Testing general relativity during the cruise phase of the BepiColombo mission to Mercury

General relativity (GR) predicts that photons are delayed and deflected by the space curvature produced by any mass. The Post-Newtonian (PN) parameter controlling the curvature induced by a gravitational field is γ, with bending and delay effects proportional to (γ + 1). γ = 1 in GR. The most accurate estimation of this PN parameter γ = (1 + (2.1 ± 2.3)) · 10-5, has been obtained by the NASA mission Cassini [1] exploiting the frequency shift of radio signal during a Superior Solar Conjunction (SSC) in 2002, while the spacecraft was in cruise to Saturn. The crucial element of the experiment was an advanced radio system providing a highly stable multi-frequency radio link in X and Ka band (8.4 and 32.5 GHz), and a nearly complete cancellation of the plasma noise introduced by the solar corona in Doppler measurements. The ESA-JAXA mission BepiColombo to Mercury will improve the Cassini radio instrumentation by enabling the ranging function also in the Ka band radio link used by the Mercury Orbiter Radio science Experiment (MORE). The fully digital architecture of the transponder provides a pseudo-noie modulation of the carrier at 24 Mcps and a two-way range accuracy of 20 cm. Thanks to the simultaneous tracking by means of the standard telecommunication link, both range and range rate observables will be available for new, more accurate tests of GR. This paper reports on the simulations carried out in order to assess the attainable accuracies in the estimation of γ during the cruise phase of BepiColombo. In an optimal configuration, an uncertainty of 5·10-6 may be attained

On the determination of Jupiter's satellite-dependent Love numbers from Juno gravity data

The Juno gravity experiment, among the nine instruments onboard the spacecraft, is aimed at studying the interior structure of Jupiter to gain insight into its formation. Doppler data collected during the first two gravity-dedicated orbits completed by Juno around the gas giant have already provided a measurement of Jupiter's gravity field with outstanding accuracy, answering crucial questions about its interior composition. The large dataset that will be collected throughout the remaining phases of the mission until the end in July 2021 might allow to determine Jupiter's response to the satellite-dependent tidal perturbation raised by its moons, and even to separate the static and dynamic effects. We report on numerical simulations performed over the full science mission to assess the sensitivity of Juno gravity measurements to satellite-dependent tides on Jupiter. We assumed a realistic simulation scenario that is coherent with the result of data analysis from the first gravity passes. Furthermore, we implemented a satellite-dependent tidal model within the dynamical model used to fit the simulated Doppler data. The formal uncertainties resulting from the covariance analysis show that Juno is indeed sensitive to satellite-dependent tides on Jupiter raised by the inner Galilean satellites (the static Love numbers of degree and order 2 of Io, Europa and Ganymede can be determined respectively to 0.28%, 4.6% and 5.3% at 1 sigma). This unprecedented determination, that will be carried out towards the end of the mission, could further constrain the interior structure of the planet, allowing to discern among interior models and improving existing theories of planetary tidal response

Improvement of BepiColombo's radio science experiment through an innovative Doppler noise reduction technique

The Mercury Orbiter Radio science Experiment (MORE), onboard the ESA/JAXA BepiColombo mission to Mercury, is designed to estimate Mercury’s gravity field, its rotational state, and to perform tests of relativistic gravity. The state-of-the-art onboard and ground instrumentations involved in the MORE experiment will enable to establish simultaneous X/X, X/Ka and Ka/Ka-band links, providing a range rate accuracy of 3 µm/s (at 1000 s integration time) and a range accuracy of 20 cm. The purpose of this work is to show the improvement achievable on MORE’s performance by means of the Time-Delay Mechanical Noise Cancellation (TDMC) technique. The TDMC consists in a combination of Doppler measurements collected (at different times) at the two-way antenna and at an additional, smaller and stiffer, receive-only antenna that should be located in a site with favorable tropospheric conditions. This configuration could reduce the leading noises in a Ka-band two-way link, such as those caused by troposphere and ground antenna mechanical vibrations. We present the results of end-to-end simulations and estimation of Mercury’s gravity field and rotational state considering the TDMC technique. We compare results for a two-way link from NASA’s DSS-25 (in Goldstone, CA) or from ESA’s DSA-3 (in Malargue, Argentina), while we assume APEX as the receive-only antenna. We show that in best-case noise conditions, the TDMC technique allows to obtain a factor-of-two accuracy gain on both global and local parameters, considering DSA-3 as two-way antenna. Such improvement in the scientific objectives of MORE is of geophysical interest as it could provide a constraint on the interior structure of Mercury

Generation and reduction of the data for the Ulysses gravitational wave experiment

A procedure for the generation and reduction of the radiometric data known as REGRES is described. The software is implemented on a HP-1000F computer and was tested on REGRES data relative to the Voyager I spacecraft. The REGRES data are a current output of NASA's Orbit Determination Program. The software package was developed in view of the data analysis of the gravitational wave experiment planned for the European spacecraft Ulysses

The detection of Jupiter normal modes with gravity measurements of the mission Juno

Arriving at Jupiter on July 4, 2016, NASA’s Juno mission will complete 37 orbits (14-days period) around the planet, revealing details of the interior structure and composition, a crucial aspect to understand the origin and evolution of Jupiter. A radio science experiment will help to select and validate the existing models of Jupiter internal composition, in particular the mass of the silicate core. Recently it has been proposed to exploit the Doppler data for the determination of Jupiter’s acoustic normal modes. Jupiter is a gaseous giant and its masses are subject to oscillations (normal modes) due to internal pressure waves, which cause potentially detectable disturbances in the gravity field. By displacing large masses, Jupiter’s normal modes can therefore perturb the spacecraft motion to levels that can be measured by Juno’s extremely accurate Doppler system. Theoretical models that explain these phenomena have been proposed in the past and experimental works looking for these oscillations have been carried out recently with ground-based optical telescopes. But the frequencies and the amplitudes of normal modes can in principle be modeled and estimated by means of orbit determination codes

