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Introduction to Japanese exploration study to the moon
Acta Astronautica 104 (2014) 545–551
Contents lists available at ScienceDirect
Acta Astronautica journal homepage: www.elsevier.com/locate/actaastro
Introduction to Japanese exploration study to the moon T. Hashimoto, T. Hoshino, S. Tanaka, H. Otake, M. Otsuki, S. Wakabayashi, H. Morimoto, K. Masuda n Japan Aerospace Exploration Agency (JAXA), Japan
a r t i c l e i n f o
abstract
Article history: Received 29 January 2014 Received in revised form 4 June 2014 Accepted 13 June 2014 Available online 28 June 2014
The Japan Aerospace Exploration Agency (JAXA) views the lunar lander SELENE-2 as the successor to the SELENE mission. In this presentation, the mission objectives of SELENE-2 are shown together with the present design status of the spacecraft. JAXA launched the Kaguya (SELENE) lunar orbiter in September 2007, and the spacecraft observed the Moon and a couple of small satellites using 15 instruments. As the next step in lunar exploration, the lunar lander SELENE-2 is being considered. SELENE-2 will land on the lunar surface and perform in-situ scientific observations, environmental investigations, and research for future lunar utilization including human activity. At the same time, it will demonstrate key technologies for lunar and planetary exploration such as precise and safe landing, surface mobility, and overnight survival. The lander will carry laser altimeters, image sensors, and landing radars for precise and safe landing. Landing legs and a precisely controlled propulsion system will also be developed. A rover is being designed to be able to travel over a wide area and observe featured terrain using scientific instruments. Since some of the instruments require long-term observation on the lunar surface, technology for night survival over more than 2 weeks needs to be considered. The SELENE-2 technologies are expected to be one of the stepping stones towards future Japanese human activities on the moon and to expand the possibilities for deep space science. & 2014 IAA. Published by Elsevier Ltd. All rights reserved.
Keywords: Lunar exploration Precision and safe landing Surface mobility Night survival
1. Introduction JAXA launched the Kaguya (SELENE) lunar orbiter from Tanegashima Space Center on September 2007. During the time Kaguya was in lunar orbit until it impacted into the southeast region of the Moon in June 2009, Kaguya made many contributions to science, obtaining scientific data about the origins and evolution of the Moon and developing technology for future lunar exploration [1]. As a next step, the SELENE-2 mission is set take over Japanese efforts at lunar exploration with a landing on the moon [2]. SELENE-2, the first Japanese lunar lander, is
n
Corresponding author. E-mail address: [email protected] (K. Masuda).
http://dx.doi.org/10.1016/j.actaastro.2014.06.031 0094-5765/& 2014 IAA. Published by Elsevier Ltd. All rights reserved.
expected to bring major advances in both space technology and science. From a technological perspective, SELENE-2 offers a number of capabilities—precise and safe landing, surface mobility, and overnight stay on the moon surface— all of which are key technologies for future lunar exploration activities. The lander will attempt to achieve a precise and safe, soft landing by using laser altimeters, image sensors, and landing radars. From a scientific perspective, SELENE-2 will perform in-situ observation, environmental investigations, and research for future lunar exploration at the landing site. The launch is currently targeted for 2018. To follow on from SELENE-2, work is already underway on robotic landers designated SELENE-X (where X starts from 3). Although the SELENE-X missions have not been completely defined as yet, several candidate missions such as sample return from the moon are being considered. This
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T. Hashimoto et al. / Acta Astronautica 104 (2014) 545–551
paper also introduces the current vision for SELENE-X missions leading to future human exploration of the moon. 2. The SELENE-2 mission 2.1. Why go to the moon? It is important to discover new facts about the Earth, the Moon, and other planets in order to learn about where we came from. Planetary exploration in the past has provided us with a variety of facts, and current common knowledge consists of all such facts accumulated throughout history. From the viewpoint of learning our potential for expanding the sphere of human activities, we are still missing many facts about the mechanism of planetary formation, with little known about even the Moon, which is the nearest celestial body to Earth. In terms of technology, the Moon is a best test bed for demonstrating new technologies for planetary exploration. The near side of the Moon always faces Earth, making it easy for a lander to communicate with Earth. From a social viewpoint, the Moon, like the Sun, is very close to daily life. Lunar exploration is therefore a good example for education and outreach of science and technologies. Lunar exploration is also considered an area of international collaboration, with human exploration in particular requiring extensive collaboration.
