Since its detection by Mariner 10, helium has been a key focus in studies of Mercury's exosphere. Recently, Weichbold et al. (2025), https://doi.org/10.1029/2024je008679 provided the first in situ helium measurements, inferring density from Ion Cyclotron Wave (ICW) events observed by the MESSENGER spacecraft. This approach enables, for the first time, a helium density profile across a broad altitude range without relying on prior models. We present an ab-initio model for a steady state, solar wind-driven helium exosphere, which informed the interpretation of these ICW measurements. We discuss helium release processes and evaluate whether meteoroid impacts could account for specific instances of elevated helium measurements. We developed a global, semi-analytical model based on a helium-saturated regolith and an average helium source flux of (Formula presented.) He/s from solar wind ion implantation. We calculate the helium flux distribution using an analytical lateral transport model and then generate local radial density profiles from a numerical (Monte Carlo) radial transport model. Additionally, we applied the radial transport model to estimate the scale and duration of large, sporadic helium release events and assess the likelihood of detecting these events in situ. The strong agreement between our model and the novel measurements confirms that the measurable helium exosphere is dominated by thermally recycled particles. We show that elevated helium measurements can result from the vaporization and release of helium from large (1 m) meteoroid impacts, but it is statistically unlikely that more than one impact event is captured in the given set of measurements.

The TU Delft Astrodynamics Toolbox (Tudat) is a free open-source software (FOSS) suite geared towards research and education in computational astrodynamics. It has been applied primarily to numerical simulation of the dynamics of objects in space, ranging from optimization of re-entry vehicle dynamics to the modeling of planetary spacecraft tracking and the dynamics of natural solar system bodies. The powerful and versatile estimation module of Tudat has been used for a broad range of studies for both current and future space missions. It has the capability to combine optical and radiometric tracking data from multiple spacecraft with Earth-based observations into a comprehensive estimation of the dynamics of both natural and artificial solar system bodies, as well as physical parameters of interest. Building upon this general and adaptable framework, recent developments have focused on incorporating the necessary functionality required for real tracking data analysis. In this paper, we present the integration of these capabilities into Tudat’s fully open-source framework, with a combined focus on planetary missions and Space Situational Awareness (SSA). At present, the software provides capabilities to process several categories of observational data: (i) deep-space Doppler and range tracking data of planetary missions collected by the Deep Space Network (DSN) and ESA’s ESTRACK, supporting multiple formats such as IFMS, ODF, and TNF; (ii) deep-space Doppler and VLBI tracking data of planetary missions collected by the Planetary and Radio Interferometry and Doppler Experiment (PRIDE) with radio (astronomy) telescopes; (iii) optical astrometry and radar tracking archived by the Minor Planet Center (MPC) and the Natural Satellite Data Center (NSDC). By computing observation residuals using existing orbital solutions as references, we show that our observation models are accurate to the intrinsic quality of the data (e.g., better than 0.05 mm/s for typical deep-space Doppler data). Additionally, we demonstrate that our dynamical models possess the level of fidelity necessary to enable precise orbit estimation, effectively leveraging the high quality of the available tracking data. Tudat is unique in providing modular and flexible open-source high-fidelity modeling across a broad range of orbital regimes, enabling interdisciplinary applications. We provide an overview of the data processing and estimation capabilities and give examples from various mission domains. These include high-precision orbit estimation using deep-space Doppler tracking data, orbit determination of cis-lunar/xGEO space debris in highly non-linear regimes (specifically targeting upper stages of lunar missions) from astrometric data, and estimation of small solar system bodies using astrometric data.

In 2031 the JUICE spacecraft will perform a multi-flyby tour of the Jovian system. Next to the radiometric tracking that is performed for navigation operations, the dedicated radio science instrument (3GM) collects high-accuracy radiometric measurements during the flybys. We investigate the capability of the radio science data to provide improved moon state knowledge during navigational operations. We introduce ephemeris updates from radio science data into our simulated navigation operations and examine the potential savings of statistical ΔV for corrective manoeuvres. A navigation orbit determination (OD) solution was simulated for the multi-flyby tour of JUICE, including the resulting state knowledge evolution of the Galilean moons. The OD was extended by an interface for external moon ephemeris updates, which was used to evaluate the impact of radio science generated external ephemerides on the statistical ΔV budgets for post-flyby cleanup manoeuvres. The moon state knowledge evolution during navigation operation showed a rapid reduction of the a-priori moon state uncertainty, for which the navigational tracking data coverage of the long, early tour arcs was identified as the driving factor. As a result of the longer tracking arcs, the moon state knowledge from navigation data results improves more quickly during the initial phase of the tour. Since the impact of moon state knowledge on the corrective manoeuvres is largest in this initial phase, the comparative analysis of the statistical ΔV cost shows that the adoption of radio science ephemeris products does not effectuate significant ΔV savings. Instead we showed that in order to achieve substantial ΔV savings improvements of Europa's and Ganymede's ephemerides are required ahead of JUICE's arrival. While the analysis concludes that data synergies are unlikely to benefit the navigational operations, it highlights other potential synergies between the navigation and radio science data. A comparatively strong signature of Io's dynamics was found in the simulated navigation data along the long early tour arcs, which could be leveraged for the benefit of the new global moon ephemeris solutions after JUICE.