Physical characterization of small NEAs with early-response spectroscopy

Near-Earth Objects (NEOs) are of particular interest for science, space exploration, and planetary defence, being the objects approaching most closely to the Earth. We present the results of our early-response observing program in the frame of the EU-funded NEO Rapid Observation, Characterization and Key Simulations (NEOROCKS) project, aimed to improve our knowledge on the physical properties of NEOs. A specific challenge for NEOROCKS was to keep the physical characterization up with the increasing NEO discoveries dominated by small-size objects. In this context, prompt observations close after discovery (within a month of discovery) were executed, in order to investigate the physical nature of newly discovered small tens to hundreds of meters Near-Earth Asteroids (NEAs), the smallest bodies in the Solar System possible to observe from ground. We obtained low-resolution optical spectroscopy of 41 newly-discovered NEAs at TNG-3.5m (la Palma, Spain) between April 2021 and September 2022. Visible-range low-resolution asteroid spectra allow to properly identify a number of broad absorption features (e.g., at 0.5, 0.55, 0.60- 0.65, 0.7, 0.8-0.9 m) related to the presence of different types of anhydrous and/or hydrated minerals, hence carrying information on the asteroid composition (e.g., silicaceous, carbonaceous, metallic, etc.) and thermal history. Constraining surface chemical composition of NEAs has primordial importance for planetary defense purposes, such as for risk assessment and eventual developing mitigation strategies. Taxonomic classification gives important clues on albedo, and thus permits to constrain i) asteroids size and ii) non-gravitational effects on dynamical evolution, e.g. the Yarkovsky effect. For the taxonomic classification we use the established Bus-DeMeo taxonomy (DeMeo et al. 2009, Icarus, 202, 160) and the newer Classy scheme (Mahlke et al. 2022, AA, 665, A26). We found that our sample represents a variety of taxonomies with 20% of objects belonging to rare types. 90% of our sample has an estimated diameter D < 350 m. We discuss the taxonomic distribution of NEAs with spectroscopic characterization, putting together our new dataset and published literature data, in relation to their dynamical properties. The investigation of the physical properties of the small NEAs is particularly important as it gives insights into i) the asteroidal contribution to the delivery of prebiotic material (water and organics) to our planet, and ii) the different evolutionary histories of asteroids as a function of their size. We combine taxonomic classification and accessibility information as input for future exploration /mitigation space missions.Acknowledgement: This project has received funding from the European Unions Horizon 2020 research and innovation programme under grant agreement No 870403 (project NEOROCKS).