Barycentric Ephemeris

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The goal of this project is to lay the foundations for a new solar system ephemeris. In the first funding phase, the work focused on the identification and acquisition of observational data, the improvement of relevant force models,, and on the corresponding software development. Regarding the force model, the work initially concentrated on the librational motion of the Moon as well as on perturbing effects due to individual asteroids or asteroid populations, in the form of ¿mass rings¿. The implementation of a mass ring is planned by the end of the first phase. Concerning force modeling, updates shall be implemented in the second phase, which includes an improved lunar libration model (PN3). Furthermore, we will investigate the significance of additional mass rings, representing e.g., Trans-Neptunian Objects. The explicit optional use of TCB is planned for future versions of our ephemeris, which requires the introduction of time-scale-consistent initial values and masses. Moreover, a relation with the TT time scale is essential for data reduction. So far, in this step we adopted the series expansion of Hirayama. To enable direct comparisons with other ephemerides, this analytical approach shall be replaced by simultaneous numerical integration of the differential equation for the time-scale difference TT-TDB together with the equations of motion for the solar system bodies. The check of the accuracy of our ephemeris will continue via cross-comparisons with renowned ephemerides (DE, INPOP, EPM). On the other hand, we will exploit new available observations (DVLBI, Gaia). For the first time, these allow a firm tie of the dynamical reference frame (realized by the motion of celestial bodies) w.r.t. the kinematical reference frame (via D-VLBI and Gaia observations of stars and solar system objects). A direct relation between the involved reference frames is given by planned VLBI observations of the Gaia spacecraft (PN4). After inclusion of any available observational data, a re-determination of relevant solar system parameters will be performed. This requires the development of a suitable combination strategy and careful weighting scheme for different observation types, and the selection of an optimal set of parameters for a common adjustment. In addition, the new ephemeris will support tests on fundamental physics parameters. The high computational burden of the ephemeris calculation shall be met by parallel computing techniques. Benefitting from our experiences, efforts in this direction will be intensified in the second funding period. For the software implementation we will request support from highperformance computer centers affiliated with the partner institutions. Users of our new ephemeris within the research group will be the colleagues from the projects PN2, PN3, PN4, and PN5. Further collaborations are planned with external users, such as with the asteroid and comet group at DLR.