Lunar Interior Exploration by Japanese Lunar Penetrator Mission, LUNAR-A Hitoshi Mizutani Institute of Space and Astronautical S

J. Phys. Earth, 43, 657-670, 1995

Lunar Interior Exploration by Japanese Lunar Penetrator Mission, LUNAR-A

Hitoshi Mizutani Instituteof Spaceand Astronautical Science, Sagamihara 229, Japan

TheInstitute of Spaceand AstronauticalScience (ISAS), Japan, plans to undertake a lunar mission,named as LUNAR-A,which is to be launchedin 1997.The scientific objectiveof the missionis to explorethe lunar interiorusing seismometry and heat-flow measurementto obtaina betterunderstanding of the originand evolutionof the moon. The M-V,the newestversion of the Mu serieslaunch vehicles, will be used to send a 53 kg spacecraftinto a lunar transferorbit. Three penetrators containing ultra-sensitive seismometersand heat-flowprobes will be deployedfrom a spacecraftonto the lunar surface,and willconstitute a seismicand heat-flowmeasurement network of a much largerspan than the ApolloALSEP network. The seismicobservations are expected to providekey data on the sizeof the lunar core and its physicalproperties, as well as data on deep lunar mantle structure.The heat flowmeasurements at three penetrator landingsites will also provide important data on thermalstructure and concentrations of heat-generatingelements in the moon. Combiningthese data, it is expected that we willbe ableto obtainmuch stronger geophysical constraints on the originand evolution of the moonthan has beenpreviously obtained.

1. Introduction In spite of the large amount of scientific data collected in the Apollo mission by the United States and Luna missions by the former Soviet Union, the structure and composition of the lunar interior are still very poorly known. As reviewed by Hood (1986),geophysical evidence presently available on the lunar interior is not yet adequate to strongly constrain models of lunar origin and evolution. Geophysical observations, however, can in principle provide clues to some key questions relevant to models of lunar origin and evolution, such as the global MgO/(MgO+Fe)) ratio, and the abundances of siderophile elementsand refractory elements of the moon (Wood, 1986; Larimer, 1986; Drake, 1986). The current lunar seismological model up to a depth of 1,000km does not provide us with good constraints on the mineralogical and chemical composition of the lunar mantle (Hood, 1986;Nakamura et al., 1982;Tanaka et al., 1990).Even the most recent and possibly the best seismic model by Nakamura (1983) indicates a rather large uncertainty at depths below 270km. In Nakamura's model, the complete set of seismic arrival times from the Apollo lunar seismic network was inverted to estimate the average

ReceivedOctober 21, 1993;Accepted March 7, 1994

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velocities in the three sections of the lunar mantle: two for the upper mantle (58-270km, and 270-500 km) and one for the middle mantle (500-1,000 km). The P-velocities and their uncertainty in the above three sections are 7.74•}0.12, 7.47•}0.25, and 8.26•} 0.40km/s, respectively. As pointed out by Nakamura (1983), there is no reason to believe that the velocities are actually constant in any of the above three zones, or that the indicated velocity discontinuity actually exist. Thus it is clear that we have very limited knowledge of the seismic structure of the moon at depths below 270km. If we had a better seismological model of the lunar internal structure than the currently available ones (Nakamura et al., 1982; Nakamura, 1983; Goins, 1981), we would have more reliable answers on the bulk composition of the moon, which is essential to understand the origin of the moon. In particular, determination of the lunar core size is important to interpret the observed depletion of siderophile elements in the lunar mantle (Newsome and Drake, 1983; Newsome, 1986; Ringwood and Seifert, 1986), as well as the mantle density distribution (Hood and Jones, 1985), which in turn constrains the Mg number of the mantle. The bulk abundance of refractory elements is also tightly connected with the

geophysical observation of heat flow determination, because the global lunar U concentration may be estimated from heat flow measurements. The Apollo 15 and 17 heat flow probes gave heat flow values of 21 and 16mW/m2 respectively (Langseth et al., 1976), from which Keihm and Langseth (1977) estimated a mean heat flow of 14 to 18 mW/m2. This range of heat flow values corresponds approximately to a global lunar U concentration of 33 to 44ppb, if a steady-state balance between heat loss (flow) and heat production is assumed. The estimate of the bulk U concentration is much

