This year marks the tenth anniversary of the end of the Rosetta mission at comet 67P/Churyumov–Gerasimenko [1], and the community is already preparing for the next step in cometary exploration with ESA's Comet Interceptor mission, scheduled for launch in 2029 [2]. Despite the extensive insights gained from Rosetta regarding coma structure and its impact on spacecraft dynamics, accurately and efficiently modelling the coma for orbit analysis remains a major challenge.The cometary coma exhibits a highly complex structure driven by the nucleus shape, time-varying illumination from rotation, and changing heliocentric distance [3]. These factors produce a strong day–night asymmetry, with gas number densities differing by several orders of magnitude between the sunlit and shadowed hemispheres [4]. The nearly radial outgassing velocity shows a similar angular structure: day-side bulk speeds reach up to ~800 m/s, almost twice those on the night side. Because orbiting spacecraft move at only cm/s to low m/s relative to the nucleus, this radial gas velocity dominates the aerodynamic force and must therefore be modelled with high fidelity.Uniform, homogeneous coma models [5] can capture only bulk quantities such as the total water production rate and lead to large propagation errors when applied to orbit determination. Direct Simulation Monte Carlo (DSMC) approaches reproduce the structure of the coma far more faithfully by accounting for nucleus shape and illumination [3], but are computationally prohibitive for a temporally continuous representation and are typically restricted to the inner coma (≲10–20 km). Consequently, neither approach is directly suitable for orbit analysis, where thousands of force evaluations are required along candidate trajectories.To bridge this gap, we introduce a 4-D Spherical-Harmonic Polynomial Framework for empirical gas-field modelling. The framework provides a joint radial, temporal, and angular continuous expansion of the coma. Any scalar gas-field quantity, number density, bulk speed, an individual velocity component, or temperature, is expanded in surface spherical harmonics whose Stokes coefficients depend on radius and time. Diurnal variability is captured through a discrete Fourier representation over the nucleus rotation period, while the radial evolution is described by compact polynomials. Once fitted, the resulting surrogate is operationally tractable: it delivers continuous, differentiable, millisecond-scale field evaluations directly compatible with high-throughput orbit propagation routines.In this contribution, we present the formulation of the framework and demonstrate its application to comet 67P as a case study, illustrating its ability to reproduce reference fields with high accuracy using an open-source implementation in Tudat software [6] and its utility for trajectory analysis around active small bodies in support of upcoming missions such as Comet Interceptor. Glassmeier K.-H., et al. (2007), Space Sci. Rev.Lee, S., et al. (2015), Astronomy and AstrophysicsGeraint H. Jones, et al. (2023), Encyclopedia of AstrobiologyMarschall et al. (2024), in Comets III Haser, L. (1957), Bulletins de l’Academie Royale de Belgiquie Dirkx, D, et al. (2022), EPSC2022-253

Abstract Two-way satellite time and frequency transfer (TWSTFT) is among the most accurate techniques for comparing clocks over (inter)continental baselines, exchanging bidirectional signals through a geostationary satellite—but the cost and complexity of an active (Tx & Rx) terminal restrict it to a handful of Coordinated Universal Time (UTC)(k) laboratories. We show that a passive, receive-only user can reach those same laboratories by reducing a one-way pseudorange for the satellite-motion, atmospheric, and equipment delays. Using active-network observations over 130 d, the satellite ephemeris is reconstructed by orbit determination (OD). A portable terminal—a commercial TV dish and a software-defined-radio receiver—was calibrated in common-clock mode at INRiM and deployed to the Côte d”Azur Observatory (OCA). Over a 40 d campaign, time transfer to three UTC(k) laboratories agreed with independent GNSS and active-TWSTFT links at the nanosecond level; despite larger short-term noise from solar-radiation-pressure and wet-tropospheric mismodelling, the overlapping Allan deviation reaches ≈ 10 − 14 at one day, matching GNSS performance. The accuracy is bounded by an ≈ 7 ns ( 1 σ ) budget, and an INRiM–OCA–INRiM closure sequence confirmed robustness to relocation and traceability to UTC(k). Passive TWSTFT thus offers a scalable, interference-resilient, and low-cost route to UTC(k) dissemination, with future work targeting automated OD for quasi-real-time operation.

