The Small Solar Power Sail Demonstrator IKAROS launched by H-IIA rocket No. 17 on May 21, 2010, passed near Venus on December 8, 2010, as scheduled.
Solar power sail technology explored by IKAROS consists of four main pillars: (a) automatic deployment of large-area sails in space (sail-deployment technology); (b) power generation demonstration by thin-film solar cells on sail (power-sail technology): (c) verification of acceleration by solar light pressure; and (d) guidance and navigation technology by solar sail. During the six months to IKAROS's passing by Venus, we successfully completed all four technological demonstrations.
This article discusses (c) and (d), deep-space guidance and navigation technology by solar sail realized by IKAROS.
Attitude and orbit control system of IKAROS
IKAROS is a spin-stabilized spacecraft. Its unique feature is a huge solar sail of an ultra-flexible structure, 14m x 14m in size and 7.5µm in thickness (Fig. 1). Before and after the success of full deployment of the sail on June 9, 2010, public attention was concentrated on the sail itself and its deployment system. However, the role of the attitude-control system is critical to allow IKAROS to perform "solar sailing" using its deployed sail without problems. At the same time, we were under pressure to implement the IKAROS mission at low cost. Under the circumstances, we tried a new inconspicuous yet highly capable attitude system. Let me introduce four examples here (Fig. 2).
The first example is the "gas-liquid equilibrium thruster," a new type of propulsion system onboard IKAROS for attitude control. A nontoxic alternative to the chlorofluorocarbon is used as propellant of the thruster. Since the propellant can be stored as a liquid and fired as a gas, it allows us to build a propulsion system of low pressure and small volume storage tank. Thus, the propulsion system of IKAROS, which was launched as a piggyback payload, was very easy to handle in terms of both launch-site operation and simplification of rocket interface.
The second is an attitude-detection system. Usually, for a spin-stabilized satellite, spin-axis direction in outer space is determined uniquely by earth sensor, or by a combination of sun sensor and star sensor. IKAROS uses a sun sensor and a low-gain antenna (LGA) for communication to determine its attitude. This is adopted to lower the cost of the spacecraft itself. When radiowaves transmitted from the LGA are received on the ground, the frequency is Doppler-shifted due not only to orbital movement but also its spin. The Doppler shift level is determined by the relation of the earth to IKAROS's spin axis. Accordingly, by measuring the amount of Doppler-shift and then calculating back, we can know the "earth angle." This technique evolved from one used in HAYABUSA in an emergency. Since IKAROS does not carry a high-gain antenna, we are forced to operate it through very thin line. This method that references the carrier waves without decoding the telemetry allowed us to estimate IKAROS's attitude to some degree. It proved very handy for us in actual operation of IKAROS.