higher than that of Earth's mantle. However derivation of globally representative averages of the heat flow from the two Apollo sites is necessarily difficult, so that the estimated global U abundance should have a large uncertainty, as pointed out by Rasmussen and Warren (1985), who claimed that the lunar bulk concentration of U may be as low as that in Earth's mantle. Since the bulk abundance of refractory elements in the moon, as compared with that in the Earth, is important for delineating the lunar

origin, more measurements of heat flow at different geological settings are desirable to derive a reliable global heat flow average and hence a global U abundance in the moon. The Japanese lunar mission has been planned to provide some important clues to solve the above-mentioned problems. In order to achieve the scientific objectives, penetrators were thought to be the most effective, because they can deploy scientific instruments on different sites in one mission. In the present paper, first we describe the outline of the Japanese lunar mission.

Some details of the instruments used in the mission are then given, and we discuss how the scientific observations will be made during the mission. In the last section, we discuss prospects and problems of future planetary seismology in a more general framework.

2. Outline of the Japanese Lunar Mission "LUNAR-A"

A spacecraft for the Japanese lunar penetrator mission called LUNAR-A will be launched in the summer of 1997 from the Kagoshima Space Center of the Institute of

Space and Astronautical Science, Japan (abbreviated hereafter as ISAS), using the M-V

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launch vehicle,which is being developedat present at ISAS and will be completedin 1996.The M-Vlaunch vehicle can senda spacecraftof 530kginto a lunar transferorbit. The spacecraft,which has a cylindricalshape, is designedto be spin-stabilized. It is 2.2m in maximumdiameter and 1.7m in height. The attitude and spin rate of the spacecraftis controlledby an N2H4mono-propellant Reaction Control System(RCS), while orbital maneuveringnear the moon is accomplishedby a bi-propellant(N2O4 and N2H4)engine. As shown schematically in Fig. 1, the spacecraftwill be first insertedinto an elliptic lunar orbit (i=30deg), using lunar-solargravity assist (Dunham et al., 1989;Uesugi et al., 1989).The technologyof maneuveringa spacecraftusing the lunar-solargravity assisthas been alreadydemonstrated in the ISAS'sHiten and Geotailmissions (Uesugi et al., 1992).The gravity-assistmaneuvering of the spacecraftsignificantly reduces the propellantrequired to insert the spacecraftinto lunar orbit and allowsan increasein the payload mass. The spacecraftorbit will then be controlledusing a bi-propellant engineto lower the periluneto about 40km from the lunar surface,from which a penetratorwill be deployedone by one. The penetrator is a missile-shaped instrument carrier, which is about 14cm in diameterand 90cm in length.The penetrator is deorbitedby a small solid-propellant motor from the spacecraft,canceling completely the spacecraftorbital velocity,and fallsfreely onto the lunar surface.During the freefall descent,the penetratoris reoriented to becomevertical to the lunar surfaceusing a sidejet, as shown in Fig. 2. The deorbit motor and attitude controller attached to the penetrator will be jettisonedafter they becomeuseless and before the penetrator hits the lunar surface. Thejettison is consideredto be necessaryto ensureproper penetration of the penetrator into the lunar regolith.

Fig. 1. Schematic diagram of the mission profile of the Japanese Lunar Mission, LUNAR-A. LPM stands for the Lunar Penetrator Module. Although only one of the three penetrators are shown in this diagram (left), three penetrators are deployed on the lunar surface and constitute a large triangle network of the seismometer and heat-flow probe.

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Fig. 2. Schematic diagram of deployment of the lunar penetrator module . The deorbit motor and the attitude control system are attached at the rear end and lateral side of the penetrator respectively.

The final impact velocity of the penetrator will be about 250 to 300m/s; it will encounter a shock of about 10,000G at impact on the lunar surface. It is known that the lunar surface is covered by a lunar regolith of about 10m thick (McKay et al., 1991). According to numerous experimental impact tests (e.g., ISAS Lunar Penetrator Team, 1990, 1991, 1993) using model penetrators and a lunar-regolith analog target, each penetrator is predicted to penetrate to a depth 1 to 3 m, depending on the hardness and/or particle-size distribution of the lunar regolith. The penetration depth is important for ensuring the temperature stability of the instruments in the penetrator and heat flow measurements. According to the results of the Apollo heat flow experiment

(Langseth and Keihm, 1976), an insulating regolith blanket of only 30cm is sufficient to dampen out the 280 K lunar surface temperature fluctuation to •}3K variation. Thus the penetrator instruments do not require any temperature controller which saves significantly the total power consumption.

