Joint Radiometric and Altimetric MESSENGER Orbit Solutions for Mercury Geodetic Applications

Accurate spacecraft orbit reconstruction is essential for geodetic investigations of Mercury. MESSENGER-based estimates of key parameters, including Mercury’s spin-axis orientation, obliquity, and physical libration amplitude, have shown discrepancies between solutions derived from different datasets and analysis strategies [1,2]. Part of these differences may arise from the orbit solutions used to geolocate surface measurements. For instance, orbit errors can propagate into altimetric crossover residuals and into recovered orientation parameters.

Recent orbit-determination efforts have improved the modeling of non-gravitational accelerations acting on MESSENGER, reducing systematic errors in the reconstructed trajectory [3,4]. However, these solutions do not include orbits at low Sun–Probe–Earth angles, due to the strong effect of solar plasma noise on radiometric tracking data. This restriction ensures a good overall orbit quality but also limits temporal and spatial coverage, which in turn reduces the number and distribution of usable altimetric crossovers.

Here we present new MESSENGER orbit solutions obtained by jointly processing radiometric tracking data and Mercury Laser Altimeter observations. The altimetric component leverages high-resolution digital elevation models of Mercury to better constrain the spacecraft positioning to the surface. By carefully incorporating altimetry into the orbit-reconstruction process, we aim to improve the internal consistency of the trajectory solution while accounting for the practical limitations imposed by the available tracking geometry.

We assess the quality of these new orbits, e.g., through an altimetric crossover validation, and compare their performance with existing orbit solutions. Crossover residuals provide a sensitive diagnostic of orbit-dependent geolocation errors. These improved orbit solutions will support an updated determination of Mercury’s geodetic parameters. By reducing orbit-related systematic errors, they may help clarify the origin of discrepancies between geodetic solutions based on surface measurements and constraints from interior-structure studies. In the longer term, this validated approach to combined radiometric and altimetric processing will provide a useful framework for orbit determination and geodetic analyses of future Mercury datasets, including future joint exploitation of MESSENGER and BepiColombo observations.

References:

[1] Genova et al. (2019) GRL, 46, 3625–3633. [2] Bertone et al. (2021) JGR-Planets, 126(4).  [3] Andolfo et al. (2024) J. Guid. Control Dyn., 47(3), 518-530. [4] Zurria et al, private communication.