Abstract. This paper presents the results of Planetary Radio Interferometry and Doppler Experiment (PRIDE) observations of the JUICE spacecraft during its lunar and Earth gravity-assist manoeuvres on August 19 and 20, 2024, using a network of VLBI radio telescopes. Prior to the flyby sequence, VLBI phase‑referencing observations were conducted to establish astrometric measurements and Doppler frequency calibration during the near-Earth approach. During the flyby events, the experiment successfully captured the signal ingress and egress during the lunar occultation, as well as the signal behaviour at closest approach to the lunar surface. Signal-diagnostic measurements were performed to characterise the received carrier signal and assess instrumental and propagation effects. The observed Doppler frequency measurements were subsequently compared with predicted values derived independently using the WebGeocalc service and the Tudat software framework. The close agreement between the measured Doppler observables and model predictions confirms that the tracking results are consistent with expectations for JUICE's cruise-phase configuration and the high-velocity dynamics of the Earth flyby. Finally, these measurements were also used to obtain complementary measurements of the terrestrial ionosphere and space weather conditions, and are being used for refining solar wind models using planetary spacecraft radio tracking.

The largest volcano in our solar system, Olympus Mons, is part of the Tharsis rise volcanic complex. Studies have shown that the free-air gravity anomaly at the Tharsis rise that goes up to 3,540 mGal at Olympus Mons, cannot only be explained by a flexure in the lithosphere and requires a substantial mass anomaly in the mantle to account for the heat needed to sustain Tharsis (Root et al., 2026; Redmond and King, 2004). Corroborating this, Le Maistre et al. (2023) detected a long-term acceleration in Mars' rotation rate from InSight's RISE experiment, suggesting ongoing redistribution of mass in the Martian interior that cannot be attributed to atmospheric processes alone, pointing to active internal dynamics.Previous work has demonstrated that time-variable gravity measurements from satellite tracking offer a promising path toward detecting active mantle flow on Mars. Low viscosity and deep plumes with high density contrast with the surrounding mantle produce the strongest gravity-rate signals, reaching up to ~20 nGal/year, values that are at the edge of current observational capabilities. However, the wide uncertainty range in plume and mantle properties, including size, temperature excess, depth, and viscosity structure, translates into a broad spread of predicted signals, preventing definitive interpretation.To move beyond this limitation, we investigate the long-term geodynamical evolution of the Tharsis mantle plume from its initiation to the present day. We model it as the product of a multi-billion-year thermal and dynamic history, to track how the plume head and tail develop, stall, and potentially persist as a thermal anomaly in the Martian mantle. The signal detectable by satellites tracking today does not necessarily reflect an actively rising mantle plume. Instead, it could be an imprint of a residue structure, whose characteristics are shaped by Mars' long thermal history.A central focus of our study is the role of mantle viscosity, which governs the timescales of thermal diffusion, the longevity of plume structures, and the amplitude of time-variable gravity signals. We systematically explore a range of depth-dependent viscosity scenarios based on the literature, to assess how sensitively the present-day plume state depends on these assumptions, and to identify which configurations remain consistent with existing observational constraints from InSight and orbital gravity data.By forward-modeling the gravity-rate signatures of different plume evolution scenarios, we narrow the plausible parameter space of present-day plume states and evaluate their detectability with current and future satellite missions. Our results provide refined constraints on the physical characteristics of a possible Martian mantle plume or its remnant and offer an explanation of the gravity anomaly of the Tharsis rise. This work advances our understanding of the present-day thermal and geodynamic state of Mars, with broader implications for the long-term interior evolution of terrestrial planets.Bibliography:Alkahal, R., Root, B. C., Dirkx, D., Thieulot, C., Fayolle, S., Goossens, S. (under review) Investigating gravity trends from realistic simulated satellite orbits. Icarus.Le Maistre, S., et al. (2023). Spin state and deep interior structure of Mars from InSight radio tracking. Nature, 619, 733–737. https://doi.org/10.1038/s41586-023-06150-0Redmond, H. L., and King, S. D. (2004). A numerical study of a mantle plume beneath the Tharsis Rise: Reconciling dynamic uplift and lithospheric support models. Journal of Geophysical Research: Planets, 109, E09008. https://doi.org/10.1029/2003JE002228Root, B. C., Qin, W., van der Tang, Y., Thieulot, C. (2026). Describing the global gravity field of Mars with lithospheric flexure and deep mantle flow. Journal of Geophysical Research: Planets, 131, https://doi.org/10.1029/2024JE008765