meet the mission requirements and to optimize the spacecraft design, some trade-offs are being considered. For example, whether to use direct lunar transfer orbit (LTO) insertion, geosynchronous transfer orbit (GTO), or another special launch orbit is under discussion. The selection depends on the launch vehicle performance, efficiency of the spacecraft propulsion system, launch window requirements, and other factors. The size of the orbiter is also a design factor. The smallest case is a spin-stabilized communication relay orbiter without a propulsion system like Kaguya's small satellites. In this case, a relatively large lander would descend to the surface, but this would require much fuel. By comparison, the biggest case is an orbiter of a few tons with a propulsion system for insertion into lunar orbit. This gives the smallest lander size and also minimizes the fuel required. In general, as the total spacecraft system becomes larger and the orbiter becomes bigger, a smaller lander becomes more advantageous for maximizing the mission payload. A tentative mass budget for the SELENE-2 spacecraft is shown in Table 1. The Japanese H2A rocket can carry a lander of around 1 t (dry weight) to the moon surface. This corresponds to a payload of a few hundred kilograms including a 100-kg rover. An artist's drawing of the SELENE-2 configuration is shown in Fig. 1.
2.2. Mission definition SELENE-2 is the first Japanese lunar lander and since the purpose of the mission is lunar exploration, the mission objectives are as follows [3]: I. Development and demonstration of key technologies for future exploration. Safe and accurate landing technologies. Surface mobility: rover. Night survival technologies without using radio isotope energy. II. In-situ observation and investigation for science and future lunar utilization. Detailed and sub-surface geological observation. Geophysics for discovering the interior structure of the moon. Measure the moon environment for future utilization. III. Contribution to international space exploration activity and meeting the public interest. International payload. Outreach or educational payload. In order to achieve these objectives, a configuration consisting of a 1-ton class lander and a 100-kg class surface mobile robot (rover) is being considered. From among the Japanese launch vehicles, the H-2 A rocket is satisfactory for the SELENE-2 configuration.
Table 1 Tentative mass budget of the spacecraft (GTO launch case). Orbiter
Lander
Bus system Mission payload Fuel Total Bus system Mission payload Rover Bus system Mission payload Total Fuel Total
Total
600 kg 100 2400 3100 700 200 80 20 100 1700 2700 5800
Orbiter
Rover
Lander
2.3. Spacecraft configuration The SELENE-2 consists of a lander and an orbiter. The lander carries a rover and mission payloads. In order to
Night survival units Fig. 1. Artist's drawing of the SELENE-2 configuration.
T. Hashimoto et al. / Acta Astronautica 104 (2014) 545–551
2.4. Precise and safe landing system An engine of approximately 3000 N with throttling capability is required for the descent propulsion system of a one-ton class lander (Fig. 2). Since developing a new engine takes a long time and costs a lot of money, a cluster of several space-proven 500 N thrusters employing onand off-modulation is to be used for the SELENE-2 landing. The spacecraft carries a laser altimeter, image sensors, and a landing radar for precise and safe landing. JAXA has built upon its experience with KAGUYA's laser altimeter (LALT) [4], HAYABUSA's laser altimeter (LIDAR) [5], and HAYABUSA's optical navigation camera (ONC) [5]. The development of the landing radar has been going on for years. A breadboard model (BBM) of the radar has been completed and field-tested on board a helicopter [6]. Landmark optical navigation is needed for a precise pin-point landing system. A study has found that accuracy to within a few hundred meters will be able to be accomplished using matching between landmarks in onboard images and those from a lunar surface map [7]. The reconstruction of three-dimensional information from two-dimensional images is a key technology for obstacle detection. Some image processing algorithms have been examined [8]. Landing legs have been developed based on years of studies on touchdown dynamics and stability analysis, and knowledge of soil mechanics has been essential for their development.
547
manipulator arm. We are developing a lightweight durable arm that employs ultra-sonic motors. A grinding tool is essential for geological observation because the internal structure of rocks provides very important information for geologists. A tool changer is essential for obtaining grinding, microscopic imaging, and other observation capabilities from a single arm. 2.6. Night survival We are developing various kinds of night survival technologies for surviving the long, cold night on the moon surface. One approach is to use a sophisticated thermal design. At night, heat emissions are minimized by using multi-layer insulator (MLI) shielding [10]. In laboratory experiments, the survival unit is able to stay above zero degrees Celsius for 2 weeks using a heater of a few watts. The weight of the required battery is then around a few tens of kilograms. Key technologies for this option are reduction of instrument power consumption, thermal isolation during the nighttime for more than 2 weeks, and efficient daytime heat radiation. Another option is to “survive in sleep”. The instruments are switched off at night and designed so as to avoid damage at very low temperatures. When the temperature increases again after sunrise, the instruments resume operation. We are now conducting very low temperature tests on various instruments and parts Fig. 3.
