The study of near-Earth asteroids (NEAs) is very important, due to three main reasons. First, they pose an impact risk for Earth, and damage produced by a possible collision will depend on the speed of the impact, the size of the object, and its composition. Second, given their proximity, they are easy access targets for the visit of space missions. And third, these objects are possible sources of in-situ resources for future space missions (or ISRU, for in-situ resource utilization), that would enable the affordable establishment of extraterrestrial exploration and operations by minimizing the materials carried from Earth.
Space missions OSIRIS-REx (NASA, launched on 2016) and Hayabusa 2 (JAXA, launched on 2014) will visit two primitive NEAs: (101955) Bennu and (162173) Ryugu, respectively. The main goal of these missions is to collect a sample of surface material from these NEAs and bring it back to Earth for analysis. These asteroids are considered “primitive” due to their low albedo and carbonaceous composition, inferred from their spectral characteristics, similar to those of the carbonaceous chondrites (the most primitive meteorites, that have undergone almost no evolution since the early stages of the Solar System). Several dynamic studies suggest that these NEAs originate in the inner part of the main asteroid belt, within the low-albedo low-inclination families (i<8º): the Polana–Eulalia complex, and the Erigone, Sulamitis, and Clarissa families.
There is a significant lack of compositional information for asteroids in the primitive families in the main belt, and therefore any spectroscopic or photometric survey in this sense will be a major contribution. The link between primitive NEAs and primitive families is suggestive but not definitive, so it needs confirmation and detailed study. The main objective of this thesis work is to study and characterize the most likely origin of (101955) Bennu and (162173) Ryugu, targets of the sample-return space missions OSIRIS-REx and Hayabusa 2, as well as to provide information about other possible sources of primitive asteroids throughout the main belt.
This thesis work is a direct follow-up of the studies on the Polana–Eulalia complex, the low-albedo (pv < 0.1) part of the Nysa family, located in the inner region of the main belt. This initial study showed that the Polana–Eulalia complex is mainly composed of C- and B-type asteroids (primitive objects), presenting spectral homogeneity.
This thesis is a compendium of three papers, published in the Astronomy & Astrophyisics journal. Two different approaches to the analysis have been used: (1) through visible spectroscopy (0.5–0.9 μm), using data obtained with the Gran Telescopio Canarias (GTC), we analayzed three asteroid families located in the inner region of the main belt (Erigone, Sulamitis, and Clarissa); and (2) through near-infrared photometry (0.9–2.4 μm) we analyzed the asteroid families observed in the MOVIS (Moving Objects from VISTA) catalog.
The results regarding the Erigone collisional family showed that, approximately, 87% of the observed objects have typical primitive asteroid spectra, consistent with the spectrum of (163) Erigone, the parent body. In addition, we found a significant percentage of family members (50%) with spectral features related to the presence of hydrated minerals on their surfaces. After the analysis of the Sulamitis and Clarissa families, the smallest of the possible NEAs sources, we found that about 60% of the observed asteroids in the Sulamitis family show signs of aqueous alteration on their surfaces, while almost all of the objects in the Clarissa family do not show any evidences of hydration. Also, according to the visible spectra obtained for the members of the primitive families in the inner main belt studied so far, we can differentiate between two groups: the Polana-like group (Polana and Clarissa), which presents homogeneous, featureless spectra in a continuum of slopes from blue to moderately red, and no aqueous alteration, and the Erigone-like group (Erigone and Sulamitis), showing a spectral diversity among primitive taxonomic classes and a majority of spectra with the aqueous alteration band. The study on the near-infrared data of the MOVIS catalog allowed us to characterize 43 families through the whole main belt: 15 primitive, 19 rocky, and 9 that were defined as “mixtures” of the proposed theoretical compositions.
The results of this thesis constitute one of the most significant contributions that have been made up to now to the compositional study of primitive asteroid families. Besides, the investigations about the most probable origins of NEAs (101955) Bennu and (162173) Ryugu will be of great help to test the dynamical models which explain the asteroidal transport routes from the main belt to the near-Earth space. Our results will be specially relevant for the interpretation of the images and spectra obtained in situ by the spacecrafts OSIRIS-REx and Hayabusa 2 during their encounters with their targets, placing the mission’s data within the big picture of the evolutionary history of the Solar System
