Planetary Astrophysics: Dynamics of Post-Fission Asteroids
Author:
Seth Jacobson & Daniel Scheeres
Abstract:
We derive a realistic model for the evolution of tidally perturbed
binary asteroids to examine systems immediately after a spin-up
fission event. The spin rate of an asteroid can be increased by the
YORP effect?thermal re-radiation from an asymmetric body, which
induces torques that can rotationally accelerate a body. If the
asteroid is modeled as a "rubble pile", a collection of
gravitationally bound boulders with a distribution of size scales and
no tensile strength between them, increasing the spin rate leads to an
eventual fission of components, determined by the largest separation
between the mass centers of the asteroid. We note that these
post-fission binaries are always unstable and initially evolve
chaotically. We model the shapes of these bodies as tri-axial
ellipsoids with a gravitational potential expanded up to second order.
Our model applies instantaneous tidal torques to both members of the
binary system to determine energy dissipation that could provide
enough loss to settle the system into a stable orbit.
We find that most systems experience a period of chaotic evolution
with rapid energy dissipation followed by a classical quasi-steady
state tidal evolution. The mass ratio of the system plays a dominant
role in determining the final state after the chaotic evolution.
Systems can be divided into distinct evolutionary tracks that
determine the outcome of their chaotic behavior in terms of timescale
and total dissipated energy during the rapid phase. Secondary bodies
of low mass ratio systems may proceed through their own spin-fission
events. Systems with higher mass ratios approach a fully synchronous
state after this fast energy dissipation period. After these systems
emerge from their chaotic behavior they evolve according to the much
slower classical theory, where other effects such as Binary YORP could
play a more significant role.