2.5. Surface mobility
2.7. Observation instruments
The rover is designed to be able to travel over a wide area and to observe featured terrain using scientific instruments. For geological science, observation of the internal structure and material of rocks is essential. In terms of rover development, mobile gear, navigation sensors, path planning algorithms, and environmental tests have been studied [9]. Slope climbing on lunar regolith is one particular challenge, and we have developed and compared the performance of both wheel-type and crawler-type mobile gear. Observation support tools for on board the rover are also being developed. One is a
Detailed geological analyses of representative areas such as the Procellarum KREEP Terrane (PKT) and Feldspathic Highlands Terrane (FHT) may enable us to clarify the formation process of magma oceans [11]. For this purpose, a multi-band camera or several spectrometers are required to detect minerals and elements. Geophysical observations are also indispensable for determining the interior structure of the moon. Seismic, electromagnetic, and heat flow observations are important. Since seismic observations require a global “network”, the possibility of establishing international coordinated multi-point
Fig. 2. Landing radar test over Mount Aso in Japan.
Fig. 3. Rover test at Nakatajima beach in Japan.
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observations needs to be discussed. Geodetic observations for detecting the precise motion of the moon are another promising method for finding out about the interior. Research on the lunar surface environment is also important especially for future human activity. Various instruments for measuring the environment are being considered. The third kind of mission objectives for SELENE-2 are for political, educational, cultural, and diplomatic purposes. HDTV from the Kaguya spacecraft provided clear and wonderful video of the Moon and the Earth, including the famous “Earth rise”. SELENE-2 plans to capture similar lunar surface videos for public outreach. The priority of onboard instruments was discussed during the Phase A study. Mission instrument candidates and the suitable locations for them on board SELENE-2—on the lander (L), rover (R), or orbiter (O)—are categorized as follows: 1. Geological observation of particular areas: Multi-band panoramic camera (L and/or R). Gamma-ray spectrometer (R). Microscopic camera with grinding mechanism (R). Active X-ray spectrometer (R). Laser induced brake-down spectrometer (R). 2. Geophysical and geodetic observation for determining the interior structure: Broad-band seismometer (L). Heat flow probe (L). Electromagnetic measurement (L and O). Reflector for lunar laser ranging (L). Radio source for VLBI measurement (L and O). 3. Astronomy from the moon: Low frequency radio measurement (O). 4. Research on the moon surface environment: Radiation monitor on the moon surface (L or O). Mechanical characterization of soil (L and/or R). Space dust detector (O). 5. Outreach payload: HDTV (L and/or O and/or R). International payload (TBD, L).
Fig. 4. Drop test of the landing legs.
Fig. 5. Hill climbing test of the rover.
3. Technology development status 3.1. Landing gear We have been conducting drop tests by changing the parameters of the horizontal velocity, vertical velocity, pad size, and surface hardness (Fig. 4). Shock acceleration, sinking depth, and spacecraft touch-down dynamics are being observed.
3.2. Rover and onboard tools We have developed prototypes of mobile gear and performed some hill climbing tests (Fig. 5) and tests of getting over rocks. We are designing the optimum mechanics of the mobile gear in consideration of the weight and power resources. Development of onboard tools for observation support is also important. A prototype model of
Fig. 6. Tool changer.
a tool changer is shown in Fig. 6. The instruments installed on it are a rock abrasion tool, brush, optical camera, infrared imagery camera, vane shearing measure, hand, and force torque sensor. Motors that are durable in a high and low temperature environment need to be developed for the
T. Hashimoto et al. / Acta Astronautica 104 (2014) 545–551
rover mobile gear as well as a manipulator including a tool changer. We have confirmed the thermal resistance of each part of the motors in vacuum, that is, 130 to þ220 1C in operational and 200 to þ220 1C in storage conditions. 3.2.1. Night survival units To confirm the feasibility of night survival units, we have been conducting both numerical simulations and thermal vacuum tests. Fig. 7 shows the results of numerical simulation of the temperature of the survival unit and the moon surface. Although the temperature of the moon surface outside the unit varies in the range of –200 1C to þ100 1C, the temperature inside the unit is kept in the range of 0–50 1C. Fig. 8 shows a photo of a BBM of the night survival unit used for thermal vacuum tests. Another key technology for night survival is high performance batteries [12]. The lithium-ion battery used in the system requires a very high energy density. In
K 400 380 360 340 320 300
549
addition, it needs to be able to operate over a wide temperature range. However, only a few charge and discharge cycles are required. Since the total period of day plus night on the lunar surface is around 1 month, there are only 13 charge and discharge cycles in a year. Fig. 9 shows a photo of the prototype cell. The target energy density of the cell is more than 200 Wh/kg. Table 2 summarizes the specifications and performance of the cell. The measured nominal capacity was 58 Ah, which is 1.4 times the capacity of a conventional cell. In addition, the mass is lower than that of a conventional cell. As a result, the energy density of 211 Wh/kg, which is 1.5 times the density of the conventional cell, was achieved. The cycle life performance has also been tested. 3.2.2. Roadmap for Japanese lunar exploration International cooperation and coordination for moon exploration is discussed within the framework of the International Space Exploration Coordination Group (ISECG), through bilateral agency-to-agency discussion, or through bottom-up collaboration at the researcher level. To accomplish the purposes of our lunar exploration, JAXA proposes the step-by-step exploration shown in Fig. 10, where in each of the areas of, for example, technology development, science and investigation, and social matters, the progress is considered in steps (Table 3). The next step after SELENE-2 is further upgraded missions, with the successor SELENE-X missions potentially involving a large-scale lander, return technology, or night survival with hundreds of watts. Samples retrieved from the moon should give us many as yet unknown
280 260 240 220 Fig. 7. Numerical simulation result.
