The flux of Tunguska-sized fragments from the main asteroid belt

Abstract

The numerical model described in Menichella et al. (Earth, Moon and Planets 72, 133–149, 1996) is used to investigate the flux of Tunguska-sized asteroid fragments into chaotic resonant orbits leading them to attain an Earth-crossing status. The assumed main-belt size distribution is derived from that of known asteroids, extrapolated down to sizes ≈ 1 m and modified in such a way as to yield a quasi-stationary fragment production rate over times ≈ 100 Myr. Collisional physics consistent with the results of laboratory hypervelocity impact experiments and evidence from asteroid families are used, and the sensitivity of the results to the most critical poorly known parameters is analysed. The results of simulations show that the main asteroid belt on average can inject into the resonant escape hatches about one Tunguska-sized fragment per year, with an uncertainty of about a factor of ∓3. Owing to their limited dynamical and collisional lifetimes (as inferred from the better-known behaviour of km-sized near-Earth asteroids), only a fraction ≈ 1% of the Tunguska-sized near-Earth fragments are likely to hit the Earth, yielding an average flux of the order of one impact per century, consistent with observations (within the existing uncertainties). Large-scale stochastic collisions in the main belt can enhance this fragment flux by a factor of up to 6 over intervals ≈ 1 Myr, assuming that this corresponds to the typical dynamical timescale in the resonances. Such enhanced-flux episodes are expected to occur every several tens of Myr.