblique impacts are also key to understanding the geology of Mathilde. Relative to normal impacts, oblique impacts are less likely to disrupt a target, produce less ejecta, and most often do not create elongated craters, consistent with the observed morphology of giant craters on Mathilde. For a realistic projectile population, the probability of giant cratering on Mathilde is 2.1 to 2.6 times greater than that of disruption, and the probability of Mathilde's surviving five giant impacts is 1 in 5 to 1in 7.