Trans. JSASS Aerospace Tech. JapanVol. 14, No. ists30, pp. Pk_23-Pk_28, 2016

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Copyright© 2016 by the Japan Society for Aeronautical and Space Sciences and ISTS. All rights reserved.

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Penetration Dynamics of an Asteroid Sampling System

Inspired by Japanese Sword Technology

By Takeo WATANABE,1) Hironori A. FUJII,2) Takeshi SAKAMOTO3) and Genrokuro MATSUNAGA4)

1) Teikyo University, Utsunomiya, Japan 2)Tokyo Metropolitan University & TMIT Research Center, Tokyo, Japan

3) National Institute of Technology, Ariake College, Omuta, Japan 4)Matsunaga Japanese Swords Forging Laboratory, Arao, Japan

(Received July 31st, 2015)

In this study, we proposed a tether-type system for the surface sampling of primeval astronomical bodies in future asteroid explorations. In particular, we experimentally investigated the collision dynamics and penetration of the sampling apparatus. The experiment adopted the simple method of dropping the corer from a height and considering gravity and the kinetic energy imparted on impact. The masses and initial velocities of the corer were widely varied. These intrusion tests were performed on lightweight concrete that emulated an asteroid surface. From the results, we could relate the penetration depth to the kinetic conditions of the corer. In addition, to achieve the sharpness and plasticity demand of the corer tip, we borrowed the techniques of traditional Japanese swordsmithing in fabricating the corer samples.

Key Words: Sample Return, Tether, Penetrator, Japanese Sword, Asteroid Exploration

1. Introduction In this study, we propose a tether-type system for collecting surface samples from asteroids in future asteroid exploration. The proposed method aims at sampling the stratigraphic sequence. For this purpose, the sampler is equipped with a corer that penetrates into the asteroid surface after the sampling device is released from the spacecraft onto the asteroid.1, 2)

The Hayabusa spacecraft retrieved fine particle samples from the asteroid Itokawa by firing a bullet at the asteroid surface and recovering the dust formed by the impact. 3, 4, 5) In a subsequent mission, Hayabusa 2 employed a sampling mechanism that will dig into the asteroid surface with a collision device, but this mechanism is essentially the same as that of Hayabusa 1.6) While these missions sampled the surface layer only, the mission subsequent to Hayabusa 2 (Hayabusa Mk-2) is expected to sample stratigraphic sequence.7) After various proposals, we devised a sampling system that accounts for the lack of gravity (which makes drilling difficult) and ensures the safety of the spacecraft by launching a tether-type sampling device into an asteroid. The sampling procedure was tested in an experimental study, focusing on the dynamics of the sampler from impact to penetration. Various methods were adopted in the experiments such as dropping a corer from high above the surface. Penetration experiments were performed on a simulated asteroid-like surface for a wide range of corer masses and initial velocities. The penetration depth of the corer was found to be roughly proportional to the mass and three half power of velocity. As a guide for designing the penetration system, we also optimized the shape and structure of the tip. The sampling method in this research employs a corer

equipped with a blade that wedges into the target object. For very hard surfaces, the blade cannot penetrate sufficiently. In previous experiments, the tips of stainless steel corers became deformed. Japanese swords are superior cutting tools due to their unusual hardness, created through the repeated heating of uniquely wrought steel. Although the scientific findings are limited, these swords have been known to cut through metal and stone. This information led to the idea of improving the corer by employing similar technology. To achieve the sharpness and plasticity demand of the corer tip, we borrowed the techniques of traditional Japanese swordsmithing in fabricating the corer samples. The first goal of this research is to empirically verify the dynamic parameters that determine the penetrative performance of the corer and to produce a guideline for the design of the sampling mechanism. The second goal is to enhance the penetrative capability of the corer by employing the technique used to fabricate Japanese swords and to fabricate a prototype of the corer.

2. Tether-type Sampling System Several sampling methods have been designed by the Japan Aerospace Exploration Agency and the Hayabusa series pre-projects, namely, the “bullet bombardment-type” system devised by Hayabusa and “explosive charge-type penetrator,” “birdlime-type,” and “drilling-type” systems.8) The present study considers the “tether-type sampling system” as a novel and more advantageous form of the explosive charge-type penetrator system.9) Because the spacecraft detaches the sampling mechanism without landing on the asteroid, the tether-type system is expected to be more secure than the

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methods requiring touchdown. Moreover, the sampler can collide with the asteroid at a sufficiently high speed to sample the stratigraphic sequence. The sequence enclosed in the corer is then retrieved via the tether. The collection procedure, in which the tether recovers the corer after penetration, is shown in Fig. 1.

