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The science objectives of the SELENE-II mission as the post SELENE mission
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Advances in Space Research 42 (2008) 394–401 www.elsevier.com/locate/asr
The science objectives of the SELENE-II mission as the post SELENE mission Satoshi Tanaka *, Hiroaki Shiraishi, Manabu Kato, Tatsuaki Okada, Science Working Group of Post SELENE Missions ISAS/JAXA, Yoshinodai 3-1-1, Sagamihara, Kanagawa 229-8510, Japan Received 30 November 2006; received in revised form 29 June 2007; accepted 3 July 2007
Abstract Further study for the planning of the post SELENE mission has been discussed by a dedicated working group. As the extension of the SELENE-B study [Okada, T., Sasaki, S., Sugihara, T., et al. Lander and rover exploration on the lunar surface: a study for SELENE-B mission. Adv. Space Res. 37, 88–92, 2006] which proposed in-situ geological investigations using a robotic rover and a static lander, this report newly proposes a revised configuration which enhances the scientific field of view. The spacecraft of this mission, ‘‘SELENE-II’’, is designed as a full payload of H-II launch vehicle, while the former study was designed as a half payload of the same vehicle. This expansion of capacity enabled us to increase the payload mass of the lander to deploy geophysical instruments and to land on a wider region on the Moon including polar regions. We also gained the opportunity to deploy two penetrators in order to make a wide network for geophysical observations. In the new configuration, this mission can install three stations of a global seismic network, which will be able to refine the deeper structure of the Moon. In this study also, a new type of deployment system, whose mechanical interface is much simpler than that of the LUNAR-A mission, is preliminarily designed. The selection of the landing site is still undergoing discussion, but the lander is required to operate as long as about one year and more for the geophysical observations, especially for seismology. In order to realize this, one possible idea is to land in permanently sunlit regions. Polar regions also have a benefit from the geological point of view; the north polar region is a typical high land area and the south one is a part of or adjacent to the South Pole Aitken (SPA), where the deeper part of the crust or the mantle material are expected to be collected. In addition to the lander scientific instruments designed previously (Okada et al., 2006.) for the geological survey, a broad band seismometer is considered to be deployed prior to other geophysical instruments and we expect it to provide us with information about the bulk layered structure with only one station if free oscillations are successfully detected. Even if the free oscillations cannot be detected, the dispersion of surface waves not affected by scattering of the regolith or megaregolith layer brings information to understand the crustal and upper mantle structures. Several landing missions are planned by NASA, CNSA, ISRO, and ESA by 2010–2015 during which the operational period is possible to be overlapped by the different missions. This must be a great opportunity to make larger network observations in the future. It must be a great opportunity to start international collaboration in various ways for the upcoming lunar exploration era. Ó 2007 COSPAR. Published by Elsevier Ltd. All rights reserved. Keywords: Lunar exploration; Lander; Rover; Penetrator; Seismic observation
1. Introduction
*
Corresponding author. Tel.: +81 42 759 8198; fax: +81 42 759 8516. E-mail address: [email protected] (S. Tanaka).
Recently, lunar exploration programs are being planned or being developed in many space agencies. Some missions are planned to be launched in a few years such as SELENE (Japan) (Konishi et al., 2005), Chandrayaan-1 (India)
0273-1177/$34.00 Ó 2007 COSPAR. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.asr.2007.07.002
S. Tanaka et al. / Advances in Space Research 42 (2008) 394–401
(Bhandari, 2005), Chang’e-1 (China) (Zhi-jian et al., 2005), and Lunar Reconnaissance Orbiter (NASA) (Chin et al., in press). In Japan, the Moon has been assigned as the priority of the planetary missions since the early 90’s, when the Japanese first lunar mission, HITEN(1990), was successfully inserted into lunar orbit. Almost 90 spacecrafts have been launched since 1958, and about 50 missions were successfully achieved. Among them, about 20 missions supplied us with scientific data so far. With the Apollo and Luna missions, some key scientific concepts such as magma ocean theory have been derived. The giant impact theory, which was originally deduced to give the Earth the present angular momentum of the Earth–Moon system, was also supported by some chemical data of the returned samples. Aside from the Earth, the Moon is the only terrestrial body for which the age of geological resurfacing can be estimated because we have samples of known spatial context. The method of crater chronology is widely used for geological age determination for other planets and asteroids. Although, Clementine and Lunar Prospector missions both of which were successfully achieved by NASA in the middle 90’s, brought us new aspects of lunar sciences after the Apollo and Luna era (e.g., Lucey et al., 1998; Lawrence et al., 1999; Jolliff et al., 2000). Detailed analysis of lunar meteorites, about 40 different kinds of which have been collected at present, also brought us unexpected data concerning the crustal evolution of the Moon (e.g., Korotev, 2005; Takeda et al., 2006). In general, these results gave us a possibility of much wider variety of lunar crusts. On the other hand, continuous analysis of Apollo data, by using new analytical instruments for the returned samples, and by new analytical methods and progress in computer technology for the geophysical data set, revealed much precise data about chemical composition, returned sample ages, and a new vision of the internal structure (e.g., Khan et al., 2000; Lognonne´ et al., 2003; Chenet et al., 2006; Lucey et al., 2006). Therefore, we should note that the Moon is still an amazing science target and the traditional concepts are expected to be changed drastically in the future works. Reanalysis of previous data will still bring new results which change the understanding of the Moon and new remote sensing data are indeed needed for a more precise/global description of the surface. In addition to these, in situ measurements are compulsory with geophysical observations related to the internal structure in order to address some of the major remaining questions. In 2005, Japan Aerospace Exploration Agency (JAXA) has published its future program and space activity entitled ‘‘Long-Term Vision’’. In this long-term plan for the next two decades, JAXA announced that future lunar exploration is one of the mainstreams of Japanese space programs. It is of virtue to investigate the future lunar exploration continuously from multilateral viewpoints. In this report, based on the previous study of SELENE-B mission (Okada et al., 2006), we propose the concept of the SELENE-II
