Strategy of JAXA’s Lunar Exploration Using a Lunar Rover

Shin-Ichiro Nishida, Sachiko Wakabayashi

Lunar and Planetary Exploration Center, Japan Aerospace Exploration Agency e-mail: nishida.shinichiro@jaxa.jp Abstract JAXA is carrying out research and development of a mobile robot (rover) aimed at base construction and searching for rocks and soils on the lunar surface. The target areas for base construction and lunar exploration are mainly in mountainous zones, and the moon’s surface is covered by regolith. Achieving a steady run on such irregular terrain is the big technical problem for rovers. A newly developed lightweight crawler mechanism is good for driving on such irregular terrain because of its low contact force with the ground. This was determined considering the mass and expected payload of the rover. This paper describes the technical issues of the rover designed for lunar exploration and base construction work, and presents the results of study into methods of dynamics testing and analysis which are needed in its development. This paper gives an overview of the SELENE-2 lunar exploration project using a lunar rover and the composition of its mobility and control system.

1. Introduction high vacuum and strong radiation. There is a large One of the goals in JAXA’s long-term vision is to temperature range with a sharp transition between lit and advance the exploration of the moon’s surface, and unlit areas. This is a far more severe environment for a missions to survey and investigate the lunar surface space machine than, for example, earth orbit. On the following on from SELENE are being studied. The other hand, Mars has an atmosphere, albeit thin, which southern polar region of the moon is the leading candidate allows for easy lubrication of mechanisms, the location for the construction of a lunar base because temperature difference between day and night is lower sunshine conditions are good and there is a high than on the moon and the temperature changes at a slow possibility that resources such as water are present. rate. On Mars, surface particles are roundish due to Missions are expected to use robotic technologies such as weathering, while the regolith which covers the moon’s mobile rovers. However, the lunar surface is a severe surface is sharp. And since the regolith which has environment that presents many technical challenges for covered the moon's surface on the other hand is in the exploration and survey activities, particularly since there state which lay soft, it is not easy for a rover to run on it. are many unknowns about the geographical features and Thus, the moon’s surface is severe environment for environment in the south polar region. machines, and designing mechanisms that can operate This paper introduces the strategy of SELENE-2 lunar under such conditions is technically challenging. exploration. And it presents the main technical issues for lunar exploration rovers, technical roadmap of their 2.2. Polar Zones development and the results of studies into a rover system In the polar regions of the moon, heights such as crater configuration. rims may be permanently sunlit while the bottoms of

2. The Lunar Surface Environment 2.1. Environment Comparison Table 1 compares the surface environments of the moon and Mars with that of the earth. The lunar surface is a severe environment, having a

Table1 Comparison of environments with Earth Parameter Earth Moon Mars Gravity (G) 1 0.17 0.38 Pressure (Pa) 1 10-9 0.007 Temp. range (°C) –15/+40 –120/–20 –100/+15 Figure 1 Artist concept of SELENE-2 rover Soil particles Round Irregular Round craters may be permanently shaded, especially in the 4. Rover Technical Issues and System south polar region. Although it is assumed that a lunar Configuration base would be sited at a permanently sunlit location, The concept of SELENE follow-on mission rover is since these are in mountainous areas there are many illustrated in Fig. 1. surface undulations and the surface inclination is The technical issues facing robots that operate on the expected to range from level to up to 30 degrees (the rest lunar surface and possible solutions are described below. angle of regolith), with an average slope of about 15 4.1. Electrical Power and Thermal Control degrees. The inner walls of craters are even steeper. As explained above, a rover cannot be expected to be Moreover, since the incident sunlight shines almost continuously exposed to sunlight and moreover, horizontally, slight surface undulations produce large temperatures are always low at the south pole. This shadowed domains. presents a severe challenge for the power supply and For these reasons, the south polar region contains the thermal control of a rover operating in shadowed regions. most areas where the surface receives the least sunlight. Since sunlight is incident horizontally, a solar array raised Therefore, it is expected that it cannot fully receive vertically above the rover may come out of shadow and be sunlight even if a low height vehicle like a rover chooses used for photovoltaic power generation. However, it is and runs a path. Moreover, there are strong local thought that it would be necessary to raise such a solar temperature variations, ranging from between –30 and array fairly high. –50 Celsius in sunlit areas down to –230 Celsius in Since there is at present inadequate detailed shadows. geographical feature data, such as on undulations of the

