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Dawn Status

Christopher T. Russell
Dawn Principal Investigator, UCLA

The Dawn mission is presently in the formulation phase, pre-paring to demonstrate at its Preliminary Design Review (PDR) that it is ready to proceed with the implementation phase. As part of the preparation for PDR, design reviews of four of the five scientific instruments have been completed, (Visible and Infrared Mapping Spectrometer - VIR; Gamma Ray and Neutron Detector - GRaND; magnetometer, and laser altimeter). All instruments passed their reviews with flying colors. The review of the fifth instrument, the framing camera, will take place in September. Much of the work on the camera will now take place at the Max Planck Institut fur Aeronomie (MPAe) in Katlenburg-Lindau. In conjunction with the new division of labor, Dawn welcomes two new members from MPAe to its science team, Uwe Keller, who will lead the camera development and Uli Christensen, MPAe Director, who will assist with the science planning and analysis.

This summer we also completed testing of the solar cells that will power the Dawn spacecraft and its ion propulsion system. Mission success depends greatly on the efficiency of these cells. When the spacecraft journeys away from the Sun, the illumination drops and the arrays cool. The cooler arrays are more efficient, compensating somewhat for the drop in illumination, but the combined effects have not previously been well characterized. Since most outer solar system missions have used radioactive thermal generators, the data on this low intensity, low temperature effect (LILT) is sparse. Fortunately, our LILT testing confirmed the efficiencies that were assumed when Dawn was proposed to NASA.

The project opted to add a fifth solar panel to each wing to increase Dawn's power margin at Ceres (at which time the spacecraft could reach 2.9 AU). The project has also been asked to increase its financial reserve, necessitating other mission trades and descopes. Since the mission is well along in its design phase, there are limited options to do this. Our optimized solution to meet this challenge is to launch on a standard Delta 2925, rather than a heavy, using Mars for a gravity assist. This mission plan also provides much needed calibration data for the instruments, and an early test of the science operations and data analysis systems. There will be some delay in arrival at Vesta but the very interesting southern polar region will still be illuminated. After acquiring its Vesta observations, the spacecraft departs for Ceres. This plan requires some rephasing of the budget profile and has not yet been approved by NASA HQ, but we are hoping and expecting it will, and that Dawn will move into the implementation phase late in the calendar year.