Space weathering and tidal effects among near-Earth objects

The spectral slope of sunlight reflected by an asteroid surface may change with time due to several physical processes acting on the surface (``space weathering''). We present a general and significant relation between the spectral slope of silicate--rich near-Earth and Main Belt asteroids and the "exposure" to space weathering. We find strong evidences that the Sun-related processes dominate the space weathering. Moreover, by using a large data set of spectroscopic observations of planet--crossing asteroids (near--Earth objects and Mars--crossers), we found a statistically significant correlation between their spectral slope and the perihelion distance. The correlation could be due to tidal effects: during a close encounter with a terrestrial planets tides may partially, or even completely, remove the old and the weathering process. The effects have to be combined to understand the evolution of optical properties of NEAs with time.