Osamu Mori

© 2018 Elsevier Ltd An engineering mission OKEANOS to explore a Jupiter Trojan asteroid, using a Solar Power Sail is currently under study. After a decade-long cruise, it will rendezvous with the target asteroid, conduct global mapping of the asteroid from the spacecraft, and in situ measurements on the surface, using a lander. Science goals and enabling instruments of the mission are introduced, as the results of the joint study between the scientists and engineers from Japan and Europe.

© 2018 The Japan Society for Aeronautical and Space Sciences. 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.

© 2017 The Author(s). Due to its important role in the sorting of particles on microgravity bodies by size, Brazil nut effect (BNE) is a major subject of study for understanding the evolution of planetesimals. Recent studies have revealed that the mechanism for the BNE on microgravity bodies is the percolation of particles or void-filling, rather than granular convection. This study also considers the mechanism for the BNE under 'less-convective' conditions and introduces three categories of behaviour for particles that mainly depend on the dimensionless acceleration of vibration Γ (ratio of maximum acceleration to gravitational acceleration), using a simplified analytical model. The conditions for Γ proposed by the model for each category are verified by both numerical simulations and laboratory experiments. 'Less-convective' conditions are realized by reducing the friction force between particles and the wall. We found three distinct behaviours of the particles when Γ > 1: the (i) 'slow BNE', (ii) 'fast BNE', and (iii) 'fluid motion' (the reverse BNE may be induced), and the thresholds for Γ correspond well with those proposed by the simplemodel. We also applied this categorization to low-gravity environments and found that the categorization scales with gravity level. These results imply that laboratory experiments can provide knowledge of granular mobility on the surface of microgravity bodies.

<p>IKAROS is a solar power sail demonstrator launched by JAXA in 2010. IKAROS successfully deployed its large sail by a centrifugal force due to the spin motion of the spacecraft body and obtained a solar power sail navigation. In the evaluation of IKAROS membrane shape, some unexpected phenomena were observed; (1) tether connecting IKAROS membrane and main body got loose in spite of normal spin rate, and (2) the membrane was not warped by photon pressure in spite of low spin rate. The purpose of this paper is to understand generation mechanisms about these unexpected phenomena. Because thin-film devices of thin-film solar cell and reflectivity control device are multilayer film structures, curves occur. The bending stiffness of IKAROS membrane is increased due to the curve. This paper presents the multi-particle model of the membrane considering curve and bending moment of thin-film device. The membrane shape is estimated by numerical simulations and the influence of the thin-film device is made clear.</p>

© 2017 IAA 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.

<p>JAXA has developed a Jovian Trojan asteroid sample return mission using a solar power sail. Jovian Trojan asteroids are among the few remaining frontiers in our solar system and may hold clues to its formation and evolution. However, visiting Jovian Trojans is much more difficult than reaching near-Earth objects because of the large amount of fuel required to reach them. Moreover, large distance from the sun makes power generation difficult. Solar power sails offer an effective way of realizing such challenging exploration. This paper outlines a solar power sail spacecraft and discusses the design of a trajectory for a sample return mission to a Jovian Trojan asteroid. The time of flight is long, but a large payload can be delivered to the asteroid by using a solar power sail. Reducing the duration of a sample return mission is difficult, but it is possible for a one-way mission. This paper presents a trajectory design for such a one-way mission.</p>

<p>The latest re-acquisition of IKAROS, which rebooted after hibernation mode, was achieved on April 23, 2015. In this re-acquisition, post-processing of open-loop tracking data was performed to search for IKAROS. The open-loop method enables signal processing, even under difficult communication conditions. The algorithm for processing open-loop tracking data is proposed and is applied for actual signal from IKAROS. It is demonstrated that the signal can be well analyzed by removing the Doppler shift effect because of the orbital and spinning motion of IKAROS, even when the signal is too weak to be acquired by real-time operation. Using the method proposed in this study, estimating the spinning motion of a spacecraft and acquiring various data are possible as well as detecting signal. This research can be applied not only to a spinning-type spacecraft, such as IKAROS, but also to those with uncertain attitudes. </p>

Copyright © 2015 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved. This paper gives a detailed evaluation of the solar and thermal radiation accelerations acting on the IKAROS spacecraft consisting of a solar sail and a central body during its operational mission from June to December 2010. In particular, the predicted temperatures are compared with actual in-flight measurementsonthe sail membrane and on the body. The results show good correspondences in most cases, but a few appreciable deviations have been observed as well. The simulation results indicate that the magnitude of the thermal radiation perturbations on the solar sail trajectoryis below1%ofthose inducedby the solar radiation. The accelerations causedby the thermal radiation turn out tobe insignificant because their magnitudes are within the rangeofuncertaintyof the accelerations inducedbythe solar radiation forces.

