久保 勇貴

This paper reports on the manufacturing and evaluation of a solar power sail membrane prototype for the OKEANOS project. The in-house prototype was built by the Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency. Mechanical and electrical evaluation tests were conducted. The membrane, thin-film solar cells, reflectivity control devices were good condition after the manufacturing and handling. The improvements in the manufacturing process and design were found. The manufacturing process and design were fundamentally established. After the prototype, improvement plans for the manufacturing process and design were tried. We have a prospect of manufacturing the flight model sail and continue to the development.

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.

JAXA and other organizations are considering transformable spacecraft. The transformable spacecraft is a spacecraft that can change its structure. Utilizing these characteristics, this spacecraft has an engineering challenge to perform attitude control without fuel using the non-holonomic properties and the solar radiation pressure. The mission that is being considered is an on-orbit interferometer, which requires highly accurate attitude stability. In transformable spacecraft, the effects of solar radiation pressure can be controlled by structural changes. This allows the solar radiation pressure to be used for attitude control. In addition, an equilibrium point can be created by structural changes. The equilibrium point is an advantage in stabilizing the posture because the torque caused by solar radiation pressure due to the posture change is reduced. It is considered that attitude control around the stable equilibrium point is relatively easy. If a control method around the unstable equilibrium point can be established, the mission range will be greatly expanded. In this study, we propose a method for attitude stabilizing control of transformable spacecraft at unstable equilibrium point using solar radiation pressure. The proposed method identifies the equilibrium point as well as stabilizes the attitude. Numerical analysis confirmed the effectiveness of the proposed method under certain constraints. However, the proposed method has a large manual aspect and is limited only to certain constraints. Therefore, in this study, in addition to the proposed method, we will consider stabilizing the attitude of the spacecraft by reinforcement learning.

This paper provides the thruster control approach for micro-satellites to realize autonomous orbit and attitude control. The Space Robotics Laboratory (SRL) of Tohoku University and ALE Co. Ltd. are currently developing a micro-satellite called "ALE-2" to generate artificial meteors. This satellite is required to keep the sun-synchronous orbit at the altitude of 375-400 km due to operational safety and mission constraints. ALE-2 is equipped with the High Density Cold Gas Jet System (HDCGJ) developed by Patchedconics, LLC for orbit and attitude control. Although a variety of thruster control methods have been proposed, this research focuses on a design of a reliable, uncomplicated and robust algorithm using the low-thrust, 4 nozzle propulsion system. A precomputed thruster selection table is utilized to reduce the processing power and to optimize the thrust force and torque distribution. The evaluation result from the full-software closed-loop simulation demonstrated that the proposed approach was valid for the orbit control of ALE-2 while high attitude control stability could be maintained in the long term.

Transformable spacecraft under development is an innovative system that consists of several structural components, such as panels, connected together by internal force actuators. The spacecraft can change its structure drastically by driving installed actuators and achieve the following four features simultaneously. The first feature is "attitude change by internal force using non-holonomic characteristic of the system". It is possible to orient the spacecraft to an arbitrary direction by repeating the deployment of the panel in an appropriate order by the internal force actuator. The second feature is that "change of the structure enables the multiple functions by switching modes". Two telescopes will be installed for scientific missions utilizing the features of the transformable spacecraft and used to realize two different observation modes. One is a mode in which each telescope is oriented to different directions to perform wide-field observation (single telescope mode). The other is a mode in which two telescopes are pointed in the same direction. This mode enables the spacecraft to work as an interferometer (interferometer mode). The third feature is "orbit control and orbit keeping by controlling the solar radiation pressure on the spacecraft with the use of change of spacecraft structure". Since the spacecraft can change its structure by the internal force actuator, the orbit control and orbit keeping are achieved without fuel consumption. By utilizing this feature, the spacecraft will be injected into an artificial halo orbit around Sun-Earth Lagrangian point L2, and the technology demonstration of the transformable spacecraft and the observation mission will be performed in the orbit. The fourth feature is "passive cooling of observation equipment by use of panels as sunlight shield". In the observation mode, observation in the infrared region is performed and sufficient cooling is required. Appropriate arrangement of panels enables shielding of sunlight, and then the passive cooling of the observation equipment is realized. As a result, disturbance due to refrigerator is eliminated, which contributes to precise observation in addition to the contribution by non-holonomic attitude control without disturbance. This paper shows the analysis and experimental results for feasibility studies and conceptual designs of above four features. Furthermore, development status of the system and each subsystem to realize the spacecraft are introduced.

This paper proposes the control method which achieves fuel-free station-keeping around Sun-Earth L2 point (SEL2). This station-keeping is driven with only solar radiation pressure (SRP), and therefore the orbit is maintained without fuel consumption by controlling attitude of a spacecraft toward the sun. Furthermore, this attitude control is also achieved without fuel by using the technique of non-holonomic turn. The target orbit is the artificial orbit around SEL2, the size of which is much smaller than typical halo orbits, and thus provides more stationary thermal condition to spacecrafts.

As an innovative spacecraft system, a transformable spacecraft is proposed, which consists of multiple bodies connected with each other. They are equipped with actuators that move them relatively within a certain range of angle. The shape of the bodies is arbitrary, and the simplest is panel shape, for example. Supposing a transformable spacecraft consisting of a number of panels, they can be folded to be compact as a whole, unfolded to configure a large plane, and recomposed to configure various kinds of three-dimensional shape. The system enables a single spacecraft to have multiple functions by transforming the shape. A distinctive characteristic of a transformable spacecraft is its capability of performing nonholonomic attitude control. By transforming to another shape and transforming back to the original shape in a different path, the attitude is changed even if the shape is unaltered. This nonholonomic control is realized only by internal torque, and does not require any fuel consumption. We introduce the concept of a transformable spacecraft and its applications to missions, utilizing the nonholonomic control. For example, combining the function of variable-shape structure with the nonholonomic control, a multifunction space telescope can be realized orbiting around the Sun-Earth Lagrange point.

A phase equilibrium propulsion system is a kind of cold-gas jet in which the phase equilibrium state of the fuel is maintained in a tank and its vapor is ejected when a valve is opened. One such example is a gas-liquid equilibrium propulsion system that uses liquefied gas as fuel. This system was mounted on the IKAROS solar sail and has been demonstrated in orbit. The system has a higher storage efficiency and a lighter configuration than a high-pressure cold-gas jet because the vapor pressure is lower, and is suitable for small spacecraft. However, the system requires a gas-liquid separation device in order to avoid leakage of the liquid, which makes the system complex. As another example of a phase equilibrium propulsion system, we introduce a solid-gas equilibrium propulsion system, which uses a sublimable substance as fuel and ejects its vapor. This system has an even lower vapor pressure and does not require such a separation device, instead requiring only a filter to keep the solid inside the tank. Moreover, the system is much simpler and lighter, making it more suitable for small spacecraft, especially CubeSat-class spacecraft, and the low thrust of the system allows spacecraft motion to be controlled precisely. In addition, the thrust level can be controlled by controlling the temperature of the fuel, which changes the vapor pressure. The present paper introduces the concept of the proposed system, and describes ejection experiments and its evaluation. The basic function of the proposed system is demonstrated in order to verify its usefulness.