Fig. 1. Concept of the tether-type sampling system.

Surface samples are collected at the launch, penetration, and recovery phases. The present study focuses on the penetration phase; specifically, the dynamics of the corer’s collision and penetration into the asteroid surface are investigated in a series of experiments. 3. Gravity Drop Penetration Experiment 3.1. Simulating the materials of an asteroid’s surface This subsection describes the materials used to simulate the asteroid surface in the penetration experiments. Although the experimental material should ideally simulate a D-type asteroid or a comet-asteroid transition object, the details of the D-type asteroid structure are insufficiently known. Therefore, we could only estimate the asteroid’s dynamic composition.10) Here, we adopted a foam-processed lightweight concrete, as shown in Table 1 and Fig. 2. The density, porosity, and other properties of this lightweight concrete can be arbitrarily controlled by changing the ratio of cement and foaming obsidian, which replaces gravel.

Fig. 2. Foam-processed lightweight concrete.

Table 1. Properties of foam-processed lightweight concrete.

Material Lightweight concrete

Density (g/cm3) 0.5

Porosity (%) 78

Compressive strength (MPa) 16.8

Size (mm) 250×250×100

3.2. Corer design and fabrication Corers for the gravity drop penetration experiment were fabricated by milling stainless steel pipes. Specifically, a stainless steel pipe (SUS304) with an outer diameter of 25 mm and a thickness of 2 mm was milled. The specifications are shown in Table 2. The corer design is illustrated in Fig. 3.

Fig. 3. Corer (short).

Table 2. Specifications of corer.

Material SUS304

Cutting edge angle (deg) 10

Diameter (mm) 25

Thickness (mm) 2

Length (mm) 150

Mass (kg) 0.153

3.3. Gravity drop experiment To determine the effect of the corer mass and velocity on the penetration into lightweight concrete, we varied the corer mass and terminal velocity to yield the mechanical energies of 10 to 60 J (in 10 J increments). The corer mass was altered by stacking a 1 kg shaft body with multiple 0.5 kg weights, giving a total mass of 1–3.5 kg (in 0.5 kg increments). In addition, the terminal velocity was adjusted by altering the free-fall height of the dropped object. By this method, we determined the changes in the penetration depth while varying energy and maintaining constant energy but varying the corer mass and velocity. 3.3.1. Drop tube For safety purposes, the dropped object was guided through a drop tube that was constructed by joining a series of polyvinyl chloride pipes to reach the desired height. The building permitted a drop height of up to 9 m. A diagram of the experiment and photographs of the experiment setup are shown in Fig. 4(a–c). The initial height of the corer was adjusted by altering the length of the polypropylene cord; the cord was then released, dropping the corer through the guide tube. The landing area was enclosed to prevent debris from scattering, and the impact was captured by multiple cameras that included a high-speed camera.

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methods requiring touchdown. Moreover, the sampler can collide with the asteroid at a sufficiently high speed to sample the stratigraphic sequence. The sequence enclosed in the corer is then retrieved via the tether. The collection procedure, in which the tether recovers the corer after penetration, is shown in Fig. 1.

Fig. 1. Concept of the tether-type sampling system.

Surface samples are collected at the launch, penetration, and recovery phases. The present study focuses on the penetration phase; specifically, the dynamics of the corer’s collision and penetration into the asteroid surface are investigated in a series of experiments. 3. Gravity Drop Penetration Experiment 3.1. Simulating the materials of an asteroid’s surface This subsection describes the materials used to simulate the asteroid surface in the penetration experiments. Although the experimental material should ideally simulate a D-type asteroid or a comet-asteroid transition object, the details of the D-type asteroid structure are insufficiently known. Therefore, we could only estimate the asteroid’s dynamic composition.10) Here, we adopted a foam-processed lightweight concrete, as shown in Table 1 and Fig. 2. The density, porosity, and other properties of this lightweight concrete can be arbitrarily controlled by changing the ratio of cement and foaming obsidian, which replaces gravel.

Fig. 2. Foam-processed lightweight concrete.

Table 1. Properties of foam-processed lightweight concrete.

Material Lightweight concrete

Density (g/cm3) 0.5

Porosity (%) 78

Compressive strength (MPa) 16.8

Size (mm) 250×250×100

3.2. Corer design and fabrication Corers for the gravity drop penetration experiment were fabricated by milling stainless steel pipes. Specifically, a stainless steel pipe (SUS304) with an outer diameter of 25 mm and a thickness of 2 mm was milled. The specifications are shown in Table 2. The corer design is illustrated in Fig. 3.

Fig. 3. Corer (short).

