  While the number of asteroids with known shapes has drastically increased
over the past few years, little is known on the the time-evolution of shapes
and the underlying physical processes. Here we propose an averaged abrasion
model based on micro-collisons, accounting for asteroids not necessarily
evolving toward regular spheroids, rather (depending on the fall-back rate of
ejecta) following an alternative path, thus confirming photometry-derived
features, e.g. existence of large, relatively flat areas separated by edges. We
show that our model is realistic, since the bulk of the collisions falls into
this category.
