OR3-3
微小重力下での着陸パッド動的貫入における柔軟地盤の機械的特性
Mechanical Properties of Soft Ground
during Dynamic Penetration of a Landing Pad
in a Microgravity Environment
大槻真嗣 1, 前田孝雄 2, 小林泰三 3, 尾崎伸吾 4, 石上玄也 5Masatsugu OTSUKI1, Takao MAEDA2, Taizo KOBAYASHI3, Shingo OZAKI4 and Genya ISHIGAMI5
宇宙航空研究開発機構，Japan Aerospace Exploration Agency,
東京農工大学，Tokyo University of Agriculture and Technology,
3 立命館大学，Ritsumeikan University,
4 横浜国立大学，Yokohama National University,
5 慶應義塾大学，Keio University
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2

1. Introduction
Since the world's first soft landing on the moon with the former Soviet Union's Luna-91), 120 spacecraft
have attempted celestial landings to date2), starting with the U. S. Ranger-3 attempt3). A total of 44 lunar landers
were launched until the former Soviet Union successfully returned the sample from the lunar surface with
Luna-24 in 1976. However, there was a gap of nearly 40 years until China successfully landed on the moonwith Change'34) in 2014. Although spacecraft by several private companies have attempted to land on themoon in recent years, with no success, India successfully accomplished this feat5) as the fourth nation in August2023. In the meantime, the United States has shifted its focus towards exploring Mars and is methodically
deploying its lander and rover to the surface of the Red Planet6-10). In Japan, the OMOTENASHI (Outstanding
Moon exploration Technologies demonstrated by Nano Semi-Hard Impactor) spacecraft11, 12) was launchedaboard SLS (Space Launch System) in November 2022 and operated in orbit as a full-scale landing on a highgravity object, following the touch-and-go exploration of small bodies by the Hayabusa series13). Also, in 2023,
the Smart Lander for Investigating Moon (SLIM) spacecraft14) is set to launch for lunar landing while aspacecraft, Martian Moons eXploration (MMX)15), is scheduled to be launched to a Martian moon in 2024. Thus,
landing explorations on the surface layer of celestial bodies are gaining momentum not only in Japan but also
in other countries worldwide.
MMX plans to collect more than 10 grams of surface samples from the Martian moon and bring them backto Earth, with the goal of landing safely and staying for several hours on the microgravity surface multiple
times. From these results, it is expected that a clearer understanding of the origin of the Martian moon will beobtained through close observation and sample analysis. Two prevailing theories will be investigated todetermine their accuracy. The two theories exist for the origin of Mars’ moons: the capture of an asteroid after
the Mars formation and the accumulation of debris scattered by a giant impact of Mars. For this MMX mission,
it is imperative to design a landing gear that will enable safe landing, extended stay and smooth take-off. The
landing gear is currently being developed for planetary probes, including small celestial bodies. The intention
is to prevent destruction during landing due to impact and kinetic energy of the spacecraft, as well as togradually reduce surface missions due to regolith. Currently, many landers employ a combination of reversethrust rockets with solid or liquid fuel and shock-absorbing materials such as aluminum honeycombs that rely
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on plastic deformation to protect the spacecraft from impact and tripping over upon landing. Most of planetarylanders have a free-fall period at the end of the landing sequence to avoid regolith fly-up contamination causedby the reverse injection manoeuvre. Upon touchdown, it is standard practice to cushion the impact with legs
equipped with the shock-absorbing materials utilizing aluminum honeycombs. Especially in aluminumhoneycomb, the acceleration can be controlled, and the total energy absorption can be passively adjusted bydesigning its crash cross section and volume. On the other hand, Luna-9 achieved the world's first soft landingof an observation instrument on the lunar surface by releasing the lander, covered in an airbag, just prior tolanding on lunar surface after taking the reverse injection for deceleration. Recently, the Skycrane method9)
has been developed for lowering a large rover, such as Curiosity and Perseverance, by actively controlling it
in a suspended form. Such advanced methods are being developed on the moon and Mars, but when exploringsmall bodies with extremely low gravity, measures such as avoiding ascent immediately after taking samples
within reach are taken due to uncertainties in surface properties. This is the case with both the Hayabusa series
spacecraft and OSIRIS-REx, neither of which involve active landing and staying. On the contrary, Philae16),
which endeavoured to delicately land on the comet, transported multiple landing equipment such as a reverse
injector, a harpoon, drills, and a shock-absorbing damper. However, except for the damper, the equipment
did not work, and Philae severally rebounds, resulting in landing in the shade of rocks at where the sunlight
does not reach. It significantly decreases the worth of the mission, and it is undisputed that the equipment
should function normally even for landing on a small body. Therefore, it is essential for the MMX spacecraft
to quantitatively assess the interaction with the soft ground composed of regolith covering the surface ofcelestial body in advance and to design the landing gear appropriately.
To test equipment in a microgravity environment on Earth, free fall in a drop tower and parabolic flights
are required. However, due to cost limitations, both the time and frequency of execution are restricted.
Therefore, it is essential to develop a methodology for predicting the mechanical properties of the soft groundon the surface layer of small celestial body at an early design stage and use it for proactive verification in thelanding gear design. In this context, the Hourglass mission17) was undertaken on the International Space
Station with the aim of acquiring a comprehensive understanding of the behaviour of the regolith particles
that comprise the ground in low gravity. However, analysis to improve accuracy is limited to predicting thescattering of regolith and attachment of floating regolith. Evaluating the behaviour of these particles is
intricate. Thus, it is considered that the deeper testing is necessary to predict the reaction force when a large
spacecraft touches a small body. In other words, it is crucial to obtain the necessary information for designfrom microgravity tests conducted on the ground.
In this paper, we present the results of the dynamic penetration test of the landing pad under microgravity
using the drop tower and the obtained findings. Firstly, we report that dynamic penetration tests have been
carried out on two types of granular media in the drop tower, and that the sinking tendency of the specific
granular media does not differ from the result of the 1G test. In addition, the relationship between the drop
tower test with a scale model and the 1G test with a full-scale model is also mentioned, as the drop tower test
has significant dimensional limitations.
2.

