IN January 1991, the 0.9-m Spacewatch telescope made the first observation1 of an asteroid outside Earth's atmosphere but in the neighbourhood of the Earth-Moon system. Since then, more than 40 Earth-approaching asteroids (defined as objects with perihelia of less than 1.3 AU) have been discovered, including 13 smaller than 50 m. Using these data, one of us (D.L.R.) has shown2 that there is an excess of Earth-approaching asteroids with diameters less than 50 m, relative to the population inferred from the distribution of larger objects. Here we argue that these smaller objects - characterized by low eccentricities, widely ranging inclinations and unusual spectral properties - form a previously undetected asteroid belt concentrated near Earth. The recent discovery of additional small Earth-approaching asteroids supports this conclusion.
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