Solar power sail mission of OKEANOS

The solar power sail is an original Japanese concept in which electric power is generated by thin-film solar cells attached on the solar sail membrane.Japan Aerospace Exploration Agency(JAXA)successfully demonstrated the world’s first solar power sail technology through IKAROS(Interplanetary Kite-craft Accelerated by Radiation of the Sun)mission in 2010.IKAROS demonstrated photon propulsion and power generation using thin-film solar cells during its interplanetary cruise.Scaled up,solar power sails can generate enough power to drive high specific impulse ion thrusters in the outer planetary region.With this concept,we propose a landing or sample return mission to directly explore a Jupiter Trojan asteroid using solar power sail-craft OKEANOS(Oversize Kite-craft for Exploration and AstroNautics in the Outer Solar System).After rendezvousing with a Trojan asteroid,a lander separates from OKEANOS to collect samples,and perform in-situ analyses in three proposed mission sequences,including sending samples back to Earth.This paper proposes a system design for OKEANOS and includes analyses of the latest mission.

Self-excited oscillation of spinning solar sails utilizing solar radiation pressure

The present paper proposes a control method to excite spinning solar sail membranes for three-dimensional use.Using optical property switching,the input is given as the change in magnitude of the solar radiation pressure.The resonance point of this system varies with the vibration state due to its nonlinearity and the change in equilibrium state.To deal with this,a state feedback control law that automatically tracks the resonance point is developed in the present study.The proposed method enables decentralized control of the actuators on the sail,each of which determines the control input independently using only the information of vibration state.The proposed method is validated using numerical simulations.The results show that the nonlinear system behaves differently from the linear system,and the vibration grows using the decentralized control regardless of resonance point variation.

Improvement of sail storage and deployment mechanism for spin-type solar power sail

Deployable membrane structures are expected to be used for large area space structures,such as solar propulsion sails,magnetoplasma sails,drag-deorbiting sails,membrane antennas,and solar power sails.They are lightweight and can be compactly stored at launch.One achievement of the Japan Aerospace Exploration Agency(JAXA)was the successful deployment of a 200 m2 sail using centrifugal force in the IKAROS mission,which was the first solar propulsion sail-craft in history.JAXA has long been studying the technology of spin deployment of sail membranes,and is currently planning the spin deployment of a class sail larger than 2000 m2 as the next step in the development of the IK AROS technology.This paper discusses the unexpected behaviors during on-orbit sail deployment by IKAROS,as well as problems with the sail holding method,and proposes an improved sail storage structure and deployment mechanism for the OKEANOS mission.

Influence of thin-film device with curvature on natural frequency of rectangle membrane under uniaxial tension

A solar power sail demonstrator“IKAROS”demonstrated solar sailing technology in 2010.The membrane of the spinning solar sail IKAROS is estimated to be deformed toward the Sun.The deformation was kept even under low spin-rate.Previous studies suggest that curvature of thin-film solar cells on the membrane increases the out-of-plane stiffness by finite element analysis.Shape,out-of-plane stiffness,and natural frequency of membranes have to be predicted for solar sails with thin-film devices,such as thin-film solar cells,dust counters,and reflectivity control devices in order to reduce the margins of sail size and propellant mass against disturbance solar pressure torque acting on the membrane.In this paper,the effect of a curved thin-film device on the natural frequency of a rectangle membrane under uniaxial tension was investigated.Three types of membranes were evaluated:a membrane with a curved thin-film device,a membrane with a flat thin-film device,and a plane membrane.Geometric nonlinear finite element analysis and eigenvalue analysis were conducted to investigate the natural frequencies under varying tension.The simulations were verified by vibration experiments.It was found that under low tension,the natural frequency of the membrane with the curved thin-film device is significantly higher than that of the others and that under high tension,the natural frequency of the membrane with the thin-film device is slightly lower than that of the plane membrane.In addition,parametric analysis on the curvature of the thin-film device shows that natural frequency at low tension is sensitive to the curvature.The eigenvalue analysis of a whole solar sail with the curved thin-film devices also suggests that the curvature remarkably affects the vibration modes.In conclusion,curved thin-film devices have a significant impact on the out-of-plane stiffness of a membrane under low tension.

Hayabusa2’s station-keeping operation in the proximity of the asteroid Ryugu

The Japanese interplanetary probe Hayabusa2 was launched on December 3,2014 and the probe arrived at the vicinity of asteroid 162173 Ryugu on June 27,2018.During its 1.4 years of asteroid proximity phase,the probe successfully accomplished numbers of record-breaking achievements including two touchdowns and one artificial cratering experiment,which are highly expected to have secured surface and subsurface samples from the asteroid inside its sample container for the first time in history.The Hayabusa2 spacecraft was designed not to orbit but to hover above the asteroid along the sub Earth line.This orbital and geometrical configuration allows the spacecraft to utilize its high-gain antennas for telecommunication with the ground station on Earth while pointing its scientific observation and navigation sensors at the asteroid.This paper focuses on the regular station-keeping operation of Hayabusa2,which is called“home position”(HP)-keeping operation.First,together with the spacecraft design,an operation scheme called HP navigation(HPNAV),which includes a daily trajectory control and scientific observations as regular activities,is introduced.Following the description on the guidance,navigation,and control design as well as the framework of optical and radiometric navigation,the results of the HP-keeping operation including trajectory estimation and delta-V planning during the entire asteroid proximity phase are summarized and evaluated as a first report.Consequently,this paper states that the HP.keeping operation in the framework of HPNAV had succeeded without critical incidents,and the number of trajectory control delta-V was planned fficiently throughout the period.

Mechanism-free control method of solar/thermal radiation pressure for application to attitude control

Solar radiation pressure(SRP)impinging on spacecraft is usually regarded as a disturbance for attitude motion,but it can be harnessed to solve the very problem it creates.Active SRP control is possible with solar radiation powered thin-film devices such as reflectivity control devices or liquid crystal devices with reflective microstructure.Thermal radiation pressure(TRP)can likewise be used to solve flight attitude problems caused by SRP,TRP,or other factors.TRP on solar cells can be controlled by switching regulators under the control of them,resulting in temperature change.These SRP/TRP controls are free from mechanisms,such as reaction wheels,and thus they do not produce internal disturbances.In addition,the magnitude of SRP/TRP torques is generally much smaller than internal disturbance torques produced by reaction wheels,which creates a potential for precision far beyond that achieved with mechanical controls.This paper summarizes how SRP/TRP can be used by means of numerical simulations of typical control methods.The usefulness of this mechanism-free attitude control is verified for future use on both Earth orbiting satellites and interplanetary spacecraft including solar sails.