Forming Binary Near-Earth Asteroids From Tidal Disruptions

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Walsh, Kevin John
Richardson, Derek C
We present simulations and observations as part of a model of the binary near-Earth asteroid population. The study of binary asteroid formation includes a series of simulations of near-Earth asteroid (NEA) tidal disruption, analyzed for bound, mutually orbiting systems. Discrete and solid particles held together only by self-gravity are employed to model a ``rubble pile'' asteroid passing Earth on a hyperbolic encounter. This is accomplished via N-body simulations, with multiple encounter and body parameters varied. We examine the relative binary production rates and the physical and orbital properties of the binaries created as a function of the parameters. We also present the overall relative likelihoods for possible physical and dynamical properties of created binaries. In order to constrain the shape and spin properties of the bodies that feed the NEA population, an observing campaign was undertaken to observe lightcurves of small Main Belt asteroids (D < 5 km, SMBAs). Observations of 28 asteroids increases the overall number of SMBAs studied via lightcurves to 86. These observations allow direct comparison between NEAs and MBAs of a similar size. The shape and spin for the SMBAs are incorporated into a Monte Carlo model of a steady-state NEA population, along with the binaries created by tidal disruption simulations. Effects from tidal evolution and binary disruption from close planetary encounters are included as a means of altering or disrupting binaries. We find that with the best known progenitor (small Main Belt asteroids) shape and spin distributions, and current estimates of NEA lifetime and encounter probabilities, that tidal disruption should account for approximately 1-2% of NEAs being binaries. Given the observed estimate of an ~15% binary NEA fraction, we conclude that there are other formation mechanisms that contribute significantly to this population.