Table 2. Specifications of corer.

Material SUS304

Cutting edge angle (deg) 10

Diameter (mm) 25

Thickness (mm) 2

Length (mm) 150

Mass (kg) 0.153

3.3. Gravity drop experiment To determine the effect of the corer mass and velocity on the penetration into lightweight concrete, we varied the corer mass and terminal velocity to yield the mechanical energies of 10 to 60 J (in 10 J increments). The corer mass was altered by stacking a 1 kg shaft body with multiple 0.5 kg weights, giving a total mass of 1–3.5 kg (in 0.5 kg increments). In addition, the terminal velocity was adjusted by altering the free-fall height of the dropped object. By this method, we determined the changes in the penetration depth while varying energy and maintaining constant energy but varying the corer mass and velocity. 3.3.1. Drop tube For safety purposes, the dropped object was guided through a drop tube that was constructed by joining a series of polyvinyl chloride pipes to reach the desired height. The building permitted a drop height of up to 9 m. A diagram of the experiment and photographs of the experiment setup are shown in Fig. 4(a–c). The initial height of the corer was adjusted by altering the length of the polypropylene cord; the cord was then released, dropping the corer through the guide tube. The landing area was enclosed to prevent debris from scattering, and the impact was captured by multiple cameras that included a high-speed camera.

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Fig. 4(a). Setup of the gravity drop experiment.

Fig. 4(b). Setup of the gravity drop experiment.

Fig. 4(c). Setup of the gravity drop experiment.

3.3.2. Shaft The corer and weight were fixed and hung by a 1 kg shaft fabricated using metal. Fig. 5 illustrates the shaft and the corer-shaft-weight-cord system. This shaft connects and fixes the sharp edge of the corer to a weight, which determines the dynamic characteristics.

Fig. 5. Shaft and corer mounted onto the shaft.

3.4. Results of the gravity drop penetration experiment Photographs of the corer after penetration in the gravity drop penetration experiment are shown in Figs. 6 and 7. The energy (mass) of the corer in Figs. 6 and 7 is 60 J (1 kg) and 30 J (3.5 kg), respectively. As shown in Figs. 8 and 9, the corer mass and penetration depth are the increasing functions of energy, but even at the same energy, increasing the corer mass increases penetration. In Fig. 8, we observe that as the kinetic energy is varied from 10 to 60 J, the penetration increases regardless of energy. Fig. 9 plots the penetration as a function of velocity for different kinetic energies. The penetration depth decreases with increasing velocity at constant energy. Collectively, these results imply that at constant energy, the penetration depth increases at a higher mass and lower velocity. The experiment was conducted once for each parameter setting, and the resulting data was not sufficient for statistical analysis. However, the results gave enough detail to suggest trends in the dynamics of penetration. We are preparing to conduct more experiments in the future.

Fig. 6. Energy = 60 J; height = 6.12 m; penetration = 105 mm.

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breakage of the structure may occur for very hard objects. A stainless steel corer was used for earlier experiments and was observed to deform under certain conditions. It has been known since ancient times that Japanese swords possess a high degree of hardness produced by heat treatment of their unique steel structure. It has been reported that under the right combination of dynamic conditions, the swords cut through hard materials such as metal and stone. The fabrication process for Japanese swords should produce a high-performance corer.13) Japanese sword corers were developed using Japanese sword process technology—a traditional technology which has been cultivated for 1000 years or longer. As such, this technology is among the most reliable industrial technologies known. In applying Japanese sword processing to the development of planet sampling corers, we considered the materials, refinement, and accuracy of form, heat treatment, and surface lapping. Japanese swords are made from “Tama-hagane,” a mixture of iron sand and charcoal. The carbon content of the steel is optimized during forging by the swordsmith. The rough form is shaped simultaneously. The rough sword is then polished into a sharp, beautiful artwork by a Japanese sword-polishing technician. The forms and sizes of our developed Japanese sword corers are indicated in Fig. 11. The types of our Japanese sword corers are listed in Table 3. 4.1. Material As the corer material, we employed 100% Tama-hagane, an alloy of 80% Tama-hagane and 20% SK105 (carbon tool steels), and 100% SK105. The SK105–Tama-hagane alloy ensures little oxidative consumption under forging and was experimentally confirmed to yield a harder blade edge than pure Tama-hagane. The corer constructed from 100% JIS SK105 was designed to resemble Tama-hagane with carbon content. 4.2. Form and size The forged Tama-hagane is contoured with line structures. To investigate whether the performance was altered by the contouring direction, we fabricated two types of corers—parallel and vertical—to the following line structures. The material was fabricated by machining, and the contouring was performed by a wire-cut electric discharge machine (Fig. 12). For compatibility with past designs and experiments, the outside diameter of the corer was specified as 25 mm. Because the point angle of a Japanese sword is 31°–33°, the point angle of the blade edge was originally designed as 15°. However, in experiments, cores with edge angles below 15° penetrated more deeply. Therefore, we ultimately adopted a 10° blade edge (Fig. 11). The sharpness of the corer tip is a key to increasing the penetration capability, but at the same time, it is necessary that the blade be sufficiently thick in order to have the rigidity needed to withstand the impact. Manufacturing cost is another factor that needs to be considered. 4.3. Heat treatment The fundamental heat treatment was trialed on product type E (SK105). The heat treatment proceeds as follows. An extended pipe (STKM13A) and corer is treated by brazing,