Basic principle

In the Martian moon landing sequence of the MMX spacecraft, the state in which the spacecraft first
touches the surface of Phobos is defined as ‘grounding’, the state in which the spacecraft is expected to remain
stationary is defined as ‘landing’, and the state in which the spacecraft leaves the surface is defined as 'takeoff'. Here we show what the issues are and how to deal with them when interacting with the celestial surface
layer in a low-gravity environment, i.e., the landing dynamics that occur during the transition from‘grounding’ to ‘landing’.
First, the MMX spacecraft is larger than the conventional landers for small celestial bodies, and it remains
on the surface of the Martian moon for several hours after landing. The landing gear plays a crucial role in
exploration missions as it safeguards the spacecraft against damage, ensures a secure landing, and eliminates
hindrances to mission success. Specifically, a landing gear is required that yields a low impact to the spacecraft,avoids tripping it over upon landing, and prevents scattering of regolith covering the celestial body’s surface.
Moreover, it must retain a stable posture for an extended period. The MMX spacecraft intends to perform acontrolled free fall from a 10 meters altitude to avoid the scattering of regolith resulting from reverse injection.
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However, the velocity upon ‘grounding’ is expected to reach 0.5 m/s, with a corresponding kinetic energy ofapproximately 200 J. This would be equivalent to landing on a high-gravity celestial body like the moon orMars. Therefore, it is essential to examine the incidence of falls and rebounds in the surface layer of the Martianmoon where the restoring force by gravity is inadequate. On the contrary, as the Martian moon possesses agreater gravity than other small celestial bodies, the influence on the surface layer upon ‘grounding’ is greaterthan that of its counterparts. Accordingly, the landing deceleration encountered by the spacecraft in the
deployed state should be adequately mitigated.
Figure 1 (left) shows the configuration of the MMX spacecraft’s leg. The MMX spacecraft is equipped withthe four landing legs, which are designed to be able to control acceleration and dissipate energy with shockabsorbing materials, to suppress sinking and reduce the amount of regolith scattering with tapered landing
pads, to stabilize the attitude during stay with the leg extension function and to absorb shock with the same
shock characteristics multiple times. Other features include dynamic load measurement on the chassis (shownon the right in Figure 1) and contact detection at the bottom of the landing pad.

ShockabsorberinsideActuator-telescopic main leg

Spacecraft body

Main Leg
structure

Sub leg

Lengthadjustable
shock
absorber

Schematic of function
block for load sensing

Sub leg

Coupling
Sensor top mass

Single axis
force sensor
Landing pad

Foot padSoft ground

Reaction force

Figure 1. Configuration of the landing leg of the MMX spacecraft and composition for load
measurement18) by the main landing leg.

