Dawn is the first mission to attempt to orbit two distant planetary bodies. The objects chosen, 4 Vesta followed by 1 Ceres, are the two most massive members of the asteroid belt that appear to have been formed on either side of the dew line in the early solar nebula. This paper describes the present status of the mission development and the plans for operation at Vesta and Ceres.
The Dawn project is progressing toward its 2006 launch on a mission to orbit main belt asteroids (4) Vesta and (1) Ceres. Designed to provide insights into important questions about the evolution of the solar system, Dawn will spend more than 0.5 years in orbit about each of these bodies. This challenging mission is enabled by an ion propulsion system. In contrast to missions that use conventional chemical propulsion, the use of this system couples flight system mass and power , thereby requiring different methods of managing these and other technical resources. Now that the project is nearing launch, the refinement of resource estimates allows the identification of excess margin, which is being applied in novel ways to increase the scientific potential of the mission. The unusual relationship of the margins is described, and progress in preparing for the mission is presented.
The UARS Microwave Limb Sounder (MLS) measures ClO (~25-45 km), O3 (~15-80 km), H2O (~15-85 km), and pressure (~30-60 km). ClO is the dominant form of chlorine which destroys ozone in the stratosphere; its measurement on a global scale, which will be done by the MLS, is essential for understanding and monitoring the depletion of stratospheric ozone by chlorine from industrial products. Simultaneous measurements of O3 and H2O provide additional important information on stratospheric ozone chemistry. The H2O and O3 measurements will be to higher altitudes than previously explored on a global basis. The pressure measurements provide the vertical reference for composition measurements. Secondary MLS measurement goals include H2O2, HNO3, temperature, and one component of wind in the mesosphere. Measurements are made by observing millimeter-wavelength spectral line thermal emission as the instrument field-of- view is vertically scanned through the atmospheric limb. All are made simultaneously and continuously, at all times of day and night. Prior development for the UARS MLS included an aircraft [1,2] and balloon [3,4,5] MLS. Development has also started on an MLS experiment for the future Earth observing system (Eos); the Eos MLS is described in a separate paper presented at this meeting.
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