Sample of Paper for Journal of the Japan Society

Trans. JSASS Space Tech. Japan Vol. 7, No. ists26, pp. Tk_13-Tk_16, 2009

An Investigation of Lunar Mission of Shallow Structure Exploration Using Artificial Moonquake

Junya TERAZONO1), Yukihito KITAZAWA2,3), Hiroshi KANAMORI4), Satoshi TANAKA3) and Tatsuaki OKADA3)

1)The University of Aizu Aizu-Wakamatsu, Japan, 2)IHI Corporation, Tokyo, Japan 3) The Institute of Space and Astronautical Science, JAXA, Sagamihara, Japan 4) Shimizu Corporation, Tokyo, Japan

(Received May 2nd, 2008)

Manned lunar explorations are planned in several countries for future goals of their space exploration. Many scientists and engineers are considering that the lunar human outpost will be established no later than early 2020s. The manned lunar mission should be considered as the real problem. To construct rigid lunar bases, the investigation of shallow structure of the moon is required. Here the authors present current technology and understandings of investigation method of lunar shallow structure, and proposes plausible exploration scenario for forthcoming unmanned landing explorations.

Key Words: Lunar Structure, Moonquake, Lunar Outpost, Regolith

1. Introduction occurrence of these minerals are considered to be closely related to the large-scaled topographical structure. By By the successful launches of Japanese lunar explorer knowing subsurface conditions, we will be able to know “Kaguya” and Chinese lunar explorer “Chang’e-1”, the distributions of resources for future utilization. lunar exploration has entered the third period of In this paper, we will describe the concept of lunar exploration boom, after “moon race” in 1960s and subsurface study using the artificial moonquakes. First, exploration by small satellites in 1990s. On 2008, two we will address on past and current result of lunar more lunar explorers, “Chandrayaan-1” by India, and subsurface explorations, and compare the method “Lunar Reconnaissance Orbiter” by the United States will conducted and proposed. Second, we will focus on the be launched. These lunar explorers will fly over the moon surface mechanics experiment and the deliverable. And and will collect information on lunar surface including we will propose future lunar subsurface mission based on mineral composition and topographical features. these settings. On the other hand, many scientists and engineers are anticipating for the lunar surface mission as the next step 2. Exploration Method of Lunar Subsurface of the lunar orbiting explorations. Several countries have Structure started basic study on the exploration. Such activities include Japanese SELENE-21), Russian “Luna-Glob” and There are many geophysical exploration techniques of British “MoonLITE”2,3). The concentration of the mission the shallow subsurface. Seismic techniques (passive in 2012 and around clearly shows that lunar landing will methods, active methods, analysis of surface waves), be the next main method of exploration in early 2010s. boring techniques (auger, core samplers, penetrators, etc.), electrical resistivity (ER) surveys, and electromagnetic These landing missions are regarded as a precursor of (ex. Rader) sounding techniques are mainly used for the coming lunar manned exploration. Manned lunar outpost Earth and the lunar surface exploration. vision has been proposed by several space organization Recently, Lunar Rader Sounder onboard Japanese lunar such as NASA4) and JAXA5). Other countries such as explorer “Kaguya” revealed detailed subsurface structure6). China and India have been reported by media that they However, in point of view of regolith utilization (ex. have a plan to set lunar bases. 3 The investigation of lunar environment before setting meteoroid and radiation shielding, extraction of He, and the lunar outpost is a pressing theme. As we have a few manufacturing of cement), information of regolith layers’ and old information on the materials and behavior of lunar thicknesses and those distributions around a human lunar surface, more detailed information is required to construct base are needed. The information does not need to include large structures endurable for human habitation. Also, the information of deep layers of the moon, but need to subsurface structure must be examined to ensure the include distributions of regolith amounts and densities of installation of large and heavy structures. layers (regolith’s size) of shallow layers. The subsurface structure is also a key to clarify the In case of the Apollo project, two experiments, the distribution of valuable mineral resources, as the Active Seismic Experiment on Apollo 14 and 16 and the