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It will require about a month to deploy three penetrators at widely spaced sites. Two penetrators will be placed on the near-side of the Moon and one on the far-side. The site of one of the near-side penetrators is located near the Apollo 12 or 14 site, enabling us comparison of the LUNAR-A data with Apollo network data. The far-side penetrator will be placed at a position near the antipodal point of the Apollo 12 site. After releasing all of the penetrators, the spacecraft will make trajectory control maneuver and will be injected onto a low altitude near-circular orbit: the altitude is about 200km from the lunar surface. The data gathered by the scientific instruments will be numerically compressed and stored in a recorder within the penetrator and then will be transmitted to the. Earth via the carrier spacecraft which will come over each penetrator every about 15 days. A UHF (f=400MHz) hybrid telemetry system will be used for the communication between the deployed penetrators and the relay spacecraft, while communication between the spacecraft and ground station will be by S-band (f=2GHz). Since the lunar regolith is relatively transparent to the radio-wave (e.g., Carrier et al., 1991), there will be essentially no attenuation of the radio signal from the antenna, which will also be buried at 1 to 3m depth in the regolith. The data transfer rate from the penetrator to the mother spacecraft is about 1kbits/s and that from the spacecraft to the ground station is 8kbits/s. Besides the scientific instruments within the penetrator, the orbiting spacecraft also will carry a monochromatic imaging camera. The spatial resolution will be about 30m; we intend to take images of the lunar surface near the terminator so that subtle topographic variation will be captured.

3. Science Instruments in Penetrator The penetrator itself, excluding a retro-motor and an attitude control system, is of a cylindrical shape with an ogive-shape nose. It is about 90cm long and 14cm in maximum diameter, and weighs about 13kg. It contains the scientific instruments of a two-components seismometer and a heat flow probe together with other supporting instruments such as a tiltmeter and an accelerometer. The tiltmeter is used to know the attitude of the penetrator in the regolith and the accelerometer is used to judge the, depth of the penetrator, by integrating the recorded deceleration at impact of the penetrator. After intensive experimental studies (see e.g., ISAS Lunar Penetrator Team, 1993), we proved that the scientific instruments as well as the various electronics and batteries described above can withstand the very high impact load, if proper care is taken in the design and fabricating procedures. The seismometer system consists essentially of two short-period electro-magnetic seismometers with a resonant period of about 1s, each of which is aligned orthogonally. Since the penetrator will in general not be placed in the lunar regolith in a pure vertical direction, a rotation mechanism is installed to reorient the seismometers to the desired direction. However the specific horizontal direction of the horizontal-component seismometer will not be known, because the penetrator hits on the lunar surface spinning and we will not have any good means to determine which direction is north or south

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in the lunar regolith. We might be able to determine the direction later after analyzing seismic data. The seismometers are designed to be approximately 10 times as sensitive as either the Apollo short-period or long-period seismometer at a frequency of around 1Hz

(Latham et al., 1969), as shown in Fig. 3. It will be able to detect a seismic disturbance of 0.1A amplitude. This high gain is achieved by using a new rare-earth magnet and a newly developed electro-magnetic coil wound with 20ƒÊCu wire. The high shock- durability is attained by reducing movable components to a minimum. The ground motion will be recorded for signals larger than a threshold level in order to reduce

power consumption. The threshold level can be changed later in operation by a command from the ground. Although the lunar seismic signal for one event usually lasts more than 1 h, we will record the signals for only a limited period for each event, to save memory size and power consumption in the nominal operation mode. The recording duration will depend on the size of the moonquakes, which will be judged 256s after

Fig. 3. Response curve of the LU- NAR-A seismometer. Response of the Apollo short-period (SPZ) and long- period (LP) (peaked response and flat Fig. 4. Schematic diagram of the ther- response cases) seismometers are also mal conductivity instrument used on shown for comparison. LUNAR-A penetrator.

