Scaling Forces to the Asteroid Surface: The role of cohesion
The surfaces and interiors of asteroids are characterized by their low ambient gravitational accelerations which range from milli-Gs on the surface of Eros to much less than micro-Gs at the equator of rapidly spinning bodies such as 1999 KW4 Alpha. We report research on performing a full accounting for all relevant physical forces, in addition to a particle's weight, in such regimes and find that cohesive van der Waals forces become significant for macroscopic grains (Scheeres et al., Icarus in press). For example, theory predicts that on the surface of Itokawa the cohesive attraction between two centimeter-sized grains in contact should exceed their weight, implying that a collection of such gravels should behave more like a cohesive powder (such as bread flour) than as a terrestrial gravel pile. Extending the theory to the self-gravitation of two boulders in contact, such as in a rubble pile, predicts that mutual cohesive forces dominate mutual gravitation for component sizes less than a few meters. One implication of this is that higher rotation rates are necessary to cause smaller grains to fission from an asteroid, while larger aggregates should be less affected and fission at lower rotation rates, introducing a size-scale into an otherwise scale-invariant physical process. A simple model for this cohesive effect is presented and its implications for asteroid science are discussed.