Lunar Seismic Profiling Experiment on Apollo 17, were performed to determine the detailed structure of the subsurface of the Moon. Both experiments involved deto- nation of a series of small explosives. The seismic waves or ground motions caused by these explosions were measured by a network of geophones. On Apollo 14 and 16, up to 19 explosions were detonated by an astronaut using a device called a "thumper" along a 90-meter-long geophone line. On Apollo 16, three mortar shells were also used to lob Fig. 1. Trade-off study of typical concepts of active seismic experiments. Concepts of No.1 and explosive charges to distances No.2 use a rover. No.3 and No.4 use a lander and (small) penetrators. (ASS: Artificial Seismic of up to 900 meters from the Source. MP: Measurement Point) ALSEP (Apollo Lunar Surface Experimental Package). The idea No.1 and No.2 are suitable for exact On Apollo 17, eight explosive charges were positioned measurement, but those methods needs a rover and it is during the three EVAs (Extra-Vehicular Activities) at not expected to explore spread area quickly. The idea distances of up to 3.5 kilometers from the Lunar Module. No.3 and No.4 does not need a rover and it is expected to 7) These charges had masses of 57 grams to 2.7 kilograms . get information of spread area, but determination of These experiment results showed that the seismic locations and attitudes of penetrators (geophones) has experiments were suitable to estimate subsurface structure. difficulties in present technologies. For the preliminary In early phases of Japanese lunar exploration missions, exploration phase, we selected No.2 for the exploration of the explorations using unmanned rovers and/or landers are the shallow sub-surface, since it has simple architecture. planned. Here we discuss effective active seismic experiments on unmanned robotic missions. Dropping of a mass, explosive devices and mechanical vibrators are typical artificial seismic sources for Fig. 1. describes trade-off study of typical concepts of subsurface exploration. For general spacecrafts design, active seismic experiments using a rover and/or a lander. low weight is required. It is difficult to use the dropping No.1 of Fig. 1. shows the conceptual exploration tech- of a mass and lunar low gravity reduces the energy of the niques using a rover and geophones (and seismographs). dropping. The rover performs setting of a network of geophones and Explosive devices are not specific for spacecrafts but it also has active seismic source. After the setting up the is difficult to control frequencies of the seismic source. As network by the rover, it makes active seismic signals on a result, mechanical vibrators are suitable for the exact the surface. subsurface explorations. Although the designs of vibrators No.2 shows a combination idea of one geophone (or a are under discussions, we discuss scale model experiments of few geophones) and one rover. One geophone is (or a few the lunar subsurface exploration in the next section. geophones) located on the exploration point and the rover repeats dislodging and making seismic signals. In the 3. Simulation of Lunar Sub-surface Structures concept of No.3, small penetrators with geophones and a lander are used. During the landing phase of the lander, 3.1 Previous studies penetrators are released from the lander. After landing the Properties of lunar regolith and lunar ground have lander makes seismic signals. been considered as very important factors for developing In case of No.4, lander launches small penetrators as various devices to be operated on the moon. Studies on active seismic sources after landing. The lander has a the following issues have been therefore conducted from geophone or a seismograph and records the seismic the view point of terramechanics up to this point. signals caused by penetrators. Each method which 1. Evaluation and simulation of lunar soil properties described in Fig. 1. has merits and demerits. 2. Evaluation and simulation of lunar ground

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3. Rover mobility dynamic response of lunar ground at the landing impact. 4. Landing dynamics A soil container was about 100cm in diameter and 100cm 5. Excavation efficiency in depth. FJS-1 was placed only in the upper 40cm layer, A lunar soil simulant called FJS-1 was first developed, and lower 40cm layer was filled with gravels with because it was the most fundamental material and required maximum particle size of 2cm to simulate a highly for any of the studies listed above. FJS-1 simulates the compacted regolith, which was observed in Apollo core mechanical properties and approximates the chemical samples. compositions of the actual lunar soil carried back in the Landing pad models with various sizes, shapes, and US Apollo missions. Table 1 shows mechanical properties weights were dropped down to the surface of the layer, of FJS-1, and Fig. 2. indicates grain size distribution of and accelerations in several places were measured in the FJS-1. impact test.

Table 1. Mechanical Properties of FJS-1

100 ： Range of Lunar Sample 80 ： FJS-1

40 20 Fig. 4. Test Bed for Landing Impact

0 3.2 Artificial moonquake test 0.001 0.01 0.1 1 10 Several experimental studies will be required to design Particle Size (mm) the exploration mission of subsurface structure, and to Fig. 2. Grain size Distribution of FJS-1 determine the following conditions. 1. Vibration generator

Guide Rail > Vibration applying method (device) > Target frequency and amplitude Laser Displacement Sensor Counter Weight 2. Wave detector 6-axis Force & Torque Sensor > Vibration detecting method (device) Angle Sensor > Influence of surface wave Inclinable Container One of the recommended tests is a vibration parameter test, which use a soil container similar to the test bed for Torque Sensor landing impact as shown in Fig. 5. A vibration generator

