2019AJ....158...69K

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.