A quarter of known asteroids is associated with more than 100 distinct asteroid families, meaning that these asteroids originate as impact fragments from the family parent bodies. The determination of which asteroids of the remaining population are members of undiscovered families, or accreted as planetesimals from the protoplanetary disk, would constrain a critical phase of planetary formation by unveiling the unknown planetesimal size distribution. We discovered a 4-billion-year-old asteroid family extending across the entire inner part of the main belt whose members include most of the dark asteroids previously unlinked to families. This allows us to identify some original planetesimals, which are all larger than 35 kilometers, supporting the view of asteroids being born big. Their number matches the known distinct meteorite parent bodies.

Visible-wavelength color and reflectance provide information about the geologic history of planetary surfaces. We present multispectral images (0.44 to 0.89 microns) of near-Earth asteroid (101955) Bennu. The surface has variable colors overlain on a moderately blue global terrain. Two primary boulder types are distinguishable by their reflectance and texture. Space weathering of Bennu surface materials does not simply progress from red to blue (or vice versa). Instead, freshly exposed, redder surfaces initially brighten in the near-ultraviolet (become bluer at shorter wavelengths), then brighten in the visible to near-infrared, leading to Bennu’s moderately blue average color. Craters indicate that the timescale of these color changes is ~105 years. We attribute the reflectance and color variation to a combination of primordial heterogeneity and varying exposure ages.

Aims. The near-Earth asteroid population suggests the existence of an inner Main Belt source of asteroids that belongs to the spectroscopic X-complex and has moderate albedos. The identification of such a source has been lacking so far. We argue that the most probable source is one or more collisional asteroid families that escaped discovery up to now. Methods. We apply a novel method to search for asteroid families in the inner Main Belt population of asteroids belonging to the Xcomplex with moderate albedo. Instead of searching for asteroid clusters in orbital elements space, which could be severely dispersed when older than some billions of years, our method looks for correlations between the orbital semimajor axis and the inverse size of asteroids. This correlation is the signature of members of collisional families, which drifted from a common centre under the effect of the Yarkovsky thermal effect. Results. We identify two previously unknown families in the inner Main Belt among the moderate-albedo X-complex asteroids. One of them, whose lowest numbered asteroid is (161) Athor, is ∼3 Gyrs-old, whereas the second one, whose lowest numbered object is (689) Zita, can be as old as the Solar System. Members of this latter family have orbital eccentricities and inclinations that spread them over the entire inner Main Belt, which is an indication that this family could be primordial, i.e. it formed before the giant planet orbital instability. Conclusions. The vast majority of moderate-albedo X-complex asteroids of the inner-Main Belt are genetically related, as they can be included into few asteroid families. Only nine X-complex asteroids with moderate albedo of the inner Main Belt cannot be included in asteroid families. We suggest that these bodies formed by direct accretion of the solids in the protoplanetary disk, and are thus surviving planetesimals.