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the recognition of an event. Although the time window is sufficiently long to observe most of the body-wave phases even at ƒ¢=180deg, it may be too short for surface wave analysis. Therefore we will have an operation mode that permits us to record the full wave form, once we record an especially large event. The heat-flow probe consists of fourteen temperature sensors and five thermal- conductivity measurement equipments attached to the wall of the penetrator (ISAS LUNAR-A Team, 1990; Tanaka et al., 1992). Although the lunar surface temperature varies from 100 at night to 400K in day time, the temperature oscillation is damped out very quickly with depth due to the very low thermal diffusivity of the lunar regolith. According to the Apollo results by Langseth et al. (1976), the temperature oscillation from night to day is less than 1K at depth of about 50cm. Therefore it will be possible to determine the subsurface temperature gradient by measuring the temperature at several points deeper than 50cm from the surface. The thermal conductivity of the lunar regolith is measured by recording the temperature variation associated with the heat output from a point source attached to the interface between the penetrator and regolith (Horai et al., 1991). The schematic diagram of the device for the thermal conductivity measurement is shown in Fig. 4. The principle of this method allows us to determine the thermal inertia of the lunar regolith but not the thermal conductivity. But because it is known that the density and

Fig. 5. The theoretical temperature field around the penetrator 27 days after penetration into regolith (after Tanaka et al., 1992). Although the temperature gradient along the penetrator is not the same as the background selenotherm, we will be able to determine the background temperature gradient through analysis of the temperature data along the penetrator axial direction.

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heat capacity of the upper lunar regolith do not vary from place to place, we probably can use the Apollo data to estimate these values for the regolith at the penetrator site. Then we can estimate the thermal conductivity with a reasonable accuracy from the thermal diffusivity value. The temperature field in the lunar regolith around the penetrator will be significantly disturbed by the penetrator itself, as shown in Fig. 5 where the results of a model calculation are shown. The determination of the heat flow values from temperature measurements on the surface of the penetrator and thermal-conductivity measurement is not a very simple task. A detailed analysis of the temperature field within and around the penetrator will be required for a quantitative determination of the heat-flow values. Although the accuracy of the determined heat flow depends on the accuracy of the modeling, preliminary assessment using the finite element method on the temperature field around the penetrator indicates that we can estimate the heat-flow value within 10% error (Tanaka et al., 1992). All the instruments are powered by Li-SOCl2 (super Lithium) batteries which have a power density of about 430Wh/kg. Although the instruments are designed to be of very low power-consumption type, the life-time of the instruments are limited to 1 year due to the limitation of the battery mass allotted.

4. Scientific Observation of Lunar-A

Three penetrators will be deployed at three different sites: the first candidate site is near the Apollo 12 or 14 landing sites, the second is a site in Mare Mendeleev on the farside which is antipodal to the first site, and the third is a site in Mare Crisium on the near side. The three penetrators constitute a seismic network of a much larger span than that of the Apollo seismic network. The network will cover a rather wide range of angular distances if we can observe the seismic waves from the same general variety of deep moonquakes listed by Nakamura

(1978). Figure 6 shows how many data can be obtained at a particular angular distance from the deep moonquakes assuming the three candidate sites (337.2•‹E, — 3.0•‹N),

Fig. 6. Predicted statistics of events observed by LUNAR-A network as a function of angular distance (after Terazono, 1993).

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(154.2•‹E, 14.5•‹N), (60.2•‹E, 12.8•‹N) of the penetrator seismometers and the catalogued location of the deep moonquake sources. Because a deep moonquake from the each source occurs about once per month, we will be able to accumulate a rather com- prehensive data set over a wide range of angular distance for 1 year observation period. Since each deep moonquake has its own characteristic waveform and an origin-time versus tidal-phases correlation (Toksoz et al., 1977; Nakamura, 1978), the focus of a deep moonquake may be determined by data obtained at a single station near Apollo 12 or 14 station. Figure 7 shows the waveforms received at Apollo 12 site for twelve events from one deep moonquake source which is located at (16.6•‹S, 39.8•‹W) by Nakamura et al. (1982). It is notable that even fine-details of the later phases in the

Fig. 7. Lunar seismograms of Al deep moonquake source observed at Apollo 12 site. Note steady similarity of the wave form of each event (after Terazono, 1993).