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mission from the scientific viewpoint as the follow up to SELENE which will be launched in 2007, and as the recovery of LUNAR-A which was officially cancelled in 2007 (Shiraishi et al., 2008). 2. Brief summary of recent Japanese lunar missions and their objectives As described briefly above, JAXA was undertaking two lunar missions. In this section, we summarize briefly outline, current status and scientific objectives of these two missions. The LUNAR-A mission (Fig. 1) is characterized by the investigation of the internal structure of the Moon by two hard landers, so called ‘‘penetrators’’ (Mizutani et al., 2003). The penetrator is a missile-shaped instrument carrier, which is about 14 cm in diameter and 80 cm in length. The final impact velocity of the penetrator will be about 280 m/s, and it is expected to bury below 1 to 3 m from the surface. The LUNAR-A mission has been suspended since 2004 mainly because of a recall and replacement of some thruster bulbs of the spacecraft, and a major malfunction was found in the penetrator system during the qualification test (QT) performed in November 2003 (Mizutani et al., 2005). Following the recommendations of the technical and mission management review boards held in 2004, we made a decision to concentrate on the accomplishment of the LUNAR-A penetrator technologies prior to the development of the spacecraft. A three year program has been proceeding since fiscal year 2005, and the launch feasibility is not determined at present (Tanaka et al., 2006). Technical development of the penetrator is still be undertaken, although, the mission was officially canceled on February 2007 (Shiraishi et al., 2008). In this mission, the main scientific objective is to make clear the existence of a core and its radius because this could become a strong constraint when inferring the bulk composition of the Moon (Mizutani et al., 2003). Nevertheless, LUNAR-A can only deploy two seismic stations, so we should not expect to obtain sufficient precision of the velocity structure in the mantle and crustal thickness, both of which are also crucial information relevant to bulk abundance and internal evolution of the Moon (e.g., Khan et al., 2000; Lognonne´ et al., 2003; Taylor, 2005). Beside these scientific goals, this mission would demonstrate the usefulness of the penetrator technology for future planetary missions. In this respect, the LUNAR-A mission should be seen as the first step of the technical challenge toward the long-range goal of investigation of the planetary internal structure. We believe that improvement and modification of the LUNAR-A type penetrators will expand the horizons of future planetary exploration. The other mission under development is SELENE (Fig. 2) which was started in 1998 as a joint mission of ISAS and National Space Development Agency of Japan (NASDA) (e.g., Kato et al., 2000; Sasaki et al., 2003; Koni-
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Fig. 1. Flight model of LUNAR-A spacecraft and the penetrator module. The height of the spacecraft is 1.7 m.
shi et al., 2005). This mission consists of three separate units: the main orbiter, a small relay satellite (RSAT), and a small VLBI (Very Long Baseline Interferometry) Radio (VRAD) satellite. The scientific instruments on board the main orbiter are used for global mapping of the lunar surface, magnetic field measurements, and gravity field measurements together with the instruments on the Relay Satellite and VRAD Satellite. Fourteen science instruments are classified into three major categories such as ‘‘science for the Moon’’, ’’science on the Moon’’ and ‘‘science from the Moon’’ (Sasaki et al., 2003). Some types of instruments (e.g., visible camera, X-ray fluorescence spectrometer, gamma ray spectrometer) are planned to fly onboard future missions, Chandrayaan-1 (India), Chang’e-1 (China), and LRO (NASA). The quality of remote sensing data can be improved by mutual calibration of the instruments and by complementary observations. The key technologies, such as lunar orbit insertion and attitude/orbit control of the orbiter will also be verified for future lunar exploration. Launch target is rescheduled for 2007, and the final tests are now being undertaken. Global maps both of topography and chemical abundances were obtained by the Clementine and Lunar Prospector missions. These results generally lead to changes of
the classical concept of the magma ocean and revealed more heterogeneous structure of the Moon (e.g., Lawrence et al., 1999; Jolliff et al., 2000). The results also lead to the possible existence of outcrops originating from the deep interior of the Moon. In conjunction to recent results of chemical analyses of lunar meteorites, the Apollo and Luna returned samples are not thought to be representative of the whole Moon but only of specific locations. (e.g., Korotev et al., 2005; Takeda et al., 2006). Therefore, our knowledge is still insufficient to understand the lunar evolution at a large scale. In the near future, we expect from SELENE and other remote sensing missions more precise data, both in spatial and chemical abundance, that can enhance our understanding of the lunar evolution scenario. On the other hand, orbital remote sensing has some limitations. For example, it is very difficult to speculate on mineral assemblage, crystalization or impact age, elemental abundance of trace elements, and so on (Okada et al., 2006). Especially, the surface of the Moon consists of regolith and breccias, both of which are strongly affected by the impact brecciation and alteration. Among them, we need to find out specified samples which have not suffered from these impact effects.
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Fig. 2. Flight model of SELENE spacecraft.