This study estimates the effect of creases, or plastic wrinkle lines, on the out-of-plane stiffness of solar sails. A method to simulate the crease effects using reduced-order finite-element (FE) models is proposed, and is applied to a practical solar sail architecture. A detailed crease shape is determined by a geometrically nonlinear FE analysis using a simple model first; the effect of creases is then replaced by beam elements. This method substantially reduces the computational effort required for the FE analysis of a solar sail model. The result suggests the significant impact of creases on membranes out-of-plane stiffness when the tension level in the membrane is small.

The solar power sail is a deep space probe that will be powered by a hybrid propulsion system with solar photon acceleration and ion engines to explore the outer planetary regions of the Solar System without having to rely on nuclear power. The Japan Aerospace Exploration Agency (JAXA) launched the worlds first solar sail demonstration spacecraft, "IKAROS" (Interplanetary Kite-craft Accelerated by Radiation Of the Sun), in 2010. This spacecraft successfully demonstrated several key technologies related to the use of a solar power sail in a deep space flight environment. JAXA is currently planning an outer Solar System exploration mission using the demonstrated solar power sail technology, where the spacecraft will fly to Jupiter and perform a swing-by for a Jovian Trojan asteroid. This study undertook a trajectory design and system feasibility analysis for this mission. Candidate target asteroids were selected based on ballistic trajectory analysis, and then, electric-propulsion, continuous-thrust trajectory design was conducted to verify the conditions assumed for the ballistic analysis (e.g., estimate of steering loss due to the low-thrust trajectory). It was found that out of over 4000 Trojan asteroids, only 7 are feasible candidates considering the preliminary system design results. To broaden the choice of target asteroids, it will be necessary to reduce the weight of the solar power sail itself, its deployment mechanism, and the bus electronics.

Japan Aerospace Exploration Agency (JAXA) are planning an outer solar system exploration and sample return mission from a Jovian Trojan asteroid using a 3000 m² solar power sail. The difficulty of this mission is a severe restriction on the weight; only 300 kg is allocated for sampling and returning to Earth. In this weight, fuel for trajectory and attitude control, sampling mechanism, re-entry capsule, and other systems required to return to the Earth are included. In this paper, a preliminary analysis of this sample return mission is conducted. Three scenarios for sampling are proposed; sampling with a 3000 m² solar power sail, with a detachable small solar power sail using electric propulsion systems and with a small probe using chemical propulsion systems. The mission analysis shows that the most feasible configuration is to conduct the sampling with the 3000 m² solar power sail using an extension mast.

This paper proposes a method to store a large solar-sail membrane while ensuring repeatability of its stored configuration. Large membranes used as a solar sail should be stored compactly to save the launch volume; in addition, their stored configuration should be sufficiently predictable in order to guarantee reliable deployment in orbit. However, it is difficult to store a large membrane compactly because of the finite thickness of the membrane. This paper demonstrates the feasibility of the proposed "bulging roll-up" method experimentally using 10m-size membranes, and evaluates the repeatability of its stored configuration quantitatively.

A solar power sail demonstrator &ldquo;IKAROS&rdquo; deployed its square large membrane successfully by centrifugal force. The deployment consisted of two stages. The flight data suggest that the membrane deployment in the second stage was remarkably non-synchronous. This paper reproduces the non-synchronous deployment behavior using a small experimental model deployed in a vacuum chamber. Then, two kinds of finite-element analyses are conducted to show that the effect of membrane creases may have caused the non-synchronous deployment.

Piecewise Straight Fold is proposed for large solar sail membranes to simultaneously realize the high packaging efficiency and the simple folding. The fold pattern is based on Spiral Fold to reduce the deviation of the fold line, which improve the packaging efficiency. In addition, the fold pattern consists of piecewise straight fold lines, which approximate the Spiral Fold line, in order to simplify the folding. A simple manufacturing process of Piecewise Straight Fold is developed based on the Z-fold membrane. The preliminary experimental results show the feasibility of the Piecewise Straight Fold and its simple manufacturing process, where the high packaging efficiency is also verified. The wrapping fold experiments for the solar power sail membrane is demonstrated by using the Piecewise Straight Fold. In the wrapping fold experiments, the applicability of the Piecewise Straight Fold to the solar power sail membrane is verified.

