Spin-up of rubble-pile asteroids: Disruption, satellite formation, and equilibrium shapes

We present results from numerical experiments testing the behavior of cohesionless gravitational aggregates experiencing a gradual increase of angular momentum. The test bodies used in these numerical simulations are gravitational aggregates of different construction, distinguished by the size distribution of the particles constituting them, parameterized in terms of the angle of friction (ϕ). Shape change and mass loss are found to depend strongly on ϕ, with results ranging from oblate spheroids forming binary systems to near-fluid behavior characterized by mass shedding bursts and no binary formation. Bodies with the highest angle of friction, ϕ ∼ 40°, evolve to shapes with average axis ratios of c/a ∼ 0.70 and b/a ∼ 0.90 (a ⩾ b ⩾ c), and are efficient at forming satellites. Bodies with lower angle of friction, ϕ ∼ 20°, evolve to shapes with average axis ratios of c/a ∼ 0.61 and b/a ∼ 0.83, and are less efficient at forming satellites. The most fluid-like bodies tested, with ϕ near zero, become very elongated, with average axis ratios c/a ∼ 0.40 and b/a ∼ 0.56, and do not form satellites in any simulation. In all but 2 fluid-like cases out of 360, no more than 5% of the total mass was ejected in a single event. Bodies with substantial cores were also tested under slow spin-up, and cases with cores larger than ∼30% of the total mass were successful at forming binaries.The binary systems created in all simulations are analyzed and compared against observed binary near-Earth asteroids and small Main Belt asteroids. The shape and rotation period of the primary, orbital and rotational period of the secondary, and the orbital semi-major axis and eccentricity are found to closely match the observed population.

► We model the slow spin-up of rubble pile asteroids.
► Asteroids with high angles of friction (>40°) can maintain spherical/oblate shapes at critical rotation.
► Asteroids that can maintain spherical/oblate shapes can build satellites in orbit via mass lost from their equators.