Spectral studies in view of the Hera mission

Didymos is a binary asteroid system composed of (65803) Didymos and its small satellite, Dimorphos. The NASA’s DART planetary defense mission demonstrated the possibility of asteroid deflection by changing the Dimorphos orbit through an artificial impact. Understanding the physical and compositional characteristics of the target is crucial for analyzing the outcomes of this impact experiment. The Hera mission, developed by the European Space Agency (ESA), was launched in October 2024, to study the Didymos system and to investigate the aftermath of the DART impact on Dimorphos, assessing any changes to its orbit and surface characteristics. The mission will gather critical data regarding the composition and structure of both Didymos and Dimorphos, providing invaluable insights into the nature of these small solar system bodies. By enhancing our knowledge of these asteroids, Hera will play a pivotal role in refining our planetary defense strategies and improving our understanding of the building blocks of the solar system. This dissertation aims to conduct spectroscopic studies in preparation for the Hera mission, with a particular focus on the ASPECT spectrometer. It begins with the search for meteorite which best reproduce Didymos; this selection will be very useful in preparation for Didymos studies, in particular to assess space weathering effects (i.e., solar wind, micrometeoroid bombardments and cosmic ray effects) and photometric behaviors (i.e., changes in the spectral shape due to different phase angles of observation) on spectroscopic data. Additionally, identifying these analogues may assist in calibrating the instruments aboard the Hera mission. We utilized two online databases of meteorite spectra along with recent spectra of Didymos captured both before and after the DART impact. Our initial selection was carried out by comparing the band center values of the silicate absorption bands, localized at 1 and 2 μm, between Didymos and the meteorites. In the second selection a four-dimensional parameter space was defined, where the dimensions corresponded to the band depth and the slope of the two absorption bands, each normalized to Didymos values. We then identified the meteorites closest to Didymos within this space. A final selection took into account multiple spectra of the same meteorite (especially those from different databases), the match in the Mid Infrared (MIR), and the comparison with the literature. One of the results of this work is a list of six meteorites that are the most analogous to the Didymos system. We also found out that Didymos is most probably mainly composed of L/LL ordinary chondrites, with a preference for the LL sub-type. From our list of meteorites, we were able to estimate the normalized abundance of olivine and pyroxene of Didymos. Finally, a match between Didymos and OC meteorites was also found in the Mid-InfraRed (MIR) range. Another goal of this thesis is the definition of new spectra parameters to maximize the scientific return from the ASPECT spectrometer on board Hera, by inferring the Didymos system composition of the olivine and pyroxene relative abundance (olv/(olv +pyx)). ASPECT data range from the visible up to shortwave infrared (SWIR) wavelengths (650-2500 nm) using four different channels. The SWIR channel, being a single-pixel channel, has a much lower spatial resolution than the other three channels. Consequently, for the highestresolution mapping of the Didymos surface, we concentrated on the first three channels (650–1600 nm). So, new spectral parameters should be defined within this limited spectral range. For this reason, we first analyzed spectral parameters over a broader range commonly used in the literature to determine the composition of S-type asteroids. We then developed new methods specific to the ASPECT instrument’s spectral range to determine the mineral composition of Didymos and, more broadly, of S-type asteroids or other type of asteroids with a silicate surface (such as Vesta). These methods were initially established using binary mineral mixtures, then calibrated and validated with ordinary chondrite meteorites. To further validate these methods, we applied them to other asteroids previously visited by spacecraft, Vesta and Itokawa. The result of this work is a comprehensive analysis procedure, designed for ASPECT data, that enables a detailed mapping of mineral distributions across the asteroid’s surface. Finally, a method to retrieve and study hydroxyl signatures was defined (in view of the Didymos exploration) and applied to Vesta. We took advantage of the newly calibrated data of the VIR spectrometer onboard the NASA’s Dawn mission to characterize spectral features not or weakly detected in previous studies thanks to the improved signal-to-noise (S/N) ratio for these spectra. The goals were to confirm, reinforce and characterize the OH distribution on Vesta by studying the 2.8 μm band and to look for OH combination bands around 2.2–2.4 μm by defining a spectral analysis procedure that could be applied also to the ASPECT data. The analysis of hydroxyl absorption bands revealed an anti-correlation between the abundance of hydroxyl-bearing molecules and surface reflectance, supporting the link between hydroxyl presence and Vesta’s dark units. This finding reinforces the hypothesis that the dark areas on Vesta are due to exogenous carbonaceous chondrites, which are often characterized by hydroxyl-bearing molecules. Otherwise, no 2.2–2.4 μm feature associated with OH was detected, indicating that it is not related to hydroxyl presence. Nonetheless, the method developed for investigating this feature will still be applied to ASPECT data for further analysis of the Didymos asteroid. In conclusion, this research not only provides critical insights into the composition and characteristics of Didymos but also establishes methodologies that can be instrumental in analyzing the data that will be obtained from the Hera mission. The findings and techniques developed herein serve as a foundation for enhanced understanding of small solar system bodies and their roles in planetary science.