Observational Catalogs And Numerical Ephemerides For The Satellite System Of Neptune

Developing high-precision ephemerides for Neptunian satellites requires not only the continuation of observing campaign but also the collection and improvements of ex-isting observations.So far,no complete catalogs of observations of Neptunian satellites are available.The goals of our study on observations are to provide new precise positions,and to compile the first complete observational catalog including all available ground-based and Hubble Space Telescope(HST)astrometric observations of Neptunian satellites.The observations are tabulated in a single and consistent format and given in the same time scale(Terrestrial Time(TT))and reference system(International Celestial Refer-ence System(ICRS)),including necessary changes and corrections.On one hand,new CCD observations of Triton and Nereid are made at Lijiang 2.4-m Telescope and Yaoan 0.8-m High Precision Telescope in 2013-2020,and then reduced based on Gaia-DR2.On the other hand,the first complete observational cat-alog of Neptunian satellites called OCANS-8-2020 is compiled,after recognizing and correcting errors and omissions.Furthermore,in addition to what have been considered for the COSS08 catalog for eight main Saturnian satellites,all observed absolute and relative coordinates are converted to the ICRS,with corrections for star catalog biases with respect to Gaia-DR2.New debiasing tables for both the modern and old star cata-logs previously not given based on Gaia-DR2 are developed and applied.Treatment of missing positions of comparison bodies in conversions of observed relative coordinates is proposed.OCANS-8-2020 and the new debiasing tables are publicly available online.OCANS-8-2020 includes all available astrometric observations obtained between 1847 and 2020.All observations are given in TT and ICRS.The star catalog biases are re-moved,which are significant for Nereid and small outer satellites of Neptune.Among all the observed coordinates of Triton,2400(12%)obtained in 103 nights(3%)are our new ones,and,of Nereid,1174(20%)in 75 nights(13%).Compared with similar ground-based observations in the world,our new ones have relatively high precisions,which are about 0’.’03 for Triton and about 0’.’06 for Nereid.This observational cata-log is the basis for subsequent research on the motion theory of the Neptunian satellite system and related dynamical studies.New observations and new planetary and satellite ephemerides provide new oppor-tunities to improve ephemerides for Neptunian satellites as well as relevant parameters.In particular,the observations include a lot of new accurate Earth-based positions re-duced with Gaia catalogs and accurate positions obtained from HST.We aim to reliably improve ephemerides for Triton,Nereid,and small outer satel-lites of Neptune,and some parameters by using all available astrometry in 1847-2020 and updating existing dynamical models.Orbits are determined by fitting dynamical and observational model parameters to observations in a weighted least-squares sense.For this,we developed a N-body numerical orbit determination tool called FORCES.The numerical dynamical and ob-servational models make use of the latest Jet Propulsion Laboratory ephemerides for planets and the small inner satellites of Neptune,DE440 and NEP090.In addition to the gravitational effects considered by NEP081,the dynamical model also includes pertur-bations from the small inner satellites and a revised model for the motion of Neptune’s pole orientation.Moreover,a method for improving the computational efficiency of these perturbations is used.Since Neptune’s pole orientation is possibly improvable according to the preliminary post-fit sensitivity analysis,it is adjusted together with the satellite state vectors.The linear mapping of covariance matrix is a measure of formal uncertainties of our orbit and pole solutions.However,to obtain more reliable accu-racy estimations,it is necessary to consider the uncertainties in both observations and unadjusted model parameters.For this,a BR-RS method performing both Bootstrap resampling of observations(BR)and random sampling of unadjusted parameters(RS)is used.The new ephemerides for Triton and Nereid,FORCES-8-MAIN-2020,covering years 1600-2650 are available online in SPK file format as recommend by the Interna-tional Astronomical Union(IAU).The orbits are well determined with positional uncer-tainties expected to be within 200 km(about 10 mas as seen from the Earth)for Triton and 1000 km(50 mas)for Nereid for the next 100 years as estimated by the BR-RS method.In particular,the correction in Nereid mean orbit motion from the NEP081 solution is+4’.’9 yr-1,with a BR-RS uncertainty of 0’.’24 yr-1.In the fitting process,we also determine Neptune’s pole orientation.At the initial epoch 1989 September 1,00:00:00 TDB(Barycentric Dynamical Time),the right ascension and declination of the new pole orientation referred to the ICRS are ap=299°.339±0°012(formal)/±0.°014(BR-RS)and δp=42°.985±0.°016(formal)/±0°.045(BR-RS),respectively.In 1800-2200,the BR-RS 1-σ uncertainty of the pole orientation is well constrained within about 0.05.The new ephemerides for small outer satellites of Neptune,FORCES-8-OUTER-2020,covering years 1600-2650 are also available online in the SPK file format.Re-sults show that Psamathe and Neso orbits still have large uncertainties,which indicate the risks of loss.This is mainly caused by the insufficient orbital coverage of the obser-vations of these two satellites.In order to reduce the loss risks,we suggest observing these satellites as faint as 25 R-magnitude with the Wide Field Survey Telescope and the Survey Space Telescope of China Manned Space Program(formerly known as China Space Station telescope).The present paper also provides analytical expressions,which are geometrically intuitive and easy to extrapolate,for the new Neptunian satellite orbits and Neptune’s pole orientation.Among them,the approximation error of Triton orbit in 1800-2200 can reach～5 km.For Neptune’s pole orientation,the widely used model defined by IAU Working Group on Cartographic Coordinates and Rotational Elements is used to establish the analytical expression.The approximation error of the obtained expression in 1600-2650 does not exceed 0’.’08.It is found that there is a long-term change in the Neptune’s pole orientation,which is consistent with the long-term change in the angular momentum direction of Neptune system.