Early mission power assessment of the Dawn solar array

NASA's Discovery Mission Dawn was launched in September 2007. Dawn will be the first to orbit two asteroids on a single voyage. The solar array for the Dawn mission will provide power under greatly varying illumination and temperature conditions. Dawn's ion propulsion system (IPS) will provide the spacecraft with enough thrust to reach Vesta and Ceres and orbit both. The demanding mission would be impossible without ion propulsion - a mission only to the asteroid Vesta (and not including Ceres) would require a much more massive spacecraft and, a much larger launch vehicle. Although typical Earth orbiting solar arrays experience changes in power over the mission lifetime due to environmental conditions, these changes are minuscule when compared to the Dawn solar array power change throughout their mission. The array capability at 1 AU is of the order of 10.5 kW, and the array capability at 3 AU is estimated at 1.4 kW. Although analysis estimates the power margins to be generous, uncertainty in the calculations may reduce these margins by a factor or 2. The only way to determine the array capability is to use the full spacecraft load (~3.1 kW) to move towards the peak power point. At present, the total spacecraft load will not match the full capability of the array for several years. In order to calibrate the array, two 'off-pointing' exercises were performed on the spacecraft, one at 45° and one at 60° 'off-sun'.