moon’s surface, it cannot be guaranteed that any 3. Missions and means for their Realization photovoltaic power generated will be adequate, so 3.1. Lunar Robotic missions alternative means of power supply must be examined. The following are set as major objectives for robotics If the use of radioisotopes is ruled out, then in areas in lunar exploration missions after SELENE. without detailed topographical and sunshine information, a. Survey for lunar exploitation power may be supplied from a lander by a cable, and the b. Scientific exploration rover would be equipped with a reel holding the cable. c. Demonstration of robot technology for This appears to be a promising solution, especially for the outpost construction and operation initial check-out period. However, if power from the d. International collaboration lander is used to charge a battery in the rover which then Astronomical observation from the moon is also being disconnects and moves away to explore, and the battery considered as a science objective. then runs low, a system which must return to the lander to Investigating soil and foundation characteristics will recharge has a high possibility of discharging before be important for outpost construction. Validation of rover reaching the lander if unexpected difficulties are technology and trials of position measurement encountered, and recovery may be impossible. Such a technologies to detect surface movement are also possible solution is therefore not acceptable. However, if the mission objectives. upper part of the lander is a few dozen meters above the moon’s surface and can receive sunshine for long periods, 3.2. Mission Scenarios a system [2] which directs sunlight toward the rover using To realize each of the missions mentioned above, a a heliostat would have merits of low mass and high number of task elements can be identified as shown in efficiency. Table 2. The following methods can be considered for the In addition, at prospective outposts, it is thought that a analysis of the soil and rock samples collected by a rover. power generation tower that generates large amounts of A. Analyse and process samples on the rover. electric power from sunlight and a regenerative type of B. Return samples to a lander for analysis and fuel cell will be essential. processing System B increases mission time drastically if multiple 4.2. Traction Mechanism sites are to be surveyed and exposes the rover to much Since the regolith has piled up 10cm or more, the greater mechanical strain, and so a study of system A is support force of the ground is weak. For this reason, when prioritized although it requires greater complexity on the a rover runs the moon's surface, slips and subductions may rover. occur. Table 3 shows the results of a study comparing various Table 2 Task elements of lunar rover types of traction systems. The performance over Run to the target regolith-covered slopes and the traveling ability over Mapping of terrain rough terrain are thought to be important criteria, and Measurement of environment and position system selection and development are furthered. Abrasion rocks In the hill-climbing performance of the slope covered Observation rocks and soils with the regolith, the good testing result is obtained by Pick-up rocks and soils the traction system of Locker-crawler type. This is based Coring rocks and soils on the effect of low grounding pressure and bundle Installation mission equipments hardening by the crawler belt. Optimization of the belt and suspension mechanisms are furthered.[3]- [4] (An outdoor run testing situation: Fig. 2) sun sensor. Dead reckoning based on the odometeric Since regolith has sharp edges and contains many fine information from the traction system would not be particles, it acts as an abrasive and measures are needed sufficiently accurate due to errors from sliding over the to protect components in contact with regolith from regolith. Using a radio emitter or optical reference on the strong abrasion. Moreover, it is necessary to seal moving lander or star tracker and a sensor on the rover is components against dust. A prototype rotary seal is effective for position determination but relies on an shown in Fig. 3. optical or radio line of sight being available. Therefore, determining changes in position by reference to Table 3 Comparison of Traction Systems geographical features, using for example range finding and/or stereo image sensors, in considered essential. Methods Merits Demerits Promise Equipping the rover with a stereo imaging sensor is Rigid Simple High ground pressure seen as essential, and since there are many domains in wheel shadow, LED or flash lighting will be required for it to operate. Obstacles can also be directly detected using a Flexible Simple －Middle ground pressure rangefinder, and equipping the rover with a small laser Wheel －Large dia. rangefinder to enable three-dimensional measurements is －Low controllability desirable. From these considerations, the rover course Crawler Low ground Complex mechanism measurement / control system will ideally have the pressure components shown in Table 4, although resource Light －Low ground × restrictions may force some items to be omitted. The Crawler pressure concept of the component allocation on a rover is shown in Fig. 4. －Simple mehcanism Table 4 Sensors for course measurement/control system Fiber optical gyro Star tracker Stereo camera Wide field camera Laser range finder

Figure 2 Outdoor run testing situation

Figure 4 Conponent allocation

4.4. Operation and Control To allow earth-based controllers to monitor the rover, it will be necessary to transmit images from a forward-facing sensor on the rover at a fixed rate. It will also be necessary to construct a continuously updated three-dimensional map of the lunar surface on the ground based on images or measurements received from the

