A Pluto–Charon Sonata: Dynamical Limits on the Masses of the Small Satellites

Abstract

Abstract During 2005–2012, images from Hubble Space Telescope (HST) revealed four moons orbiting Pluto–Charon. Although their orbits and geometric shapes are well-known, the 2σ uncertainties in the masses of the two largest satellites—Nix and Hydra—are comparable to their HST masses. Remarkably, gravitational n-body computer calculations of the long-term system stability on 0.1–1 Gyr timescales place much tighter constraints on the masses of Nix and Hydra, with upper limits ∼10% larger than the HST mass. Constraints on the mass density using size measurements from New Horizons suggest Nix and Hydra formed in icier material than Pluto and Charon.