Mercury Orbiter Radio Science Experiment Research Articles

Pseudo-Drag-Free System Simulation for Bepicolombo Radio Science Using Accelerometer Data

The Mercury Orbiter Radio Science Experiment onboard the European Space Agency/Japan Aerospace Exploration Agency (JAXA) BepiColombo mission aims at determining the gravity field and the rotational state of the planet to provide insight into its internal structure and at performing tests of general relativity. The experiment will rely on accurate radiometric data provided by the onboard Ka-band transponder and on measurements of the nonconservative perturbations acting on the spacecraft, provided by the Italian Spring Accelerometer. This paper presents a software implementation of a pseudo-drag-free system which includes the accelerometer measurements in the orbit determination process. Numerical simulations focus on the identification of a suitable calibration strategy to fulfill the experiment goals pertaining to geodesy and geophysics. The achievement of the expected scientific results will depend meaningfully on the quality of the accelerometer data. Perturbative analysis aided in the identification of a calibration strategy for the accelerometer data processing that allows obtaining an unbiased solution and compensating for accelerometer errors.

Comparison of light-time formulations in the post-Newtonian framework for the BepiColombo MORE experiment

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.

The BepiColombo solar conjunction experiments revisited

BepiColombo ESA/JAXA mission is currently in its 7 year cruise phase towards Mercury. The Mercury orbiter radioscience experiment (MORE), one of the 16 experiments of the mission, will start its scientific investigation during the superior solar conjunction (SSC) in March 2021 with a test of general relativity (GR). Other solar conjunctions will follow during the cruise phase, providing several opportunities to improve the results of the first experiment. MORE radio tracking system allows to establish precise ranging and Doppler measurements almost at all solar elongation angles (up to 7–8 solar radii), thus providing an accurate measurement of the relativistic time delay and frequency shift experienced by a radio signal during an SSC. The final objective of the experiment is to place new limits to the accuracy of the GR as a theory of gravity in the weak-field limit. As in all gravity experiments, non-gravitational accelerations acting on the spacecraft are a major concern. Because of the proximity to the Sun, the spacecraft will undergo severe solar radiation pressure acceleration, and the effect of the random fluctuations of the solar irradiance may become a significant source of spacecraft buffeting. In this paper we address the problem of a realistic estimate of the outcome of the SSC experiments of BepiColombo, by including in the dynamical model the effects of random variations in the solar irradiance. We propose a numerical method to mitigate the impact of the variable solar radiation pressure on the outcome of the experiment. Our simulations show that, with different assumptions on the solar activity and observation coverage, the accuracy attainable in the estimation of γ lays in the range [–] ×10−6.

Report on First Inflight Data of BepiColombo's Mercury Orbiter Radio Science Experiment

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 the 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/s (at 1000 s integration time) for the range-rate measurements and 20 cm for the range (at a few seconds of the 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 the range and range-rate data collected by ESA's deep-space antenna (DSA) during the initial cruise phase of BepiColombo. The novel 24 Mcps pseudonoise (PN) modulation of the Ka-band carrier, enabled by MORE's Ka-band transponder, built by Thales Alenia Space Italy, Rome, Italy, provided two-way range measurements to the centimeter-level accuracy, with an integration time of 4.2 s at 0.29 astronomical units. In tracking passes with favorable weather conditions, the range-rate measurements attained an average accuracy of 0.01 mm/s at 60 s integration time. Data from May 20–24, 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 the 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 the range measurements of unprecedented accuracy for a planetary mission, and that, in general, all target accuracies for radio-metric measurements were exceeded.

Addressing some critical aspects of the BepiColombo MORE relativity experiment

The Mercury Orbiter radio Science Experiment (MORE) is one of the experiments on-board the ESA/JAXA BepiColombo mission to Mercury, to be launched in October 2018. Thanks to full on-board and on-ground instrumentation performing very precise tracking from the Earth, MORE will have the chance to determine with very high accuracy the Mercury-centric orbit of the spacecraft and the heliocentric orbit of Mercury. This will allow to undertake an accurate test of relativistic theories of gravitation (relativity experiment), which consists in improving the knowledge of some post-Newtonian and related parameters, whose value is predicted by General Relativity. This paper focuses on two critical aspects of the BepiColombo relativity experiment. First of all, we address the delicate issue of determining the orbits of Mercury and the Earth-Moon barycenter at the level of accuracy required by the purposes of the experiment and we discuss a strategy to cure the rank deficiencies that appear in the problem. Secondly, we introduce and discuss the role of the solar Lense-Thirring effect in the Mercury orbit determination problem and in the relativistic parameters estimation.

