This paper presents the major results from a study of an unmanned asteroid belt probe powered by solar electric propulsion (SEP) and launched by an Atlas/Centaur or a Titan IIIC. The SEP comprises roll-out solar cell arrays, mercury electron-bombardment ion thrusters, and modular solid-state power-conditioning equipment. With a 10-kw solar array (rated at 1 a.u.), the spacecraft would gather information about the flux distribution, size, velocity, and direction of asteroidal and cometary particles as small as 10~ g. The flux data would be accurate to within 10% standard error. The spacecraft would coast in the asteroid belt, between 2 and 3.5 a.u., for almost 1000 days. The trajectory has an aphelion of 3.5 a.u.; thrust is cut off at 210 days, just as the spacecraft reaches 2 a.u.