Context. Consistently modelling the effects of the tides raised on a satellite on the dynamics of the satellite itself and on those of a nearby spacecraft (either in orbit or performing a flyby) requires accounting for the instantaneous tidal deformation of the satellite's gravitational potential. For synchronous satellites, the spin-orbit resonance causes perfect commensurability between the orbital and rotational periods, and the main tidal forcing frequency. This imposes stringent consistency requirements on the modelling of the delicate interplay between the satellite's orbital motion, rotational dynamics, and tidal deformation. These three aspects of satellite dynamics are typically handled separately (at least partially) in classical modelling approaches, which are therefore highly inconsistency-prone for the specific spin-orbit resonance case.Aims. Modelling inconsistencies can lead to the under- or overestimation of tidal parameters when dissipation signatures are extracted simultaneously from both spacecraft and moon dynamics, a combined approach that is increasingly critical for current and upcoming mission analyses. As a promising alternative, we propose a coupled integration of the satellite's orbit, rotation, and tidal deformation.Methods. Integrating the satellite's deformation requires introducing an additional set of differential equations for its degree-two gravity coefficients to complement the translational and rotational equations of motion. A concurrent integration ensures that the satellite's instantaneous tidal response is fully consistent with its orbit and rotation, while automatically accounting for all dynamical couplings at play. In this paper, we present a two-dimensional implementation of this coupled propagation framework and investigate its ability to produce realistic dynamics with expected tidal dissipation signatures. As a proof-of-concept, we validated the physical self-consistency of the results using the Earth-Moon and Mars-Phobos systems as conceptual test cases.Results. Our coupled propagation naturally maintains the spin-orbit resonance while producing the expected orbital migration and circularisation rates (including libration-induced tidal dissipation enhancement in the case of Phobos), a delicate balance that is hard to achieve with decoupled modelling strategies. The time history of the satellite's gravitational tidal response (obtained as a direct output of our integration) is also in agreement with analytical predictions derived from the tidal potential theory.Conclusions. Our coupled approach thus provides a unified and consistent way to model orbit-rotation-tide interactions. Crucially, it is equally applicable to representing tidal effects on the satellite itself and on a nearby spacecraft. This is critical for planetary missions such as Juice and Europa Clipper, where tidal dissipation signatures can (and will) be extracted from both the spacecraft's and moons' dynamics.

Ground-based radio astronomy and space science get together in the Planetary Radio Interferometry and Doppler Experiment (PRIDE), which provides the precise determination of the lateral position of spacecraft on the celestial sphere using phase referencing near-field Very Long Baseline Interferometry (VLBI).As part of the ESA Jupiter Icy Moons Explorer (JUICE) mission, PRIDE is complementary to on-board radio science experiments. The technique consists of radio astronomical observations of the JUICE radio signal alongside calibrating quasars to deliver high-resolution lateral position measurements within the International Celestial Reference Frame. PRIDE requires no extra onboard instruments and relies on the spacecraft's existing telecommunications system and ground-based telescope arrays, like the European VLBI Network or the Australian Long Baseline Array.Since JUICE launch, PRIDE observations have tracked the spacecraft on several occasions. During the Lunar-Earth Gravity Assist (LEGA), multiple radio telescopes observed JUICE to independently validate the spacecraft trajectory to verify PRIDE internal pipelines. In August 2025, the European VLBI Network ran an observing campaign during the Venus gravity assist manoeuvres. In this presentation, the status and the results of the PRIDE-JUICE cruise phase activities will be presented.

