First spectral characterization at different rotational phases for the first target of a kinetic impactor mission: (65803) Didymos.

Binary NEOs represent a great opportunity for space missions towards small bodies, enabling the investigation of nature and origin of two interesting bodies together and maximizing the scientific return of the mission. Moreover, they can be used as a testing ground for planetary defense purposes. In this context, the NASA Double Asteroid Redirection Test (DART, Rivkin et al. 2021) has been approved to be the first demonstration of kinetic impactor as an hazard mitigation technique. The target chosen for this first full scale test is the (65803) Didymos binary NEO system. The DART spacecraft, launched on November 23rd 2021, will impact Dimorphos, the secondary member of the Didymos binary asteroid system, in late September 2022. Observational campaigns from Earth will help measure and characterize the actual deflection from its orbit around the primary member. The DART mission will host the ASI Light Italian Cubesat for Imaging of Asteroids (LICIACube, Dotto et al. 2021) that will be released 10 days before the impact. It will reach several important scientific goals, such as: i) witness with its optical payloads the impact of DART, ii) study the structure and evolution of the ejecta plume, and iii) acquire images of the event's aftermath on the impacted hemisphere, as well as characterizing the non-impacted one. In order to maximize the scientific results and optimize the planning of the DART/LICIACube mission, it is crucial to obtain a detailed characterization of the surface of this NEO prior to the DART impact. Remote characterization of the system will help disentangle the contribution of the primary from the secondary body and assess the heterogeneity of the surface composition. The limited compositional data available for this binary NEO suggests a possible silicate composition (De Leon et al. 2006), similar to L- and LL-ordinary chondrites (Dunn et al. 2013), the most common meteorites retrieved on Earth. During the observational window in 2021 we obtained for the first time a complete rotational characterization of the system via visible spectroscopy. While the observations confirm an affinity with silicate material and ordinary chondrites, data analysis shows a subtle but persistent spectral slope variation, computed in this case between 0.5 and 0.7 µm. This slope variation is also confirmed by comparing our most recent data with spectra obtained during the previous 2002/2003 and 2019 passages. While this variability can have multiple causes (contribution of the secondary, different viewing geometries, different degrees of surface alterations) our comparison with laboratory data possibly hints at a different concentration of hypersthene and olivine, the principal components of L-/LL ordinary chondrites. New spectral characterization in 2022 (when the system will be brighter than the last two decades) and ideally in the unexplored NIR range will be helpful to confirm these promising results, while RGB-images obtained from LUKE on board of LICIACube have a great chance to resolve this interesting conundrum.