April 4th, 2014: Are “active asteroids” experiencing mass shedding or breakup?

Recent observations have discovered several objects that are evidently experiencing some sort of mass shedding or breakup. The interpretations of such objects can be across this spectrum; for example, P/2013 P5 may be experiencing mass shedding, while P/2013 R3 may be breaking up into several components. P/2010 A2, on the other hand, could be some combination of these effects. Detailed observations of these bodies can be interpreted as being consistent with rotational break-up of the objects, based on the spectrum and magnitude of ejecta speeds plus some aspects of the morphology of their debris tails. An interesting hypothesis is whether all of these outcomes could have singularly arisen from the YORP effect. Specifically, could rotational instability lead to such different failure modes? The present study considers failure modes of irregularly shaped rubble-pile bodies due to a YORP-type spin up, comparing perfectly ellipsoidal shapes with actual shape models (based on radar observations and spacecraft imaging). Failure modes may be categorized into two different modes: structural failure, a process of plastic deformation finally leading to fission or global redistribution in a catastrophic way, and surface shedding at which particles sitting on the surface are lofted from there due to centrifugal accelerations exceeding gravitational accelerations prior to structural failure. We have developed dynamical and structural analysis techniques to determine the failure modes of rubble pile objects given their shape, mass, and spin rate. This talk discusses correlations between the shape and the failure mode due to a YORP-type spin up, using 21 sample asteroid models. The primary result of this study shows that irregular asteroids can be categorized into four shape classes: (i) spherical bodies undergoing structural failure, (ii, iii) ellipsoidal bodies experiencing either structural failure or surface shedding, and (iv) bifurcated bodies failing by structural failure. We discuss what the observational implications of these failure modes may be.

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