Doppler tracking of planetary missions is crucial for science objectives related to understanding, among others, solar system interiors and evolution. These analyses conventionally rely on closed-loop Doppler data from networks such as ESTRACK and the DSN. The Planetary Radio Interferometry and Doppler Experiment (PRIDE) can provide complementary data by extracting open-loop Doppler and VLBI observables from spacecraft radio signals recorded by radio-astronomical telescopes [1,2,3]. These (three-way) observables are generated using the existing downlink from the spacecraft and therefore generated concurrently with the typical (usually two-way) closed-loop tracking data. While PRIDE data of planetary missions have been used for various applications, their direct use in a full orbit-estimation problem had not yet been demonstrated. Here, we report on the first end-to-end PRIDE-based orbit estimation of a planetary mission, using PRIDE Doppler data.We use the 2013 Mars Express flyby of Phobos, for which both conventional closed-loop Doppler data and PRIDE open-loop Doppler (EVN experiment GR035, PI: Pascal Rosenblatt) data are available [4]. Using the open-source TU Delft Astrodynamics Toolbox (Tudat) [5,6], we independently estimate the Mars Express trajectory and the gravitational parameter of Phobos from the closed-loop and open-loop datasets. The solutions are cross-validated against the Mars Express orbit computed by the Royal Observatory of Belgium (ROB) team from closed-loop Doppler data using the GINS software.The closed-loop solution gives RMS position differences with respect to the ROB Mars Express ephemerides of approximately 0.27 m radial, 17 m along-track, and 130 m cross-track, consistent with the expected uncertainty of the reference solution. For the PRIDE open-loop Doppler data, we form 27 different three-station combinations of PRIDE datasets, with one station on each of three continents, such that each combination spans the full tracking interval. From each combination, we estimate an independent orbit of Mars Express, leading to separate orbit estimates, each of comparable quality to those derived from closed-loop data, as quantified by the difference with respect to the ROB reference. The estimated gravitational parameter of Phobos is likewise consistent between the open-loop and closed-loop analyses and with reference values from the literature.To further quantify the uncertainty of the PRIDE-based orbit estimates, we compute the RMS dispersion of the 27 station-combination solutions at each point along the estimated trajectory. This dispersion peaks at approximately 0.4 m radial, 15 m along-track, and 100 m cross-track. Its temporal behaviour closely matches the difference between our closed-loop solution and the ROB reference ephemeris, indicating that the spread between PRIDE station-combination solutions provides a measure of the true orbit-estimation error that is comparable to the difference with respect to an external reference solution. This is a key advantage of PRIDE’s multi-station redundancy: it enables an internal assessment of solution uncertainty, without relying exclusively on formal covariance estimates, which are often too optimistic, or on comparison with an external reference solution, which is not always available.Assessing the true uncertainty of radio-tracking-based estimation is crucial for robustly using estimated parameters (for instance the gravitational parameter of Phobos) to improve our understanding of planetary interiors and Solar System evolution. PRIDE observations are particularly valuable for selected tracking intervals of high scientific interest, where the multi-station redundancy can provide validation and error quantification beyond what is available from conventional tracking alone. For events such as flybys of the Galilean moons [3], PRIDE Doppler data can therefore provide a complementary source of information for assessing the reliability of orbit and parameter estimates. Future work will include the processing of PRIDE Doppler data during the JUICE flyby of Venus, and spacecraft orbit estimation using a combination of open- and closed-loop tracking data. [1] Duev, D.A., et al. "Spacecraft VLBI and Doppler tracking: algorithms and implementation." Astronomy & Astrophysics 541 (2012): A43.[2] Molera Calvés, G., et al. "High spectral resolution multi-tone Spacecraft Doppler tracking software: Algorithms and implementations." Publications of the Astronomical Society of Australia 38 (2021): e065.[3] Gurvits, L.I., et al. "Planetary radio interferometry and Doppler experiment (PRIDE) of the JUICE mission." Space Science Reviews 219.8 (2023): 79.[4] Bocanegra-Bahamón, T.M., et al. "Planetary Radio Interferometry and Doppler Experiment (PRIDE) technique: A test case of the Mars Express Phobos Flyby: II. Doppler tracking: Formulation of observed and computed values, and noise budget." Astronomy & Astrophysics 609 (2018): A59.[5] Dirkx, D., et al. "The open-source astrodynamics Tudatpy software–overview for planetary mission design and science analysis." EPSC2022 EPSC2022-253 (2022).[6] Gisolfi, L, et al. "Open-Source High-Fidelity Orbit Estimation for Planetary Science and Space Situational Awareness Using the Tudat Software." arXiv preprint arXiv:2510.23179 (2025).

The recent discovery of strong tidal dissipation in Saturn’s interior has radically changed our view of the Saturnian system. While some questions are naturally answered by the new paradigm, others are emerging and require further measurement. This article presents the next key questions to be addressed by future space missions and analysis. Suggestions for space measurements to discriminate between different scenarios concerning the formation, evolution and internal state of the Saturnian system are given.

High-precision inter-satellite ranging is critical for formation flying, autonomous navigation, and scientific measurements in small-satellite missions. Laser communication terminals (LCTs) offer an opportunity to perform both data transfer and ranging, but their dual-use imposes stringent requirements on onboard clocks and timing electronics. This paper investigates the impact of clock-induced timing errors on two-way LCT-based ranging between CubeSats operating around the near-Earth asteroid 99942 Apophis. A methodology is developed to unify clock noise specifications provided in datasheets, generating realistic timing errors across microsecond-to-hour integration periods. Using high-fidelity orbital simulations, two orbital configurations—coplanar and non-coplanar—are analyzed to evaluate how relative satellite geometry influences the propagation of clock errors into range measurements, orbit determination, and the estimation of Apophis’ gravitational parameter. Results demonstrate that inter-satellite links (ISLs) can reduce orbit determination errors along directions weakly constrained by Earth-based Doppler—from 1–3 m to 0.1–0.3 m in coplanar formations, and even further in non-coplanar formations—corresponding to improvements of one to two orders of magnitude. Subsystem-level noise, such as detector jitter and time tagging, can still limit achievable precision, even with high-performance clocks. The methodology provides a framework applicable to a broad range of small-satellite missions, guiding the selection of clocks, formation geometry, and system design to optimize both navigation performance and science return.

