VLT/X-Shooter spectroscopic characterisation of the Didymos system, target of the ESA Hera mission

The Didymos--Dimorphos system was the target of the NASA Double Asteroid Redirection Test (DART) mission and is also the destination of the ESA Hera spacecraft. Ground-based spectroscopy provides an insight into its overall surface composition pre- and post-impact. We aim to characterise the Didymos system by analysing ultraviolet (down to 0.32 ) to near-infrared (up to 2.40 ) m m ground-based reflectance spectra obtained before and after the DART impact, expanding the pool of available Earth-based observations of the asteroid. We present the spectra of the Didymos system over four consecutive nights (September 26--29, 2022) acquired by the X-Shooter instrument of the Very Large Telescope. We taxonomically classified the spectra and analysed their main spectral features, evaluating the characteristic band parameters of the 1 and 2 broad absorption bands, as well as the spectral slopes. m We infer the following spectral properties: an average Band I centre at 0.944 , a Band II centre at 1.995 , and a band area ratio of 0.94 , we find that the surface exhibits a degree of space weathering comparable to that of main-belt asteroids. Furthermore, a meteorite-based ordinary chondrite classification method that isolates absorption band profiles from continuum reddening favours an H-chondrite affinity for the Didymos system. Post-impact spectra show no fundamental change in mineralogy. m m Comparing these to other S-type asteroid spectra The Didymos system shows spectroscopic signatures consistent with H-type ordinary chondrites, viewed through a mature space-weathered surface. The DART impact does not reveal a different subsurface composition, in agreement with the hypothesis that Dimorphos formed from material shed by Didymos.