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The Hera mission contributes to the international effort towards the validation of the kinetic impactor asteroid deflection technique by retrieving all the physical and dynamical properties of Dimorphos in order to validate numerical impact codes. In particular, Hera’s core objectives from the point of view of the deflection demonstration are the following: (i) Measuring the mass of Dimorphos to determine the momentum transfer efficiency from DART to the asteroid; (ii) Investigating the crater in detail to improve our understanding of the cratering process and the mechanisms by which the crater formation drives the momentum transfer efficiency; (iii) Observing subtle dynamic effects (e.g. libration imposed by the impact, orbital and spin excitation of the secondary) that are difficult to detect for remote observers; (iv) Characterising the physical properties of Dimorphos (including size, shape, volume, density, porosity, size distribution of surface material) to allow scaling of the momentum transfer efficiency to different asteroids. In addition, the Hera mission will allow the demonstration of two key technologies for future deep-space missions: (v) The use of CubeSats for multipoint investigations operated via an inter-satellite network link via Hera; (vi) Autonomous visual-based navigation for very low altitude flybys over the surface of Dimorphos. The Hera spacecraft will launch in October 2024 onboard an Ariane 6 launcher with an 18-days launch window. The trajectory foresees a Mars swing-by in mid-March 2025 and the rendezvous phase with Didymos starting end-December 2026. Following the operationally safe capture sequence, the asteroid close-proximity operation phase will start from a gate position of about 30 km. Operations will continue for about 6 months and allow for detailed investigations of the Didymos surface down to few kilometres or less from the surface of Dimorphos depending on the performance of the onboard feature-tracking navigation system. The mission is currently in phase B2 with OHB System as prime contractor together with GMV, QinetiQ, Spacebel and OHB-I as core team members. The preliminary design review is schedules in October 2020. The paper will provide an overview of the mission together with its latest system and payload configuration.

The Hera mission is the planetary defence mission approved under the ESA Space Safety Programme. Together with the NASA DART mission, it is the first Asteroid Deflection Demonstration. DART will impact Dimorphos, the natural satellite of Near Earth Asteroid Didymos, in 2022. Using the kinetic impactor technology, it will demonstrate the ability to change the trajectory of a small asteroid. Hera will launch in 2024 and arrive in the Didymos system at the end of 2026. Its primary goals are to accurately measure the mass of Dimorphos and to characterise the crater and effect of the DART impact on this small asteroid. Hera will spend over 2 years matching orbits with Didymos, travelling through near-Earth space and the inner main-belt. This potentially allows Hera to perform an additional asteroid flyby enroute to Didymos. For such a flyby, a number of factors need to be considered for target selection. Primary factors are the accuracy of the orbit of the asteroid, and distance from the nominal trajectory. Resolution of geomorphology requires a minimum number of spatial resolution elements at the desired flyby distance. Previous observations on the form of lightcurves, colours or spectra are important for flyby science planning. Finally, a dynamical class of asteroid may be targeted for specific science priorities. Analysis of the nominal trajectory shows approximately 100 asteroids passing the nominal position of Hera within 0.02 au. Ten of these potential targets are Near-Earth Asteroids, the rest being Mars-crossers and main-belt asteroids. We will present our current knowledge on these targets, and highlight the highest priority targets for further observation in the next 2-3 years. Although ground-based reconnaissance has started on some targets, further observations are required to support the flyby planning process. Figure 1. Osculating orbital elements of asteroids passing close to Hera on its nominal trajectory to the Didymos system, without any additional trajectory adjustment. Red dots indicate a current flyby distance d < 0.01 au, blue circles indicate a current flyby distance 0.01 au < d < 0.02 au. The solid line indicates a perihelion distance of 1.3 au, asteroids to the left of this line are Near-Earth Asteroids.