MESSENGER Power and Thermal System Operations

Abstract

The MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft launched on 3 August 2004 and is en route to orbit Mercury. MESSENGER is passively cooled, uses a peak-power tracking system, and is able to operate both near Earth and at Mercury. The operations of the MESSENGER power and thermal systems, from launch through three Mercury flybys, provide lessons for the exploration of the inner solar system. Since launch, MESSENGER has experienced solar distances ranging from 1.0 AU to 0.3 AU, equivalent to 1 to 11 Suns. The widely varying solar distance makes for a challenging operational environment for the power and thermal systems. MESSENGER’s sunshade provides the main thermal protection from solar heating. For most solar distances, spacecraft attitude is maintained to keep the sunshade pointed within 12 to 15° of the sunward direction. However, at near-Earth solar distances, having the shade between the Sun and the spacecraft resulted in excessive radiative cooling of the spacecraft. To counteract this effect, a series of attitude maneuvers referred to as “flips” and “flops” were performed to point the spacecraft’s sunshade respectively away from and back toward the Sun. These “flips” and “flops” were a complex set of maneuvers involving not only changes to the spacecraft attitude but also updates to on-board autonomy rules meant to keep the sunshade pointed at the Sun as well as rotations of the solar array panels by 180°. These maneuvers were considered high-risk events, as they temporarily disabled on-board protection systems designed to preserve spacecraft safety, and they required coordination and sequencing among multiple subsystems (power, guidance and control, and autonomy) to complete execution without violating flight constraints. These activities were not developed until the need arose after launch. As the solar distance changes, the solar arrays are operated to balance power production with thermal protection for the array panels. This balance involves commanding the solar arrays to maintain a fixed offset from pointing directly at the Sun. The limits on off-pointing angles that yield sufficient power and yet protect the solar arrays thermally change with solar distance, so periodic commanded updates are required. For solar distances less than 0.4 AU, the safe off-pointing-angle range increasingly narrows. At its lowest, the range is only 8° wide. Within such a narrow range, operations requiring the spacecraft attitude to change in the same direction as the solar-array rotation can become difficult, because small excursions outside the off-pointing range can cause solar-array temperatures to rise quickly. Since launch, one Earth flyby, two Venus flybys, and three Mercury flybys have been performed. Each of these encounters has been challenging because of solar eclipses and rapid changes in the spacecraft attitude to permit targeted science observations while fully operating the science payload and communication equipment, a combination of circumstances that potentially could cause a greater demand for power than the solar arrays can supply. The Mercury flybys have been especially challenging because they took place at a solar distance of ~0.3 AU, at which the range in off-Sun pointing angle for the solar arrays