Simulated recovery of Europa's global shape and tidal Love numbers from altimetry and radio tracking during a dedicated flyby tour

The fundamental scientific objectives for future spacecraft exploration of Jupiter's moon Europa include confirmation of the existence of subsurface ocean beneath the surface ice shell and constraints on the physical properties of the ocean. Here we conduct a comprehensive simulation of a multiple-flyby mission. We demonstrate that radio tracking data can provide an estimate of the gravitational tidal Love number k2 with sufficient precision to confirm the presence of a liquid layer. We further show that a capable long-range laser altimeter can improve determination of the spacecraft position, improve the k2 determination (2 (3-4% error), which is directly related to the amplitude of the surface tidal deformation. These measurements, in addition to the global shape accurately constrained by the long altimetric profiles, can yield further constraints on the interior structure of Europa. Key Points A multiple-flyby mission to Europa can recover key geophysical parameters Laser altimetry can uniquely and accurately recover the global shape of Europa Laser altimetry enables the recovery of h2 to constrain the ice shell thicknes

On the determination of post-Newtonian parameters with BepiColombo radio science experiment

One of the main goals of the Mercury Orbiter Radio science Experiment (MORE), onboard the ESA-JAXA BepiColombo mission to Mercury, is to perform a test of gravitational theories by means of high precision radio-observables, constraining several Post-Newtonian (PN) parameters. This will be performed in two steps: (i) with a superior solar conjunction experiment during the cruise phase of the mission; (ii) by reconstructing the orbit of Mercury around the Sun once the spacecraft will be arrived at Mercury. In this work we present the results of numerical simulations of the MORE relativity experiment, carried out in a realistic scenario, showing how the experiment can improve over current estimates

BepiColombo’s geodesy and relativity experiments from an extended mission

The Mercury Orbiter Radio science Experiment (MORE) of the ESA-JAXA BepiColombo mission to Mercury consists of ground and onboard instrumentation enabling a highly stable, multi-frequency radio link at X and Ka band (8.4 and 32.5 GHz). Range rate measurements obtained from this advanced radio link will be unaffected by plasma noise and are expected to attain accuracies of 3 micron/s (at 1000 seconds integration time) at nearly all elongation angles. Thanks to a novel wideband ranging system, based on a 24 Mcps pseudo-noise modulation, the spacecraft range will be measured to an accuracy 20 cm (two-way). The MORE investigation will greatly benefit from a direct measurement of the vectorial non-gravitational accelerations by means of the Italian Spring Accelerometer (ISA). The high quality radio-metric observables will provide a precise reconstruction of the spacecraft orbit and an accurate estimation of the gravity field and rotational state of the planet. Thanks to the dedicated onboard instrumentation, MORE is expected to improve significantly the already outstanding MESSENGER results, limited by plasma noise and the difficulty of modeling non-gravitational accelerations. In addition, BepiColombo will carry out tests of general relativity by reconstructing the orbit of the planet and the propagation of photons in the solar gravitational field. Indeed, since the orbit of Mercury is affected more than any other planets by relativistic effects, the relativity experiment aims at improving the determination of several Post-Newtonian (PN) parameters. Further physical parameters such as the rate of change of the gravitational constant G and the oblateness factor J2 of the Sun will be estimated as well. Several numerical simulations of the MORE experiment have been carried out over the past years. In this work we present a new set of simulations under the latest mission scenario and instrument performances, as obtained from ground tests of the instrumentation. Our simulation setup solves simultaneously for gravity harmonic coefficients, rotational state elements and relativistic PN parameters. The paper reports on the results obtained under the nominal, one year, mission duration, and shows the improvements attained by an extended mission of one or two years. Indeed, the pericenter of BepiColombo’s planetary orbiter will drift from 15 degree N to 13, 41, 70 degree S respectively in one, two and three years. In addition the pericenter altitude will decrease from 480 to 250 km in three years. This will allow a more comprehensive and homogeneous reconstruction of the gravity field and rotational state of Mercury. We show also that an extended mission would be greatly beneficial also to the relativity experiment

Precise modeling of non-gravitational accelerations of the spacecraft BepiColombo during cruise phase

The ESA/JAXA (Japan Aerospace Exploration Agency) BepiColombo mission is currently in its cruise phase and is set to reach Mercury in late 2025. The spacecraft is equipped with advanced radio-tracking instrumentation that provides accurate radiometric data for precise orbit determination (POD). During the cruise phase, the radio link enables tests of general relativity (GR) and measurements of the solar corona properties. To fully exploit the accuracy of the radiometric measurements (Doppler and ranging) and to obtain better accuracies in the GR tests, a comprehensive dynamical model of the spacecraft is needed. A reliable and precise model of the non-gravitational accelerations would maximize the scientific return of the radio-science experiments and prevent unmodeled dynamic perturbations from degrading the POD solution. In this work, we investigate the effects of non-conservative forces acting on BepiColombo during three different radio-science campaigns conducted in November 2020, March 2021, and February 2022. The last two periods correspond to the first two GR tests of BepiColombo. We design a method for modeling these forces using telemetry measurements, radiometric observables, and mathematical models; and we analyze their characteristics in relation to the different environments encountered by the spacecraft during the three periods. This work is a preparatory and unavoidable step for the data analysis of the first two GR experiments of BepiColombo and the next radio-science campaigns, which will be performed in an even more challenging dynamical environment