. MotivationTides are a key driver of planetary system evolution: they govern the intensity of tidal heating in the moons' interiors, drive their rotations towards equilibrium states, and set the migration and circularisation rates of their orbits. A detailed characterisation of the satellites' current response to tidal forcing offers invaluable insight into their interior structure and properties, as well as into the present-day evolution of the system's orbital configuration. This is, in turn, essential for placing constraints on the long-term thermal-orbital evolution of the system and, for icy moons in particular, on the history of their internal oceans.In the context of the upcoming Juice and Europa Clipper missions, radio-science tracking of both spacecraft during their close encounters with the Galilean moons (flybys and orbital phase) will be critical in this regard. Such measurements will constrain the moons' dynamics, and in particular the dynamical signatures of tidal effects, at an unprecedented level of detail and precision [1,2]. At the accuracy levels anticipated post-missions, however, the limiting factor for the inferred solution might no longer be the precision of the radio-science tracking, but instead the physical fidelity and self-consistency of the dynamical models that underpin the data analysis.More specifically, tidal effects manifest themselves both in the spacecraft's trajectory and in the satellites' own dynamics. Reliably extracting these signatures from radio-science data therefore requires that tidal contributions be incorporated into the dynamical models of the spacecraft and moons in a fully consistent way. For synchronous satellites, the intricate coupling between tide, orbit, and rotation makes this particularly challenging: any mismodelling of these interactions leads to an erroneous dissipation signature and ultimately affects the recovered estimates [3]. Achieving such consistency in the Galilean system is further complicated by the Laplace resonance, which requires Io, Europa, and Ganymede to be modelled as a single, unified dynamical system to ensure that the strong gravitational couplings between them are properly captured. 2. ApproachTo address this challenge, we proposed a unified dynamical framework [4] in which each satellite's gravitational deformation is propagated through an ordinary differential equation derived from a prescribed rheology, along with the orbital and rotational dynamics (building on previous works focussing on exosystems [5,6]). Embedding the internal response directly within the integrated dynamics guarantees that all orbit-rotation-tide couplings are accounted for, at all forcing frequencies. Critically, the proposed model establishes a direct and physically motivated connection between interior properties (e.g., viscosity, rigidity) and tidal dissipation signatures in the moons’ dynamics, and ensures full consistency between the orbits, rotations and tidal deformation of all moons. We expanded our original two-dimensional proof-of-concept [4] into a complete N-body implementation that includes higher-order effects, such as moon-moon interactions, and three-dimensional effects in the moons’ orbits and rotations (non-zero inclination and obliquity).3. Results and outlookApplied to the Galilean system, the model successfully reproduces expected dynamical features: it maintains the Laplace resonance, yields realistic spin-orbit resonant rotations in a Cassini state, and recovers the orbit expansion and circularisation rates predicted by tidal theory [7]. Crucially, the propagated dynamics also captures coupling signatures that fall outside the reach of conventional formulations. Multi-frequency and indirect forcing and response of both tides and rotation emerge naturally from the concurrent integration of the coupled equations of motion, with additional forcings at the frequencies of the other Galilean moons appearing directly in each satellite's rotational and deformational response. This is particularly promising for Ganymede, whose gravitational deformation will be characterised with unprecedented detail, including sampling of the moon’s response at different frequencies [8]. More generally, these results demonstrate the potential of a fully coupled modelling approach for future analyses of Juice and Europa Clipper data, where the Galilean satellites’ orbits, rotations, and tidal interactions are self-consistently modelled as an integrated dynamical system.References[1] Magnanini, A. et al. Astronomy & Astrophysics 687 (2024): A132.[2] Fayolle, S. Dissertation (2025)[3] Magnanini, A., Zannoni, M., and Lainey, V. Astronomy & Astrophysics 707 (2026): A96.[4] Fayolle, S., et al. Astronomy & Astrophysics 707 (2026): A224.[5] Correia, A. et al. Astronomy & Astrophysics 571 (2014): A50.[6] Boué, G., Correia, A., and Laskar, J. Celestial Mechanics and Dynamical Astronomy 126.1 (2016): 31-60.[7] Goldreich, P., and Soter, S. Icarus 5.1-6 (1966): 375-389[8] De Marchi, F., et al. Icarus 386 (2022): 11515