Fig. 9. Prototype lithium-ion battery.
Table 2 Specifications and performance of the prototype cell.
Fig. 8. Breadboard model of the night survival unit.
Terms
Prototype
Conventional
Nominal capacity [Ah] Nominal voltage [V] Dimensions W D H [mm] Mass [kg] Energy density [Wh/kg]
58 3.88 98 37 159 1.07 211 (@25 1C)
41 3.70 98 37 159 1.10 138
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Fig. 10. Lunar exploration roadmap proposed by JAXA. Table 3 Japan's lunar exploration strategy. Kaguya (SELENE) Technology development
SELENE-2
SELENE-X
Soft landing surface mobility Large-scale lander return night survival with a few watts technology night survival with hundreds of watts Planetary science Remote sensing observation In-situ observation of geology In-situ observation of geology and geophysics on the near side and geophysics on the polar of surface material, global region and sample return terrain, gravity field, magnetic field, etc. Demonstration of lunar Investigation for future Remote sensing observation Observation of radiation observatory detailed environment, surface soil lunar utilization of sunlit/shadow, material observation of polar region mechanics resources, etc. International collaboration Date exchange Payload co-development System level collaboration Public interest and outreach
Lunar orbit insertion communication relay
High definition TV from orbit. (Earth rise)
High definition TV from the surface
materials, which should bring major breakthroughs in planetary science. Moreover, the technologies resulting from the SELENE series missions are expected to contribute to making human exploration of deep space a reality.
What else?
Remaining objectives Manned spacecraft Life support system Sample return from other interesting areas including the far side seismometer network In-situ resource utilization Program level collaboration Astronaut activity
launch date in 2018. Technologies accumulated from the SELENE series of missions is expected to contribute to future human exploration of space. References
4. Conclusions The preliminary design of the spacecraft is being worked on while the development of key technologies and optimization of mission details continues. We are now evaluating the trade-offs of optimizing the spacecraft configuration. The SELENE-2 study team expects the project to proceed to Phase B in the near future, targeting a
[1] Yoshisada Takizawa, Susumu Sasaki, Manabu Kato, Kaguya (SELENE) mission OVERVIEW, in: 26th ISTS, 2008-k-20, Hamamatsu, Japan, 2008. [2] Tatsuaki Hashimoto, Takeshi Hoshino, Satoshi Tanaka, Kohtaro Matsumoto, Jun'ichiro Kawaguchi, Japanese moon landing mission SELENE-2, in: 26th ISTS, 2008-k-55, Hamamatsu, Japan, 2008. [3] Tatsuaki Hashimoto, Takeshi Hoshino, Satoshi Tanaka, Masatsugu Otsuki, Hisashi Otake, Japanese moon lander SELENE-2 as A robotic
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[5]
[6]
[7]
[8]
precursor mission, in: Proceedings of the Global Space Exploration Conference, GLEX-2012.03.1.7 12702, Washington, DC, USA, 2012. Hiroshi Araki, Seiichi Tazawa, Hirotomo Noda, Emiko Migita, Izumi Kamiya, Nobuyuki Kawano, Sho Sasaki, Preliminary results of the lunar topography by Kaguya–Lalt mission, in: 26th ISTS, 2008-k-48, Hamamatsu, Japan, 2008. Takashi Kubota Tatsuaki Hashimoto, Takahide Mizuno, Light weight sensors for the autonomous asteroid landing of MUSES-C mission, Acta Astronaut. 52 (2003) 381–388. T. Mizuno, S. Fukuda, T. Sakai, Y. Okada, T. Sato and K. Kishimoto, Dynamic performance of a landing radar bbm in field experiments, in: 26th ISTS 2008-d-01, Hamamatsu, Japan, 2008. T. Hashimoto, Y. Imamura, T. Kubota, K. Ninomiya, Accurate navigation for lunar lander/orbiter based on crater identification, Adv. Astronaut. Sci. 108 (2001) 667–676. Yoshifusa Demizu, Masatsugu Otsuki, Tatsuaki, Hashimoto, Hazard recognition methods for planetary lander with single camera,