3. Science by a lander mission – Progress in the planning of lander-based scientific observations Science concepts for the next lunar exploration missions have been discussed in a working group (WG), which is authorized in the steering committee for space science of ISAS/JAXA. Previously, a landing mission ‘‘SELENE-B’’ was proposed as ISAS’s next mission, but it was not ranked as the top priority for ISAS’s next engineering mission (Okada et al., 2006). The SELENE-B mission was designed to be launched as a half payload of H-II launch vehicle, then the scientific investigations were restricted to in-situ geological observations and the landing site was limited to the near side in equatorial to mid-latitude because of the limited mass budget (about 500 kg total dry mass and 50 kg science payload for the lander, respectively). Taking into account of the missions following SELENE and recovering LUNAR-A, it is worthwhile to perform not only the geological but also the geophysical observations simulta-
neously. In order to include both, we assume a spacecraft of full payload size of the H-II launch vehicle. We should also consider the mission life which must be extended to about 1 year to collect scientifically meaningful data for the geophysical observations. One possible idea to survive longer duration is to deploy the lander in an area permanently illuminated by sunlight (Shoemaker et al., 1994; Bussey et al., 1999). Polar regions have a benefit from a geological point of view, the north polar region is a typical high land area and the southern polar region is a part of or adjacent to the South Pole Aitken (SPA), where the deeper part of the crust or the mantle material are expected to be sampled. Our proposal in this study is a combination of a lander, two penetrators and a relay satellite as shown in Fig. 3. The dry mass of the lander is 830 kg (which is about 300 kg heavier than that of the previous SELENE-B study), mainly by addition of a geophysical observation package and extensive solar panels in order to supply adequate power to maintain the system landed in the polar regions.
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Fig. 3. Image view of SELENE-II spacecraft (lander, relay satellite, and two penetrators) and three-axis maneuvering penetrator deployment system.
As for the penetrator system, we propose a new type of deployment system of the penetrator. For the LUNAR-A mission, high accuracy of the mechanical interface between the penetrator and the spacecraft is required, the orbit and the attitude of the spacecraft must be controlled with precision, and the spacecraft must be spun up at a high rate (2 rps) at the time of deployment. This is not the appropriate system to include the penetrator system in future lunar and planetary missions because the spacecraft itself must be designed for the penetrator(s). Alternatively, we propose a three-axis maneuvering carrier system whose interface is much easier than that of the LUNAR-A mission (Fig. 3). The spacecraft dose not require severe control of attitude
nor spinning up. In other words, this is a kind of lander system without deceleration of vertical velocity. This technical innovation has an advantage in providing the penetrator system for other planetary missions in general even though the system requires more weight for the deployment system. For the SELENE-II mission in this case, two penetrators (100 kg/1 system) and relay satellite (about 200 kg) can be deployed from our preliminary estimation. Combined with the seismometry carried out by the lander, we can establish a three stations global seismic network which enables us to determine the seismic hypocenter and velocity structure. The basic concept of this seismic network by one lander at polar and two penetrators at equatorial regions is
S. Tanaka et al. / Advances in Space Research 42 (2008) 394–401
similar to that of the proposed Russian Luna-Glob mission (Galimov, 2005). The velocity profile is a key information for inferring the formation of the Moon. For example, some different explanations were discussed about the seismic gap at 500 km depth (Wieczoreck et al., 2006), but we cannot conclude this discussion unambiguously because the uncertainty is too large to bring further arguments about the depth of magma ocean, initial status at the formation, and regional heterogeneity.
4. Key technologies of the scientific instruments on board the lander In order to obtain the maximum scientific benefit from the seismic observation by the lander and the penetrators, we propose a broad band seismometer with a frequency range up to 0.01 Hz on board the lander and a short period seismometer on the penetrator, with a sensitivity peak around 1 Hz where the maximum power of seismic waves was observed by Apollo observations. The seismometer on board the penetrator is designed to have shock durability for the penetration event at around 5000 to 10000 G. In addition to the shock durability, the seismometer is a passive type which dose not require any battery consumption to operate (Yamada et al., 2007). On the other hand, the broadband seismometer requires a feedback system which consumes some power, to extend the bandwidth and linearity. Taking into account the requirement of long observation duration, a feedback system was not applied to the penetrator system, but this will be able to be deployed to the lander system which can supply enough power by solar or other energy resources. The lander observations yield the possibility to detect free oscillations excited by large shallow moonquakes or meteoroid impacts. If we successfully detect free oscillations, the averaged velocity structure will be obtained by one seismic station while the velocity discontinuity can be detected by the short period observations. It is controversial whether the free oscillations could be successfully detected by the Apollo observations (Khan and Mosegaard, 2001; Gudkova and Zharkov, 2002; Gagnepain-Beyneix et al., 2006). In general, even the surface waves of the Moon could not be detected by the Apollo long period (LP) seismometers, and the sensitivity of the seismometer is required to be 10 times larger than that of Apollo seismometers to detect the surface waves excited by one of the largest impact events (Lognonne´, 2005). There is another case for recording long period seismic waves for lunar seismology. Apollo seismic observations revealed that the seismic waves were intensely scattered as an effect of the regolith layer. As a result, the S/N ratio of the seismic waves is too low to identify precisely the P-S time and the later phase. The long period seismographs are expected to record undisturbed high S/N waves because they are not affected by the shallow structure. The dispersion of surface waves without the scattering effect of the regolith or megar-
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egolith layer should then yield clear information of the crustal and upper mantle structures. Therefore, the development of the broadband seismometer is one of the key technologies for the scientific observations by the lander, and one possible candidate is the instrument that has been developed in France by IPGP (e.g., Lognonne´, 2005; Mimoun et al., 2007). As for the geological investigations, the scientific concept and the instrumental configuration is basically the same as proposed in the SELENE-B study (Okada et al., 2006). The roles of in situ geological observations are complementary to remote sensing studies and in order to prepare the upcoming sample return mission as a next stage, it is indispensable to obtain the knowledge beforehand precise information about the chemistry and mineralogy as a ground truth to corroborate or re-evaluate the pre-existing interpretation of remote sensing results. Geological samples around the lander and using a robotic rover for distant crops will be taken with a manipulator and brought back to the lander for detailed analyses with a mini-laboratory called SIP (Science Instrument Package). The SIP will have a microscope, measure X-ray fluorescence and diffraction, carry out IR spectrometry and so on. For the in-situ observations, a control of sample surface conditions is one of the crucial technologies in order to analyze mineralogical and chemical composition of fragments a few millimeters in size in a breccia which was formed by ejecta related to the crater formation. We have developed a sample polishing system using ultrasonic technique. The Apollo and Luna returned samples indicated that some important fresh fragments which have not suffered from impact alteration must have remained in the breccias. These were frequently found as ‘‘rake’’ samples of a few centimeters in size, buried in the regolith. For these reasons, the sample collection system on board the lander or rover is also a fundamental technology which is also under development with priority (Fig. 4). 5. Summary We proposed a configuration of the SELENE-II mission as the follow-up of SELENE, and as the recovery of LUNAR-A missions. By the expansion of the payload mass compared to the SELENE-B mission, we preliminarily designed a mission outline which combines geological and geophysical sciences. Long operational duration (about one year) required by the seismic observations suggests a landing in polar regions, where it is possible to supply electrical power constantly. Although, it should be noted that the mission should be designed not only for the scientific purposes but also for the future utilization and for the human activity on the Moon; some important scientific aspects described in this report must be taken into account as the first priority for at least several next missions.