Context. Upcoming science missions to Phobos will potentially provide unprecedented observations of Phobos's orbit in the form of orbiter and/or lander tracking data. This will likely require an updating of the dynamical models currently used to invert this data, with the coupling between the satellite's orbit and rotation being of particular importance. State-of-the-art ephemerides estimations for tidally locked satellites rely on a decoupled approach where translational models are combined with a simplified analytical representation of the moon's rotation (typically a single-frequency periodic variation superimposed to a synchronous rotation). Aims. This paper investigates the coupled propagation of Phobos's translational and rotational dynamics, and assesses the extent to which the most commonly used uncoupled model can emulate the results of the coupled integration, and what consequences the mis-modeling has on the products of data inversion. Methods. We considered two models: a coupled model that propagates Phobos's translational and rotational dynamics simultaneously, and an uncoupled model that assumes Phobos to be in a fully locked configuration with a once-per-orbit longitudinal libration. By simulating the dynamics for about ten years, first in a coupled and then in an uncoupled manner, we compared the results and used the coupled trajectory as simulated observations for an estimation of the different parameters using uncoupled translational dynamics. Results. For identical initial states, differences between the coupled and uncoupled trajectories were found to accumulate to 40 m, most predominantly in Phobos's direction of motion. Longitudinal librations were misrepresented by the uncoupled model particularly around the frequencies of the normal mode, where forcings are amplified up to 3.6 x 10(-3) degrees. Long-term latitudinal librations also arise from forcings due to coupling-induced changes in orbital inclination. The use of uncoupled models in data inversion results in true errors in the estimated parameters. In this case, we performed estimations of different lengths up to 1000 days to estimate Phobos's initial state, once-per-orbit libration amplitude, and harmonic coefficients & Cdot;(over bar) (2,0) and & Cdot;(over bar) (2,2). Errors in dynamical parameters were found to be on the order of 10(-3) degrees for the physical libration amplitude and of 10(-5) for the harmonic coefficients (relative errors of around 0.1%). Conclusions. These true errors are one to three orders of magnitude above the formal errors expected from laser ranging measurements to a Phobos lander, which indicates that the typical single-frequency uncoupled model is not suitable for the proper inversion of such data. Refined rotation models will therefore be required, either by expanding the uncoupled model to multiple frequencies or by performing a fully coupled orbital-rotational propagation as proposed in this paper. We discuss the theoretical and practical limitations of an extended analytical parametrization in the specific case of tidally locked satellites, and advocate for the use of a fully coupled approach.

The radio telescopes of the European VLBI Network (EVN) and the University of Tasmania (UTAS) conducted an extensive observation campaign of the European Space Agency's (ESA) Mars Express (MEX) spacecraft between 2013 and 2020. The campaign, carried out under the Planetary Radio Interferometry and Doppler Experiment (PRIDE) framework, aimed to study interplanetary phase scintillation and assess the noise budget in the closed-loop Doppler observations. The average closed-loop Doppler noise was determined to be approximately 10 mHz at a 10-s integration time, reaffirming the technique's suitability for radio science experiments. We evaluated how different observational parameters such as the solar elongation, antenna size, and elevation angle impact the Doppler noise. A key part of the analysis involved comparing results from co-located telescopes to investigate system noise effects. Co-located telescopes at both Wettzell and Hobart provided highly consistent results, with any deviations serving as diagnostic tools to identify station-dependent issues. Additionally, the use of phase calibration tones during spacecraft tracking showed that the instrumental noise contribution is of the order of 5 $\%$ of the total noise. This study provides a detailed noise budget for closed-loop Doppler observations with VLBI telescopes while emphasizing the effectiveness of the co-location method in isolating system-level noise. These findings are important for optimizing future radio science and VLBI tracking missions using stations outside the the Deep Space Network (DSN) and European Space Tracking (ESTRACK) network.