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IAC-10.A3.2C.18, in: Proceedings of the 61st International Astronautical Congress, Prague, Czech Republic, 2010. S. Nishida and S. Wakabayashi, Strategy for JAXA’s lunar exploration using a lunar rover, i-SAIRAS ‘08, Los Angeles, U.S.A., 2008. Kazunori Ogawa, Yu-ichi Iijima, Naoya SakataniSatoshi Tanaka, Lowpower long-term survival system for scientific instruments on Moon, in: Proceedings of the 9th Low Cost Planetary Mission Conference, Laurel, MD, USA, 2011. Hiroaki Shiraishi Satoshi Tanaka, Manabu Kato, Tatsuaki Okada, The science objectives of the SELENE-II mission as the post SELENE Mission, Adv. Space Res. 42 (2008) 394–401. Takeshi Hoshino, Hitoshi Naito, Kazunori Ogawa, Tatsuaki Hashimoto, Technologies for lunar night survival powered by solar arrays, in: Proceedings of the 10th International Energy Conversion Engineering Conference, AIAA-2012-3976, Atlanta, Georgia, USA, 2012.
Contents lists available at ScienceDirect
Acta Astronautica journal homepage: www.elsevier.com/locate/actaastro
Introduction to Japanese exploration study to the moon T. Hashimoto, T. Hoshino, S. Tanaka, H. Otake, M. Otsuki, S. Wakabayashi, H. Morimoto, K. Masuda n Japan Aerospace Exploration Agency (JAXA), Japan
a r t i c l e i n f o
abstract
Article history: Received 29 January 2014 Received in revised form 4 June 2014 Accepted 13 June 2014 Available online 28 June 2014
The Japan Aerospace Exploration Agency (JAXA) views the lunar lander SELENE-2 as the successor to the SELENE mission. In this presentation, the mission objectives of SELENE-2 are shown together with the present design status of the spacecraft. JAXA launched the Kaguya (SELENE) lunar orbiter in September 2007, and the spacecraft observed the Moon and a couple of small satellites using 15 instruments. As the next step in lunar exploration, the lunar lander SELENE-2 is being considered. SELENE-2 will land on the lunar surface and perform in-situ scientific observations, environmental investigations, and research for future lunar utilization including human activity. At the same time, it will demonstrate key technologies for lunar and planetary exploration such as precise and safe landing, surface mobility, and overnight survival. The lander will carry laser altimeters, image sensors, and landing radars for precise and safe landing. Landing legs and a precisely controlled propulsion system will also be developed. A rover is being designed to be able to travel over a wide area and observe featured terrain using scientific instruments. Since some of the instruments require long-term observation on the lunar surface, technology for night survival over more than 2 weeks needs to be considered. The SELENE-2 technologies are expected to be one of the stepping stones towards future Japanese human activities on the moon and to expand the possibilities for deep space science. & 2014 IAA. Published by Elsevier Ltd. All rights reserved.
Keywords: Lunar exploration Precision and safe landing Surface mobility Night survival
1. Introduction JAXA launched the Kaguya (SELENE) lunar orbiter from Tanegashima Space Center on September 2007. During the time Kaguya was in lunar orbit until it impacted into the southeast region of the Moon in June 2009, Kaguya made many contributions to science, obtaining scientific data about the origins and evolution of the Moon and developing technology for future lunar exploration [1]. As a next step, the SELENE-2 mission is set take over Japanese efforts at lunar exploration with a landing on the moon [2]. SELENE-2, the first Japanese lunar lander, is
n
Corresponding author. E-mail address: [email protected] (K. Masuda).
http://dx.doi.org/10.1016/j.actaastro.2014.06.031 0094-5765/& 2014 IAA. Published by Elsevier Ltd. All rights reserved.