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Fig. 4. Photographic view of sampling and fixing system for the lander on board which is under development.
In addition to the in-situ geological survey by the lander, an extended lunar seismological network to investigate the deep structure in detail using penetrators and landers must be recognized as an important goal of the scientific contribution. A feasibility study is under way for the deployment of both the lander and the penetrators in one mission. Since landing missions are planned by other agencies and countries at around the same time, it is notable to have a great opportunity to make an ‘‘international seismic network observation’’. Then the network observation should also be considered by this international framework. In order to realize this, it is the proper opportunity to start the international collaborations in various ways for the upcoming lunar exploration era. Acknowledgments The authors would like to thank Pr. Yoshio Nakamura, Dr. Axel Hargermann, Dr. Nozomu Takeuchi, Dr. Y. Iijima, and Dr. Naoki Kobayashi for their helpful discussions of the seismic observations in the future lunar missions. We would also like to thank anonymous reviewers for constructive comments to the manuscript. References Bhandari, N. Chandrayaan-1: science goals. J. Earth Syst. Sci. 114 (6), 699–710, 2005. Bussey, D.B.J., Spudis, P.D., Robinson, M.S. Illumination conditions at the lunar south pole. Geophys. Res. Lett. 26 (9), 1187–1190, 1999. Chenet, H., Lognonne´, Ph., Wieczorek, M., et al. Lateral variations of lunar crustal thickness from the Apollo seismic data set. Earth Planet. Sci. Lett. 243, 1–14, 2006. Chin, G., Brylow, S., Foote, M., et al. Lunar reconnaissance orbiter overview: the instrument suite and mission. Space Sci. Rev. (in press). doi:10.1007/s11214-007-9153-y. Gagnepain-Beyneix, J., Lognonne´, P., Chenet, H., et al. A seismic model of the lunar mantle and constraints on temperature and mineralogy. Phys. Earth Planet. Int. 159, 140–166, 2006.
Galimov, E.M. Luna-Glob project in the context of the past and present lunar exploration in Russia. J. Earth Syst. Sci. 114 (6), 801–806, 2005. Gudkova, T.V., Zharkov, V.N. The exploration of the lunar interior using torsional oscillations. Planet Space Sci. 38, 565–578, 2002. Jolliff, B., Gillis, J., Haskin, L., et al. Major lunar crustal terrains: surface expressions and crust-mantle origins. J. Geophys. Res. 105, 4197–4216, 2000. Kato, M., Sasaki, S., Iijima, Y., et al. Science instruments and their development in SELENE mission. In: ICEUM-4, Proceedings of Fourth International Conference on the Exploration and Utilization of the Moon. ESA SP-462, pp. 119–123, 2000. Khan, A., Mosegaard, K., Rasmussen, K.L. A new seismic velocity model for the Moon from a Monte Carlo inversion of the Apollo lunar seismic data. Geophys. Res. Lett. 27, 1591–1594, 2000. Khan, A., Mosegaard, K. New information on the deep lunar interior from an inversion of lunar free oscillation periods. Geophys. Res. Lett. 28, 1791–1794, 2001. Konishi, H., Kato, M., Sasaki, S., et al. SELENE project status. J. Earth Syst. Sci. 114 (6), 771–775, 2005. Korotev, R. Lunar geochemistry told by lunar meteorites. Chemie der Erde 65, 297–346, 2005. Lawrence, D.J., Feldman, W.C., Barraclough, B.L., et al. High resolution measurements of absolute thorium abundance on the lunar surface. Geophys. Res.Lett. 26 (17), 2681–2684, 1999. Lognonne´, P., Gagnepain-Beyneix, J., Chenet, H. A new seismic model of the Moon: implications for structure, thermal evolution and formation of the Moon. Earth Planet. Sci. Lett. 211, 27–44, 2003. Lognonne´, P. Planetary seismology. Annu. Rev. Planet. Sci. 33, 571–604, 2005. Lucey, P., Blewett, D., Hawke, B. Mapping the FeO and TiO2 content of the lunar surface with multispectral imagery. J. Geophys. Res. 103, 3679–3699, 1998. Lucey, P., Korotev, R., Gillis, J., et al. Understanding the lunar surface and space–moon interactions. Rev. Mineral Geochem. 60, 83–219, 2006. Mimoun, D., Lognonne´, P., Giardini, D., et al. The seis experiment: a planetary seismometer for mars and the moon. 38th Lunar Planet. Sci. Conf. Abstract No. 2204, 2007. Mizutani, H., Fujimura, A., Tanaka, S. LUNAR-A mission: goals and status. Adv. Space Res. 31 (11), 2315–2321, 2003. Mizutani, H., Fujimura, A., Tanaka, S., et al. LUNAR-A mission: outline and current status. J. Earth Syst. Sci. 114 (6), 763–768, 2005.
