A SECOND ASTEROID BELT, possibly as big as the one between Mars and Jupiter, might exist between the orbits of Uranus and Neptune. Matthew Holman's (University of Toronto) computer simulation places collections of test particles in various graviational niches throughout the solar system and then tracks the particles over billions of years. Many of these pretend asteroids are captured by planets or ejected from the solar system. Holman finds that the region between 24 and 27 astronomical units undergoes major asteroid attrition but could harbor a large enough population of survivors to make an observational search worthwhile. (Nature, 19 June 1997.)