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Jeffrey F. Bell (Hawaii Institute of Geophysics and Planetology)
Binzel and Xu (Science 260, 186) discovered many small main-belt asteroids with visible spectra resembling that of Vesta (class V), which they suggested were ejecta from a large crater in Vesta's basaltic crust. However, later near-IR observations by Burbine and Binzel (BAAS 29, 964) of many of these objects revealed very reddened spectra in the 0.7-1.7 micron region, grossly different from known V-class asteroids but similar to the common S-class asteroids. These authors have attempted to preserve the genetic link with Vesta by proposing that these objects have basaltic V-like compositions concealed by a "weathered" S-like regolith. This model is inconsistent with the real regolith effects seen in the howardite breccias, spectral changes with age on the lunar maria, and laboratory space weathering experiments on basalts. More fundamentally, it is grossly inconsistent with the original identification of the family on the basis of spectral similarity. Instead, I propose that most of the supposed Vestoids are in fact background non-family S-class asteroids which appeared V-like in the original visible data due to unusually deep silicate absorption bands. Gaffey et al. (Icarus 106, 573) showed that band depths increase with decreasing size among the S asteroids. This known "weathering" effect within the S-class accounts for the the new IR observations in a more logical and economical manner than the unknown V-to-S process postulated by Burbine and Binzel. Unfortunatly, it also implies that the Vesta family is much smaller than now supposed, or perhaps completely absent.