Tetsuo YOSHIMITSU

大気球シンポジウム 平成27年度（2015年11月5-6日. 宇宙航空研究開発機構宇宙科学研究所 (JAXA)(ISAS)）, 相模原市, 神奈川県資料番号: SA6000044003レポート番号: isas15-sbs-003

大気球シンポジウム 平成26年度（2014年11月6-7日. 宇宙航空研究開発機構宇宙科学研究所 (JAXA)(ISAS)）, 相模原市, 神奈川県著者人数: 18名資料番号: SA6000021014レポート番号: isas14-sbs-014

JAXA is planning to launch Hayabusa 2 in December 2014. Hayabusa 2 will be equipped with an asteroid exploration rover which is developed by the consortium of universities. A goal of the rover is establishment of the technology for locomotion on asteroid surface. In our previous work, we proposed a new moving mechanism called "Environment-Driven Torquer, EDT." The EDT is driven by environment temperature with bimetal. In other words, the EDT can move the rover without both battery and CPU. We have developed an engineering model with magnet latch. In this paper, performance evaluation of the EDT is executed by free-fall experiments in ZARM.

Asteroid probe Hayabusa 2 is developed by JAXA. Asteroid exploration rovers will be carried by Hayabusa 2. In our research project, we have developed an environmental-temperature-driven buckling actuator with a bimetal thin plate for a hopping mechanism of the rover. In this study, to characterize the hopping capability of the actuator, we conducted microgravity examination in ZARM drop tower. We fabricate a test rover for the microgravity examination with a rapid heating system for actuation in short free-fall time. Although reaction force from baseplate was incomplete in take-off, the rover hopped with a vertical velocity of 3.12 mm/s with 0.035 rad/s rotating rate.

Asteroids have some tips to know the origin of the solar system. In recent years, asteroid exploration by surface explorers has been studied actively. Conventional rovers can move under microgravity, but the rovers have not considered thermal control. In this paper, a new type of rover is proposed to move under microgravity and to control its temperature. The effectiveness of the proposed rover is investigated by simulations.

Japan has decided to launch the second sample return mission "Hayabusa-2" to the Near Earth asteroid 1999JU3 in 2014. The predecessor spacecraft "Hayabusa" made a great success when it returned to the Earth in June 2010 with a capsule containing some particles obtained around the S-type asteroid "Itokawa." The authors installed a tiny hopping rover called "MINERVA" into Hayabusa spacecraft. The rover was deployed at the vicinity of the asteroid in 2005, but failed to make a surface exploration since the human operator on the ground made a critical failure in deploying the rover. The second spacecraft also involves a plan to have a tiny rover system which will make a surface exploration over the 1km-sized asteroid. With the past experience in developing a rover, the authors are again working to install some rover packages to Hayabusa-2. The total concept is the same but this time multiple rovers are considered. Many of the aspects of the mother spacecraft come from the heritage of the previous explorer which was build using the technologies more than ten years ago. But the rover system is a completely brand-new one, based on the lessons learned from the previous rover system. Since the target asteroid parameters are different from the previous target, the rover design has to be made from the beginning. We also face to the another technically challenging matters arisen from the point of the distance from the Sun as well as the surface cruel temperature of low-albedo body. This paper describes the system configuration of the rover system currently designed and developed for the launch in 2014.

The authors are involved in the development of a tiny rover system onboard "Hayabusa-2" spacecraft which is scheduled to launch in 2014 for the Near Earth asteroid "1999JU3." The development of Hayabusa-2 started very urgently after its predecessor spacecraft "Hayabusa" made a self-sacrificing reentry into the Earth in 2010. The authors have a heritage how to develop a surface exploration rover through the experience on "MINERVA" rover in Hayabusa mission. But the rover system for Hayabusa-2 is a brand-new one since it is designed to fit the new target asteroid, of which surface environment and conditions are different from those of the previous one. The mobile system of the primary rover has the same strategy on its hopping movement as MINERVA, but is slightly different on the configuration of the actuators. This paper describes the mobile system of the primary rover onboard Hayabusa-2.

In lunar or Martian exploration, compact and ultra-light rover is required or Smart Lander for Investigating Moon (SLIM) as the optional rover from the viewpoint of cost reduction and short-term development. Inflatable structure technology is most appropriate way for such ultra-light rover. However, there are not enough researches on rovers with shock absorbing inflatable wheel. Basic architecture of the ultra-light rover wheel with shock absorbing using inflatable structure technology is clarified.

The planetary surfaces are covered with soft soil, so the wheel of planetary rover easily slips and loses the traction. This paper presents a terramechanics-based wheel dynamics model to improve the rover mobility via considering the dynamic wheel sinkage. Terramechanics is a study of soil properties and an interaction between the wheel and soft soil. Since terramechanics-based wheel model considers only the static state of wheel sinkage, the wheel model is not applicable to the dynamic condition. Understanding the traction mechanics in the dynamic wheel sinkage is important in order to predict the wheel motion in the static state. We propose the dynamic normal force model to treat the dynamic wheel sinkage via considering the state variation of soil condition. Besides we present the wheel driving model in the dynamic wheel sinkage by considering the shear deformation modulus. We evaluate the proposed models by simulations.

Planetary surface is covered by soft soil. Thus wheel slips and gets stuck easily. Wheel slippage causes wheel stuck, navigation error, and difficulty in climbing slopes. Purpose of this research is to develop wheel control system for efficient travelling on soft soil. The efficient travelling is that a rover is able to travel on high speed and small wheel slippage on soft soil. Considering strong control system for disturbance, it is important to incorporate terramechanics theory. In this paper, wheel dynamics model based on terramechanics is proposed, and the dynamics model is verified it is appropriate for dynamic motion by simulation analysis.

Many space missions are being undertaken to improve our understanding of the solar system and how to use space resources. Nowadays, since MER's success, exploration of planetary surfaces is being focused on. Rovers have the capability of reaching important scientific areas on planetary surfaces. Rovers traverse unknown environments in limited missions. Therefore, rovers are expected to have the ability to explore as widely as possible and to navigate themselves efficiently. The conventional navigation systems based on stereo vision or laser range finder are not wide enough to obtain distant area environment information. Since understanding wide environment makes rover navigation more efficient, detailed terrain maps are not necessarily required. This paper proposes an efficient image- based navigation by a single camera.

Investigations into small planetary bodies are becoming more attractive, and some missions to small bodies have already been sent. As such bodies have numerous geographical features, the next requirement is direct surface investigations by rovers. Localization of rovers is required to navigate them to specific points. However, conventional methods of localization are not adequate on small bodies because of the small body's irregular shape and small mass. We proposed an efficient method of localizing rovers on small bodies. It uses two-way range measurements between the rover and mother spacecraft. The measurements are conducted repeatedly. This method can be used over the whole surface of the investigating body, and it only needs one spacecraft. Numerical simulations demonstrated the effectiveness of the proposed method assuming an Itokawa-sized-asteroid, whose maximum diameter is 600[m]. In this paper, the sensitivity direction is analyzed, which indicates the preferable orbit of spacecraft and the sufficient number of measurement.

Lunar or planetary exploration robots (rovers) move to long-distance destination under the limited conditions such as unknown environment and time delay. Rovers are required to move autonomously and efficiently. Rovers need the ability to recognize the environment widely and to plan route and sensing depending on the situation. This paper proposes a behavior planning scheme for rovers based on environment understanding. The proposed method includes route planning and sensing planning.