Motor Encoder will be placed on top of the layer of lunar soil simulant, and pressure gauges or accelerometers will be embedded Wheel Specimen at some specified depths. Testing parameters will be the Fig. 3. Testing Apparatus for Rover Mobility frequency and amplitude of applied vibration, and bulk density of lunar soil simulant packed in the container. The Fig. 3. shows a testing apparatus for evaluating optimum vibration conditions including reference wave 8) mobility of lunar rovers . velocities will be obtained for every bulk density of the Superstructure of the apparatus consists of a wheel soil by analyzing detected wave data. The most efficient specimen, a driving system, a gravity compensation method of applying and detecting vibrations will be system, and sensors, while the substructures consists of a determined from these tests. container equipped with an inclination system, a plowing The other test is a surface wave test to investigate the device, and FJS-1. FJS-1 was placed in the container in influence of surface wave spreading on the lunar soil as about 10 cm lift thickness, and leveled by a plowing shown in Fig. 6. In this test, lunar soil simulant will be device before the mobility tests to create a constant loose packed in a large soil container like a swimming pool. A condition. vibration generator will be placed at the center of Fig. 4. shows a testing apparatus for measuring container on the surface, and wave detectors will be set on

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geophone is a simple device, we will be able to carry Lunar Soil Vibration Generator the geophone to the moon with slightly modification of Simulant earth models. z Several short-range penetrators should be installed in the lander to set seismometers. It should be smaller than ones which has been developed for the LUNAR-A mission. z Detailed structure of vibrators and required duration period of vibration should be determined for sub-scaled experiments shown in Fig. 6. Since the manufacturing technology of Lunar Soil Simulant is already established, the experiments should be Wave performed as soon as possible. Detector 5. Conclusion

We showed our research on the future lunar shallow structure exploration mission. We need to examine the following issues to make this mission concept more Fig. 5. Conceptual Schematic of Vibration Parameter Test precisely. z Extent of duration to detect shallow structure of the the surface at various distances from the generator. moon. This duration will be closely related to the whole Characteristics and influences of surface wave on the data mission duration. required to analyze subsurface structures will be z Light-weight device for vibration generators. It should investigated from this test. be modified based on terrestrial models. They also should be worked properly in airless and low-gravity Vibration Generator Wave Detector environment. z Theoretical and experimental studies of seismic wave propagation using lunar sub-surface models. Lunar Soil Simulant We will continue to study the mission plan and to perform sub-scale experiments using Lunar Soil Simulant. And we Fig. 6. Conceptual Schematic of Surface Wave Test aspire for the embarkation of the instruments for coming lunar landing missions. Issues to be examined before conducting these tests are; References 1. Scale effect and boundary conditions of the testing bed model. 1) http://www.jspec.jaxa.jp/activity/selene2.html 2. Treatment of the effects of secondary wave 2) Crawford I. A., Ball A. J., Wilson L., Smith A., Gao Y., UK reflected from the bottom and/or side walls of the soil Penetrator Consortium: MoonLITE: The Scientific Case, 39th Lunar and Planetary Science Symp., 2008 container. 3) Smith A., Crawford I. A., Ball A. J., Barber S. J., Church P., 3. Validity of test results and their correspondence to Gao Y., Gowen R. A., Griffiths A., Hagermann A., Pike W. T., the actual lunar conditions. Phipps A., Sheridan S., Sims M. R., Talboys D. L., Wells N.: MoonLITE — Technological Feasibility of the Penetrator 4. Mission Strategy Concept, 39th Lunar and Planetary Science Symp., 2008 4) http://www.nasa.gov/mission_pages/exploration/mmb/

5) http://www.jaxa.jp/about/2025/index_e.html Considering these results and deliberation of mission 6) T. Ono, A. Kumamoto, Y. Yamaguchi, Y. Yamaji, T. method and experiments, we here propose some mission Kobayashi, Y. Kasahara, H. Nakagawa, S. Oshigami, H. Oya; strategy for lunar shallow structure survey. Initial results of the Lunar Radar Sounder (LRS) experiment on-board the KAGUYA (Selene) spacecraft, European z The ideas style will be lander-based. Rovers are prefer- Geosciences Union. General Assembly 2008, Vienna, Austria, able, but not mandatory. To minimize the payload, we 13 – 18 April 2008, (Abstract EGU2008-A-03758) should concentrate on more scientific instruments 7) Apollo Program Summary Report, section 3.2.10 Active Seismic delivery. Experiment, JCS-09423, April, 1975 8) Kanamori, H., Aoki, S., and Nakashima, H.: ‘Terramechanics of z The seismic source should be geophones. Installations a Micro Lunar Rover’, Engineering, Construction, and for plural points are preferable. If we can use rovers, Operations in Challenging Environments: Earth & Space 2004, they can carry geophones for suitable positions. As the 2004.

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