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

During 2005-2012, images from Hubble Space Telescope (HST) revealed four moons orbiting Pluto-Charon (Weaver et al 2006, Showalter et al 2011, 2012). Although their orbits and geometric shapes are well-known, the 2$\sigma$ uncertainties in the masses of the two largest satellites - Nix and Hydra - are comparable to their HST masses (Brozovic et al 2015, Showalter & Hamilton 2015, Weaver et al 2016). Remarkably, gravitational $n$-body computer calculations of the long-term system stability on 0.1-1 Gyr time scales place much tighter constraints on the masses of Nix and Hydra, with upper limits $\sim$ 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.