The S-type asteroid – ordinary chondrite controversy and discoveries by the Hayabusa mission to asteroid 25143 Itokawa

Abstract

Concerning the controversy that the visible and near-infrared (NIR) reflectance spectra of the S-type asteroids and ordinary chondrites which are the most abundant in the near-Earth region do not match with each other, especially the hypothesis of space weathering had been strongest as the explanation in the past. As the result of visible and NIR observations of an S-type asteroid 25143 Itokawa by the Hayabusa Mission, such idea has become even more assuring. Itokawa consists of bright and dark portions in general, and both of those regions in any scale show spectra indicating a mineral assemblage consistent with only those of LL chondrites among all known meteorites, wherein the only differences are the mean optical path length and the degree of space weathering. The fact that an asteroid which is made of an LL-chondrite material and space-weathered to look as an S-type asteroid suggests that many more S asteroids made of ordinary-chondrite materials (including H and L types) exist in the near-Earth region.