Figure 2 presents an example of the time history of the reaction force applied to the landing pad during‘grounding’ with celestial surface. When the time history of force during ‘grounding’ of the pad with the soft
ground is divided into three phases, firstly, in Phase-1, the pad begins to settle into the soft ground. Then, in
Phase-2, the shock-absorbing material receives the reaction force at a constant load within the Plateau region,achieving limited acceleration and the dissipation of the kinetic energy. Due to the considerable dissipation of
the kinetic energy in MMX, most of the time taken for landing dynamics is occupied by this phase, in which
the act of settling onto the ground does not make progress. Then, in Phase-3, rebound occurs or stays with theremaining energy after dissipating as much energy as possible. Therefore, it is necessary to predict themechanical properties of the soft ground during Phase-1 until the shock-absorbing material begins to collapse,
due to the dynamics interacting with the celestial surface. Excessive sinkage in Phase-1 increases theconstraints of planetary protection and increases the risk of pad failure. In essence, determining Phase-1 holds
significant importance in deciding the size of the landing pad and subsequent response, especially in theamount by which terminal sinkage must decrease before the shock-absorbing material begins to collapse.
The verification for the MMX spacecraft is advancing two strategies for predicting mechanical properties
at the surface of celestial bodies. One is to create a simulated regolith of the surface layer of small celestialbodies, obtain the response of the scale model in a microgravity environment such as a drop tower test, collectdata to match the responses as closely as possible, and directly verify the design. 6 types of simulated regolith
assuming a Phobos surface layer were produced19), and the validity of the settling condition is evaluated by
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microgravity testing using the simulated regolith with the smallest reaction force among them (Phobos
simulant, Model 3-1), which will be mentioned in this paper. However, since it is difficult to reproduce bothgravity and instrument dimensions in experiments, interpolation by analysis is also essential.
Phase-1
Landing dynamics

Phase-3
Stabilized or reboundPhase-2Compression of shock
absorber with constant load

Soft ground case
(Scaling factor : 0.1)

Figure 2. Typical load history on the force sensor located at the top of the landing pad.

Another strategy is to select and use macro models of interactions, and to analyse them to see if they holdintentionally over a wide parameter range. Settlement is predicted using methods based on the Hyperbolic
Load-Settlement model20) and the model based on the Resistive Force Theory (hereinafter called RFT) 21). The
latter can be easily introduced into simulators as it can also be used as an explicit method to calculate when
the pad angle or penetration direction changes. The scaling factor (hereafter referred to as SF), which is aparameter of the RFT, is measured by aircraft experiments using the other simulant (Phobos simulant, Model2-2) and the difference in bulk density and reaction force due to different dimensions is measured to limit therange used in the design. As a result, by limiting the scope of the verification in the simulator and clarifying
the calculation method, including the expression expansion, we have been able to construct a system in which
the verification can be widely carried out, including the development manufacturers.
However, in the microgravity tests with the simulant as described above have confirmed somephenomena with different governing physical laws, which limit the perfect reproduction in RFT. Since thecompressive state of the shock-absorbing material accounts for most of the spacecraft behaviour in the MMX
spacecraft at the time of ‘grounding’, it is assumed that the effect of different amplitudes, e.g., a peak position,other than the amplitude, is poor for the behaviour in a very short time until the settling stops, considering
the response speed of the spacecraft. Therefore, it is assumed that the spacecraft behaviour to be verified canbe reproduced by swinging the parameter range of the RFT, and the complete reproduction of the time history
is a future issue, and the reproduction of the landing dynamics using the RFT model has already been
performed22).
In this presentation, to investigate the first strategy, the reaction force response in Phase-1 until the shock
absorber begins to collapse is captured in the microgravity environment, and the results provide guidance on
how to conduct a full-scale leg performance evaluation test at 1G. However, the correction of the reaction force
prediction equation of the second strategy with pads of different sizes is beyond the scope of this paper, andonly the experimental results with the pad of one size are mentioned.
3.

Experiments and results

3.1. Experimental setup and conditions
Figure 3 shows the experimental setup used for the dynamic penetration test of the landing pad in a
microgravity environment. Table 1 shows the specifications of the experimental setup and the experimental
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conditions. Using Tohoku Silica Sand No.8 (hereinafter called TK8) and Phobos Simulant Model 3-1(hereinafter called PS) as granular media samples, experiments are conducted in which an aluminum discsimulating a landing pad is dropped into the soft ground at a constant speed. A weight with measurement
instruments is connected to the top of the pad and the sample is placed under reduced pressure in an acrylic
vacuum chamber. The behaviour of the pad is monitored by a high-speed visible light camera from outside
the chamber, and the flight position of the pad and the position of the ground surface are always measured by
two laser displacement sensors. In addition, a triaxial accelerometer is mounted inside the weight, and a highsensitivity force sensor (Kistler 9217A) and a 6-axis force-torque sensor (ATI Mini 40) are mounted on the leg,uniaxially restrained by guide rails. Furthermore, a separation mechanism using an extrusion spring and ashape memory alloy device (E500 from the former TiNi company) enables timely and constant speed injection.
The bulk density is then deliberately controlled by hammering a sand bucket filled with the sample, and thetest measurement is the bulk density calculated from the position of the ground surface when the pad finallystrikes. Lastly, the vacuum chamber is retracted into the drop capsule, and the capsule is allowed to free fall
for approximately 2.5 seconds without any resistance from the outside air, and then collides with the inside of
the capsule, which is slowed down by an air damper, with an impact of approximately 40 G on landing.

