Development: Realization of IKAROS as a System
To complete IKAROS as exploration spacecraft system actually flying in outer space, we needed to overcome a number of challenges in addition to sail deployment. The reasons why such a small team could successfully complete IKAROS include: excellent communications between development team members, active and voluntarily contributions outside their own specialty, and good teamwork based on strong trust with dedicated subsystem staff and hardworking manufacturing companiesEstaff.
The biggest challenge of the spin-type solar sail as a navigation system is how to flexibly manage and control its angular momentum generated by the rotation of the large-area film. We had to solve the following problems: (1) predict and manage wrinkles arising on the sail after deployment and the disturbance in solar-light pressure caused by the wrinkles; and (2) control of the large angular-momentum and elastic sail to realize fuel-saving and keep it stable. Please refer to ISAS News, 2011 January edition (No. 358), for details on many technological ideas to solve problem (2).
Problem (1) was difficult. It is in fact impossible to predict what wrinkles would emerge on the flexible sail after deployment. Furthermore, the impact of the disturbance torque generated by solar-light pressure varied greatly depending on the shape and condition of the wrinkles. After the launch of IKAROS, a great discovery was made.
Operation: Discovery of Spiral Motion
In addition to the structure and material groups, we initiated an acceleration research groupEto evaluate IKAROSs solar-sail navigation after deployment. The group consisted of a dozen specialists and students in orbital mechanics from inside ISAS and outside. The group examined solar radiation pressure acceleration performance based on data obtained from the actual operation. Further, it worked on a wide range of related research in solar-sail orbit, guidance, and navigation.
In solar-sail navigation, orbital controlEmeans control of the sail orientation and also attitude control. Understanding attitude motion is critical. The acceleration research group anticipated two properties of IKAROSs attitude motion before launch: the windmill effectEand Sun-tracking property of spin axis.E/span>
The windmill effect is a behavior where the spin rate changes as sunlight collides with the wrinkled sail surface. This is just like windmill rotation by receiving wind. The effect was observed immediately after IKAROSs sail deployment.
On the other hand, the Sun-tracking property of spin axis is a behavior whereby light pressure on spin satellite causes the axis to track the Sun by plotting arc. Using this property, we can keep the spacecraft pointing at the Sun without fuel. This technique was used to help the asteroid explorer HAYABUSA when it lost fuel.