The rubble pile hypothesis for small asteroids in the Near Earth and Main Belt populations have been driven by several factors, including the observed high porosity of those bodies whose mass have been measured, the evident limitation on spin rate of asteroids larger than
~500 meters, and direct observation of the surface morphology of these bodies. Given these observations, it has been presumed that small asteroids should evolve as if they were cohesionless collections of grains. Detailed geophysical analysis of these bodies by Holsapple (Icarus 2010) show that cohesionless bodies will evolve under the addition of angular momentum by the YORP effect into more distended and, paradoxically, more slowly rotating bodies. Additional analysis in Holsapple (Icarus 2007) has shown that cohesional strength within a rubble pile could strengthen a collection of grains to the point where they could sustain rapid rotation.
In our current talk we use the above as a starting point and incorporate new observations of asteroid morphology driven by recent analysis of asteroid Itokawa by the Hayabusa science team. The sample collected by that spacecraft confirmed the presence of fine grains on the surface of that asteroid, and thus implies the presence of such fines distributed throughout the body itself. This has important consequences for the strength of a rubble pile due to natural cohesion forces that are known to be present for such small grains, as evidenced by terrestrial experience. We calculate and simulate the effect of these forces for the strength of a rubble pile constituted of larger boulders embedded in a matrix of finer grains. This simple model predicts a yield strength that varies inversely with the mean particle size, and provides sufficient strength to a rubble pile to account for many of the fast spinning bodies seen in the population.
If rubble pile asteroids are strengthened through cohesion between dust fines, there are several implications that can be tested and compared against the existing asteroid data. These involve the observed size cutoff for binary asteroids, the presence of rapidly rotating tumbling asteroids, and other features in the data. We present these tests and evaluate whether this model is consistent with them. Finally, based on the theoretical work behind this research we propose a reinterpretation of the spin limit for asteroids with size between 0.5 - 10 km in terms of mechanics models of strength and failure, predicting a larger mean density than the currently derived ~2.1 g/cm^3 limit.
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