The study of primitive asteroids has gained much momentum in the last decade. The term "primitive" usually refers to objects with spectral characteristics similar to those of the carbonaceous chondrites, the most primitive meteorites. These meteorites contain pristine materials that have undergone very little processing since their formation, some of then even predating the formation of the Solar System. Therefore, the study of primitive asteroids can reveal information about the origin and evolution of our planetary system since its early stages.

Our objective in this thesis is to characterise key physical properties of a selection of primitive asteroid groups in order to advance our knowledge about their nature. We derived asteroid sizes, visible (0.55 micron) and infrared (3.4 micron) albedos by fitting an asteroid thermal model to their infrared data provided by the Wide-field Infrared Survey Explorer (WISE). A 40-cm space telescope launched by NASA in 2010, WISE obtained infrared flux measurements of more than 150,000 asteroids in four broad-band filters with wavelengths given by W1 = 3.4, W2 = 4.6, W3 = 12, and W4 = 22 micron.

We focused on three selected groups of primitive asteroids and a primitive near-Earth asteroid. First, we studied the statistical properties of those asteroids taxonomically classified as B-types, an interesting and poorly known group of asteroids with negative (blue) visible spectral slopes, some of which have been spectrally linked to the most primitive meteorites. These asteroids present puzzling properties, such as their significantly high density compared to other C-complex asteroids, or the fact that they have recently been found to have a wide range of spectral slopes in the near-infrared wavelength range. By combining their 3-micron reflectances computed from WISE W1-band data with published spectral observations from visible to near-infrared wavelengths, we found that the W1 reflectances of the B-type population tend to be lower than their 2.5-micron reflectances. Our interpretation of these statistics, which has been recently confirmed from spectroscopic observations, is that a substantial fraction of these objects have 3-micron absorption features caused by the presence of water on their surfaces. Said water can either be in the form of ice or bound within the lattice structure of the minerals.

We also examined the Pallas collisional family, the only collisional family whose very few observed members have been consistently classified as B-types. We found that this family must be compositionally different from the rest of B-types, since they have significantly higher visible albedos and their 3-micron albedos are homogeneously low, in contrast with the wide range of values shown by the rest of the objects in the spectral class.

Second, our research group's involvement in the sample-return MarcoPolo-R mission proposed to the European Space Agency motivated us to characterise the thermal inertia of (341843) 2008 EV5, the primitive near-Earth object selected as the mission's primary target. Related to the conductivity, the density, and the specific heat capacity of the surface material, the thermal inertia governs the temperature distribution across the surface of the asteroids and hence their thermal emission. We used the resulting thermal inertia value, derived by fitting a sophisticated thermophysical model to the asteroid's WISE infrared data, and a model of thermal conductivity to constrain the mean size of the particles on the surface of (341843) 2008 EV5. This knowledge would have been not only relevant for the mission's design but it will also contribute to the characterisation of the primitive near-Earth asteroid population and will help enhance the scientific return gained from other missions.

Finally, we studied the Hildas and the Jupiter Trojans. The Hildas are located outside the main asteroid belt at 4 AU in a constructive mean motion resonance with Jupiter. The Jupiter Trojans librate around the L4 (leading) and L5 (trailing) Lagrangian points of Jupiter. The orbits of both populations have been stable over most of their histories, and since their configuration is directly linked to Jupiter's formation and migration, improving our understanding of their nature will shed light on the Solar System's distant past. We studied the W1-reflectance statistics of these objects and found that a spectrally redder subgroup within the L4 Trojans (27 objects) shows a statistically significant shortage of reflected flux in this band. Again, our interpretation is that this is due to the presence of water-related 3-micron absorption bands in a substantial fraction of these objects.  The statistics for the Hildas would also lead to the same inference, but in this case our sample is not sufficiently large for these results to be statistically significant. On the other hand, the fact that all four Hildas with spectroscopic observations in the 3-micron region have been previously found to present these bands adds plausibility to this conclusion.