1,721,100 research outputs found
The BepiColombo solar conjunction experiments revisited
No full textBepiColombo 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 [6-13] ×10−6
On the determination of Jupiter's satellite-dependent Love numbers from Juno gravity data
Full text linkThe 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
JUICE's 3GM gravity experiment around ganymede - Comparison between nominal and extended mission
No full textThe JUpiter Icy Moons Explorer (JUICE) is a European Space Agency (ESA) mission dedicated to investigating Jupiter's icy satellites and Jovian environment. The mission will be launched in 2022 from Kourou, French Guyana, on an Ariane 5 and it will arrive in the Jovian system in 2029. The mission will perform a series of flybys of the icy moons Europa, Callisto and Ganymede before being inserted into a 9-month orbit around Ganymede. The Ganymede orbital phase is divided into a 5-month elliptical orbit (GEO) and a 4-month circular orbit at an altitude of approximately 500 km (GCO-500). JUICE is endowed with a suite of instruments that will investigate the moon's icy crust, interior structure, magnetic field, and exosphere. The 3GM (Geodesy and Geophysics of Jupiter and the Galilean Moons) experiment on board the spacecraft will exploit accurate Doppler and range measurements to determine the moons' orbits, gravity fields, tides and therefore infer features of their internal structures. In this paper, we compare the expected results of the nominal GCO-500 phase with a possible extended mission to a 200 km circular orbit (GCO-200). The simulations of the nominal mission (GCO-500) reveals that 3GM can provide a gravity map of the moon's up to degree and order 40. The Love number k2, modelling the tidal response, is determined with an accuracy of 10-4 (1-s), which will allow us to set a constraint on the internal structure of the moon. The obliquity, f, and the libration at orbital period, ?, can be retrieved with a level of uncertainty of 1 and 2 μrad, respectively. In this paper, we compare the expected results of the nominal GCO-500 phase with a possible extended mission to a 200 km circular orbit (GCO-200). At a lower altitude, the gravity field can be recovered up to degree and order 80, thus, revealing more details about the superficial structures with a resolution of 207 km at the equator. The potential effect of spacecraft drag due to Ganymede's tenuous exosphere, which ranges between 10-17 - 10-16 kg/m3, in the GCO-500 phase is very faint making not possible for 3GM to estimate it. During the extended mission, the drag becomes 2 orders of magnitude higher, thus, 3GM will be able to provide an estimation of the exospheric density
On the determination of post-Newtonian parameters with BepiColombo radio science experiment
Full text linkOne 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
Covariance Analysis Applied to the MESSENGER and BepiColombo Relativity Experiments
No full textMESSENGER (MErcury Surface, Space ENvironment, GEochemistry, and Ranging) and BepiColombo are joint NASA and ESA/JAXA missions devoted to the exploration of Mercury. MESSENGER operated between 2008 and 2015, while BepiColombo will be launched in October 2018 and will orbit around Mercury for at least one year starting from March 2026. Both missions have the aim to study characteristics of Mercury in terms of its geology, magnetosphere, exosphere, etc. BepiColombo will improve some of the MESSENGER results regarding the rotation state and gravity field of Mercury and it will perform important tests of the General Relativity. In this work we investigate the benefits of the joint dataset of the two missions for the Relativity experiment. The very large baseline (about 19 years) together with BepiColombo's high precision measurements will be a key factor for the improvement of the accuracies. To this aim, we developed a semi-analytical model for a covariance analysis to estimate the formal uncertainties of the parameters of our interest: post-Newtonian parameters, Nordtvedt parameter, the Sun's GM, its rate of change and the Sun's gravitational oblateness. Non-gravitational forces have also been modeled. Finally, particular attention has been paid to the repercussions on the Relativity experiment due to the uncertainties on the GMs and ephemerides of planets and minor bodies of the Solar System
Environmental disturbances on missions for precise tests of relativistic gravity and solar system dynamics: the bepicolombo case
No full textBepiColombo is an ESA/JAXA mission directed toward Mercury, which has been successfully launched on 20 October 2018. The spacecraft will perform accurate general relativity tests both during the 6-years cruise phase and the orbital phase around Mercury. The on-board microwave instrumentation enables very precise ranging and Doppler measurements even when approaching superior solar conjunctions, offering the possibility to improve the classical relativistic test of time delay and frequency shift of radio signals. Random accelerations caused by unpredictable solar irradiance fluctuations could negatively affect this experiment. No measurements of these fluctuations are available for the purposes of the experiment and we show that no direct compensation strategies based on the on-board accelerometer can be applied because of the extremely low frequency components of the disturbance. We analyse the effect of the dynamical noise introduced by fluctuations in the solar irradiance showing by means of numerical simulations that suitable stochastic models of the noise source can still guarantee an estimate of the relativistic time delay with improved accuracy with respect to current knowledge
The rotational dynamics of Titan from Cassini RADAR images
No full textBetween 2004 and 2009 the RADAR instrument of the Cassini mission provided 31 SAR images of Titan. We tracked the position of 160 surface landmarks as a function of time in order to monitor the rotational dynamics of Titan. We generated and processed RADAR observables using a least squares fit to determine the updated values of the rotational parameters. We provide a new rotational model of Titan, which includes updated values for spin pole location, spin rate, precession and nutation terms. The estimated pole location is compatible with the occupancy of a Cassini state 1. We found a synchronous value of the spin rate (22.57693. deg/day), compatible at a 3-σ level with IAU predictions. The estimated obliquity is equal to 0.31°, incompatible with the assumption of a rigid body with fully-damped pole and a moment of inertia factor of 0.34, as determined by gravity measurements. © 2016 Elsevier Inc
Bepicolombo gravity and rotation experiment in a pseudo drag-free system
No full textThe Mercury Orbiter Radioscience Experiment (MORE) on-board the BepiColombo mission was designed to provide an accurate estimation of the gravity field and the rotational state of Mercury. The Mercury Planetary Orbiter (MPO) is equipped with on-board instrumentation that allows highly stable, multi-frequency radio links in X and Ka band in order to achieve range rate and range coherent two-way measurements accurate to 0.003 mm/s (at 1000 s integration time) and 20 cm, respectively. Precise trajectory reconstruction allows us to estimate accurately the spherical harmonic coefficients of the Hermean gravity field, at least up to degree 35, the tide and the rotational parameters (right ascension and declination of the pole and physical librations in longitude). The determination these parameters provides crucial information on the interior structure of the planet. A full numerical simulation of the radioscience orbit determination process has been carried out taking into account the data provided by the on-board Italian Spring Accelerometer (ISA). This allows us to realize a software version of a drag-free system. In this paper, we report on the results of these numerical simulations aiming at a realistic assessment of the attainable accuracy in the determination of the gravity field and the rotation of Mercury with the implementation of a pseudo drag-free orbit determination process
Analysis of the 3GM gravity experiment of ESA’s JUICE mission
No full textThe ESA’s JUICE mission will provide a thorough investigation of the Jupiter system and the Galilean moons through a suite of ten different scientific instruments. JUICE will perform flybys of Europa and Callisto, and will orbit around Ganymede in the last phase of the mission. The 3GM experiment will exploit accurate Doppler and range measurements to determine the moons’ orbits and gravity fields (both static and tidal) and to infer their interior structure. This paper presents the attainable accuracies from the 3GM geodesy experiment under the current mission scenario. Our analysis includes the use of a high-accuracy accelerometer to remove the dynamical noise induced by propellant sloshing
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