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Context. At present, near-Earth objects (NEOs) are being discovered at an ever-increasing rate. However, their physical characterisation is still significantly lagging behind. In particular, the taxonomic classification of newly discovered NEOs is of great importance with regard to improving our understanding of the population of NEOs. Aims. In this context, our goal is to probe potential links between orbital properties of NEOs and their composition. We investigate whether we can make a reasonable guess about the taxonomic class of an NEO upon its discovery with a decent orbital accuracy. Methods. We used a G-mode multivariate statistical clustering method to find homogeneous clusters in a dataset composed of orbital elements of NEOs. We adopted two approaches, using two sets of variables as inputs to the G-mode method. In each approach, we analysed the available taxonomic distribution of resulting clusters to find potential correlations with several unique parameters that distinctively characterise NEOs. We then applied a dynamical model on the same clusters to trace their escape regions. Results. Approach 1 (A1) led us to obtain NEO clusters that can be linked to a primitive composition. This result was further strengthened by the dynamical model, which mapped outer-belt sources as escape regions for these clusters. We remark on the finding of a cluster akin to S-type NEOs in highly eccentric orbits during the same approach (A1). Two clusters, one with small NEOs in terrestriallike orbits and one with relatively high inclinations, were found to be common to both approaches. Approach 2 (A2) revealed three clusters that are only separable by their arguments of perihelion. Taken altogether, they make up the majority of known Atira asteroids. Conclusions. For an NEO whose orbit is relatively well determined, we propose a model to determine whether the taxonomy of an NEO is siliceous or primitive if the orbital elements of the NEO fall within the presented combinations of inclination, eccentricity, and semi-major axis ranges.

Introduction The study of near-Earth objects (NEOs) is crucial to better understand the origin, formation and the evolution of the solar system. In particular, compositional, morphological and orbital characterisation of NEOs sheds light on the delivery of water and organics [1,2,3] to the prebiotic Earth, while ironically, some NEOs could be potential hazards for life on Earth [4], as it has been witnessed in the past during impacts. Furthermore, these objects are also of interest for the future of humankind, for they could be useful as vital resources during interplanetary travel. Given this context, we apply the G-mode multivariate statistical clustering method [5,6,7] on the orbital parameters of the currently available NEOs population, to probe potential associations with their spectral classification [8,9,10]. Data and methods We apply the G-Mode multivariate statistical clustering analysis to selected orbital elements of NEOs to determine any dynamical clustering of objects. Once the clusters of objects are found, we proceed to investigate whether they have any correlations with spectral classes. The G-mode method leads to an automatic statistical clustering of a sample containing N objects (NEOs in this case), described by M variables (orbital elements) with the only control imposed by the user being the confidence level q1, expressed in terms of σ. Our sample consists of 10669 NEOs belonging to the dynamical groups Atiras, Atens, Apollos and Amors, available from the Minor Planet Center, filtered based on their orbital uncertainty (excluding those with an uncertainty parameter > 4). Our input parameters to the G-mode method are twofold. First, we use three variables: inclination (i), eccentricity (e) and semi-major axis (a) of the orbit as these are the main three parameters that define an orbit around the Sun. Secondly, we include three pseudo-parameters: mean orbital intersection distance with respect to the Earth (eMOID), perihelion distance (q) and aphelion distance (Q) of the orbit, in addition to the aforementioned three parameters, thus using six variables. Preliminary results Using i, e and a of the NEOs in our sample as inputs for G-mode, we obtain three clusters of NEOs at q1=1.9σ (with an accurate classification probability of 94.26%). The mean parameter values of each cluster with the median absolute deviation are given in Table 1. We have also reported some other parameters of interest, which include, Tisserand parameter with respect to Jupiter (TJ) and the absolute magnitude H. At this criterion, the vast majority of objects are clustered in the cluster #1. The cluster #2 with only 20 objects, appears interesting, as it is constrained by low-inclined, quasi-circular Earth-like orbits. The objects of the final cluster #3 are constrained by their relatively larger inclinations. We next used six variables: i, e, a, eMOID, q and Q as inputs for G-mode, while still holding q1 fixed at 