Orbits and resonances of the regular moons of Neptune

We report integrated orbital fits for the inner regular moons of Neptune based on the most complete astrometric data set to date, with observations from Earth-based telescopes, Voyager 2, and the Hubble Space Telescope covering 1981-2016. We summarize the results in terms of state vectors, mean orbital elements, and orbital uncertainties. The estimated masses of the two innermost moons, Naiad and Thalassa, are GM_Naiad = 0.0080 ± 0.0043 km³ s^-2 and GM_Thalassa = 0.0236 ± 0.0064 km³ s^-2, corresponding to densities of 0.80 ± 0.48 g cm^-3 and 1.23 ± 0.43 g cm^-3, respectively. Our analysis shows that Naiad and Thalassa are locked in an unusual type of orbital resonance. The resonant argument 73λ˙_Thalassa - 69λ˙_Naiad - 4Ω˙_Naiad ≈ 0 librates around 180° with an average amplitude of ~66° and a period of ~1.9 years for the nominal set of masses. This is the first fourth-order resonance discovered between the moons of the outer planets. More high precision astrometry is needed to better constrain the masses of Naiad and Thalassa, and consequently, the amplitude and the period of libration. We also report on a 13:11 near-resonance of Hippocamp and Proteus, which may lead to a mass estimate of Proteus provided that there are future observations of Hippocamp. Our fit yielded a value for Neptune's oblateness coefficient of J₂ = 3409.1 ± 2.9 × 10^-6.