High-speed camera(outside chamber)

Vacuum chamber
Deployment mechanism(Non-Explosive Actuator as aseparation device and a spring)
Measurement system(including a 3-axis accelerometer,a 2-axis gyroscope, temperature
sensors, data loggers, electricalbatteries, a proximity sensor anda force sensor amplifier)Leg unit

High precision single
axis force sensor
with guide rails

6-axis force
torque sensor
Bulk density adjusted
regolith ground

Landing pad

Thermistor asa touch sensor

Laser displacement sensing spot

Sand and box

Figure 3.
(left) Overall configuration of the experimental setup for the landing pad; (right) enlarged view of the leg unit andits measurement configuration.
Table 1. Summary of the experimental setup parameters and the experimental conditions.

Terms
Pad diameter

70mm

Flying mass

2.5

kg

Impact speed> 0.50m/s

Degree of vacuum

> 100

Pa

Granular media

Value

Unit

Tohoku silica sand #8 (TK8)
Phobos simulant (PS)

Bulk density (TK8)
Bulk density (PS)

1211~1474
1034~1299

kg/m3
kg/m3

Scaling factor (SF)

0.1~9

-

3.2. Results and considerations
Figure 4 shows a graph plotting the amount of terminal sinkage that occurred when a landing pad collided
with the granular media sample in the microgravity environment and the bulk density at that time in eachsample. There are two types of results for each sample in vacuum at 1G and vacuum at G, and totally fourtypes of results appear. However, self-weight compensation, which is not applied to the G results, is
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Sinkage [mm]subtracted from the 1G results. As a result, in TK8, the difference in the amount of sinkage is large due to thedifference in gravity, while in PS, the result is almost the same. Figure 5 also shows the behaviour of sand(regolith) during pad collisions in the microgravity environment. The results show that TK8 has scattering fineparticles, whereas PS has no scattering particles and appears to have collapsed in situ with no effect on thesurrounding granular media. A similar trend was observed for the conditions with different bulk densities.

Figure 4. Terminal sinkage (settlement) when the pad penetrates the ground at the given bulk densities(1G/G & TK8/PS).

Tohoku Silica
sand #8(TK8)
Phobos simulant Model 3-1 (PS)

Figure 5. The impact of the pad on each test ground in microgravity (left: TK8, right: PS).

These results are attributed to the interaction with the surrounding granular media. At 1G, the granular
media pushed by the flat plate will try to move in a direction away from the compression point and gradually
flow upwards to distribute the force, but the weight of the surrounding granular media will become a large
resisting force and the amount of settling will be suppressed. On the other hand, at low gravity, the weight ofthe surrounding granular media becomes almost zero, so the force to hold the granular media in place by theflat plate compression is no more than a small inertial or adhesive force, and as the result it sinks greatly. Thiswas observed in the case of highly compressible granular media such as regolith in static penetration tests(measurement of ultimate bearing capacity) in the reference23), and the same result was obtained there. As aresult, the same phenomenon was thought to occur even at the time of dynamic penetration, and the behaviourof the granular media in Fig. 5 seemed to reinforce the correctness of the phenomenon.
In summary, gravity independent results can be obtained by evaluating the amount of dynamic
compression penetration using granular media such as regolith forming the test bed. Conducting a full-scale
compression penetration test at 1G would be equivalent to performing the same test at G. The same amountof subsidence means that the changes in the amplitude and time direction of the force history are very similar.
Consequently, don't different sizes have different phenomena? Since the current behaviour is a granularparticle phenomenon and the particle size does not change when the pad size is changed, it can be judged thatthe same result can be obtained when viewed locally.
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4.

Conclusions

In this paper, we presented the results of dynamic penetration tests to investigate the gravity dependenceof the landing pad behaviour under microgravity and the findings obtained. As the result, it was found thatfor highly compressible granular media such as regolith, the settling tendency is no longer different from theresult of the 1G test. This result suggests that the 1G test can predict the response results without performinga size-constrained microgravity test, which would extremely reduce the resources for the verification of thespacecraft design. In the future, we will try to compare the dynamic penetration results at G with different
size pads.
5.

Acknowledgements

We would like to thank to Professor H. Miyamoto of the University of Tokyo for producing and providing
us with the Phobos Simulant (Model 3-1) for this study.
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