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Tidal effects in planetary systems are the main driver in the orbital migration of natural satellites. They result from physical processes occurring deep inside celestial bodies, whose effects are rarely observable from surface imaging. For giant planet systems, the tidal migration rate is determined by poorly understood dissipative processes in the planet, and standard theories suggest an orbital expansion rate inversely proportional to the power 11/2 in distance, implying little migration for outer moons such as Saturn's largest moon, Titan. Here, we use two independent measurements obtained with the Cassini spacecraft to measure Titan's orbital expansion rate. We find Titan migrates away from Saturn at 11.3 $\pm$ 2.0 cm/year, corresponding to a tidal quality factor of Saturn of Q $\simeq$ 100, and a migration timescale of roughly 10 Gyr. This rapid orbital expansion suggests Titan formed significantly closer to Saturn and has migrated outward to its current position. Our results for Titan and five other moons agree with the predictions of a resonance locking tidal theory, sustained by excitation of inertial waves inside the planet. The associated tidal expansion is only weakly sensitive to orbital distance, motivating a revision of the evolutionary history of Saturn's moon system. The resonance locking mechanism could operate in other systems such as stellar binaries and exoplanet systems, and it may allow for tidal dissipation to occur at larger orbital separations than previously believed., Published in Nature Astronomy, SharedIt link: https://rdcu.be/b4I3Q

Doppler tracking is nowadays an essential tool in deep space orbit determination and radio science investigations. One of the most critical steps in this tracking process is the frequency estimation, and the variance of this estimation process is directly linked to the accuracy of spacecraft range-rate estimation, therefore possibly limiting it. In this paper, an FFT-based algorithm is presented and characterized using signals corrupted by additive noise channels of varying intensity. A comparison to the current operational tracking method for Open Loop data, based on Phase-Locked Loops, is presented and discussed. Monte Carlo simulations confirm that the estimation accuracy of the proposed method is appropriate for radio science experiments and achieves better accuracies than the alternative method already in use, in case of low signal-to-noise ratios.

The Juno spacecraft reached the mid‐point of its nominal mission in December 2018, after completing 17 perijove passes. Ten of these were dedicated to the determination of the gravity field of the planet, with the aim of constraining its interior structure. We provide an update on Jupiter's gravity field, its tidal response and spin axis motion over time. The analysis of the Doppler data collected during the perijove passes hints to a non‐static and/or non‐axially symmetric field, possibly related to several different physical mechanisms, such as normal modes or localized atmospheric or deeply‐rooted dynamics.

During its mission in the Saturn system, Cassini performed five close flybys of Dione. During three of them, radio tracking data were collected during the closest approach, allowing estimation of the full degree-2 gravity field by precise spacecraft orbit determination. The gravity field of Dione is dominated by $J_{2}$ and $C_{22}$, for which our best estimates are $J_{2} \times 10^6 = 1496 \pm 11$ and $C_{22} \times 10^6 = 364.8 \pm 1.8$ (unnormalized coefficients, 1-$\sigma$ uncertainty). Their ratio is $J_{2}/C_{22} = 4.102 \pm 0.044$, showing a significative departure (about 17-$\sigma$) from the theoretical value of $10/3$, predicted for a relaxed body in slow, synchronous rotation around a planet. Therefore, it is not possible to retrieve the moment of inertia directly from the measured gravitational field. The interior structure of Dione is investigated by a combined analysis of its gravity and topography, which exhibits an even larger deviation from hydrostatic equilibrium, suggesting some degree of compensation. The gravity of Dione is far from the expectation for an undifferentiated hydrostatic body, so we built a series of three-layer models, and considered both Airy and Pratt compensation mechanisms. The interpretation is non-unique, but Dione's excess topography may suggest some degree of Airy-type isostasy, meaning that the outer ice shell is underlain by a higher density, lower viscosity layer, such as a subsurface liquid water ocean. The data permit a broad range of possibilities, but the best fitting models tend towards large shell thicknesses and small ocean thicknesses.

The latest estimation of Jupiter’s gravitational field was obtained by processing the Doppler data from two gravity orbits of NASA’s Juno mission, using the Jet Propulsion Laboratory software monte. In this work, we present the results of the analysis of the same measurements employing the orbit determination software orbit14, developed at the University of Pisa, used here for the first time with real data. We found that the estimated values of Jupiter’s spherical harmonic coefficients from the two solutions are consistent within the formal uncertainty. The analysis is complemented with a discussion on the results obtained with alternative set-ups.