expected to bring major advances in both space technology and science. From a technological perspective, SELENE-2 offers a number of capabilities—precise and safe landing, surface mobility, and overnight stay on the moon surface— all of which are key technologies for future lunar exploration activities. The lander will attempt to achieve a precise and safe, soft landing by using laser altimeters, image sensors, and landing radars. From a scientific perspective, SELENE-2 will perform in-situ observation, environmental investigations, and research for future lunar exploration at the landing site. The launch is currently targeted for 2018. To follow on from SELENE-2, work is already underway on robotic landers designated SELENE-X (where X starts from 3). Although the SELENE-X missions have not been completely defined as yet, several candidate missions such as sample return from the moon are being considered. This
546
T. Hashimoto et al. / Acta Astronautica 104 (2014) 545–551
paper also introduces the current vision for SELENE-X missions leading to future human exploration of the moon. 2. The SELENE-2 mission 2.1. Why go to the moon? It is important to discover new facts about the Earth, the Moon, and other planets in order to learn about where we came from. Planetary exploration in the past has provided us with a variety of facts, and current common knowledge consists of all such facts accumulated throughout history. From the viewpoint of learning our potential for expanding the sphere of human activities, we are still missing many facts about the mechanism of planetary formation, with little known about even the Moon, which is the nearest celestial body to Earth. In terms of technology, the Moon is a best test bed for demonstrating new technologies for planetary exploration. The near side of the Moon always faces Earth, making it easy for a lander to communicate with Earth. From a social viewpoint, the Moon, like the Sun, is very close to daily life. Lunar exploration is therefore a good example for education and outreach of science and technologies. Lunar exploration is also considered an area of international collaboration, with human exploration in particular requiring extensive collaboration.
meet the mission requirements and to optimize the spacecraft design, some trade-offs are being considered. For example, whether to use direct lunar transfer orbit (LTO) insertion, geosynchronous transfer orbit (GTO), or another special launch orbit is under discussion. The selection depends on the launch vehicle performance, efficiency of the spacecraft propulsion system, launch window requirements, and other factors. The size of the orbiter is also a design factor. The smallest case is a spin-stabilized communication relay orbiter without a propulsion system like Kaguya's small satellites. In this case, a relatively large lander would descend to the surface, but this would require much fuel. By comparison, the biggest case is an orbiter of a few tons with a propulsion system for insertion into lunar orbit. This gives the smallest lander size and also minimizes the fuel required. In general, as the total spacecraft system becomes larger and the orbiter becomes bigger, a smaller lander becomes more advantageous for maximizing the mission payload. A tentative mass budget for the SELENE-2 spacecraft is shown in Table 1. The Japanese H2A rocket can carry a lander of around 1 t (dry weight) to the moon surface. This corresponds to a payload of a few hundred kilograms including a 100-kg rover. An artist's drawing of the SELENE-2 configuration is shown in Fig. 1.
2.2. Mission definition SELENE-2 is the first Japanese lunar lander and since the purpose of the mission is lunar exploration, the mission objectives are as follows [3]: I. Development and demonstration of key technologies for future exploration. Safe and accurate landing technologies. Surface mobility: rover. Night survival technologies without using radio isotope energy. II. In-situ observation and investigation for science and future lunar utilization. Detailed and sub-surface geological observation. Geophysics for discovering the interior structure of the moon. Measure the moon environment for future utilization. III. Contribution to international space exploration activity and meeting the public interest. International payload. Outreach or educational payload. In order to achieve these objectives, a configuration consisting of a 1-ton class lander and a 100-kg class surface mobile robot (rover) is being considered. From among the Japanese launch vehicles, the H-2 A rocket is satisfactory for the SELENE-2 configuration.
Table 1 Tentative mass budget of the spacecraft (GTO launch case). Orbiter
Lander
Bus system Mission payload Fuel Total Bus system Mission payload Rover Bus system Mission payload Total Fuel Total
Total
600 kg 100 2400 3100 700 200 80 20 100 1700 2700 5800
Orbiter
Rover
Lander
2.3. Spacecraft configuration The SELENE-2 consists of a lander and an orbiter. The lander carries a rover and mission payloads. In order to
Night survival units Fig. 1. Artist's drawing of the SELENE-2 configuration.
T. Hashimoto et al. / Acta Astronautica 104 (2014) 545–551
2.4. Precise and safe landing system An engine of approximately 3000 N with throttling capability is required for the descent propulsion system of a one-ton class lander (Fig. 2). Since developing a new engine takes a long time and costs a lot of money, a cluster of several space-proven 500 N thrusters employing onand off-modulation is to be used for the SELENE-2 landing. The spacecraft carries a laser altimeter, image sensors, and a landing radar for precise and safe landing. JAXA has built upon its experience with KAGUYA's laser altimeter (LALT) [4], HAYABUSA's laser altimeter (LIDAR) [5], and HAYABUSA's optical navigation camera (ONC) [5]. The development of the landing radar has been going on for years. A breadboard model (BBM) of the radar has been completed and field-tested on board a helicopter [6]. Landmark optical navigation is needed for a precise pin-point landing system. A study has found that accuracy to within a few hundred meters will be able to be accomplished using matching between landmarks in onboard images and those from a lunar surface map [7]. The reconstruction of three-dimensional information from two-dimensional images is a key technology for obstacle detection. Some image processing algorithms have been examined [8]. Landing legs have been developed based on years of studies on touchdown dynamics and stability analysis, and knowledge of soil mechanics has been essential for their development.