followed by high-frequency hardening. Tempering is performed at 180°, and subzero treatment is performed at −70°. Finally, tempering is repeated at 180°. The edges of blades subjected to such heat treatment are break-resistant. However, at the high injection speeds of corers, the heat-treated blades may break. The heat treatment imposed on the Japanese sword corers must be carefully determined from the experimental results. Based on the experience and intuition of the swordsmith, numerous prototypes will be fabricated and tested. Data will be collected through trial and error to determine the optimal conditions for heat treatment.

(a) Appearance （top: blade; bottom:

whole body） (b) Blade size

Fig. 11. Japanese sword corer. 4.4. Lapping Whether the surface roughness of a corer influences its penetration depth is yet to be known. The relationship between surface smoothness and penetration depth will be investigated shortly. For this purpose, we produced four types of Japanese sword corers and corers made from high-carbon steel material (SK105). The relation between injection speed and penetration depth is currently investigated on corers made from stainless steel piping. However, stainless steel may not withstand very high injection speeds. We expect that Japanese sword corers will prove worthy under these circumstances.

(a) Image of contoured alloy 80% “Tama-hagane”, 20% JIS

SK105

(b) Contouring direction

Fig. 12. Contouring.

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Fig. 7. Energy = 30 J; height = 0.87 m; penetration = 73 mm.

Fig. 8. Plot of penetration vs. mass.

Fig. 9. Plot of penetration vs. velocity.

Kneubuehl11) fired bullets from a handgun into dry fir wood and recorded the penetration depths of the bullets. He derived the following empirical relationship:

2

23

298.0d

mvX (1)

where X is the penetration depth of the bullet into the fir material (mm), and d, m, and v are the diameter (mm), mass (g), and velocity (m/s) of the bullet, respectively. The dynamic parameter of this penetration χ is

23mv (2) We now analyze our present results in terms of these parameters. Hereafter, we refer to Eq. (2) as “the penetration parameter.” Penetration is a physical phenomenon in which mechanical energy is transformed into destruction or deformation. It is a complex mechanism, and the theoretical analysis is difficult, making the empirical method the only viable option. However, considering that there is no phase change and that the deformation will be generated in the porosity, we have concluded that experiments similar to those done on wood will be helpful. The results of earlier experiments confirmed this idea.

Fig. 10. Plot of penetration vs. mv3/2.

Fig. 10 plots the penetration depth as a function of the penetration parameter (Eq. (2)). The results of this experiment are closely correlated with Kneubuehl’s empirical formula. The proportionality factor depends on various physical properties such as the thickness and geometry of the corer, the sharpness of the tip, the composition of the target object, and the porosity of the object. However, the amount of penetration can be increased simply by increasing the kinetic energy. Increasing the mass would be one option, but this is limited by the size of the spacecraft and the payload. And we can calculate the velocity and kinetic energy required to achieve the target depth, providing important parameters for the final design. There were some instances in which the sample fell off during the extraction of the corer. A mechanism to prevent samples from falling off during the extraction and recovery phase needs to be devised. For future experiments, the mass of collected samples will also be measured and analyzed. There were also instances when the corer gripped onto the samples when penetrating deeply. Quantitative measurement has not yet been implemented. However, a reduction in gripping force and its control needs to be researched in the future. 4. Form, Size, and Fabrication of Japanese Sword Corers12)

The proposed sampling method will cut into the target object with a blade at the tip of the corer, and deformation or

