Ultraviolet to near-infrared spectroscopy of the potentially hazardous, low delta-V asteroid (175706) 1996 FG3. Backup target of the sample return mission MarcoPolo-R

Context. Primitive near-Earth asteroids (NEAs) are important subjects of study for current planetary research. Their investigation can provide crucial information on topics such as the formation of the solar system, the emergence of life, and the mitigation of the risk of asteroid impact. Sample return missions from primitive asteroids have been scheduled or are being studied by space agencies, including the MarcoPolo-R mission selected for the assessment study phase of ESA M3 missions. Aims: We want to improve our knowledge of the surface composition and physical nature of the potentially hazardous, low delta-V asteroid (175706) 1996 FG3, backup target of MarcoPolo-R. This intriguing object shows an as-yet unexplained spectral variability. Methods: We performed spectroscopic observations of 1996 FG3 using the visible spectrograph DOLORES at the Telescopio Nazionale Galileo (TNG), and the UV-to-NIR X-Shooter instrument at the ESO Very Large Telescope (VLT). Results: We find featureless spectra and we classify 1996 FG3 as a primitive Xc-type in the Bus-DeMeo taxonomy. Based on literature comparison, we confirm the spectral variability of this object at near-infrared (NIR) wavelengths, and find that spectral variations exist also for the visible spectral region. Phase reddening cannot explain such variations. Obtained with the same observational conditions for the whole 0.3-2.2 ?m range, our X-Shooter spectrum allowed a proper comparison with the RELAB meteorite database. A very good fit is obtained with the very primitive C2 Tagish Lake carbonaceous chondrite (pressed powder), confirming 1996 FG3 as a suitable target for a sample return mission from primitive NEAs. Conclusions: We hypothesize a compacted/cemented surface for 1996 FG3, like that observed by the Hayabusa mission on (25143) Itokawa, with the possible presence of regions showing different degrees of surface roughness. This variegation could be related to the binary nature of 1996 FG3, but to check this hypothesis further observations are necessary. Based on observations carried out at the European Southern Observatory (ESO), Chile (programme 088.C-0695), and with the Italian Telescopio Nazionale Galileo (TNG) operated on the island of La Palma by the Fundacion Galileo Galilei of the INAF (Istituto Nazionale di Astrofisica) at the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofisica de Canarias (programme AOT25/TAC13).