NASA's Deep Space One was a very successful test of a prototype ion drive, which fired for a total of 678 days and enabled the probe to run down Comet Borrelly, a feat which would have been impossible for a chemical rocket. "Dawn", the first NASA operational (i.e., non-technology demonstration) mission to use an ion drive for its primary propulsion, successfully orbited the large main-belt asteroids 1 Ceres and 4 Vesta. A more ambitious, nuclear-powered version was intended for a Jupiter mission without human crew, the Jupiter Icy Moons Orbiter (JIMO), originally planned for launch sometime in the next decade. Due to a shift in priorities at NASA that favored human crewed space missions, the project lost funding in 2005. A similar mission is currently under discussion as the US component of a joint NASA/ESA program for the exploration of Europa and Ganymede.