An analysis of the geodesy and relativity experiments of BepiColombo

BepiColombo is a ESA-JAXA mission aimed to a comprehensive exploration of Mercury, the innermost planet of the solar system. The Mercury Orbiter Radio science Experiment (MORE) will exploit a state of the art microwave tracking system, including an advanced Ka-band transponder, to determine the gravity field and the rotational state of the planet, and to perform extensive tests of relativistic gravity. In this work we analyze all the aspects of the radio science investigation, which include: (i) the solar conjunction experiment in cruise; (ii) the gravimetry and rotation experiments; (iii) the fundamental physics test. We report on the results of numerical simulations based on the latest mission scenario, with launch in October 2018 and arrival at Mercury in December 2025. We show that the gravity and rotation measurements expected from BepiColombo will allow to better characterize the size of an inner solid core inside the outer liquid core, and the properties of the outer mantle and the crust. We discuss how the current estimate of several parametrized post-Newtonian (PPN) parameters can be improved by MORE through the determination of the heliocentric motion of Mercury and by measuring the propagation time of radio waves. We also assess in a quantitative way the benefits of an extended mission.

Determining the amplitude of Mercury's long period librations with the BepiColombo radio science experiment⋆

The Mercury Orbiter Radio science Experiment (MORE) is one of the experiments on-board the ESA/JAXA BepiColombo mission to Mercury. A crucial goal of MORE is to determine the gravity field and rotational state of Mercury in order to enable a better understanding of the planet's geophysics. The authors have recently reported on the results of a set of simulations of the MORE gravimetry and rotation experiments, carried out with the dedicated ORBIT14 software. Since that time, the launch date has been postponed twice, leading to a shift of more than one year in the orbital phase of the mission. Actually, the updated schedule results in a more suitable planetary configuration to determine the amplitude of the forced librations in longitude induced by Jupiter. In fact, the amplitude can be considerably enhanced due to a near-resonance with the free librations period, a key parameter to constrain the interior structure of Mercury. We show that the newest launch date allows the measurement of the long period librations amplitude forced by Jupiter with an accuracy of some tenth of arcseconds, a significant improvement with respect to the results with the previous mission schedule.

Testing General Relativity with the Radio Science Experiment of the BepiColombo mission to Mercury

The relativity experiment is part of the Mercury Orbiter Radio science Experiment (MORE) on-board the ESA/JAXA BepiColombo mission to Mercury. Thanks to very precise radio tracking from the Earth and accelerometer, it will be possible to perform an accurate test of General Relativity, by constraining a number of post-Newtonian and related parameters with an unprecedented level of accuracy. The Celestial Mechanics Group of the University of Pisa developed a new dedicated software, ORBIT14, to perform the simulations and to determine simultaneously all the parameters of interest within a global least squares fit. After highlighting some critical issues, we report on the results of a full set of simulations, carried out in the most up-to-date mission scenario. For each parameter we discuss the achievable accuracy, in terms of a formal analysis through the covariance matrix and, furthermore, by the introduction of an alternative, more representative, estimation of the errors. We show that, for example, an accuracy of some parts in 10 − 6 for the Eddington parameter β and of 10 − 5 for the Nordtvedt parameter η can be attained, while accuracies at the level of 5 × 10 − 7 and 1 × 10 − 7 can be achieved for the preferred frames parameters α 1 and α 2 , respectively.

Constraining the Nordtvedt parameter with the BepiColombo Radioscience experiment

BepiColombo is a joint ESA/JAXA mission to Mercury with challenging objectives regarding geophysics, geodesy, and fundamental physics. The Mercury Orbiter Radioscience Experiment (MORE) is one of the on-board experiments, including three different but linked experiments: gravimetry, rotation, and relativity. The aim of the relativity experiment is the measurement of the post-Newtonian parameters. Thanks to accurate tracking between Earth and spacecraft, the results are expected to be very precise. However, the outcomes of the experiment strictly depend on our ``knowledge'' about solar system: ephemerides; number of bodies (planets, satellites, and asteroids); and their masses. In this paper we describe a semianalytic model used to perform a covariance analysis to quantify the effects on the relativity experiment, due to the uncertainties of Solar System bodies' parameters. In particular, our attention is focused on the Nordtvedt parameter $\ensuremath{\eta}$ used to parametrize the strong equivalence principle violation. After our analysis we estimated $\ensuremath{\sigma}[\ensuremath{\eta}]⪅4.5\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}5}$, which is about 1 order of magnitude larger than the ``ideal'' case where masses of planets and asteroids have no errors. The current value, obtained from ground-based experiments and lunar laser ranging measurements, is $\ensuremath{\sigma}[\ensuremath{\eta}]\ensuremath{\approx}4.4\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}$. Therefore, we conclude that, even in the presence of uncertainties on Solar System parameters, the measurement of $\ensuremath{\eta}$ by MORE can improve the current precision of about 1 order of magnitude.