BepiColombo's Mercury Orbiter Radio-science Experiment (MORE) was conceived to enable extremely accurate radio tracking measurements of the Mercury Planetary Orbiter to precisely determine the gravity field and rotational state of Mercury, and to test theories of gravitation (e. g. Einstein's Theory of General Relativity). The design accuracy of the radio tracking data was 0.004 mm/sec (at 1000 s integration time) for range-rate measurements and 20 cm for range (at a few seconds of integration time). These accuracies are attained due to a combination of simultaneous two-way microwave links at X (7.2-8.4 GHz) and Ka-band (32-34 GHz) to calibrate the dispersive plasma noise component. In this letter, we present the first analysis of range and range-rate data collected by ESA's deep space antenna (DSA) during the initial cruise phase of BepiColombo. The novel 24 Mcps pseudo-noise (PN) modulation of the Ka-band carrier, enabled by MORE's Ka-band Transponder (KaT), built by Thales Alenia Space Italy, provided two-way range measurements to centimeter-level accuracy, with an integration time of 4.2 s at 0.29 astronomical units. In tracking passes with favorable weather conditions, range-rate measurements attained an average accuracy of 0.01 mm/s at 60 s integration time. Data from 20 to 24 May 2019 were combined in a multi-pass analysis to test the link stability on a longer timescale. The results confirm the noise level observed with the single-pass analysis and provide a preliminary indication that the MORE PN ranging system at 24 Mcps is compatible with the realization of an absolute measurement, where the need to introduce range biases in the orbital fit is much more limited than in the past. We show that in the initial cruise test the BepiColombo radio link provided range measurements of unprecedented accuracy for a planetary mission, and that, in general, all target accuracies for radio-metric measurements were exceeded.

The Mercury Orbiter Radio Science Experiment (MORE) of the ESA mission BepiColombo will provide an accurate estimation of Mercury’s gravity field and rotational state, improved tests of general relativity, and a novel deep space navigation system. The key experimental setup entails a highly stable, multi-frequency radio link in X and Ka band, enabling two-way range rate measurements of 3 micron/s at nearly all solar elongation angles. In addition, a high chip rate, pseudo-noise ranging system has already been tested at 1-2 cm accuracy. The tracking data will be used together with the measurements of the Italian Spring Accelerometer to provide a pseudo drag free environment for the data analysis. We summarize the existing literature published over the past years and report on the overall configuration of the experiment, its operations in cruise and at Mercury, and the expected scientific results.

In preparation for the ESA/JAXA BepiColombo mission to Mercury, thematic working groups had been established for coordinating the activities within the BepiColombo Science Working Team in specific fields. Here we describe the scientific goals of the Geodesy and Geophysics Working Group (GGWG) that aims at addressing fundamental questions regarding Mercury’s internal structure and evolution. This multidisciplinary investigation will also test the gravity laws by using the planet Mercury as a proof mass. The instruments on the Mercury Planetary Orbiter (MPO), which are devoted to accomplishing the GGWG science objectives, include the BepiColombo Laser Altimeter (BELA), the Mercury orbiter radio science experiment (MORE), and the MPO magnetometer (MPO-MAG). The onboard Italian spring accelerometer (ISA) will greatly aid the orbit reconstruction needed by the gravity investigation and laser altimetry. We report the current knowledge on the geophysics, geodesy, and evolution of Mercury after the successful NASA mission MESSENGER and set the prospects for the BepiColombo science investigations based on the latest findings on Mercury’s interior. The MPO spacecraft of the BepiColombo mission will provide extremely accurate measurements of Mercury’s topography, gravity, and magnetic field, extending and improving MESSENGER data coverage, in particular in the southern hemisphere. Furthermore, the dual-spacecraft configuration of the BepiColombo mission with the Mio spacecraft at higher altitudes than the MPO spacecraft will be fundamental for decoupling the internal and external contributions of Mercury’s magnetic field. Thanks to the synergy between the geophysical instrument suite and to the complementary instruments dedicated to the investigations on Mercury’s surface, composition, and environment, the BepiColombo mission is poised to advance our understanding of the interior and evolution of the innermost planet of the solar system.

