高尾 勇輝

Abstract A new attitude control method for solar sails is proposed using a single-axis gimbal mechanism and three-axis reaction wheels. The gimbal angle is varied to change the geometrical relationship between the force due to solar radiation pressure (SRP) and the center of mass of the spacecraft, such that the disturbance torque is minimized during attitude maintenance for orbit control. Attitude maneuver and maintenance are performed by the reaction wheels based on the quaternion feedback control method. Even if angular momentum accumulates on the reaction wheels due to modelling error, it can also be unloaded by using the gimbal to produce suitable torque due to SRP. In this study, we analyzed the attitude motion under the reaction wheel control by linearizing the equations of motion around the equilibrium point. Further, we newly derived the propellent-free unloading method based on the analytical formulation. Finally, we constructed the integrated attitude{orbit control method, and its validity was verified in integrated attitude{orbit control simulations.

To extend the usability of solar sails in the sun–Earth–moon system, we analyze the transfer trajectories from the 9:2 Earth–moon near-rectilinear halo orbit (NRHO) to halo orbits around the sun–Earth L1 and L2 points under the assumption of a future mission for a solar sail spacecraft equipped with a solar electric propulsion (SEP) system deployed from the Lunar Orbital Platform-Gateway. The dynamics are modeled using the bicircular restricted four-body problem, where the gravitational forces from the sun, Earth, and moon as well as solar radiation pressure (SRP) are considered. We propose a trajectory design method that utilizes both SRP and SEP. The method consists of initial guess generation and optimization steps. The initial guess generation comprises the forward propagation of the escape trajectory from the NRHO, the backward propagation of the stable manifold of the target halo orbits, and their apoapsis patching process. Optimization is conducted to minimize propellant consumption by effectively controlling SRP. We perform optimizations with various parameters, namely, the sail area-to-mass ratio ([Formula: see text]), specifications of SEP, target sun–Earth halo orbit, and departure [Formula: see text] direction. The results validate the proposed trajectory design method and verify that solar sail acceleration can reduce the necessary amount of propellant, which indicates that such missions can be realized by small CubeSats.

This paper proposes a new methodology for solar-sail attitude control that uses only momentum wheels. Different from conventional solar sails packaged in a central hub, the sailcraft is deployed in the direction of one side of the storage. In this single-wing configuration, the offset between the center of mass (c.m.) and center of pressure (c.p.) is large and lies in the sail plane. When specular reflection is dominant, solar-radiation-pressure (SRP) force vector points in the out-of-plane direction, thus causing an in-plane SRP torque orthogonal to the c.m./c.p. offset vector. Therefore, by placing a bias momentum in the c.m./c.p. direction, the sailcraft keeps rotating in the same plane while maintaining its orientation relative to the sun. Analysis reveals that the attitude motion of the one-winged momentum-biased solar sail is basically unstable, but the system can be stabilized in a neutral manner through minor control of the bias momentum. Furthermore, adding another control moment in the out-of-plane direction enables asymptotic stability. Control in the remaining in-plane direction makes it possible to avoid wheel saturation. Numerical simulations demonstrate that both attitude maintenance and maneuver can be performed and that the controller is robust to parameter errors.

A novel approach for shape control of membrane structures is presented to realize their use in three-dimensional and variable configurations. The shape control is accomplished by exciting a spinning membrane. The membrane forms a shape consisting of several vibration modes, depending on the input frequency, and the wave surface stands still when its frequency is synchronized with the spin rate; that is, the wave propagation and the spin cancel each other, resulting in a static wave surface in the inertial frame. This idea enables control of continuous membrane structures with large deformation using fewer actuators than conventional methods. This paper describes the general theory of the static wave-based shape control. The mathematical model of membrane vibration, the classification of control input, and the control system for exciting a static wave are summarized. The proposed method is demonstrated through a ground experiment. A 1 m large polyimide film is rotated and vibrated in a vacuum chamber, and the output shape is measured using a real-time depth sensor. It is shown that the observed shapes agree with numerical simulation results. An additional simulation that models the Japanese solar sail Interplanetary Kite-craft Accelerated by Radiation Of the Sun (IKAROS) demonstrates that the proposed method also works with a practically large-scale membrane in the space environment.

