NASA's Dawn spacecraft is a success from both a scientific and a technical standpoint. During the nearly nine years since its launch, the probe has orbited both Vesta and Ceres, two of the largest objects in the asteroid belt. For scientists, Dawn's most notable discovery is that it found spectacular craters on Ceres, the Texas-sized dwarf planet dotted with brilliant white specks.
Dawn has also demonstrated the viability of ion propulsion as a means of interplanetary travel. The spacecraft's thrusters ionize its xenon propellant, offering a considerable savings in terms of a propellant-to-thrust ratio. Ion engines get good gas mileage compared to traditional chemical rockets, although on this scale they travel more slowly. NASA may eventually use larger ion thrusters to ship large amounts of cargo to Mars in advance of human landings.
Now thanks to this efficiency, even after getting into orbits around both Vesta and Ceres, Dawn has a little bit of xenon gas left. Originally mission managers had planned to park it in a stable orbit around Ceres later this summer, creating a permanent artificial satellite. They could not crash the spacecraft into Ceres, as is customary with many similar missions, because Dawn has not been sterilized in accord with planetary protection procedures. But the extra xenon has created an additional opportunity.