The JUpiter Icy Moons Explorer is an ESA mission set for launch in 2023 April and arrival in the Jovian system in 2031 July to investigate Jupiter and its icy satellites with a suite of 10 instruments. The mission will execute several flybys of the icy moons Europa, Callisto, and Ganymede before ending the mission with a 9-month orbit around Ganymede. The 3GM experiment on board the spacecraft will exploit accurate range and Doppler (range-rate) measurements to determine the moons’ orbit, gravity field, and tidal deformation. The focus of this paper is on the retrieval of Europa’s and Callisto’s gravity field, without delving into the modeling of their interior structures. By means of a covariance analysis of the data acquired during flybys, we assess the expected results from the 3GM gravity experiment. We find that the two Europa flybys will provide a determination of the J ₂ and C ₂₂ quadrupole gravity field coefficients with an accuracy of 3.8 × 10⁻⁶ and 5.1 × 10⁻⁷, respectively. The 21 Callisto flybys will provide a determination of the global gravity field to approximately degree and order 7, the moon ephemerides, and the time-variable component of the gravitational tide raised by Jupiter on the moon. The k ₂ Love number, describing the Callisto tidal response at its orbital period, can be determined with an uncertainty σ _k2 ∼ 0.06, allowing us to distinguish with good confidence between a moon with or without an internal ocean. The constraints derived by 3GM gravity measurements can then be used to develop interior models of the moon.

Media propagation delay and delay‐rate induced by the water vapor within the Earth's troposphere represent one of the main error sources for radiometric measurements in deep space. In preparation for the BepiColombo and JUICE missions, the European Space Agency has installed and operates the prototype of a tropospheric delay calibration system (TDCS) at the DSA‐3 ground station located in Malargüe, Argentina. An initial characterization of the TDCS performance was realized using two‐way Doppler measurements at X‐band to perform the orbit determination of the Gaia spacecraft. This work will further characterize the system by analyzing two‐way Doppler and range data at X‐ and Ka‐band for 31 tracking passes of the BepiColombo spacecraft, which were recorded between March 2021 and February 2022 during the first two solar conjunction experiments. The performance exceeds the expectations based on the previous analysis, with a reduction of the Doppler noise of 51% on average and up to 73% when using the TDCS measurements in place of standard calibrations based on global navigation satellite system data. Furthermore, the campaign serves as validation of the TDCS operations during superior solar conjunctions, with most of the tracking passes at low elongation now satisfying the Mercury orbiter radioscience experiment requirements on two‐way Doppler stability. These results, which are in line with those of similar instruments installed at other Deep Space Network antennas, are obtained using a commercial microwave radiometer with significantly lower installation and maintenance costs.

The ESA/JAXA BepiColombo mission, launched on 20 October 2018, is currently in cruise toward Mercury. The Mercury Orbiter Radio-science Experiment (MORE), one of the 16 experiments of the mission, will exploit range and range-rate measurements collected during superior solar conjunctions to better constrain the post-Newtonian parameter γ . The MORE radio tracking system is capable of establishing a 5-leg link in X- and Ka-band to obtain 2-way range-rate measurements with an accuracy of 0.01 mm s ⁻¹ @ 60 s sampling time and 2-way range measurements at centimeter level after a few seconds of integration time, at almost all solar elongation angles. In this paper, we investigate if the light-time formulation derived by Moyer, implemented in JPL’s orbit determination code MONTE, is still a valid approximation, in light of the recent advancements in radiometric measurement performance. Several formulations of the gravitational time delay, expressed as an expansion in powers of GM / c ² r , are considered in this work. We quantified the contribution of each term of the light-time expansion for the first superior solar conjunction experiment of BepiColombo. The maximum 2-way error caused by Moyer approximation with respect to a complete second order expansion amounts to 17 mm. This is at the level of accuracy of the novel pseudo-noise (PN) ranging system at 24 Mcps used by MORE. A complete second order expansion is then recommended for present and future superior solar conjunction experiments. The perturbation caused by the planets in the Solar System is considered as well, resulting in significant effects due to the Jupiter, the Earth and the Saturn systems. For these bodies the classical Shapiro time delay is sufficient. The corrections due to the Sun oblateness and angular momentum are negligible. The aforementioned considerations are valid for all superior conjunction experiments involving state-of-the-art radio-tracking measurements.