The JUpiter Icy Moons Explorer (JUICE) is an European Space Agency (ESA) mission to investigate Jupiter¿s icy satellites that will launch in 2022 and arrive in the Jovian system in 2029. The mission will perform flybys of the icy moons Europa and Callisto, before being inserted into a 4-month, circular orbit about Ganymede, at an altitude of approximately 500 km to investigate the moon¿s icy crust, interior structure, magnetic field and exosphere. The 3GM experiment on board the spacecraft will exploit accurate Doppler and range measurements to determine the moons¿ orbits, gravity fields, tides and therefore infer details about their internal structures. In this work, we focus on the last phase of the mission, when JUICE aims to provide an accurate global map of Ganymede¿s gravity field. The preliminary simulations carried out reveal that the moon¿s gravity field can be determined up to degree and order 40. The Love number k2, modelling the tidal response, is determined with an accuracy of 7e-5 (1-sigma), which will allow us to set a constraint on the internal behavior of the moon. The objective of this work is to use different techniques to further improve the results and analyze different mission scenarios: 200 km circular orbit at Ganymede at the end of the missions, 2 weeks around Ganymede uniquely dedicated to gravity measurements and evaluate the impact of wheel off loading maneuver on the experiment. The knowledge of the gravity field can be improved by setting constraints on the a priori value by using Kaula's rule. This technique has already been used in previous cases such as for Mercury gravity field reconstruction by using data from the MESSENGER mission (Mazarico et al, 2014) and in the simulation of BepiColombo mission (Imperi et al. 2018). The constrained multi-arc approach (Milani et al 2010) has already been proven effective by Imperi et al (2018) to further constraint the trajectory and estimate the daily wheel off loading maneuver.
The innovation of this work is intimately related to the topic of space exploration and with the ESA's JUICE mission.
JUICE primary objective is to study the Jupiter icy satellites and the Jupiter system. The investigations that the 3GM gravity experiment will help in the understanding the moon's internal structure and differentiation, icy composition, exosphere, magnetosphere and likely the formation and evolution of the moon structure and orbit. This work is the first assessment of the 3GM team related with different mission scenarios that are may be possible in the future development of the mission. It will help also to understand the robustness and reliability of our results with respect to the nominal case. I am also responsible for a definitive assessment on the nominal case: it is a working on that is under internal review and will be submitted to a journal in the next few weeks. The current state of the art of 3GM assessment is only the JUICE mission science objective (Titov et al, 2014) that can be considered a minimal requirement. This comprehends for example that the minimum degree of the spherical harmonic expansion of Ganymede's gravity field is 12. A more accurate but still preliminary result of Cappuccio et al. (2018) shows that the 40 degree is a more realistic value of the maximum degree for the gravity field reconstruction.
If we talk about the knowledge of the moon itself then it is very poor since only a few flybys of the Galileo spacecraft let to determine only the quadrupole gravity field (degree 2) and no information about the tides have been collected (Anderson el al, 1996). 3GM and JUICE in general can do much better than this, as discussed previously. JUICE will constitute a breakthrough on the knowledge of icy moons. This class of celestial bodies have never been studied in so many details and are the most interesting objects in the solar system for studying possible habitability. As far as we know the icy moons are the only bodies in the solar system to host liquid oceans. More realistic upper and lower constraints on our experiment will help all the scientific community to be develop adequate models to understand which Ganymede's features we can actually detect.