The third is a variable-reflectivity element (liquid-crystal device). This is a kind of electric frosted glass sometimes seen in executive offices, etc. The reflectivity of the device developed by us for IKAROS varies in response to power ON and OFF. The devices were enclosed in thin polyimide film, the same material as the sail, and mounted in line around the edges of the sail. As they turn ON and OFF repeatedly in synchronization with the spin rate, solar-light pressure becomes unbalanced, eventually generating torque that affects the entire spacecraft to tilt its spin axis. With this method using light pressure, we can control IKAROS's attitude without use of fuel. As reported in our press release, this mechanism has functioned very well.
The fourth example is attitude-control logic. Since IKAROS flies hoisting a huge, ultra-flexible film at all times, we needed to satisfy contradictory requirements. Specifically, since the solar sail is orbitally controlled by changes in the sail direction, we have to maintain stably the shape of the huge, ultra-flexible structure and, at the same time, to perform quick attitude control. The traditional control law called "rhumb-line control" is usually adopted for attitude control of spin-satellites. For IKAROS we introduced a control law called "flex rhumb-line control," which added some measures needed for a flexible structure to the conventional law. However, our attempt failed. Unfortunately or fortunately, as the structural attenuation of IKAROS's sail is strong enough, the new control law did not work fully. As a control specialist, this is regrettable. But I accepted this fact positively because the new approach was introduced following the principle of space development "better safe than sorry."
Verified acceleration caused by solar-light pressure
On June 9, 2010, IKAROS successfully deployed its sail fully. The deployment status was confirmed promptly by information from gyro, then the Doppler. As IKAROS does not perform active attitude control while the sail is deployed, its spin rate is governed by the principle of conservation of angular momentum. This means that, by monitoring any decrease of spin rate by gyro data, we can see to what degree the sail deploys. Our staff in charge of IKAROS's attitude was monitoring the data.
On the other hand, staff in charge of IKAROS's orbit was monitoring the Doppler. By filtering and removing spin modulation of the Doppler appearing in the communication waves mentioned above, we were able to determine the speed at which IKAROS left the earth or the velocity in the direction-of-sight line. The data clearly indicated that IKAROS started accelerating just after sail deployment with an acceleration amount of 3.6 x 10-6m/s. This value is just that we predicted as solar-light pressure acceleration. It was this very moment that we confirmed start of cruising by solar-sail in deep space for the first time in the world.
Attitude operation strategy for IKAROS
The solar-light pressure applied to the sail also acts as a torque disturbing IKAROS's attitude in addition to translational force (i.e., light-pressure acceleration). For a solar sail requiring much light-pressure acceleration, torque disturbance by light pressure is an unavoidable issue. From the planning stage before the launch, we intended to perform attitude control positively using this light-pressure torque disturbance.
Although IKAROS is a spin-stabilized spacecraft, its spin axis fluctuates opposite to the principle of conservation of angular momentum because its sail receives huge solar-light pressure. We can forecast the fluctuation accurately if we know the shape and optical characteristics of the sail exactly. Actually, however, since, at the development stage, we were unable to estimate accurately the sail's wrinkles after deployment and temporal changes of optical characteristics, we were required to make model identification using flight data.
With IKAROS, we realized our plan to perform model identification and, then, make use of the tendency of the fluctuation reversely so as to orient IKAROS to the desired direction while saving the propellant as much as possible. In fact, almost no propellant was consumed to keep the sail's surface direction to the Sun. The amount of change of the spin axis to inertial space was 180 degree over six months, which means that the sail was tuned just to the opposite direction without use of fuel (Fig. 3). IKAROS used its fuel mainly to change the sail direction promptly or maintain the spin rate.