547
manipulator arm. We are developing a lightweight durable arm that employs ultra-sonic motors. A grinding tool is essential for geological observation because the internal structure of rocks provides very important information for geologists. A tool changer is essential for obtaining grinding, microscopic imaging, and other observation capabilities from a single arm. 2.6. Night survival We are developing various kinds of night survival technologies for surviving the long, cold night on the moon surface. One approach is to use a sophisticated thermal design. At night, heat emissions are minimized by using multi-layer insulator (MLI) shielding [10]. In laboratory experiments, the survival unit is able to stay above zero degrees Celsius for 2 weeks using a heater of a few watts. The weight of the required battery is then around a few tens of kilograms. Key technologies for this option are reduction of instrument power consumption, thermal isolation during the nighttime for more than 2 weeks, and efficient daytime heat radiation. Another option is to “survive in sleep”. The instruments are switched off at night and designed so as to avoid damage at very low temperatures. When the temperature increases again after sunrise, the instruments resume operation. We are now conducting very low temperature tests on various instruments and parts Fig. 3.
2.5. Surface mobility
2.7. Observation instruments
The rover is designed to be able to travel over a wide area and to observe featured terrain using scientific instruments. For geological science, observation of the internal structure and material of rocks is essential. In terms of rover development, mobile gear, navigation sensors, path planning algorithms, and environmental tests have been studied [9]. Slope climbing on lunar regolith is one particular challenge, and we have developed and compared the performance of both wheel-type and crawler-type mobile gear. Observation support tools for on board the rover are also being developed. One is a
Detailed geological analyses of representative areas such as the Procellarum KREEP Terrane (PKT) and Feldspathic Highlands Terrane (FHT) may enable us to clarify the formation process of magma oceans [11]. For this purpose, a multi-band camera or several spectrometers are required to detect minerals and elements. Geophysical observations are also indispensable for determining the interior structure of the moon. Seismic, electromagnetic, and heat flow observations are important. Since seismic observations require a global “network”, the possibility of establishing international coordinated multi-point
Fig. 2. Landing radar test over Mount Aso in Japan.
Fig. 3. Rover test at Nakatajima beach in Japan.
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observations needs to be discussed. Geodetic observations for detecting the precise motion of the moon are another promising method for finding out about the interior. Research on the lunar surface environment is also important especially for future human activity. Various instruments for measuring the environment are being considered. The third kind of mission objectives for SELENE-2 are for political, educational, cultural, and diplomatic purposes. HDTV from the Kaguya spacecraft provided clear and wonderful video of the Moon and the Earth, including the famous “Earth rise”. SELENE-2 plans to capture similar lunar surface videos for public outreach. The priority of onboard instruments was discussed during the Phase A study. Mission instrument candidates and the suitable locations for them on board SELENE-2—on the lander (L), rover (R), or orbiter (O)—are categorized as follows: 1. Geological observation of particular areas: Multi-band panoramic camera (L and/or R). Gamma-ray spectrometer (R). Microscopic camera with grinding mechanism (R). Active X-ray spectrometer (R). Laser induced brake-down spectrometer (R). 2. Geophysical and geodetic observation for determining the interior structure: Broad-band seismometer (L). Heat flow probe (L). Electromagnetic measurement (L and O). Reflector for lunar laser ranging (L). Radio source for VLBI measurement (L and O). 3. Astronomy from the moon: Low frequency radio measurement (O). 4. Research on the moon surface environment: Radiation monitor on the moon surface (L or O). Mechanical characterization of soil (L and/or R). Space dust detector (O). 5. Outreach payload: HDTV (L and/or O and/or R). International payload (TBD, L).
Fig. 4. Drop test of the landing legs.
Fig. 5. Hill climbing test of the rover.
3. Technology development status 3.1. Landing gear We have been conducting drop tests by changing the parameters of the horizontal velocity, vertical velocity, pad size, and surface hardness (Fig. 4). Shock acceleration, sinking depth, and spacecraft touch-down dynamics are being observed.