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breakage of the structure may occur for very hard objects. A stainless steel corer was used for earlier experiments and was observed to deform under certain conditions. It has been known since ancient times that Japanese swords possess a high degree of hardness produced by heat treatment of their unique steel structure. It has been reported that under the right combination of dynamic conditions, the swords cut through hard materials such as metal and stone. The fabrication process for Japanese swords should produce a high-performance corer.13) Japanese sword corers were developed using Japanese sword process technology—a traditional technology which has been cultivated for 1000 years or longer. As such, this technology is among the most reliable industrial technologies known. In applying Japanese sword processing to the development of planet sampling corers, we considered the materials, refinement, and accuracy of form, heat treatment, and surface lapping. Japanese swords are made from “Tama-hagane,” a mixture of iron sand and charcoal. The carbon content of the steel is optimized during forging by the swordsmith. The rough form is shaped simultaneously. The rough sword is then polished into a sharp, beautiful artwork by a Japanese sword-polishing technician. The forms and sizes of our developed Japanese sword corers are indicated in Fig. 11. The types of our Japanese sword corers are listed in Table 3. 4.1. Material As the corer material, we employed 100% Tama-hagane, an alloy of 80% Tama-hagane and 20% SK105 (carbon tool steels), and 100% SK105. The SK105–Tama-hagane alloy ensures little oxidative consumption under forging and was experimentally confirmed to yield a harder blade edge than pure Tama-hagane. The corer constructed from 100% JIS SK105 was designed to resemble Tama-hagane with carbon content. 4.2. Form and size The forged Tama-hagane is contoured with line structures. To investigate whether the performance was altered by the contouring direction, we fabricated two types of corers—parallel and vertical—to the following line structures. The material was fabricated by machining, and the contouring was performed by a wire-cut electric discharge machine (Fig. 12). For compatibility with past designs and experiments, the outside diameter of the corer was specified as 25 mm. Because the point angle of a Japanese sword is 31°–33°, the point angle of the blade edge was originally designed as 15°. However, in experiments, cores with edge angles below 15° penetrated more deeply. Therefore, we ultimately adopted a 10° blade edge (Fig. 11). The sharpness of the corer tip is a key to increasing the penetration capability, but at the same time, it is necessary that the blade be sufficiently thick in order to have the rigidity needed to withstand the impact. Manufacturing cost is another factor that needs to be considered. 4.3. Heat treatment The fundamental heat treatment was trialed on product type E (SK105). The heat treatment proceeds as follows. An extended pipe (STKM13A) and corer is treated by brazing,

followed by high-frequency hardening. Tempering is performed at 180°, and subzero treatment is performed at −70°. Finally, tempering is repeated at 180°. The edges of blades subjected to such heat treatment are break-resistant. However, at the high injection speeds of corers, the heat-treated blades may break. The heat treatment imposed on the Japanese sword corers must be carefully determined from the experimental results. Based on the experience and intuition of the swordsmith, numerous prototypes will be fabricated and tested. Data will be collected through trial and error to determine the optimal conditions for heat treatment.

(a) Appearance （top: blade; bottom:

whole body） (b) Blade size

Fig. 11. Japanese sword corer. 4.4. Lapping Whether the surface roughness of a corer influences its penetration depth is yet to be known. The relationship between surface smoothness and penetration depth will be investigated shortly. For this purpose, we produced four types of Japanese sword corers and corers made from high-carbon steel material (SK105). The relation between injection speed and penetration depth is currently investigated on corers made from stainless steel piping. However, stainless steel may not withstand very high injection speeds. We expect that Japanese sword corers will prove worthy under these circumstances.

(a) Image of contoured alloy 80% “Tama-hagane”, 20% JIS

SK105

(b) Contouring direction

Fig. 12. Contouring.

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Table 3. Type of Japanese sword corers.

Type Materials Contouring direction

A 100% Tama-hagane parallel

B 100% Tama-hagane vertical

C 80% Tama-hagane，20% JIS

SK105 alloy parallel

D 80% Tama-hagane，20% JIS

SK105 alloy verticality

E JIS SK105 (high carbon tool

steel) isotropic

5. Conclusions As a mechanism for sampling asteroid surfaces, we chose the tether-type sampling system as a study theme and experimentally evaluated the dynamics of the penetration phase. A corer was designed to collide at high speeds under various conditions. The asteroid surface was simulated by a lightweight concrete material. The penetration depth was measured as a function of the dynamic parameter. In addition, to maximize penetration performance, we borrowed the fabrication techniques of Japanese swordsmithing to produce the corer tip. From the study results, we can elucidate the basic characteristics of the sampling system. This research will contribute to realizing a sampling system that retrieves many examples of intact stratigraphic sequences. Due to the high cost of the Japanese sword corer, an experiment using the actual equipment in a penetration test has yet to be conducted.

References

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10) Consolmagno, G. J., Britt, D. T. and Macke, R. J.: The Significance of Meteorite Density and Porosity, Chemie der Erde, 68, pp. 1-29, 2008.

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