S. Tanaka et al. / Advances in Space Research 42 (2008) 394–401 Okada, T., Sasaki, S., Sugihara, T., et al. Lander and rover exploration on the lunar surface: a study for SELENE-B mission. Adv. Space Res. 37, 88–92, 2006. Sasaki, S., Iijima, Y., Tanaka, K., et al. The Selene mission: goals and status. Adv. Space Res. 31 (11), 2335–2340, 2003. Shiraishi, H., Tanaka, S., Fujimura, A., et al. The present status of the Japanese penetrator mission: LUNAR-A. Adv. Space Res. 42, 386– 393, 2008. Shoemaker, E.M., Robinson, M.S., Eliason, E. South-pole region of the Moon as seen by Clementine. Science 266 (5192), 1851–1854, 1994. Takeda, H., Yamaguchi, A., Bogard, D.D., et al. Magnesian anorthosites and a deep crustal rock from the farside crust of the Moon. Earth Planet. Sci. Lett. 247, 171–184, 2006. Tanaka, S., Fujimura, A., Shiraishi, H., et al. Present status of the development of the LUNAR-A penetrator. In: Proceedings of the
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International Symposium on Space Technology and Science, pp. 1020– 1024, 2006. Taylor, S.R. Lunar science: an overview. J.Earth Syst. Sci. 114 (6), 587– 591, 2005. Wieczoreck, M., Jolliff, B., Khan, A., et al. The constitution and structure of the lunar interior. In: Joliff, B.J., Wieczorek, M.A., Shearer, C.K., Neal, C.R. (Eds.), New Views of the Moon. pp. 221–364, 2006. Yamada, R., Yamada, IO., Shiraishi, H., et al. Seismic observation by the seismometer on board the penetrator for lunar exploration. In: 38th Lunar Planetary Science Conference Abstract No.1503, 2007. Zhi-jian, Y., Li-chang, L., Yung-chun, L., et al. Space operation system for Chang’E-1 program and its capability evaluation. J. Earth Sys. Sci. 114 (6), 787–794, 2005.
Advances in Space Research 42 (2008) 394–401 www.elsevier.com/locate/asr
The science objectives of the SELENE-II mission as the post SELENE mission Satoshi Tanaka *, Hiroaki Shiraishi, Manabu Kato, Tatsuaki Okada, Science Working Group of Post SELENE Missions ISAS/JAXA, Yoshinodai 3-1-1, Sagamihara, Kanagawa 229-8510, Japan Received 30 November 2006; received in revised form 29 June 2007; accepted 3 July 2007
Abstract Further study for the planning of the post SELENE mission has been discussed by a dedicated working group. As the extension of the SELENE-B study [Okada, T., Sasaki, S., Sugihara, T., et al. Lander and rover exploration on the lunar surface: a study for SELENE-B mission. Adv. Space Res. 37, 88–92, 2006] which proposed in-situ geological investigations using a robotic rover and a static lander, this report newly proposes a revised configuration which enhances the scientific field of view. The spacecraft of this mission, ‘‘SELENE-II’’, is designed as a full payload of H-II launch vehicle, while the former study was designed as a half payload of the same vehicle. This expansion of capacity enabled us to increase the payload mass of the lander to deploy geophysical instruments and to land on a wider region on the Moon including polar regions. We also gained the opportunity to deploy two penetrators in order to make a wide network for geophysical observations. In the new configuration, this mission can install three stations of a global seismic network, which will be able to refine the deeper structure of the Moon. In this study also, a new type of deployment system, whose mechanical interface is much simpler than that of the LUNAR-A mission, is preliminarily designed. The selection of the landing site is still undergoing discussion, but the lander is required to operate as long as about one year and more for the geophysical observations, especially for seismology. In order to realize this, one possible idea is to land in permanently sunlit regions. Polar regions also have a benefit from the geological point of view; the north polar region is a typical high land area and the south one is a part of or adjacent to the South Pole Aitken (SPA), where the deeper part of the crust or the mantle material are expected to be collected. In addition to the lander scientific instruments designed previously (Okada et al., 2006.) for the geological survey, a broad band seismometer is considered to be deployed prior to other geophysical instruments and we expect it to provide us with information about the bulk layered structure with only one station if free oscillations are successfully detected. Even if the free oscillations cannot be detected, the dispersion of surface waves not affected by scattering of the regolith or megaregolith layer brings information to understand the crustal and upper mantle structures. Several landing missions are planned by NASA, CNSA, ISRO, and ESA by 2010–2015 during which the operational period is possible to be overlapped by the different missions. This must be a great opportunity to make larger network observations in the future. It must be a great opportunity to start international collaboration in various ways for the upcoming lunar exploration era. Ó 2007 COSPAR. Published by Elsevier Ltd. All rights reserved. Keywords: Lunar exploration; Lander; Rover; Penetrator; Seismic observation
1. Introduction
*
Corresponding author. Tel.: +81 42 759 8198; fax: +81 42 759 8516. E-mail address: [email protected] (S. Tanaka).
