Computer Science – Performance
Scientific paper
Dec 2011
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011agufm.p23c1724m&link_type=abstract
American Geophysical Union, Fall Meeting 2011, abstract #P23C-1724
Computer Science
Performance
[1932] Informatics / High-Performance Computing, [6205] Planetary Sciences: Solar System Objects / Asteroids
Scientific paper
Most, if not all asteroids larger than a few hundred meters in diameter are rubble-piles - aggregates of small fragments, held together by self-gravity with virtually no tensile strength. These strenghless bodies are, ironically, very resistant to disruption by collisions, as density discontinuities in their interiors make it impossible for a shock front to propagate far from the impact point. Rubble-piles are, however, very susceptible to disruption by tidal forces. An Earth-Crossing asteroid (ECA) will experience a strong tidal force for a relatively short time, when its orbit brings it deep enough in the Earth's gravitational field. Before there is an asteroid collision with the Earth, we are likely to observe a number of tidal collisions - that is, events which cause observable changes to the asteroid. In addition, new high-resolution maps of the Moon by HiRISE record clear imprints of tidally disrupted asteroids (V. Bray, pers. comm.) So there are and will be data whereby we can understand the bulk asteroid geophysical properties that are so notoriously difficult (and expensive) to measure. Analytical expressions exist for the minimum circular orbit at which a fluid body can retain an equilibrium shape; this is the classic Roche limit. Similar expressions exist for ideal parabolic encounters by non-rotating liquid spheres (Sridhar and Tremaine, 1992). But a rubble-pile ECA will not behave like a fluid body; inter-particle forces and dilatation can prevent it from deforming. Every asteroid tidal encounter with the Earth is unique, so we need good models with the best physics, capable of exploring a wide parameter space. Richardson et al. (1998) explored the sensitivity of tidal disruption of Earth-crossing rubble-piles to the orbit periapse, encounter velocity, spin period, and deviation from sphericity; and found all four highly important in determining the extent of disruption of the progenitor body. They used an N-body code with energy dissipating collisions to simulate hundreds of encounters. They used spheres as the building blocks of a simulated rubble-pile, and therefore neglect or underestimated the effects of static friction and granular locking, effects that can only be adequately modeled with non-spherical, roughly shaped grains. The same is true of dilatation whereby granular shear requires physical expansion; spherical piles are more easily rearranged and exhibit less dilatancy. Additionally, electrostatic cohesion forces have recently been shown to be important in a milli-gravity environment (Scheeres et al., 2010). In this present work, we revisit the study of ECA disruption with an improved code that simulates cohesion and static and dynamic friction between arbitrarily shaped grains. We run many of the same initial conditions previously studied by Richardson et al. (1998) and compare our results to their original study. The inclusion of friction and cohesion forces, and of geometrical locking and dilatation (by modeling with polyhedral grains), all influence the outcome. Conversely, witnessing the tidal response of an ECA, either in the future when one comes whizzing by, or in the past recorded in the regolith of the Moon, will allow us through models to determine these parameters, so essential to understanding asteroid geophysics.
Asphaug Erik Ian
Movshovitz Naor
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