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Introduction The exploration of Callisto is part of the extensive interest in the icy moons characterization. As other missions will focus on Ganymede and Europa, mission proposals devoted to the exploration and characterization of Callisto are currently emerging [2][6]. Compared to the other Galilean moons, the outermost moon has kept the best-preserved records of the Jovian system formation, which makes it a candidate of choice to understand how the Jupiter system formed, and how it works. Led by the National Space Science Center (NSSC), Chinese Academy of Science (CAS), the Gan De mission [2] aims to send an orbiter around Callisto in order to map its gravity and magnetic field, and to characterize its exosphere, surface and interior. Callisto’s degree of differentiation will also be investigated as well as the possible existence of an internal ocean, as Galileo measurements suggested. Because the mission is at a very early planning stage, there is no definitive mission plan yet. In this context, we selected a small representative set of potential science orbits around Callisto. These orbits are tested under different assumptions in a detailed simulation environment, to analyse the orbit recovery as well as the recovery of Callisto gravity field and k2 Love number using tracking data. The effect of non-gravitational forces exerting on the orbiter is carefully considered, especially the strategy to handle them through the whole estimation process. Exploring different orbits Flybys of Callisto from the JUICE and the Europa Clipper missions will already provide further information on this moon [4]. However, the global mapping of the outermost Galilean moon will be uniquely improved by means of an orbiter at a high inclination and a low altitude. A specific difficulty in the orbit design in the Jovian system is related to the orbit stability, which is highly impacted by the influence of Jupiter as a third body exerting strong perturbations on a Callisto orbiter. Additionally, the occultations of the Sun by Callisto should be minimized to ensure a maximum solar illumination for power, which is critical this far from the Sun. This implies a minimum angle between the orbital plane w.r.t the direction of the Sun (βSun), which is larger the lower the orbit is. Depending on the position of the Earth at that time, this might have a direct impact on the angle between the orbital plane w.r.t the direction of the Earth (βEarth), which is of importance for orbit determination and gravity field recovery using Doppler tracking data. In this presentation, we selected various low Callisto orbits, which are suitable as science orbits for Gan De. These orbits have different altitudes, Earth beta angles, and injection date into orbit. Although several of them can be used sequentially, they are considered separately. A detailed force model The set of different reference orbits is propagated using the planetary extension of the Bernese Software (BSW) [3]. All the planets of the solar system, as well as the Galilean satellites, are considered as point masses. In the case of Jupiter, the zonal harmonics of the gravity field are also part of the force model. Because our knowledge of Callisto’s gravity field is restricted to the results of the Galileo mission (degree 2) [1], we have derived a synthetic gravity field as ground truth for our simulation. The scaled gravity field of the Earth’s Moon is used from degree 3 to 100. An artificial value of k2 Love number is also adopted to provide a ground truth. In order to rely on solar power this far from the Sun, the size of the solar panels is significant, which makes the effect of the solar and planetary radiation pressure on them essential to account for. However, the modelling of these forces can be challenging, particularly because of their dependency on the geometry and the optical properties of the probe. Even with a precise macro model, the optical properties of each plate can evolve with time. Several strategies are tested to handle these non-conservative forces, such as the use of pseudo-stochastics parameters and the possibility of an on-board accelerometer. Orbit determination and gravity field recovery Using a detailed noise model [5] and a realistic tracking station schedule, realistic Doppler tracking data (2-way Ka-band Doppler range rate) is simulated as measurements from the Deep Space Network. The non-gravitational accelerations in action during the orbit propagation are used to generate realistic accelerometer measurements. These observations are then used to reconstruct the orbit along with dynamical parameters in a global least-squares adjustment. Within this closed-loop simulation, we investigate the impact of the non-gravitational accelerations and orbit geometry on the orbit determination and the recovery of the geodetic parameters. The focus of this presentation is on the quality of the retrieved gravity field parameters and tidal Love number k2. Acknowledgements This study has been funded with the support of the Swiss National Foundation (SNF). References [1] Anderson, J. D., et al. "Shape, mean radius, gravity field, and interior structure of Callisto." Icarus 153.1 (2001): 157-161. [2] Blanc, Michel, et al. "Gan De: Science Objectives and Mission Scenarios for China's Mission to the Jupiter System." EGU General Assembly Conference Abstracts. 2020. [3] Dach, R. et al. "Bernese GNSS software version 5.2." (2015). [4] Di Benedetto, Mauro, et al. "Analysis of 3GM Callisto Gravity Experiment of the JUICE Mission." arXiv preprint arXiv:2101.03401 (2021). [5] Iess, Luciano, et al. "Astra: Interdisciplinary study on enhancement of the end-to-end accuracy for spacecraft tracking techniques." Acta Astronautica 94.2 (2014): 699-707. [6] Smith, David E., et al. "MAGIC, A Discovery Proposal to the Icy Moon Callisto." AGU Fall Meeting Abstracts. Vol. 2019. 2019.

