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

Coupling radio propagation and weather forecast models to maximize Ka-band channel transmission rate for interplanetary missions

Deep space (DS) missions for interplanetary explorations are aimed at acquiring information about the solar system and its composition. To achieve this result a radio link is established between the space satellite and receiving stations on the Earth. Significant channel capacity must be guaranteed to such spacecraft-to-Earth link considering their large separation distance as well. Terrestrial atmospheric impairments on the space-to-Earth propagating signals are the major responsible for the signal degradation thus reducing the link’s channel temporal availability. Considering the saturation and the limited bandwidth of the conventional systems used working at X-band (around 8.4 GHz), frequencies above Ku-band (12-18 GHz) are being used and currently explored for next future DS missions. For example, the ESA mission EUCLID, planned to be launched in 2020 to reach Sun-Earth Lagrange point L2, will use the K-band (at 25.5-27 GHz). The BepiColombo (BC) ESA mission to Mercury, planned to be launched in 2016, will use Ka-band (at 32-34 GHz) with some modules operating at X-band too. The W-band is also being investigated for space communications (Lucente et al., IEEE Systems J., 2008) as well as near-infrared band for DS links (Luini at al., 3rd IWOW, 2014; Cesarone et al., ICSOS, 2011). If compared with X-band channels, K-band and Ka-band can provide an appealing data rate and signal-to-noise ratio in free space due to the squared-frequency law increase of antenna directivity within the downlink budget (for the same physical antenna size). However, atmospheric path attenuation can be significant for higher frequencies since the major source of transmission outage is not only caused by convective rainfall, as it happens for lower frequencies too, but even non-precipitating clouds and moderate precipitation produced by stratiform rain events are detrimental. This means that accurate channel models are necessary for DS mission data link design at K and Ka band. A physical approach can offer advanced radiopropagation models to take into account the effects due to atmospheric gases, clouds and precipitation. The objective of this work is to couple a weather forecast numerical model with a microphysically- oriented radiopropagation model, providing a description of the atmospheric state and of its effects on a DS downlink. This work is developed in the framework of the RadioMeteorological Operations Planner (RMOP) program, aimed at performing a feasibility study for the BC mission (Biscarini et al., EuCAP 2014). The RMOP chain for the link budget computation is composed by three modules: weather forecast (WFM), radio propagation (RPM) and downlink budget (DBM). WFM is aimed at providing an atmospheric state vector. Among the available weather forecast models, for RMOP purposes we have used the Mesoscale Model 5. The output of the WFM is the input of the RPM for the computation of the atmospheric attenuation and sky-noise temperature at the receiving ground station antenna. RPM makes use of radiative transfer solver based on the Eddington approximations well as accurate scattering models. Time series of attenuation and sky-noise temperature coming from the RPM are converted into probability density functions and then ingested by the DBM to compute the received data volume (DV). Using the BC mission as a reference test case for the Ka-band ground station at Cebreros (Spain), this work will show the advantages of using a coupled WFM-RPM approach with respect to climatological statistics in a link budget optimization procedure. The signal degradation due to atmospheric effects in DS links in terms of received DV will be also investigated not only at Ka band, but also at X, K and W for intercomparison. The quality of the DS downlink will be given in terms of received DV and the results at different frequencies compared showing the respective advantages and drawbacks