Puzzlingly, the parent bodies of ordinary chondrites (the most abundant type of meteorites) do not seem to be abundant among asteroids. One possible explanation is that surfaces of the parent bodies become optically altered, to become the S-type asteroids which are abundant in the main asteroid belt. The process is called 'space weathering'—it makes the visible and near-infrared reflectance spectrum of a body darker and redder¹. A recent survey of small, near-Earth asteroids suggests that the surfaces of small S asteroids may have developing stages of space weathering². Here we report that a dark region on a small (550-metre) asteroid—25143 Itokawa—is significantly more space-weathered than a nearby bright region. Spectra of both regions are consistent with those of LL5-6 chondrites after continuum removal³. A simple calculation⁴ suggests that the dark area has a shorter mean optical path length and about 0.04 per cent by volume more nanophase metallic iron particles than the bright area. This clearly shows that space-weathered materials accumulate on small asteroids, which are likely to be the parent bodies of LL chondrites. We conclude that, because LL meteorites are the least abundant of ordinary (H, L, and LL) chondrites, there must be many asteroids with ordinary-chondrite compositions in near-Earth orbits.

1. Department of Geological Sciences, Brown University, Providence, Rhode Island 02912, USA
2. Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency, Sagamihara, Kanagawa 229-8510, Japan
3. Department of Earth and Planetary Science, University of Tokyo, Tokyo 113-0033, Japan
4. Graduate School of Science and Technology, Kobe University, Kobe 657-8501, Japan
5. Ithaca College, 267 Center for Natural Sciences, Ithaca, New York 14850-7288, USA
6. RISE Project Office, National Astronomical Observatory of Japan, Mizusawa, Oshu 023-0861, Japan
7. School of Earth Environmental Sciences, College of Natural Sciences, Seoul National University, Seoul 151-742, Korea
8. Johns Hopkins University Applied Physics Laboratory, Johns Hopkins Road, Laurel, Maryland 20723-6099, USA

Correspondence to: Takahiro Hiroi¹ Correspondence and requests for materials should be addressed to T.H. (Email: takahiro_hiroi@brown.edu).

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