During its tour of the Saturn system, Cassini has revealed that the icy satellites of Saturn are among the most intriguing and complex bodies in the Solar System. In particular Titan, the Saturn's largest moon, possesses a global sub-surface water ocean. This may interact with an organically rich and dynamic atmosphere and surface, with implications also for Titan's potential habitability. However, many questions are still to be addressed, so that several future missions to Titan and other icy satellites were proposed in the latest years, in the context of both NASA and ESA calls for proposals. Radio science measurements are powerful tools to determine the static and dynamic gravity field of a body, posing constraints on its radial interior structure and assessing mass anomalies. In particular, gravity investigations at Titan could provide information on the internal dynamics and evolution, ocean properties, crustal thickness and state. Traditionally, the observable quantities used by interplanetary gravity science experiments are obtained by means of spacecraft tracking at microwave frequencies from a ground antenna. The generation of these observable quantities requires the spacecraft antenna to point in the direction of the Earth and have an onboard coherent radio transponder. Using both of these simultaneously impacts spacecraft resources including mass, power, and attitude control. Moreover, the geometry and the operations of the entire mission must be carefully designed to meet all science requirements, including the ones posed by the gravity experiment, through a complex and expensive trade-off process. In recent years it was proposed to use smallsat companion missions, dedicated only to specific investigations, to complement the science observations of a traditional larger exploration spacecraft. This represents a promising option since smallsats provide relatively low-cost and versatile platforms for scientific observations. Here we propose a mission consisting of two smallsats to be released in orbit around Titan and capable of globally characterize its gravity field through Satellite-to-Satellite Tracking (SST). This mission proposal is intended to complement a traditional larger mission to Titan or Enceladus. The observables are generated and decimated on-board the smallsats and transmitted back to Earth for data analysis and gravity field reconstruction, using the main mission as a relay spacecraft. We present the expected accuracy in the estimation of Titan's static and dynamic gravity field, obtained through numerical simulations of the orbit determination of two smallsats in similar orbits around the moon. The main observable quantities used in the analysis are two-way Doppler data obtained from the frequency shift of a stable microwave carrier transmitted between the two spacecraft. White Gaussian noise was added to the simulated data, with a conservative standard deviation obtained from a preliminary noise budget. A covariance analysis was carried out using a multi-arc approach, comparing different orbital, observational, and modeling strategies.

The ESA JUICE (JUpiter ICy moons Explorer) mission is planned for launch in 2022 and arrival at Jupiter around 2030. The mission is dedicated to the study of the giant gaseous and its largest moons. While the spacecraft will probe the Jovian system it will be occulted by the atmosphere of Jupiter or its satellites as seen from antennas on Earth. Such a configuration offers a great opportunity to study remotely the physical properties of the occulting atmosphere using radio links as the probe is being occulted. Indeed, non-unity index of refraction causes the electromagnetic waves to depart from the straight line and also impacts the propagation speed of the waves. Both changes modify the wave frequency and conversely, from the time variation of the Doppler measurements the index of refraction profile can be retrieved. In the literature, there are different approaches devoted to the retrieval of the refractive profile from these observables. Let mention, i) the analytic formulation of the Abel inversion which is employed for spherically symmetric atmospheres, and ii) the ray tracing method which is a numerical integration of the fundamental equations of optics and which is well suited for atmospheres with more complicated shapes. Both possess their own advantages and inconveniences. For instance, to invert a complete set of data, the ray tracing method requires more computational time than the Abel transformation. In return, the Abel inversion is based on the spherical symmetry assumption while the ray tracing technique can handle non-radial gradient in the refractive profile. In the context of the future occultations of JUICE by Jupiter, we discuss the benefit of a new formalism based on a full reformulation of the fundamental equations of optics. This new approach let to provide a very comprehensive description of the light trajectory inside a planetary atmosphere with no assumption on the refractive profile. In the special case where the departure from the spherical symmetry is small, we present an analytic solution which is well suited for the data processing of radio occultation experiments. Indeed, this solution can handle the effect of a non-spherically symmetric atmosphere with a low computational cost. We use this solution to process the Cassini Doppler data acquired during an occultation by the oblate atmosphere of Saturn. The validity of the proposed approach is assessed comparing the results with other studies available in the literature.