Recently, lunar exploration programs are being planned or being developed in many space agencies. Some missions are planned to be launched in a few years such as SELENE (Japan) (Konishi et al., 2005), Chandrayaan-1 (India)
0273-1177/$34.00 Ó 2007 COSPAR. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.asr.2007.07.002
S. Tanaka et al. / Advances in Space Research 42 (2008) 394–401
(Bhandari, 2005), Chang’e-1 (China) (Zhi-jian et al., 2005), and Lunar Reconnaissance Orbiter (NASA) (Chin et al., in press). In Japan, the Moon has been assigned as the priority of the planetary missions since the early 90’s, when the Japanese first lunar mission, HITEN(1990), was successfully inserted into lunar orbit. Almost 90 spacecrafts have been launched since 1958, and about 50 missions were successfully achieved. Among them, about 20 missions supplied us with scientific data so far. With the Apollo and Luna missions, some key scientific concepts such as magma ocean theory have been derived. The giant impact theory, which was originally deduced to give the Earth the present angular momentum of the Earth–Moon system, was also supported by some chemical data of the returned samples. Aside from the Earth, the Moon is the only terrestrial body for which the age of geological resurfacing can be estimated because we have samples of known spatial context. The method of crater chronology is widely used for geological age determination for other planets and asteroids. Although, Clementine and Lunar Prospector missions both of which were successfully achieved by NASA in the middle 90’s, brought us new aspects of lunar sciences after the Apollo and Luna era (e.g., Lucey et al., 1998; Lawrence et al., 1999; Jolliff et al., 2000). Detailed analysis of lunar meteorites, about 40 different kinds of which have been collected at present, also brought us unexpected data concerning the crustal evolution of the Moon (e.g., Korotev, 2005; Takeda et al., 2006). In general, these results gave us a possibility of much wider variety of lunar crusts. On the other hand, continuous analysis of Apollo data, by using new analytical instruments for the returned samples, and by new analytical methods and progress in computer technology for the geophysical data set, revealed much precise data about chemical composition, returned sample ages, and a new vision of the internal structure (e.g., Khan et al., 2000; Lognonne´ et al., 2003; Chenet et al., 2006; Lucey et al., 2006). Therefore, we should note that the Moon is still an amazing science target and the traditional concepts are expected to be changed drastically in the future works. Reanalysis of previous data will still bring new results which change the understanding of the Moon and new remote sensing data are indeed needed for a more precise/global description of the surface. In addition to these, in situ measurements are compulsory with geophysical observations related to the internal structure in order to address some of the major remaining questions. In 2005, Japan Aerospace Exploration Agency (JAXA) has published its future program and space activity entitled ‘‘Long-Term Vision’’. In this long-term plan for the next two decades, JAXA announced that future lunar exploration is one of the mainstreams of Japanese space programs. It is of virtue to investigate the future lunar exploration continuously from multilateral viewpoints. In this report, based on the previous study of SELENE-B mission (Okada et al., 2006), we propose the concept of the SELENE-II
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mission from the scientific viewpoint as the follow up to SELENE which will be launched in 2007, and as the recovery of LUNAR-A which was officially cancelled in 2007 (Shiraishi et al., 2008). 2. Brief summary of recent Japanese lunar missions and their objectives As described briefly above, JAXA was undertaking two lunar missions. In this section, we summarize briefly outline, current status and scientific objectives of these two missions. The LUNAR-A mission (Fig. 1) is characterized by the investigation of the internal structure of the Moon by two hard landers, so called ‘‘penetrators’’ (Mizutani et al., 2003). The penetrator is a missile-shaped instrument carrier, which is about 14 cm in diameter and 80 cm in length. The final impact velocity of the penetrator will be about 280 m/s, and it is expected to bury below 1 to 3 m from the surface. The LUNAR-A mission has been suspended since 2004 mainly because of a recall and replacement of some thruster bulbs of the spacecraft, and a major malfunction was found in the penetrator system during the qualification test (QT) performed in November 2003 (Mizutani et al., 2005). Following the recommendations of the technical and mission management review boards held in 2004, we made a decision to concentrate on the accomplishment of the LUNAR-A penetrator technologies prior to the development of the spacecraft. A three year program has been proceeding since fiscal year 2005, and the launch feasibility is not determined at present (Tanaka et al., 2006). Technical development of the penetrator is still be undertaken, although, the mission was officially canceled on February 2007 (Shiraishi et al., 2008). In this mission, the main scientific objective is to make clear the existence of a core and its radius because this could become a strong constraint when inferring the bulk composition of the Moon (Mizutani et al., 2003). Nevertheless, LUNAR-A can only deploy two seismic stations, so we should not expect to obtain sufficient precision of the velocity structure in the mantle and crustal thickness, both of which are also crucial information relevant to bulk abundance and internal evolution of the Moon (e.g., Khan et al., 2000; Lognonne´ et al., 2003; Taylor, 2005). Beside these scientific goals, this mission would demonstrate the usefulness of the penetrator technology for future planetary missions. In this respect, the LUNAR-A mission should be seen as the first step of the technical challenge toward the long-range goal of investigation of the planetary internal structure. We believe that improvement and modification of the LUNAR-A type penetrators will expand the horizons of future planetary exploration. The other mission under development is SELENE (Fig. 2) which was started in 1998 as a joint mission of ISAS and National Space Development Agency of Japan (NASDA) (e.g., Kato et al., 2000; Sasaki et al., 2003; Koni-
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Fig. 1. Flight model of LUNAR-A spacecraft and the penetrator module. The height of the spacecraft is 1.7 m.