The small solar power sail demonstrator IKAROS was developed and launched in 2010 by JAXA and deployed its large membrane successfully. Through IKAROS's on-orbit operation, one of the most critical problems was a sail's shape imbalance during the sail deployment sequence. This paper focuses on the evaluation and consideration of the sail's imbalance through high vacuum experiments using a scale-down sail model. The experimental results obtained using the specially designed rotating mechanism and a small scale model lead to a hypothesis of the reason for the IKAROS's imbalanced deployment. The experimental results are extrapolated up to IKAROS's scale, using dimensionless analyses.

IKAROS (Interplanetary Kite-craft Accelerated by Radiation of the Sun) was injected into the Venus transfer orbit by H-IIA rocket together with AKATSUKI on 21st May 2010. IKAROS has newly developed Reaction Control System called Gas-Liquid Equilibrium thruster. Gas-Liquid Equilibrium thruster loads propellant as liquid state and eject as gas state. Gas-Liquid equilibrium thruster is the thruster system suited for small spacecraft because of its easy handling depending on the non-toxic and non-inflammable propellant, which has relatively low pressure. This paper shows the development and operation result that this new thruster system work on good conditions at interplanetary trajectory compared to the prospect of performance.

In a heater control system including a number of elements, the power consumption may exceed the capacity of the power supply when a lot of elements require the power source simultaneously. In such case, the system has some risk of power shortages, while the time-averaged power consumption is enough low for the power supply. If an equipment gathers the information about the temperature of each elements and controls each switches not to turned on simultaneously, the over consumption would be prevented. However, this system needs many cables between center equipment and each elements, and becomes complex and lacks versatility. To solve these problems, authors has proposed a novel multi-agent heater controller that maintains the temperature at each element, while the whole power consumed is kept constant, without any specific server in the system. In this paper, a new developed multi-agent heater controller for spacecraft is introduced and results of experiments to verify the control system are shown. The results are compared with the results of simulations.

Solar Power Sail is a novel spacecraft with hybrid propulsion of large-area solar sail and electric propulsion driven by thin-film solar panel. The Solar Power Sail spacecraft is currently developed to probe into the outer-solar system bodies out to the Jupiter orbit, which was not well precedently explored. This mission aims to explore Trojan asteroids and interplanetary dusts by remote sensing and in-situ sampling using orbital rendezvous or fly-by technique. Scientific objective of this mission is not only planetary science but also novel astronomy; measurement of the infrared extragalactic background light without foreground contamination of the zodiacal light thanks to low-density environment at deep space, polarization measurement of the gamma-ray burst and accurate determination of its direction based on the interplanetary network technique. The Solar Power Sail mission will thus develop a new direction of space astronomy and planetary science providing us an interplanetary telescope site and will play an important roll to form a new interdisciplinary science field.

The combination of large membranes and light-weight deployable booms, often called a gossamer structure, has enabled innovative space missions, such as solar sailing, to become possible. Though many designs have been proposed and demonstrated, two problems remain regarding the folding patterns of the membranes. The first problem involves considering the thickness of a membrane to enable uniform and compact folding. The other involves membrane-folding patterns that allow for connecting the membrane to the booms at multiple points and deploying them together while minimizing the use of complex mechanisms. This study proposes three methods that consider the thickness, and two of them can keep the crease lines straight, in contrast to the conventional non-straight crease line solutions. In addition, this study derives one effective design to integrate a membrane with diagonal booms through the systematic classification of existing membrane folding patterns. © 2013 IAA.

Two separation cameras and four fixed monitor cameras are mounted on solar sail demonstrator &ldquo;IKAROS&rdquo; in order to get panoramic view of the sail. Both cameras were designed to be wide field of view, and this cause huge lens distortion. In this paper, we describe the method to correct lens distortion and estimate the dynamics of separated cameras based on bundle adjustment, and also measure the displacement of sail tips using images taken by the two different kinds of cameras on orbit. For 3-dimensional shape estimation of the membrane, we use renormalization technique for removing statistical error bias, and find that the membrane is slightly bent to the direction of the sun.

EJSM (Europa Jupiter System Mission) is an international Jovian system mission with three spacecraft. Coordinated observation of Jovian magnetosphere is one of the important targets of the mission in addition to icy satellites, atmosphere, and the interior of Jupiter. JAXA will take a role on the magnetosphere spinner JMO (Jupiter Magnetospheric Orbiter), whereas ESA will launch JGO (Jupiter Ganymede Orbiter) and NASA will be responsible for JEO (Jupiter Europa Orbiter). One possibility is to combine JMO with a proposed solar sail mission of JAXA for Jupiter and one of Trojan asteroids. Since Trojan asteroids could be representing raw solid materials of Jupiter or at least outer solar system bodies,involvement of Trojan observation should enlarge the scope and enhance the quality of EJSM.