The Giant Asteroid, Up Close and Personal
As NASA’s Dawn spacecraft investigates its first target, the giant asteroid Vesta, Marc Rayman, Dawn’s chief engineer, shares a monthly update on the mission’s progress.
Dawn’s fourth anniversary of being in space is very different from its previous ones. Indeed, those days all were devoted to reaching the distant destination the ship is now exploring. Celebrating its anniversary of leaving Earth, Dawn is in orbit around a kindred terrestrial-type world, the ancient protoplanet Vesta.
The adventurer spent August on Vesta’s shores and now it’s ready to dive in. Dawn devoted most of this month to working its way down from the 2,700-kilometer (1,700-mile) survey orbit to its current altitude of about 680 kilometers (420 miles) and changing the orientation of the orbit. (For a more detailed discussion of the altitude, go here.) The sensationally successful observing campaign in survey orbit produced captivating views, revealing a complex, fascinating landscape. Now four times closer to the surface, the probe is nearly ready for an even more comprehensive exploration from the high altitude mapping orbit (HAMO). The plans for HAMO have changed very little since it was described on the third anniversary of Dawn’s launch.
Dawn’s spiral descent went extremely well. We have seen before that bodies travel at higher velocities in lower altitude orbits, where the force of gravity is greater. For example, Mercury hurtles around the sun faster than Earth in order to balance the stronger pull of gravity, and Earth’s speed is greater than that of more remote Vesta. Similarly, satellites in close orbits around Earth, such as the International Space Station, race around faster than the much more distant moon. When it began its spiral on August 31, Dawn’s orbital speed high above Vesta was 76 meters per second (170 mph), and each revolution took nearly 69 hours. Under the gentle thrust of its ion propulsion system, the spacecraft completed 18 revolutions of Vesta, the loops getting tighter and faster as the orbital altitude gradually decreased, until it arrived at its new orbit on schedule on Sept. 18. In HAMO, Dawn orbits at 135 meters per second (302 mph), circling the world beneath it every 12.3 hours.
When Dawn’s itinerary called for it to stop thrusting, it was very close to HAMO but not quite there yet. As mission planners had recognized long beforehand, small differences between the planned and the actual flight profiles were inevitable. Extensive and sophisticated analysis has been undertaken in recent years to estimate the size of such discrepancies so the intricate plans for completing all the work at Vesta could account for the time and the work needed to deliver the robotic explorer to the intended destination. In order to accomplish the intensive program of observations with its scientific instruments, the spacecraft must follow an orbital path carefully matched to the sequences of commands already developed with painstaking attention to detail. The beauty of Dawn’;s artistically choreographed pas de deux with Vesta depends on the music and the movements being well synchronized.
During its descent, Dawn paused frequently to allow controllers to update the flight profile, accounting for some of the variances in its course along the way. Following the completion of thrusting, navigators tracked the ship more extensively as it sailed around Vesta, measuring its orbit with great accuracy. This revealed not only the details of the orbital parameters (such as size, shape, and orientation) but also more about the character of Vesta’s gravity field than could be detected at higher altitudes. With the new information, the team designed two short maneuvers to adjust the orbit. The first, lasting four hours, was executed last night, and the second, half an hour shorter, will be completed tonight. After further measurements to verify the final orbit, the month of HAMO observations will begin on Sept. 29.