1.9σ, in which case six clusters are found as reported in Table 2. Among the reported clusters, clusters #3,4 and 6 are of particular interest, for they could be associated with Jupiter-family cometary nuclei (2 < TJ < 3) as per their Tisserand parameter with respect to Jupiter. As such, the objects in these three clusters could potentially be extinct cometary nuclei. Interestingly, these clusters also have relatively higher eccentricities. We have checked available taxonomic classifications of NEOs [11,12,13] in the literature to get an insight into the composition of the objects found in our G-Mode clusters. Although taxonomic classifications are not available for the majority of members in the clusters, we find that (i) cluster #3 contains 4 C-type objects, (ii) cluster #4 contains 1 C-type, 2 D-type, 1 L-type, 5 S-type and 1 X-type objects, (iii) cluster #5 contains 1 B-type, 1 C-type, 1 T-type and 1 X-type objects. Apart from the S-type objects, the others usually have dark and red spectra indicative of primitive origin, which does not reject a cometary composition. We will augment this on-going study with more data and final results will be presented and discussed. Acknowledgements We acknowledge the financial support from Agenzia Spaziale Italiana (ASI, contract No. 2017-37-H.0 CUP F82F17000630005). We also acknowledge funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No. 870403. References [1] Marty, B., Guillaume, A. et al. 2016, Earth and Planetary Science Letters, Volume 441, Pages 91-102 [2] Altwegg, K, Balsiger, H, Bar-Nun, A. et al. 2015, Science, Vol. 347, Issue 6220, 1261952 [3] Ehrenfreund, P. & Sephton 2006, Faraday Discuss., The Royal Society of Chemistry, 133, 277-288 [4]Perna, D., Barucci M. A., Fulchignoni M. 2013, Astronomy and Astrophysics Review, Vol. 21,65 [5] Barucci, M.A., Capria, M.T., Coradini, A. et al. 1987, Icarus 72, 304 [6] Gavrishin, A. I., et al. 1992, Earth, Moon and Planets, 59, 141-152 [7] Barucci, M.A., Belskaya, I., Fulchignoni, M. et al. 2005, AJ 130, 1291 [8] Bus, S. J., & Binzel, R.P. 2002, Icarus 158, 146 [9] DeMeo, F. E., Binzel, R. P., Slivan, S. M. & Bus, S. J. 2009, Icarus, 202, 160-180 [10] DeMeo, F. E., Alexander, C. M. O., Walsh, K. J.et al. 2015, Asteroids IV, 13-41 [11] Perna, D., Barucci, M.A. et al. 2018, Planetary and Space Science, 157, 82-95 [12]Devogèle, M., Moskovitz, N. et al. 2019, The Astronomical Journal, American Astronomical Society, 158, 196 [13] Ieva, S., Dotto, E. et al. 2020, A&A, 644, A23

This thesis is based on the spectrophotometric properties of the comet 67P/Churyumov-Gerasimenko, using the OSIRIS instrument of Rosetta space mission. Composed of two scientific cameras to observe the nucleus and the coma of the comet, OSIRIS images are acquired with multiple filters, that span the near-UV to near-IR wavelength range. They were used to study the spectrophotometric curves of the exposed bright features that appeared on the surface of the cometary nucleus, leading to a comparative study, that was carried out in collaboration with the VIRTIS spectro-imager aboard Rosetta, that demonstrated, that these exposures are related to H2O ice, using its absorption band located at 2 microns. The thesis further details a spectrophotometric study of the Khonsu region in the southern latitudes of the comet, where the seasonal variation of the spectral slope of different types of terrains is explored. Finally, the results of an extended survey of exposed bright features are presented. More than 50 individual features are presented under four morphologies along with an albedo calculation, suggesting that different activity sources are responsible for their appearance on the nucleus.; Cette thèse s'inscrit dans le cadre de la mission spatiale Rosetta et porte sur les propriétés spectrophotométriques de la comète 67P/Churyumov-Gerasimenko à l’aide de l’instrument OSIRIS. Cet instrument est composé de deux caméras pour les observations du noyau et de la coma de la comète. Elles permettent d’acquérir des images avec des filtres qui opèrent dans la gamme du proche UV au proche IR. Dans un premier temps, j'ai analysé les courbes spectrophotométriques des taches claires qui sont apparues sur le noyau de la comète. Une étude comparative de celles-ci grâce aux données du spectro-imageur VIRTIS a ainsi permis de constater que les taches claires sont liées à la glace de H2O. Dans un second temps, j’ai entrepris une étude spectrophotométrique de la région Khonsu, qui a mis en évidence les variations saisonnières de la pente spectrale de différents terrains. Par la suite, j’ai élargi mon analyse des taches à tout le noyau de la comète. J’ai détecté plus de 50 taches claires dues à la présence de glace de H2O et j’ai produit une carte pour repérer leurs emplacements sur le noyau, afin d’étudier plus en détail leur répartition et leur évolution au cours de temps. Ceci m’a permis d’identifier quatre types de taches regroupés en fonction de leur morphologie et de constater qu'elles sont dues à différentes sources d'activité cométaire.