It’s easy to forget about the massive swarm of asteroids that orbit the sun between Mars and Jupiter. There’s a river of them after all, and the overwhelming majority are little more than rocks. But Ceres and Vesta are different—the size of Texas and Arizona respectively. Ceres is so big, in fact, that it qualifies as a dwarf planet, the same as Pluto. In July 2011, the Dawn spacecraft went into orbit around Vesta and stayed there until September 2012, after which it blasted away on a course to Ceres—which it will reach in February 2015—becoming the first ship of its kind to orbit two bodies. Actually “blasted away” is not quite accurate. In truth, Dawn just putt-putted away, relying on a thin stream of propulsion from its innovative ion engine. The spacecraft’s mission is thus equal parts important science and exceedingly nifty engineering—a NASA twofer if ever there were one.