shi et al., 2005). This mission consists of three separate units: the main orbiter, a small relay satellite (RSAT), and a small VLBI (Very Long Baseline Interferometry) Radio (VRAD) satellite. The scientific instruments on board the main orbiter are used for global mapping of the lunar surface, magnetic field measurements, and gravity field measurements together with the instruments on the Relay Satellite and VRAD Satellite. Fourteen science instruments are classified into three major categories such as ‘‘science for the Moon’’, ’’science on the Moon’’ and ‘‘science from the Moon’’ (Sasaki et al., 2003). Some types of instruments (e.g., visible camera, X-ray fluorescence spectrometer, gamma ray spectrometer) are planned to fly onboard future missions, Chandrayaan-1 (India), Chang’e-1 (China), and LRO (NASA). The quality of remote sensing data can be improved by mutual calibration of the instruments and by complementary observations. The key technologies, such as lunar orbit insertion and attitude/orbit control of the orbiter will also be verified for future lunar exploration. Launch target is rescheduled for 2007, and the final tests are now being undertaken. Global maps both of topography and chemical abundances were obtained by the Clementine and Lunar Prospector missions. These results generally lead to changes of
the classical concept of the magma ocean and revealed more heterogeneous structure of the Moon (e.g., Lawrence et al., 1999; Jolliff et al., 2000). The results also lead to the possible existence of outcrops originating from the deep interior of the Moon. In conjunction to recent results of chemical analyses of lunar meteorites, the Apollo and Luna returned samples are not thought to be representative of the whole Moon but only of specific locations. (e.g., Korotev et al., 2005; Takeda et al., 2006). Therefore, our knowledge is still insufficient to understand the lunar evolution at a large scale. In the near future, we expect from SELENE and other remote sensing missions more precise data, both in spatial and chemical abundance, that can enhance our understanding of the lunar evolution scenario. On the other hand, orbital remote sensing has some limitations. For example, it is very difficult to speculate on mineral assemblage, crystalization or impact age, elemental abundance of trace elements, and so on (Okada et al., 2006). Especially, the surface of the Moon consists of regolith and breccias, both of which are strongly affected by the impact brecciation and alteration. Among them, we need to find out specified samples which have not suffered from these impact effects.
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Fig. 2. Flight model of SELENE spacecraft.
3. Science by a lander mission – Progress in the planning of lander-based scientific observations Science concepts for the next lunar exploration missions have been discussed in a working group (WG), which is authorized in the steering committee for space science of ISAS/JAXA. Previously, a landing mission ‘‘SELENE-B’’ was proposed as ISAS’s next mission, but it was not ranked as the top priority for ISAS’s next engineering mission (Okada et al., 2006). The SELENE-B mission was designed to be launched as a half payload of H-II launch vehicle, then the scientific investigations were restricted to in-situ geological observations and the landing site was limited to the near side in equatorial to mid-latitude because of the limited mass budget (about 500 kg total dry mass and 50 kg science payload for the lander, respectively). Taking into account of the missions following SELENE and recovering LUNAR-A, it is worthwhile to perform not only the geological but also the geophysical observations simulta-
neously. In order to include both, we assume a spacecraft of full payload size of the H-II launch vehicle. We should also consider the mission life which must be extended to about 1 year to collect scientifically meaningful data for the geophysical observations. One possible idea to survive longer duration is to deploy the lander in an area permanently illuminated by sunlight (Shoemaker et al., 1994; Bussey et al., 1999). Polar regions have a benefit from a geological point of view, the north polar region is a typical high land area and the southern polar region is a part of or adjacent to the South Pole Aitken (SPA), where the deeper part of the crust or the mantle material are expected to be sampled. Our proposal in this study is a combination of a lander, two penetrators and a relay satellite as shown in Fig. 3. The dry mass of the lander is 830 kg (which is about 300 kg heavier than that of the previous SELENE-B study), mainly by addition of a geophysical observation package and extensive solar panels in order to supply adequate power to maintain the system landed in the polar regions.
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Fig. 3. Image view of SELENE-II spacecraft (lander, relay satellite, and two penetrators) and three-axis maneuvering penetrator deployment system.
As for the penetrator system, we propose a new type of deployment system of the penetrator. For the LUNAR-A mission, high accuracy of the mechanical interface between the penetrator and the spacecraft is required, the orbit and the attitude of the spacecraft must be controlled with precision, and the spacecraft must be spun up at a high rate (2 rps) at the time of deployment. This is not the appropriate system to include the penetrator system in future lunar and planetary missions because the spacecraft itself must be designed for the penetrator(s). Alternatively, we propose a three-axis maneuvering carrier system whose interface is much easier than that of the LUNAR-A mission (Fig. 3). The spacecraft dose not require severe control of attitude
nor spinning up. In other words, this is a kind of lander system without deceleration of vertical velocity. This technical innovation has an advantage in providing the penetrator system for other planetary missions in general even though the system requires more weight for the deployment system. For the SELENE-II mission in this case, two penetrators (100 kg/1 system) and relay satellite (about 200 kg) can be deployed from our preliminary estimation. Combined with the seismometry carried out by the lander, we can establish a three stations global seismic network which enables us to determine the seismic hypocenter and velocity structure. The basic concept of this seismic network by one lander at polar and two penetrators at equatorial regions is
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similar to that of the proposed Russian Luna-Glob mission (Galimov, 2005). The velocity profile is a key information for inferring the formation of the Moon. For example, some different explanations were discussed about the seismic gap at 500 km depth (Wieczoreck et al., 2006), but we cannot conclude this discussion unambiguously because the uncertainty is too large to bring further arguments about the depth of magma ocean, initial status at the formation, and regional heterogeneity.
