The Dawn team is pleased to announce that on February 6, Dawn was confirmed and approved to move into its implementation phase. In response to an increasing emphasis within the NASA Office of Space Science to mitigate risks of schedule slips and growth in cost, mass and power consumption, there has been increasing scrutiny of all flight programs, and Dawn is no exception. To address this changing risk posture the team took steps to improve the mission's technical margins and the financial and schedule reserves. As a result, there have been several changes in the mission profile since the original Concept Study Report. These have increased the robustness of the mission but at some expense to the science return.

The first adjustment was an increase in the solar array output so that Dawn can always operate its ion thrusters even at Ceres aphelion, and with adequate margin (15%) that could allow for unexpected power requirements. Additionally, JPL design principles now call for 20% mass margin at this phase in the program. Originally Dawn had not used these margins because of the high degree of experience gained in DS1 and the high heritage from prior Orbital spacecraft. Now it has become imperative to comply with the recommended mass margins for a program to be confirmed by NASA. Also required were at least 25% cost reserves at confirmation. The increased mass, power margin, and cost reserve requirements could be accommodated either by moving the Ceres rendezvous into an extended mission category or by taking reductions in the science return at both target bodies. The latter option was deemed more valuable and the mission was replanned accordingly.

In order to fit the mission into the tighter mass and power box, a Mars flyby was inserted into the trajectory, but this delayed Vesta and Ceres arrival and thus increased the operations costs. Dawn could not afford to extend its time in space and remain within the cost box. The Dawn spacecraft provider, Orbital Sciences Corporation made a generous contribution to the Dawn project that allowed the project to re-balance risk across the various elements and achieve its prime measurements within the cost cap and with responsible technical and cost reserves. However, sacrifices were made to achieve this scenario. First, the stay times at Vesta and Ceres have been reduced from 11 months at each body to 7 months at Vesta and 5 months at Ceres. This amount of time allows all imaging and mapping spectrometer data to be obtained, and achieves the prime objectives of the gamma ray and neutron detector (GRaND) and gravity investigations, but the stay time does not allow the spacecraft to spiral down as far as originally planned so that resolution of GRaND and the gravity data are somewhat reduced. Part of the increase in the mass margin and cost reserve has also come from the deletion of the magnetometer and its boom. The Dawn Laser Altimeter had been previously deleted before the Preliminary Design Review. While the magnetometer was on the descope list in the approved mission, the value of this instrument had been increasing as we learned more about Vesta and Ceres. Vesta almost certainly has an iron core and may have similar crustal magnetization as Mars, while Ceres appears to be an ice planet with water-ice mantle, and rocky core similar to Europa, Ganymede and Callisto. For Ceres and these Galilean moons, thermal evolution models suggest that there may be liquid water underneath the icy crust even today. A magnetometer is the only certain way to detect the existence of such a layer.

Dawn has a continuum of descope options remaining that provide increased mass margin and still return much of the key objectives. Mass growth beyond the held margin can be accommodated by reducing the stay time at the bodies to the science floor (4 months at each body), and raising the lowest observing altitude. Conversely, if the mission performs close to or above the nominal mission specifications, then the spacecraft can stay longer and go lower in its orbits about Vesta and Ceres, increasing the science return of the mission. In other words, the remaining descopes are entirely recoverable if the flight system performance allows it.

(This article is from the February 2004 Dawn's Early Light newsletter, written by Dawn Principal Investigator Christopher T. Russell.)

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