EVIDENCE FOR A NEAR-EARTH ASTEROID BELT

IN January 1991, the 0.9-m Spacewatch telescope made the first observa tion1 of an asteroid outside Earth's atmosphere but in the neighbourho od of the Earth-Moon system. Since then, more than 40 Earth-approachin g asteroids (defined as objects with perihelia of less than 1.3 AU) ha ve 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-approac hing asteroids with diameters less than 50 m, relative to the populati on inferred from the distribution of larger objects. Here we argue tha t these smaller objects-characterized by low eccentricities, widely ra nging inclinations and unusual spectral properties-form a previously u ndetected asteroid belt concentrated near Earth. The recent discovery of additional small Earth-approaching asteroids supports this conclusi on.