Evidence for a near-Earth asteroid belt

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.