Title:Binary Asteroid Formation via Spin-up
Author:Kevin Walsh

We present results of simulations modeling the rotational spinup and disruption of rubble pile asteroids. We find that gradual spinup (such as via the YORP effect) results in binary formation as long as two requirements are met. First, the asteroid must maintain a low equatorial elongation at critical spin rates. This is only possible if the body has a high angle of friction (~40 degrees), or has a substantial core, both of which help the body resist re-shaping and becoming prolate. By contrast a fluid-like body (angle of friction near 0 degrees) becomes prolate and material that leaves the surface cannot remain in a stable orbit to form a massive satellite. Second, the material must be collisionally dissipative. The particles that escape from the surface of the primary asteroid will not accumulate into large satellites unless the coefficient of restitution governing their collision outcomes is moderately to strongly dissipative, namely with a coefficient below about 0.5 (at least 50% dissipative). In our simulations where these two constraints are met, we find rapid formation of close, sizeable secondaries (semimajor axis a = 2-4 primary radii) with low eccentricities (e<0.15).