The attitude profile observed in actual IKAROS operation, however, was not an arc with a constant radius, but a spiralEmotion with gradually decreasing radius (right in Fig. 2). Initially, it was believed to be simply the difference between the ideal mathematical formula world and the real world. Since the phenomenon was very systematic, however, we discussed it intensively by examining formulae, even in the control room(!), while simultaneously operating IKAROS.
One month later, the acceleration research group came up with an answer to the mystery. The cause was wrinkles on the sail surface. The group showed a clear formula demonstrating the relationship between the wrinkle patterns and the attitude motion. The formula correctly suggested the spiral motion (lower left in Fig. 2).
We immediately incorporated the new attitude-motion model into the operation system and revised the orbital and attitude-control plans.
The sail was originally designed to receive solar radiation pressure efficiently. Rather than considering the torque generated by light pressure as a disturbance, we had better to utilize it. By positively using spiral motion, we were able to prolong fuel storage more than twice as long as expected. Our research on this phenomenon has advanced greatly, eventually enabling us to discuss the relation between solar-sail navigation performance and optical property distribution on the sail surface or the manageable accuracy of wrinkles (Fig. 3). Thus, we are writing the worlds most advanced design theoryEfor solar sail.
Before IKAROS, our research level was about the same as the rest of the world. Now, from our experience and data gained developing and flying an actual solar-sail vehicle, we have the most knowledge compared to any research team in the world. I believe the utilization of this knowledge will lead to superiority and originality of Japanese deep-space exploration.
Currently, IKAROSs attitude and orbital motion rely entirely on solar radiation pressure because of its fuel depletion. Since its downlink signal is extremely faint because it lacks an onboard high-gain antenna, we are unable to conduct ranging to determine its orbit. Thanks to the success of modeling the spiral motion, however, we can still predict IKAROSs attitude and orbit correctly and obtain the data of its solar-sail navigation. We expect to continue to break records of light-pressure acceleration day by day and to operate IKAROS as long as possible.
IKAROS was a mission to perform a technological demonstration. With its success, we gained a foothold to travel to the outer-planet region far distant from the Sun. Our next target is exploration of the Jupiter and Trojan asteroids. We will continue our research toward travel to this uncharted area, which must be made by the combination of large-area-film sail technology obtained through IKAROS and high-specific impulse ion engine.
I am delighted to receive the Fifth Space Science Encouragement Prize. This award honors the R&D activity of our entire team. Taking this opportunity, I would like to express my sincere thanks to all members of IKAROS development and research teams who have shared the same dream.