The SOLAR attitude for the International Space Station: from a one-time experimental attitude change request to a standard ISS attitude to advance SOLAR science

摘要

The Solar Monitoring Observatory, or SOLAR in short, is a payload of the European Space Agency, mounted on one of the external platforms of the Columbus module of the International Space Station (ISS). The two operational instruments on-board the payload are designed to measure the solar irradiance in the wavelength range 17 to 3080nm. However, due to its unique location and because of the mechanical constraints of the platform, observations are only possible at most two weeks a month, for not longer than 20 minutes per ISS orbital revolution. Since the SOLAR mission will be operational for almost a complete solar cycle, it will provide data on the long-term evolution of the Spectral Solar Irradiance, important for, among others, atmospheric science. However, the monthly gap in the observations does not ease the modelling of the short-term variations in the solar EUV and UV flux, related to the development of active regions and the rotation of the Sun. During the winter and summer solstices, the time between two consecutive observation windows is the shortest. By changing the ISS attitude by only a few degrees from its standard Torque Equilibrium Attitude, this gap in the observations can be bridged. Between 30 November and 12 December, 2012, the ISS roll, and mainly yaw (about 7.5°) were modified, allowing the SOLAR instruments to monitor the Sun for more than 35 days in a row, covering a complete solar rotation. This event is historical as it was the first time ever the ISS rotated exclusively for a scientific experiment. The change of the ISS attitude was reached by solely using the Control Momentum Gyroscopes and did not negatively affect any of the other external payloads. This minimal effort resulted in a great scientific benefit. During this extended observation period, data of the solar spectrum were intensively collected. A more complete dataset of the solar irradiance will contribute to a better understanding of the effect of the solar variability on the Earth's atmosphere. After this successful first event, the SOLAR bridging operation was executed twice more, namely in July and December 2013. More bridging campaigns are envisioned, with the next one requested for the summer solstice 2014.