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R. M. Canup, H. F. Levison (Southwest Research Institute)
The currently favored theory of lunar origin is the giant-impact hypothesis. Recent work that has modeled accretional growth in impact-generated disks (Ida, Canup and Stewart 1997, Nature 389) has found that systems with two large moons are common outcomes. We have recently conducted a preliminary study of the stability of a terrestrial multiple moon system as it evolves due to mutual interactions (including mean-motion resonances) and tidal interaction with the Earth (Canup, Levison and Stewart, submitted). We find that multiple moon configurations which form from impact-generated disks are typically unstable: these systems will likely evolve into a single moon state as the moons mutually collide or as the inner moonlet crashes into the Earth.