3.2. Rover and onboard tools We have developed prototypes of mobile gear and performed some hill climbing tests (Fig. 5) and tests of getting over rocks. We are designing the optimum mechanics of the mobile gear in consideration of the weight and power resources. Development of onboard tools for observation support is also important. A prototype model of
Fig. 6. Tool changer.
a tool changer is shown in Fig. 6. The instruments installed on it are a rock abrasion tool, brush, optical camera, infrared imagery camera, vane shearing measure, hand, and force torque sensor. Motors that are durable in a high and low temperature environment need to be developed for the
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rover mobile gear as well as a manipulator including a tool changer. We have confirmed the thermal resistance of each part of the motors in vacuum, that is, 130 to þ220 1C in operational and 200 to þ220 1C in storage conditions. 3.2.1. Night survival units To confirm the feasibility of night survival units, we have been conducting both numerical simulations and thermal vacuum tests. Fig. 7 shows the results of numerical simulation of the temperature of the survival unit and the moon surface. Although the temperature of the moon surface outside the unit varies in the range of –200 1C to þ100 1C, the temperature inside the unit is kept in the range of 0–50 1C. Fig. 8 shows a photo of a BBM of the night survival unit used for thermal vacuum tests. Another key technology for night survival is high performance batteries [12]. The lithium-ion battery used in the system requires a very high energy density. In
K 400 380 360 340 320 300
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addition, it needs to be able to operate over a wide temperature range. However, only a few charge and discharge cycles are required. Since the total period of day plus night on the lunar surface is around 1 month, there are only 13 charge and discharge cycles in a year. Fig. 9 shows a photo of the prototype cell. The target energy density of the cell is more than 200 Wh/kg. Table 2 summarizes the specifications and performance of the cell. The measured nominal capacity was 58 Ah, which is 1.4 times the capacity of a conventional cell. In addition, the mass is lower than that of a conventional cell. As a result, the energy density of 211 Wh/kg, which is 1.5 times the density of the conventional cell, was achieved. The cycle life performance has also been tested. 3.2.2. Roadmap for Japanese lunar exploration International cooperation and coordination for moon exploration is discussed within the framework of the International Space Exploration Coordination Group (ISECG), through bilateral agency-to-agency discussion, or through bottom-up collaboration at the researcher level. To accomplish the purposes of our lunar exploration, JAXA proposes the step-by-step exploration shown in Fig. 10, where in each of the areas of, for example, technology development, science and investigation, and social matters, the progress is considered in steps (Table 3). The next step after SELENE-2 is further upgraded missions, with the successor SELENE-X missions potentially involving a large-scale lander, return technology, or night survival with hundreds of watts. Samples retrieved from the moon should give us many as yet unknown
280 260 240 220 Fig. 7. Numerical simulation result.
Fig. 9. Prototype lithium-ion battery.
Table 2 Specifications and performance of the prototype cell.
Fig. 8. Breadboard model of the night survival unit.
Terms
Prototype
Conventional
Nominal capacity [Ah] Nominal voltage [V] Dimensions W D H [mm] Mass [kg] Energy density [Wh/kg]
58 3.88 98 37 159 1.07 211 (@25 1C)
41 3.70 98 37 159 1.10 138
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Fig. 10. Lunar exploration roadmap proposed by JAXA. Table 3 Japan's lunar exploration strategy. Kaguya (SELENE) Technology development
SELENE-2
SELENE-X
Soft landing surface mobility Large-scale lander return night survival with a few watts technology night survival with hundreds of watts Planetary science Remote sensing observation In-situ observation of geology In-situ observation of geology and geophysics on the near side and geophysics on the polar of surface material, global region and sample return terrain, gravity field, magnetic field, etc. Demonstration of lunar Investigation for future Remote sensing observation Observation of radiation observatory detailed environment, surface soil lunar utilization of sunlit/shadow, material observation of polar region mechanics resources, etc. International collaboration Date exchange Payload co-development System level collaboration Public interest and outreach
Lunar orbit insertion communication relay
High definition TV from orbit. (Earth rise)
High definition TV from the surface
materials, which should bring major breakthroughs in planetary science. Moreover, the technologies resulting from the SELENE series missions are expected to contribute to making human exploration of deep space a reality.
What else?
Remaining objectives Manned spacecraft Life support system Sample return from other interesting areas including the far side seismometer network In-situ resource utilization Program level collaboration Astronaut activity
launch date in 2018. Technologies accumulated from the SELENE series of missions is expected to contribute to future human exploration of space. References
4. Conclusions The preliminary design of the spacecraft is being worked on while the development of key technologies and optimization of mission details continues. We are now evaluating the trade-offs of optimizing the spacecraft configuration. The SELENE-2 study team expects the project to proceed to Phase B in the near future, targeting a
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