Thanks to ions: Deep in the asteroid belt is a rock the size of Arizona. It's called Vesta, it's the second biggest asteroid in the band of rubble between Mars and Jupiter, and in July it got a visitor: the Dawn spacecraft, which settled into orbit around Vesta, where it will remain for a year, studying what is one of the oldest objects in the solar system. Dawn traveled to Vesta in a whole new way: ion propulsion, which relies on a stream of xenon ions to nudge a spacecraft along on a gentle but steadily accelerating glide. Goodbye, heavy, traditional fuel — and hello, asteroid. When Dawn is done with its work, it will puff its way out of orbit and head for Ceres, the solar system's largest (Texas-size) asteroid, and repeat the same surveillance. One ship, two stops — not half bad.