The Size of 3I/ATLAS from Non-gravitational Acceleration

Abstract The third macroscopic interstellar object detected in the solar system recently passed through perihelion, with the best-fitting models of its trajectory now featuring non-gravitational accelerations (NGAs). We assess how much mass loss is required to produce plausible NGA solutions and compare with estimates of the mass loss. We find that they are consistent when the nucleus of 3I/ATLAS is around 1 km in diameter. For a recent solution with a time lag in the acceleration from Eubanks et al., we find diameters between 820 and 1050 m, assuming an outgassing asymmetry factor ζ = 0.5 and a density of the comet nucleus ρ = 0.5 g cm −3 . The limits on the diameter scale as ( ζ / ρ ) 1/3 . Substantial extrapolation is required in general to compare NGAs to mass loss rates, so reliable estimates of the mass loss rate at other stages of the comet’s trajectory will substantially reduce the systematic uncertainty in this estimate.