  We present the results of the first-ever visible spectroscopic survey fully
dedicated to the small (absolute magnitude H>20) near-Earth asteroid (NEA)
population. Observations have been performed at the New Technology Telescope
(NTT) of the European Southern Observatory (ESO), during a 30-night Guaranteed
Time Observations programme, in the framework of the European Commission
financed NEOShield-2 project. The visible spectra of 147 objects have been
obtained and taxonomically classified. They show a peculiar taxonomic
distribution, with respect to larger NEAs. In particular, olivine-rich A-types
and organic-rich D-types are more abundant than what could be expected by
extrapolating the taxonomic distribution of larger NEAs. Such results have
implications for the investigation of the first phases of solar system history,
including the delivery of prebiotic material on the early Earth. Having been
obtained over a large range of solar phase angles, our data allowed us to
evidence peculiar phase reddening behaviours for asteroids belonging to
different taxonomic types. Low-albedo asteroids display no or limited phase
reddening, compared to moderate- and high-albedo objects. This result suggests
a promising novel way to distinguish primitive asteroids in the X-complex. In
agreement with previous laboratory experiments, olivine-rich surfaces are the
most affected by phase reddening.
