Solar Radiation Pressure-Assisted Fuel-Free Sun Tracking and Its Application to Hayabusa2

This paper describes the modeling, dynamical characteristics, and implementation of an attitude control method that actively uses solar radiation pressure. The theory behind this control method is called the generalized sail dynamics model, which was developed by the authors and successfully applied to Hayabusa2, which is a Japanese asteroid explorer launched in 2014. The quasi-stable property of the dynamics is proved, which enables the implementation of a fuel-free sun-tracking attitude using only one reaction wheel. As of August 2016, the attitude of Hayabusa2 was maintained within 10deg offset from the sun direction for 193 days in total without consuming any fuel. The auto-sun tracking, single-wheel, and fuel-free features were distinctive as compared to any other conventional control methods, such as three-axis stabilization, and brought many merits to practical spacecraft operations. The theoretical background, the prelaunch evaluation based on a finite element model analysis, the identification process of the dynamics model carried out for the Hayabusa2 mission operation, and their effectiveness are presented in this paper.