Being among the most promising candidates for potential extraterrestrial habitats within our Solar System, the Galilean satellites are going to be extensively studied by the upcoming JUICE and Europa Clipper missions. Both spacecraft will provide radio science tracking data, which will allow the satellites ephemerides to be determined to much greater accuracy than is currently the case. Yet, with no flybys of Io, these data sets will be skewed towards the three outer satellites. To mitigate this imbalance, optical space-based astrometry from JUICE will provide a valuable contribution. To quantify the contribution of JUICE astrometry, we have performed the inversion of simulated optical astrometric observations by JUICE, using suitable a priori covariance to represent the radio science-only solution. Incorporating the astrometry into the ephemeris solution requires the consideration of the offset between Io's centre-of-figure (COF, which astrometry measures) and the centre-of-mass (COM, which the ephemeris solution requires). We explicitly account for the offset between COF and COM as an estimated parameter in our model. We assess the contribution of the optical observations to the ephemeris solution as a function of the radio science true-to-formal-error ratio (describing the statistical realism of the simulated radio science solution), as well as optical data quantity and planning. From this, we discuss to which extent space-based astrometry could help to validate the radio science solution, and under which conditions the data could improve the orbital solution of Io. Significant contributions of astrometry to Io's orbital solution occur for radio science true-to-formal-error ratios of 4 and higher (for the along-track and normal direction). This shows that optical space-based astrometry can improve and/or validate the radio science solution. Reductions in the obtainable uncertainties for the COF-COM-offset range from about 20 to 50 per cent-depending on the number of observations-using suitable algorithms to select the epochs at which observations are to be simulated. In particular, observations during the high-inclination phase have proven especially beneficial. Our results show that constraints on the COM-COF offset of Io could be obtained from astrometry at the level 100 m-1 km, depending on the quantity and planning of the observations. This could provide a novel data point to constrain Io's interior. Moreover, the astrometric data will provide independent validation-and possibly improvement-of the orbital solution of Io.

Juice (JUpiter ICy moons Explorer) 3GM Radio Science Experiment will map the gravitational field of Ganymede with unprecedented accuracy and measure tidally‐induced variations. These measurements will allow the characterization of its putative ocean and may resolve lateral variations in internal structure. Lateral variations cause an additional tidal signal that depends on their wavelength and amplitude. We show that shell thickness variations of the mean thickness produce an additional tidal signal times smaller than the main tidal signal, detectable given the accuracy of Juice. Using a Bayesian framework, we show that measuring differences between and constrain equator to pole shell thickness differences. Also measuring the degree‐3 spherical harmonic signal due to degree 2 forcing constrain degree‐1 and degree‐3 structure. This demonstrates tidal tomography's potential to map three dimensional structure and supports its consideration for future missions.

The TU Delft Astrodynamics Toolbox (Tudat) is a free open-source software suite for research and education in astrodynamics. Initially focused on numerical simulations of orbital dynamics and state estimation, it enables combining optical and radiometric tracking data from multiple sources to estimate the dynamics and parameters of natural and artificial bodies. Recent developments have added functionality for real tracking data analysis, with applications to planetary missions and Space Situational Awareness (SSA). Tudat currently supports processing of (i) deep-space Doppler and range data from DSN and ESTRACK, (ii) Doppler and VLBI data from the PRIDE experiment, and (iii) optical astrometry from the Minor Planet Center (MPC) and Natural Satellite Data Center (NSDC). Using tracking data from the MRO and GRAIL spacecraft and astrometric data of the asteroid Eros, we present prefit residuals (from SPICE-based observables) and postfit residuals (from fitting data to the Tudat dynamical model). Postfit Doppler residuals reach 1-5 mHz for MRO and GRAIL, orbit differences are a few meters for GRAIL and about one meter for MRO, and MRO range residuals are a few meters. From eight years of Eros astrometry, we obtain an orbit difference from the JPL Horizons solution by several tens of kilometers, consistent with its 3-sigma formal error. Tudat's SSA capability is demonstrated by propagating the orbit of Kosmos 482, a Venus lander launched in 1972 that remained in Earth orbit, over 50 years, including its predicted re-entry on May 10 2025. These examples showcase Tudat's modular, flexible, high-fidelity modeling across diverse orbital regimes within a fully open-source framework. All example code is publicly available, and future Tudat analyses will be published with fully reproducible code, allowing anyone in the community to improve and expand upon our work.

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 dV 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 dV budgets for post-flyby cleanup manoeuvres. The moon state knowledge evolution during navigation operation showed a rapid reduction of the apriori 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 dV cost shows that the adoption of radio science ephemeris products does not effectuate significant dV savings. Instead we showed that in order to achieve substantial dV 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.

