Deep in the main asteroid belt between Mars and Jupiter, Dawn is continuing its smooth, silent flight toward dwarf planet Ceres. Far behind it now is the giant protoplanet Vesta, which the spacecraft transformed from a tiny splotch in the night sky to an exotic and richly detailed world.
The voyage from Vesta to Ceres will take the pertinacious probe 2.5 years. The great majority of spacecraft coast most of the time (just as planets and moons do), each one following a trajectory determined principally by whatever momentum they started with (usually following release from a rocket) and the gravitational fields of the sun and other nearby, massive bodies. In contrast, Dawn spends most of its time thrusting with its ion propulsion system. The gentle but efficient push from the high velocity xenon ions gradually reshapes its orbit around the sun. In September 2012, as it departed Vesta after 14 months of scrutinizing the second most massive resident of the asteroid belt, Dawn’s heliocentric orbit was the same as the rocky behemoth’s. Now they are very far apart, and by early 2015, the robotic explorer’s path will be close enough to Ceres’s that they will become locked in a gravitational embrace.
Without ion propulsion, Dawn’s unique mission to orbit two extraterrestrial destinations would be impossible. No other spacecraft has attempted such a feat. To accomplish its interplanetary journey, the spaceship has thrust more than 96 percent of the time since propelling itself away from Vesta last year. Whenever it points its ion engine in the direction needed to rendezvous with Ceres, its main antenna cannot also be aimed at Earth. Dawn functions very well on its own, however, communicating only occasionally with its terrestrial colleagues. Once every four weeks, it interrupts thrusting to rotate so it can use its 5-foot (1.52-meter) antenna to establish contact with NASA’s Deep Space Network, receiving new instructions from the Dawn operations team at JPL and transmitting a comprehensive report on all its subsystems. Then it turns back to the orientation needed for thrusting and resumes its powered flight.