The Galilean moons of Jupiter are known to have atmospheres and ionospheres, detected with both ground-based observations and spacecraft data. An oxygen-hydrogen atmosphere was discovered on Ganymede with observations by the Hubble Space Telescope (Hall et al., 1998). Ganymede is a unique object in the solar system in that it has its own intrinsic magnetic field which interacts with the Jovian magnetosphere (Kivelson et al., 1997). Within the open field line regions at higher latitudes, sputtering generates an atmosphere of molecular oxygen subject to ionization and dissociated excitation from the Jovian magnetosphere (Eviatar et al., 2001). Within closed field line regions, it is expected the atmosphere is produced by sublimation (Eviatar et al., 2001). It is thought the ionosphere is generated from the neutral atmosphere via photoionization and electron impact from the Jovian magnetosphere (Carnielli et al., 2019). Prior to Juno's encounter with Ganymede, the only direct measurements of Ganymede's ionosphere were those acquired in-situ measurements from the Galileo particle detectors and by the Galileo radio occultation experiment. Due to the flyby distance of the in-situ spacecraft measurements, radio occultation data provide valuable information about the electron densities near the surface of Ganymede.The Galileo spacecraft executed a total of eight S-band radio occultations of Ganymede throughout its mission, resulting in five non-detections, two weak detections, and one strong detection of an ionosphere (McGrath et al., 2004). To the best of our knowledge, the Galileo radio science data at Ganymede were never archived. In particular with respect to Ganymede, only occultation profiles from the G8 encounter were ever published in scientific literature. The strong ionosphere detection occurred during the Ganymede G8 egress occultation resulting in a peak electron density of ∼5,000 cm −3 near the surface (Kliore, 1998). Initially, the lack of detection was surprising, but it was hypothesized that positive detections occurred where the trailing hemisphere (where the

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<p><strong>Abstract</strong><br />The Asteroid Impact and Deflection Assessment (AIDA) is an international collaboration supported by ESA and NASA to assess the feasibility of the kinetic impactor technique to deflect an asteroid, combining data obtained from NASA’s DART and ESA’s Hera missions [1, 2]. In 2022, DART will perform a kinetic impact on the secondary of the binary near-Earth asteroid (65,803) Didymos, recently named Dimorphos. After 2 years, Hera will follow-up with a detailed post-impact survey of Didymos, to fully characterize this planetary defense technique. Additionally, Hera will deploy two CubeSats around Didymos once the Early Characterization Phase has completed, to complement the observations of the mother spacecraft and increase the scientific return of the mission. The first Cubesat, called Juventas, will complete a low-frequency radar survey of the secondary, to unveil its interior, while the second one has not yet been selected.<br />One of the main objectives of Hera is to characterize the mass and mass distribution of both Didymos primary and secondary by means of radio science investigations. This paper describes the concept of the gravity science investigations to be jointly carried out by the three mission elements, i.e. Hera, Juventas and CubeSat-2. The experiment will combine classical ground-based radiometric measurements, spacecraft-based optical images of Didymos, and Satellite-to-Satellite radiometric tracking between Hera and the Cubesats. Finally, our results and achievable accuracy for the estimation of the mass and gravity field of Didymos and Dimorphos are presented.</p> <p><strong>1. Introduction</strong><br />Most of the information about the formation processes of an asteroid lies in its interior structure. One of the very few constraints of the internal mass distribution of a celestial body is given by its gravity field, even if the inversion process is not unique. First, the bulk density can be inferred by measuring the mass of the body, combined to the volume estimated from optical images. In addition, the higher degrees of the gravity field provide information about the internal distribution of mass, such as the moments of inertia.<br />The main scientific goals of the Hera radio science investigations are:</p> <ul> <li>Determine the mass and gravity field of Didymos and Dimorphos;</li> <li>Reconstruct the motion of Dimorphos around Didymos;</li> <li>Contribute to the characterization of the energy transfer between DART and Dimorphos.</li> </ul> <p>Such objectives are a valuable contribution to the Hera mission objectives, leading to a better understanding of the formation and evolution processes of the Didymos system.</p> <p><strong>2. Technique</strong><br />The determination of the gravity field of a celestial body is an application of the orbit determination process of deep space spacecraft. In particular, the gravity of Didymos can be estimated precisely reconstructing the trajectory of Hera during a selected number of close encounters (about 10 km at closest approach). The classical observables used in the orbit determination are obtained from the X-band radio link between the spacecraft and the Earth. The microwave signal is sent to spacecraft from a ground antenna and coherently retransmitted back to Earth, where Doppler and range measurements are obtained. A previous study performed for the AIM proposed mission [4] demonstrated that gravity science at Didymos is feasible using radio tracking data only, under realistic assumptions on the technological capabilities of the space and ground segment. Shorter pericenter distances increase the attainable accuracy. However, a significant improvement can be obtained even at relatively large distances processing also optical images of Didymos and Dimorphos taken by the spacecraft.