Figure 3 Prototype rotary seal lander. If based on this information operators can transmit detailed path instructions, rover control will not 4.3. Position Determination require a high degree of autonomy. However, given a time lag of about eight seconds between sending a Determination of own position using signals from earth command signal from the earth to receiving a feedback orbiting GPS satellite is difficult on the moon, and since signal from the rover, considering a three second signal the sun may not be visible over much of a path over the round-trip time between the earth and the moon and moon’s surface, direction measurement cannot depend a adding signal processing and computing delays, the rover gravity. Furthermore, the operating temperature range is will need to autonomously steer itself to follow the very large and protection is required to prevent regolith commanded path and to handle slipping or sliding. dust from entering the mechanism. An outline of the functional assignment of the rover’s Candidates for the manipulator mechanism include DC operation / control system between the terrestrial and brushless motors, which are often used by terrestrial lunar based elements is shown in Fig. 5, and the output of robots, mechanisms from other space flight projects a prototype ground–based course generation function (ETS-VII[1][2], MFD, etc.), and harmonic drives. Since system is shown in Fig. 6. loads are high compared to apparatus installed on It will be difficult to install a powerful communication satellites, a direct drive mechanism are not considered system on the rover due to restrictions in electrical power suitable. or rover mass (50–100kg), the communication with the For structural assembly or wiring (cable construction earth serves as a line through the lander. But since the case and combination of connector) work, relative positioning where the visible nature of the lander is lost is also to other objects and force control will be needed [5]– [6]. considered, communication with the earth via Relay is Since the work will be carried out by a rover not also taken into consideration. stabilized by a scaffold, a grip interface with high grasping stability and positioning marking[7] are essential. An experimental four joint rover robot arm is shown in Fig. 7.

Figure 7 Prototype rover robot arm

Figure 5 Functional assignment of rover control

Figure 8 Rover deployment mechanism

Figure 6 Course generation function

4.5. Manipulation Although the moon’s gravity is only one sixth that of earth’s, a robot arm needs to support the weight of its wrist or a grasping payload, and needs to operate, and actuator loads will be greater than for an arm and drive mechanism operating in earth orbit. Depending on the load, the contact pressure upon bearings is increased by the reduction gear mechanism’s contact pressure and by Figure 9 Testing of hill climbing performance system are included in the operation facility and verified 4.7. Carriage and Deployment by dynamic closed-loop testing in combination with the To ensure sufficient hill-climbing capability and dynamical simulator of a the payload system. mobility of a rover operating on the regolith-covered The dynamics simulator for dynamic closed-loop lunar surface, it is necessary that the ground contact area testing is built into operation facility as it is, and is used of the rover’s traction system is large and the ground as an operation simulator used for prior verification of pressure low. For this reason, as for the traction system, it operation commands and procedures. Although various is desirable that the system be as large as possible within irregular test fields have been constructed in an indoor mass constraints. In order to transport such a rover in a laboratory (Fig.11) and are being used for closed-loop lander and to deploy it, it will be necessary to carry it in a testing of measurement / control function, outdoor field folded state to some extent, to unfold it after a landing on testing is also effective for verifying measurement / the moon and to release it from the lander onto the control performance during long-distance runs over moon’s surface. For this reason, the equipment which natural features. takes down a rover from a lander and deployment mechanism of a rover itself is needed. A concept of a rover 6. Conclusion deployment mechanism using a mainly passive hinge is This paper describes the results of work in progress on shown in Fig. 8. the technical issues related to surface exploration by If the rover must return samples to the lander for robotic rovers of the south polar region of the moon, storage or analysis, a mechanism is needed on the rover currently assumed as a major candidate for future or lander to transfer the samples. If the rover is activities. In order to realize a rover, it is important to equipped with only one robot arm, the demanded consider the lander and rover as a total system regarding performance range of the robot arm becomes large, and it electrical power, communication, division of work, and may be difficult to design one arm to meet the deployment. requirements of both sample collection and sample Work will now progress on defining the composition transfer. Therefore, these systems must be considered of the total lander and rover system. Meanwhile, individually. technical development and prototype production of elements such as the rover traction mechanism and 5. Rover Testing geographical feature detection and measurement sensors 5.1. Dynamics Simulation will continue in parallel. In computer simulation of the dynamics of the rover motion, a technical issue is modeling slipping of the traction system over the regolith surface. Quantitative evaluation of slope climbing performance was achieved efficiently using a test slope covered with regolith simulant that can be set at arbitrary incline angles, shown in Fig. 9. It is clear from results using this equipment that the rate of slippage while traveling over the surface changes according to an angle of slope. An undulating geographical feature model that simulates the lunar surface was then incorporated into the computer simulation, and a simulation in which surface friction coefficient changes according to the degree of incline was developed. The form of a model and an example of an analysis of an eight-wheeled crawler traction system with a locker mechanism are shown in Fig. 10. A dynamical simulation needs to stuff especially flexibility allocation of the suspension of the traction system and servo system parameters in detail.

5.2. Measurement / Control System Testing To testing the function and performance of measurement sensors or integrated functions, it is first necessary to adequately simulate the environment, such as lighting and geographical features, and to qualify the simulation. The test environment needs to simulate the incident solar lighting at the latitude of the assumed landing site and the interplay of the light on geographical features. Figure 10 Rover dynamics simulation model and a result Functions such as geographical feature map generation and course designation that are assigned to the ground

Figure 11. Lunar rover testing facility

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