Vol. 43, No. 5, 1995 666 H. Mizutani each waveform are very similar to each other. This demonstrates how stable the waveform is for a fixed combination of the source and receiver. If the same sources can still be observed, comparison of the waveforms obtained by the penetrator seismometers with those from Apollo seismometers for each deep moonquakes catalogued in Nakamura et al. (1982) will allow us to determine the source location of deep moonquake events. Otherwise we must utilize both P- and S-wave arrival data at two other stations in order to locate the focus of a deep moonquake. If we can determine the foci of moonquakes, the amplitude and travel times of the seismic-waves will provide an important data on the internal structure. Figure 8 shows the ray-path pattern of seismic waves from a deep moonquake. The model used in Fig. 10 is Nakamura et al.'s mantle structure (Nakamura et al., 1982) to which an iron core with 400km radius is added. If the moon has a core with radius as large as 400km, the core will act as a large lens for seismic wave propagation, causing a strong convergence of ray paths at the antipodal point of the epicenter and a rather wide shadow zone. Since the focusing effect of the core is so prominent, we will be able to observe PKP and even PKPPKP phases at an appropriate station. If the lunar core is smaller than 300km, the ray-path pattern will not have features such as those shown in Fig. 8. By observing seismic waves at three different sites from many deep moonquakes, we will be able to gradually delineate the true ray-path pattern which will eventually provide a picture of the internal structure of the moon. Heat-flow observations at stations deployed by the penetrators will provide data on the thermal conductivity and vertical temperature gradient at each site. By combining these data, we will be able to determine the heat flow. As described in the introduction, these data will be essential to understand the global average of the lunar thermal regime and hence to estimate the bulk abundance of radioactive heat-generating elements. In order to avoid thermal disturbances due to power consumption within the penetrator, the heat-flow measurement will be made before and after full operation of the

Fig. 8. Seismic ray path from a deep moonquake of the focal depth of 900km for Nakamura et al.'s model plus 400km core (Vp=5.0 km/s). Note a significant 'focusing effect of the lunar core to seismic wave , causing a large energy concentration at the antipodal point of the epicenter (after Terazono, 1993).

J. Phys. Earth Lunar Interior Exploration by Japanese Lunar Penetrator Mission, LUNAR-A 667 seismometers. As mentioned earlier, a reliable determination of the heat-flow values will be made after a detailed analysis of the temperature field around the penetrator, using the obtained data. Mapping by the imaging camera on board the spacecraft will also provide us with detailed topographic data on the lunar surface. Because the LUNAR-A imaging camera is designed to take images in the region near terminator (i.e., at low sun angle), the data obtained by the LUNAR-A mission will give a much clearer images with higher contrast than the Clementine mission UV/VIS camera (e.g., Pieters et al., 1994), enabling us to see more clearly subtle elevation change of the lunar surface. These data also may shed light on the crustal formation process at early phases of lunar history.

5. Prospects of Future Planetary Seismology The LUNAR-A mission described above should be thought to be the first step toward the more long-range goal of planetary seismology. Because of recent advances in seismological instrumentation and analysis, terrestrial seismology is entering into the stage of determining the fine structure of the Earth's interior. But our knowledge of the internal structure of planets and satellites other than the Earth is very poor and is still at a stage of the zeroth or the first order of approximation. Undoubtedly seismology can play a vital role in revealing planets' and satellites' interior in future. Since an understanding of planetary internal structure is essential to clarify planetary origin and evolution, the role of planetary seismology is thus very important. The penetrators described in this paper will provide a very cost-effective means to deploy seismometers on almost all terrestrial-type planets. We believe that application and modification of the LUNAR-A type seismometers will expand the horizons of planetary seismology. The LUNAR-A mission is the first mission dedicated to studying the lunar internal structure but this mission alone will not be sufficient to reveal the fine details of lunar structure. It is very clear that we need more seismic stations and more advanced seismometers to approach the current state of terrestrial seismology. Since the moon is the most easily accessible planet, our effort to study the lunar interior should be continued and expanded after the LUNAR-A mission. Mars is also a very interesting target for future planetary seismology. Current geophysical constraints on Mars' internal structure come only from the total mass and moment of inertia, from which we cannot determine the core size and composition uniquely. Since the core size and its composition are closely related to the dynamo mechanism of planetary magnetism, seismological study of the Martian core is essential to understand the origin of the planetary magnetism as well as Martian evolution in comparative planetological framework. The MESUR project of NASA and Mars Net Project of ESA are intended to provide seismological data of the Martian internal structure. These missions are being planned to be launched in late 1990's to early 2000's. Although these missions, if realized, will provide us with very important data on the internal structure of Mars, the network established by these missions undoubtedly will be insufficient to disclose the fine crust, mantle and core structure of Mars. If we could participate in either NASA's MESUR mission or ESA's Mars Net mission or if we could undertake our own new Mars