<br />In addition, Hera may track Juventas and CubeSat-2 by means of a space-to-space inter-satellite link (ISL), capable of determining the relative distance (ranging) and the relative line-of-sight velocity (Doppler) between the two bodies. In particular, the latter is expected to represent a very nice add-on to the gravity investigation carried out by the Hera mission, as the Doppler shift that affects the inter-satellite link contains the information on the dynamics of the system, i.e. masses and gravity field of Didymos and Dimorphos.<br />The expected accuracy in the estimation of Didymos gravity fields were obtained through numerical simulations of the orbit determination of Hera and the two Cubesats. Conservative assumptions were made in terms of both radiometric and optical measurement noises, and large a-priori uncertainties for the estimated parameters were used.  </p> <p><strong>3. Results</strong><br />As a result of the numerical simulations, the masses of Didymos and Dimorphos are expected to be estimated with relative uncertainties less than 10<sup>-4</sup> and 10<sup>-3</sup>, respectively. The addition of the ISL measurements improves the achievable accuracies but it is not required to estimate the masses. However, given the relatively large distance of Hera from the system, the higher degree gravity of Didymos and Dimorphos can be estimated only adding the ISL Doppler measurements between the Cubesats and the mother spacecraft. In this case, the gravity field of Didymos can be estimated to at least degree 3, depending on the assumptions about the ISL operations and performance. Similarly, ISL Doppler measurements allows to estimate the extended gravity field of Dimorphos up to degree 2, with an uncertainty of about 10%.</p> <p><strong>Acknowledgements</strong></p> <p>This project has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No 870377 (project NEO-MAPP). MZ, IG, ML, EG, RLM, and PT wish to acknowledge Caltech and the Jet Propulsion Laboratory for granting the University of Bologna a license to an executable version of MONTE Project Edition S/W.</p> <p><strong>References</strong><br />[1] Cheng A. F., et al., “AIDA DART asteroid deflection test: Planetary defense and science objectives,” Planet. Space Sci., vol. 157, no. February, pp. 104–115, 2018.<br />[2] Michel, P., et al. “European component of the AIDA mission to a binary asteroid: Characterization and interpretation of the impact of the DART mission”. Adv. Space Res. (2018), Volume 62, Issue 8, pp. 2261-2272.<br />[3] Lasagni Manghi, R., Modenini, D., Zannoni, M., Tortora, P., “Preliminary orbital analysis for a CubeSat mission to the Didymos binary asteroid system”, Adv. Space Res. (2018), Volume 62, Issue 8, pp 2290-2305<br />[4] M. Zannoni, G. Tommei, D. Modenini, P. Tortora, R. Mackenzie, M. Scoubeau, U. Herfort, I. Carnelli, “Radio science investigations with the Asteroid impact mission”, Adv. Space Res. (2018), Vol. 62, Issue 8, pp. 2273-2289.</p>

<p>The Asteroid Impact and Deflection Assessment (AIDA) is an international collaboration supported by ESA and NASA to assess the feasibility of the kinetic impactor technique to deflect an asteroid, combining data obtained from NASA’s DART and ESA’s Hera missions. Together the missions represent the first humankind’s investigations of a planetary defense technique. In 2022, DART will impact Dimorphos, the secondary of the binary near-Earth asteroid (65803) Didymos.  After 4 years, Hera will follow-up with a detailed post-impact survey of Didymos, to fully characterize and validate this planetary defense technique. In addition, Hera will deploy two CubeSats around Didymos once the Early Characterization Phase has completed, to augment the observations of the mother spacecraft. Juventas, the first Cubesat, will complete a low-frequency radar survey of the secondary, to unveil its interior. Milani, the second Cubesat, will perform a global mapping of Didymos and Dimorphos, with a focus on their compositional difference and their surface properties. One of the main objectives of Hera is to determine the binary system’s mass, gravity field, and dynamical state using radio tracking data in combination with imaging data. The gravity science experiment includes classical ground-based radiometric measurements between Hera and ground stations on Earth by means of a standard two-way X-band link, onboard images of Didymos, and spacecraft-to-spacecraft inter-satellite (ISL) radiometric tracking between Hera and the Cubesats. The satellite-to-satellite link is a crucial add-on to the gravity estimation of low-gravity bodies by exploiting the Cubesats’ proximity to the binary, as the range-rate measurements carried out by the inter-satellite link contain information on the dynamics of the system, i.e., masses and gravity field of Didymos primary and secondary.</p><p>We will describe the updated mission scenario for the Hera radio science experiment to be jointly carried out by the three mission elements, i.e., Hera, Juventas and Milani. To conclude, our updated analysis and latest results, as well as the achievable accuracy for the estimation of the mass and gravity field of Didymos and Dimorphos, are presented.</p>