Aims. Vesta's surface is surprisingly pristine. Although its basaltic surface is roughly similar to the lunar surface, which is intensely space weathered, its surface remains unaltered. It has been shown recently that solar-wind irradiation dominates asteroidal space weathering with a timescale on the order 10 4-10 6 years. Recent ion irradiation experiments on pyroxenes have shown significant reddening and darkening of the collected spectra with progressive irradiation. Since pyroxene is a major surface component of Vesta as determined by spectroscopy, we aimed to test whether the solar wind irradiation alters significantly the optical properties of the surface of Vesta. Methods. Consequently, we performed an ion irradiation experiment on a eucrite meteorite, which characterizes the surface of Vesta well, in order to simulate the solar wind irradiation on this asteroid. Results. Our result implies that, if solar wind ions do reach the surface of Vesta, its reflectance spectrum should be much redder and its albedo lower. Indeed, this implies that solar wind particles can not have reached the asteroid surface. This strongly suggests the presence of a magnetic field shielding the surface from solar wind ions. This is the first remote detection of the magnetic field of an asteroid based on its color.