Automated spacecraft docking is a technology that has long been pursued. Deep space explorers and small spacecraft can carry fewer resources for docking, such as navigation sensors or latching structures, than can their larger near-Earth counterparts. The concept of the probe-cone docking mechanism is an effective solution to this problem. In this approach, a probe attached to the chaser satellite is guided automatically to the connection part of the target satellite by a conical structure. It is important to have a shock attenuation mechanism at the docking interface to prevent the chaser from being bounced away from the target. In the present paper, an automated docking mechanism that uses a flexible and deployable boom as the probe is proposed, and results of an analysis of the multi-body system dynamics are presented. Although analytical investigations into docking dynamics have been reported, the dynamics depend on many interdependent design parameters, the interaction of which is yet to be investigated. The present work involved a numerical analysis of the effect of each design parameter on the satellite behavior. An energy-based index that can predict the success or failure of docking was also developed in this study. In addition, a design scheme for the parameters is presented based on the results of the analysis in which the optimal combination of the design parameters is determined by searching the solution space.

This paper proposes an autonomous image-based navigation method for estimating the target-relative position of a spacecraft for distant small body exploration. The main focus is position estimation at high altitude where the outlines of a target body can be seen in images. The asteroid explorer Hayabusa2 touched down on the asteroid Ryugu with pin-point accuracy in February 2019. For this mission, the asteroid-relative position was estimated by ground operators from 20 km to 50 m above the surface of Ryugu. For the exploration of small bodies farther than the asteroid main belt, the delay of communication with Earth is unacceptably large for feedback guidance. This situation becomes worse for larger bodies because the time constant of the dynamics becomes smaller. Therefore, real-time autonomous navigation is required for distant small body exploration even at high altitude. To accomplish high-accuracy and real-time autonomous navigation, an autonomous position estimation method based on terrain-relative navigation (TRN) that estimates deviation by comparing nominal terrain information and actual terrain information is proposed. In addition to TRN, the vector code correlation (VCC) algorithm is used for the luminance comparison of terrain information. This algorithm is a type of correlation calculation method for template matching that finds the maximum correlated region in images. With the VCC algorithm, correlation can be calculated in real time via XOR operations suitable for FPGA. The estimation accuracy and processing time of the proposed method were evaluated with a comparison to those of other methods. The results show that a high estimation accuracy, similar to the image resolution, was accomplished in real time. Finally, an evaluation using flight data from Hayabusa2 shows that the estimation accuracy and processing time of the proposed method are suitable for a real mission environment. The proposed method will be a key technology for distant small body exploration.

© 2019, Tsinghua University Press. 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.

© 2020, Tsinghua University Press. This paper presents the optical navigation results of the asteroid explorer Hayabusa2 during the final rendezvous approach phase with the asteroid Ryugu. The orbit determination of Hayabusa2 during the cruising phase uses a triangulation-based method that estimates the probe and asteroid orbits using the directions from which they are observed. Conversely, the asteroid size is available as optical information just prior to arrival. The size information allows us to estimate the relative distance between the probe and the asteroid with high accuracy, that is strongly related to the success or failure of the rendezvous. In this study, the relative distance and asteroid size in real space are simultaneously estimated in real time by focusing on the rate of change of the asteroid size observed in sequential images. The real-time estimation results coincided with those of precise analyses performed after arrival.

The near-Earth carbonaceous asteroid 162173 Ryugu is thought to have been produced from a parent body that contained water ice and organic molecules. The Hayabusa2 spacecraft has obtained global multi-color images of Ryugu. Geomorphological features present include a circum-equatorial ridge, east/west dichotomy, high boulder abundances across the entire surface, and impact craters. Age estimates from the craters indicate a resurfacing age of <inline-formula><m:math xmlns:m="http://www.w3.org/1998/Math/MathML" overflow="scroll"><m:mrow><m:mo>≲</m:mo><m:msup><m:mrow><m:mn>10</m:mn></m:mrow><m:mn>6</m:mn></m:msup></m:mrow></m:math></inline-formula> years for the top 1-meter layer. Ryugu is among the darkest known bodies in the Solar System. The high abundance and spectral properties of boulders are consistent with moderately dehydrated materials, analogous to thermally metamorphosed meteorites found on Earth. The general uniformity in color across Ryugu’s surface supports partial dehydration due to internal heating of the asteroid’s parent body.

Spinning-type membrane space structures easily deform because they have no supporting structure. This may lead to an unexpected change in the effect of solar radiation pressure (SRP) on the membranes. Since SRP is a dominant factor of the dynamics of membrane space structures, especially for solar sails, knowledge of deformation is vital. However, it is almost impossible to precisely predict and design the actual deformation of membranes. This study provides a method to actively control the deformation of spinning membrane space structures. A completely fuel-free solar sailing technique is also shown as one application of the shape-control method developed.