The exploration of Callisto is part of the extensive interest in the icy moons characterization. Indeed, Callisto is the Galilean moon with the best-preserved records of the Jovian system formation. Led by the National Space Science Center (NSSC), Chinese Academy of Science (CAS), the planned Gan De mission aims to send an orbiter around Callisto in order to characterize its surface and interior. Potential orbit configurations are currently under study for the Gan De mission proposal.As part of a global characterization of Callisto, its gravity field can be inferred using radio tracking data from an orbiter. Mission characteristics such as orbit type, Earth beta angle and solar elongation will have a direct influence on the recoverability of its gravity field parameters. In this study, we will analyse this influence from closed-loop simulations using the planetary extension of the Bernese GNSS Softwareai.A number of reference orbits with different orbital characteristics will be selected for the Gan De mission and, using an extended force model, will be propagated from different starting dates and different initial Earth beta angles. Realistic Doppler tracking data (2-way X-band Doppler range rate) will be simulated as measurements from ground stations, with a dedicated noise model. These observations will then be used to reconstruct the orbit along with dynamical parameters. The focus of this presentation will be on the quality of the retrieved gravity field parameters and tidal Love number k2.

Full list of authors: Blanc, Michel; Prieto-Ballesteros, Olga; André, Nicolas; Gomez-Elvira, Javier; Jones, Geraint; Sterken, Veerle; Desprats, William; Gurvits, Leonid I.; Khurana, Krishan; Balmino, Georges; Blöcker, Aljona; Broquet, Renaud; Bunce, Emma; Cavel, Cyril; Choblet, Gaël; Colins, Geoffrey; Coradini, Marcello; Cooper, John; Dirkx, Dominic; Fontaine, Dominique; Garnier, Philippe; Gaudin, David; Hartogh, Paul; Hussmann, Hauke; Genova, Antonio; Iess, Luciano; Jäggi, Adrian; Kempf, Sascha; Krupp, Norbert; Lara, Luisa; Lasue, Jérémie; Lainey, Valéry; Leblanc, François; Lebreton, Jean-Pierre; Longobardo, Andrea; Lorenz, Ralph; Martins, Philippe; Martins, Zita; Marty, Jean-Charles; Masters, Adam; Mimoun, David; Palumba, Ernesto; Parro, Victor; Regnier, Pascal; Saur, Joachim; Schutte, Adriaan; Sittler, Edward C.; Spohn, Tilman; Srama, Ralf; Stephan, Katrin; Szegő, Károly; Tosi, Federico; Vance, Steve; Wagner, Roland; Van Hoolst, Tim; Volwerk, Martin; Wahlund, Jan-Erik; Westall, Frances; Wurz, Peter, Europa is the closest and probably the most promising target to search for extant life in the Solar System, based on complementary evidence that it may fulfil the key criteria for habitability: the Galileo discovery of a sub-surface ocean; the many indications that the ice shell is active and may be partly permeable to transfer of chemical species, biomolecules and elementary forms of life; the identification of candidate thermal and chemical energy sources necessary to drive a metabolic activity near the ocean floor. In this article we are proposing that ESA collaborates with NASA to design and fly jointly an ambitious and exciting planetary mission, which we call the Joint Europa Mission (JEM), to reach two objectives: perform a full characterization of Europa's habitability with the capabilities of a Europa orbiter, and search for bio-signatures in the environment of Europa (surface, subsurface and exosphere) by the combination of an orbiter and a lander. JEM can build on the advanced understanding of this system which the missions preceding JEM will provide: Juno, JUICE and Europa Clipper, and on the Europa lander concept currently designed by NASA (Maize, report to OPAG, 2019). We propose the following overarching goals for our Joint Europa Mission (JEM): Understand Europa as a complex system responding to Jupiter system forcing, characterize the habitability of its potential biosphere, and search for life at its surface and in its sub-surface and exosphere. We address these goals by a combination of five Priority Scientific Objectives, each with focused measurement objectives providing detailed constraints on the science payloads and on the platforms used by the mission. The JEM observation strategy will combine three types of scientific measurement sequences: measurements on a high-latitude, low-altitude Europan orbit; in-situ measurements to be performed at the surface, using a soft lander; and measurements during the final descent to Europa's surface. The implementation of these three observation sequences will rest on the combination of two science platforms: a soft lander to perform all scientific measurements at the surface and sub-surface at a selected landing site, and an orbiter to perform the orbital survey and descent sequences. We describe a science payload for the lander and orbiter that will meet our science objectives. We propose an innovative distribution of roles for NASA and ESA; while NASA would provide an SLS launcher, the lander stack and most of the mission operations, ESA would provide the carrier-orbiter-relay platform and a stand-alone astrobiology module for the characterization of life at Europa's surface: the Astrobiology Wet Laboratory (AWL). Following this approach, JEM will be a major exciting joint venture to the outer Solar System of NASA and ESA, working together toward one of the most exciting scientific endeavours of the 21st century: to search for life beyond our own planet. © 2020, The authors received support from the sponsors of their home institutions during the development of their projects, particularly at the two institutes leading this effort: at IRAP, Toulouse, MB and NA acknowledge the support of CNRS, University Toulouse III - Paul Sabatier and CNES. At CAB, Madrid, OPB and JGE acknowledge the support of INTA and Spanish MINECO project ESP2014-55811-C2-1-P and ESP2017-89053-C2-1-P and the AEI project MDM-2017-0737 Unidad de Excelencia "Maria de Maeztu". We would also like to extend special thanks to the PASO of CNES for its precious assistance and expertise in the design of the mission scenario.