DIDYMOS AT DIFFERENT ROTATIONAL PHASES: a subtle but persistent spectral variability

Introduction: Near-Earth objects (NEOs), due to their proximity to our planet, represent one of the most accessible bodies in the whole Solar System. Their investigation can offer answers to several pressing questions in modern planetology (regarding, e.g., planetary formation, delivery of water and organics to the early Earth, and emergence of life). However, NEOs can also represent a risk for human civilization, since few of them can be potential impactors [1]. The binary Didymos system: The binary NEO Didymos is composed of a main body (68503) Didymos and its satellite, later renamed Dimorphos. Recent studies estimate that nearly 15% of large NEOs (> 200 m) should be binaries [2]. At the moment, the most current explanation involves the reaccumulation of a body following a rotational disruption, probably as a result of the Yarkovsky-O'Keefe-Radzievskii-Paddack (YORP) effect. However, other mechanisms are predicted for different sizes and populations of small bodies (capture, collision and tidal processes) [3]. Binary NEOs can also be helpful to demonstrate the applicability of the most mature technique for impact mitigation, i.e. the kinetic impactor [4]. To this purpose, the NASA Double Asteroid Redirection Test (DART) has been approved to be the first demonstration of an hazard mitigation of an asteroid by using a kinetic impactor [5]. The DART/LICIACube mission: The DART spacecraft, launched on November 23rd 2021, will impact Dimorphos, the smaller member of the Didymos binary asteroid system, on September 26th 2022. Hosted as a piggyback during the 15 months of DART interplanetary cruise there will also be LICIACube, the Light Italian Cubesat for Imaging of Asteroids [6], which is a 6U cubesat space mission supported by the Italian Space Agency (ASI). LICIACube will be released in the proximity of the target and will perform an autonomous fly-by of the Didymos system, probing the DART impact and reaching several scientific goals, such as: i) witness with its optical payloads (LUKE & LEIA, [6]) 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. Ground-based characterization of the Didymos system prior to the impact: Data available in literature for the target of the DART/LICIACube mission, the binary NEO 65803 Didymos, is scarce. Only few spectra were taken during the latest observational windows in 2003 and 2019. The limited compositional data available for this NEO suggests a possible silicate composition [7], and an affinity with its best meteorite analogue, the L- and LL-ordinary chondrites [8]. A detailed spectral characterization in this case is crucial due to its binary nature, in order to disentangle the contribution of the primary from the secondary body and asses the heterogeneity of the surface composition in the light of the DART/LICIACube mission. A potential heterogeneity on Didymos: During the latest observational window in 2021 (the last before the DART impact) 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 2003 and 2019 passages.