This study comprehensively evaluates the impact of the expected Chinese Tianwen-4 mission, in conjunction with the existing data from the Juno mission, on enhancing the understanding of Jupiter’s gravity field. Integrating simulated data from both missions. The methodology incorporates detailed simulations of Tianwen-4’s orbit, assessing its influence on Jupiter’s gravity field estimations across various orbital inclinations. It also explores the integration of multimission tracking data, combining simulated Juno and Tianwen-4 data. In addition to the static gravity coefficient, the analysis extends to include the tidal effect k _nm , which quantifies the tidal response of Jupiter’s gravity field to forcing it by the Galilean satellites. The results indicate clear potential improvements in the precision of the gravity field models compared to those derived from the Juno mission alone, particularly in the lower degree harmonics, where accuracy improves by an average factor of 20.08 in the first 12°, gradually decreasing to 2.46, with an overall enhancement of 7.43. These enhancements underscore the value of integrating data from multiple missions, which provides a more nuanced understanding of Jupiter’s gravitational properties. Improving the gravity field model is essential for gaining deeper insights into Jupiter’s internal structure and dynamics, which ultimately enhances our understanding of giant planets and their formation. Accurate gravity models are crucial for interpreting a planet’s physical and chemical properties, leading to better comprehension of planetary systems.

The JUpiter ICy moons Explorer (JUICE) of ESA was launched on 14 April 2023 and will arrive at Jupiter and its moons in July 2031. In this review article, we describe how JUICE will investigate the interior of the three icy Galilean moons, Ganymede, Callisto and Europa, during its Jupiter orbital tour and the final orbital phase around Ganymede. Detailed geophysical observations about the interior of the moons can only be performed from close distances to the moons, and best estimates of signatures of the interior, such as an induced magnetic field, tides and rotation variations, and radar reflections, will be obtained during flybys of the moons with altitudes of about 1000 km or less and during the Ganymede orbital phase at an average altitude of 490 km. The 9-month long orbital phase around Ganymede, the first of its kind around another moon than our Moon, will allow an unprecedented and detailed insight into the moon's interior, from the central regions where a magnetic field is generated to the internal ocean and outer ice shell. Multiple flybys of Callisto will clarify the differences in evolution compared to Ganymede and will provide key constraints on the origin and evolution of the Jupiter system. JUICE will visit Europa only during two close flybys and the geophysical investigations will focus on selected areas of the ice shell. A prime goal of JUICE is the characterisation of the ice shell and ocean of the Galilean moons, and we here specifically emphasise the synergistic aspects of the different geophysical investigations, showing how different instruments will work together to probe the hydrosphere. We also describe how synergies between JUICE instruments will contribute to the assessment of the deep interior of the moons, their internal differentiation, dynamics and evolution. In situ measurements and remote sensing observations will support the geophysical instruments to achieve these goals, but will also, together with subsurface radar sounding, provide information about tectonics, potential plumes, and the composition of the surface, which will help understanding the composition of the interior, the structure of the ice shell, and exchange processes between ocean, ice and surface. Accurate tracking of the JUICE spacecraft all along the mission will strongly improve our knowledge of the changing orbital motions of the moons and will provide additional insight into the dissipative processes in the Jupiter system. Finally, we present an overview of how the geophysical investigations will be performed and describe the operational synergies and challenges.

China will launch the “Tianwen-IV” mission around 2030, focusing on the orbiting exploration of Jupiter and Callisto, a moon of Jupiter. As part of this ambitious mission, a main satellite will carry another satellite that will be released in the Jupiter system to continue its journey toward Uranus. Considering the current mission planning, we propose an inter-satellite radio-observation mode that differs from the conventional observation mode of tracking from Earth to precisely determine the orbit of the satellites. Given the significance of the Callisto gravity field model in both science objectives and satellite navigation, we have conducted a series of simulation experiments to evaluate the potential of this inter-satellite range-rate data for accurately estimating the Callisto gravity field. The results obtained from the analysis demonstrate that by utilizing 40 days of ground station observations, it is possible to estimate the gravity field model of Callisto up to a degree of 70. Remarkably, when combining these ground station observations with inter-satellite observations, a comparable level of accuracy can be achieved with just 10 days of observations. Furthermore, with reduced inter-satellite observation noise, accuracy improves, enabling estimation up to 80 degrees or higher. Initial inter-satellite distance selection impacts estimation accuracy. These findings serve as a valuable test bed for the future “Tianwen-IV” mission to perform precise orbit determination and gravity field model estimation to reduce reliance on deep space stations.