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Penetrator Mission, we could clearly contribute to improvement of the Martian network and thereby increase the scientific return of all the international missions. Compared with the Moon and Mars, Venus is probably more akin to the Earth in terms of the internal structure. However recent findings by the Magellan synthetic aperture radar observation of the Venusian surface (Saunders et al., 1992) indicate that the Venus is geologically active but that there is no convincing evidence for global plate tectonics. The tectonic style of Venus seems very different from that of the present Earth. Therefore the Venusian lithosphere and athenosphere might be very different from those of the Earth in spite of the similarity of the mass and size of both planets. Considering the importance of the internal structure to understand Venusian tectonics and evolution, a seismological study of the internal structure of Venus should be undertaken in the future. Because the Venus surface conditions prohibit the ordinary application of terrestrial seismological methods, it necessary to develop seismometers which can withstand the high temperatures and high pressures prevailing on the Venusian surface. This is a very challenging technological problem but it is worthwhile challenge in view of the great scientific importance of understanding the Earth, Moon, and other terrestrial planets. Besides the Moon and other terrestrial planets, the internal structure of the many icy satellites of Jupiter, Saturn, Uranus, and Neptune are also very interesting seismological targets. These bodies also can be studied by essentially the same technique as terrestrial seismology using seismic networks. In these cases, however, another new technology for low temperature electronics must be developed, because the surface temperatures of these bodies are below 200K and conventional electronic devices thus cannot function. Finally we should briefly mention the seismology of jovian planets. Because these planets have no solid surface, we cannot deploy ordinary seismometers to build seismic networks on the planets. However we can probably apply a similar technique and analysis method used in helio-seismology (e.g., Brown et al., 1986) to the jovian planets. There is a possibility that the temporal and spatial variation of light or any other electromagnetic wave from the planetary surface is related with free oscillation of theses gaseous planets. The problem of jovian seismology will be to develop the technology to observe very weak variations of the electromagnetic waves, because the strength of excitation the free oscillation in these planets is thought to be much weaker than in the sun. I believe every effort to resolve the difficulty of observing the free oscillation of the jovian planets should be made, as this is an extremely important research topics in future planetary seismology.

This paper is a brief summary of various studies done by LUNAR-A ScienceTeam. We wish to thank Dr. A. Fujimura,Dr. M. Hayakawa, Mr. S. Tanaka, Mr. H. Araki, Mr. J. Terazono, all of who are at Institute of Space and AstronauticalScience, Dr. I. Yamada of Nagoya University, Dr. J. Koyama of Tohoku University, Dr. Y. Nakamura of Texas University and other members of the team for their contribution to this project. Special thanks go to Drs. M. Kohno, M. Hinada, S. Nakajima, K. Ninomiya, H. Matsuo, and H. Saito for their valuable contribution to engineering aspect of the present project. We are very grateful to the late Prof H. Hasegawa, and ProfessorsT. Itoh, M. Shimizu,N. Kawashima,R. Akiba, and J. Nishimura for their initial

J. Phys. Earth Lunar Interior Exploration by Japanese Lunar Penetrator Mission, LUNAR-A 669 guidance and suggestions toward putting the project into a realistic project. Without their unselfish assistance and guidance, this project would never have been realized. Finally I would like to thank Prof. Robert Geller of the University of Tokyo, and two anonymous reviewers for their constructive comments to the manuscript.

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