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: [email protected] 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.