4. Key technologies of the scientific instruments on board the lander In order to obtain the maximum scientific benefit from the seismic observation by the lander and the penetrators, we propose a broad band seismometer with a frequency range up to 0.01 Hz on board the lander and a short period seismometer on the penetrator, with a sensitivity peak around 1 Hz where the maximum power of seismic waves was observed by Apollo observations. The seismometer on board the penetrator is designed to have shock durability for the penetration event at around 5000 to 10000 G. In addition to the shock durability, the seismometer is a passive type which dose not require any battery consumption to operate (Yamada et al., 2007). On the other hand, the broadband seismometer requires a feedback system which consumes some power, to extend the bandwidth and linearity. Taking into account the requirement of long observation duration, a feedback system was not applied to the penetrator system, but this will be able to be deployed to the lander system which can supply enough power by solar or other energy resources. The lander observations yield the possibility to detect free oscillations excited by large shallow moonquakes or meteoroid impacts. If we successfully detect free oscillations, the averaged velocity structure will be obtained by one seismic station while the velocity discontinuity can be detected by the short period observations. It is controversial whether the free oscillations could be successfully detected by the Apollo observations (Khan and Mosegaard, 2001; Gudkova and Zharkov, 2002; Gagnepain-Beyneix et al., 2006). In general, even the surface waves of the Moon could not be detected by the Apollo long period (LP) seismometers, and the sensitivity of the seismometer is required to be 10 times larger than that of Apollo seismometers to detect the surface waves excited by one of the largest impact events (Lognonne´, 2005). There is another case for recording long period seismic waves for lunar seismology. Apollo seismic observations revealed that the seismic waves were intensely scattered as an effect of the regolith layer. As a result, the S/N ratio of the seismic waves is too low to identify precisely the P-S time and the later phase. The long period seismographs are expected to record undisturbed high S/N waves because they are not affected by the shallow structure. The dispersion of surface waves without the scattering effect of the regolith or megar-
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egolith layer should then yield clear information of the crustal and upper mantle structures. Therefore, the development of the broadband seismometer is one of the key technologies for the scientific observations by the lander, and one possible candidate is the instrument that has been developed in France by IPGP (e.g., Lognonne´, 2005; Mimoun et al., 2007). As for the geological investigations, the scientific concept and the instrumental configuration is basically the same as proposed in the SELENE-B study (Okada et al., 2006). The roles of in situ geological observations are complementary to remote sensing studies and in order to prepare the upcoming sample return mission as a next stage, it is indispensable to obtain the knowledge beforehand precise information about the chemistry and mineralogy as a ground truth to corroborate or re-evaluate the pre-existing interpretation of remote sensing results. Geological samples around the lander and using a robotic rover for distant crops will be taken with a manipulator and brought back to the lander for detailed analyses with a mini-laboratory called SIP (Science Instrument Package). The SIP will have a microscope, measure X-ray fluorescence and diffraction, carry out IR spectrometry and so on. For the in-situ observations, a control of sample surface conditions is one of the crucial technologies in order to analyze mineralogical and chemical composition of fragments a few millimeters in size in a breccia which was formed by ejecta related to the crater formation. We have developed a sample polishing system using ultrasonic technique. The Apollo and Luna returned samples indicated that some important fresh fragments which have not suffered from impact alteration must have remained in the breccias. These were frequently found as ‘‘rake’’ samples of a few centimeters in size, buried in the regolith. For these reasons, the sample collection system on board the lander or rover is also a fundamental technology which is also under development with priority (Fig. 4). 5. Summary We proposed a configuration of the SELENE-II mission as the follow-up of SELENE, and as the recovery of LUNAR-A missions. By the expansion of the payload mass compared to the SELENE-B mission, we preliminarily designed a mission outline which combines geological and geophysical sciences. Long operational duration (about one year) required by the seismic observations suggests a landing in polar regions, where it is possible to supply electrical power constantly. Although, it should be noted that the mission should be designed not only for the scientific purposes but also for the future utilization and for the human activity on the Moon; some important scientific aspects described in this report must be taken into account as the first priority for at least several next missions.
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Fig. 4. Photographic view of sampling and fixing system for the lander on board which is under development.
In addition to the in-situ geological survey by the lander, an extended lunar seismological network to investigate the deep structure in detail using penetrators and landers must be recognized as an important goal of the scientific contribution. A feasibility study is under way for the deployment of both the lander and the penetrators in one mission. Since landing missions are planned by other agencies and countries at around the same time, it is notable to have a great opportunity to make an ‘‘international seismic network observation’’. Then the network observation should also be considered by this international framework. In order to realize this, it is the proper opportunity to start the international collaborations in various ways for the upcoming lunar exploration era. Acknowledgments The authors would like to thank Pr. Yoshio Nakamura, Dr. Axel Hargermann, Dr. Nozomu Takeuchi, Dr. Y. Iijima, and Dr. Naoki Kobayashi for their helpful discussions of the seismic observations in the future lunar missions. We would also like to thank anonymous reviewers for constructive comments to the manuscript. References Bhandari, N. Chandrayaan-1: science goals. J. Earth Syst. Sci. 114 (6), 699–710, 2005. Bussey, D.B.J., Spudis, P.D., Robinson, M.S. Illumination conditions at the lunar south pole. Geophys. Res. Lett. 26 (9), 1187–1190, 1999. Chenet, H., Lognonne´, Ph., Wieczorek, M., et al. Lateral variations of lunar crustal thickness from the Apollo seismic data set. Earth Planet. Sci. Lett. 243, 1–14, 2006. Chin, G., Brylow, S., Foote, M., et al. Lunar reconnaissance orbiter overview: the instrument suite and mission. Space Sci. Rev. (in press). doi:10.1007/s11214-007-9153-y. Gagnepain-Beyneix, J., Lognonne´, P., Chenet, H., et al. A seismic model of the lunar mantle and constraints on temperature and mineralogy. Phys. Earth Planet. Int. 159, 140–166, 2006.
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