M.Sc. in SPACE STUDIES 2005/2006

On locomotion and grasping control of a limbed rover intended for asteroid surface exploration

Filipo Andrei Mendoza Valencia

Individual Project Report submitted to the International Space University in partial fulfillment of the requirements of the M.Sc. Degree in Space Studies

August 2006 Internship Mentor: Kazuya Yoshida Host Institution: Tohoku University, Sendai, Japan ISU Academic Advisor: Isabelle Scholl

ii

ACKNOWLEDGEMENTS My most sincere thanks go to God in the first place for blessing our paths in every step. I thank my wife Katya Selene Mariscal Leyva, my daughter Katya Giovanna, and my son Filippo Josué for their patience, force, trusts, and unconditional support deposited on me, and for being my inspiration during this masters program. I specially give my gratitude to my beloved father, Gustavo Adolfo Mendoza Avila, for believing on me, for sharing and propelling my dreams all my life and for making this venture a reality. I thank my beloved mother, Maria Otilia Valencia Rincon for her lovely support and counsel. And I thank the rest of the family and friends for their continuous moral support, wise advice and kindness in every pace. Furthermore, I want to specially thank my internship mentor, Dr. Kazuya Yoshida, for his kind invitation to his laboratory and his supportive guidance during my internship period. I wish to show my appreciation to Dr. Keiji Nagatani for lending me a hand every time I needed. I want to specially thank Dr. Marco Chacín, Dr. Eric Rohmer and Andrés Mora for all their friendly assistance and brotherhood during my stay in the laboratory. And I desire to acknowledge Tohoku University and all the members of the Space Robotics Laboratory for making my stay in Japan a pleasant and unforgettable experience. I also want to thank my ISU academic advisor, Isabelle Scholl, for her friendly advice and care, which always made difficult situations easy and fun. And finally but nevertheless, I wish to honestly gratify, the ISU Student Affairs Department, and specially Jill Ferrier, for her continuous and kind support for my family and me.

iii

ABSTRACT Near-earth minor-objects such as asteroids have a great scientific importance for space exploration. In this respect, locomotion over such bodies becomes essential for in-situ scientific measurements. However, micro-gravity environment and rough terrain conditions on these objects impose new challenges on mobility for rovers to overcome. Multi-limbed rover locomotion seems to work better in such environments. Rovers need to remain attached to the asteroid while moving over the surface. This condition requires knowledge of the forces interacting under the surface while the rover walks in grasping modes, to make the rover intelligent to calculate the position and force in its walking gates. Proper study of these forces will play an important role in developing new control algorithms. This paper presents the current research work on the analysis of the effects of hexapod rover locomotion in a simulated micro-gravity asteroid surface environment. Measuring the behavior of the friction cone forces of the rover legs when in contact with the surface performs this later. This analysis plays a fundamental step towards the development of next generation micro-gravity environment rovers capable of landing smoothly on an asteroid, and will be beneficial for producing control algorithms that can make them walk in a stable fashion while remaining attached to the surface against the dynamic reaction from the moving legs. Based on previous detailed development design of a multi-limbed rover, the author overviews the needs of such a robot in a feasible mission scenario and in extension describes experimental results proving that for a successful pre-landing phase in the micro-gravity environment of an asteroid, the rover will require to master the combined control of sensitive soft landing and grasping modes to touchdown and remain attached to the surface against being expelled into deep space by the two-bodies dynamic reaction forces.

iv

TABLE OF CONTENTS

Page

ACKNOWLEDGEMENTS ......................................................................................... II

ABSTRACT.................................................................................................................III

LIST OF ACRONYMS................................................................................................ V

1. INTRODUCTION ................................................................................................. 6

2. ASTEROIDS: WHY DO WE WANT TO VISIT THEM? .................................. 8

2.1 RECENT MISSIONS TO COMETS AND ASTEROIDS .................................................... 9 2.2 MOBILITY ON ASTEROIDS .................................................................................... 13

3. DESIGNING ROVERS FOR MICROGRAVITY ............................................. 14

3.1 THE HEXABOT SPIDY .......................................................................................... 17 3.2 CONTACT AND GRASPING FORCES ....................................................................... 21

4. EXPERIMENTS.................................................................................................. 23

4.1 TEST BED SETUP .................................................................................................. 26 4.2 RESULTS.............................................................................................................. 29

5. CONCLUSIONS.................................................................................................. 34

REFERENCES............................................................................................................ 36

v

LIST OF ACRONYMS A AU: Stands for astronomical unit and is based on the mean distance from Earth to the

Sun, 9.3×10 7 miles (1.5×108 km). J JPL: Jet Propulsion Laboratory L LINEAR: Lincoln Near-Earth Asteroid Research N NASA: National Aeronautics and Space Administration NEO: Near Earth Object, a space object in a high proximity encounters with Earth S SRL: Space Robotics Laboratory

6

1. INTRODUCTION Since the beginning, space activities have been characterized by the human pursuit of increasing our scientific knowledge of the universe, as a parallel reason for the exploration of outer space. There are countless numbers of scientific findings that relate to space in our daily lives; of which, though, not everybody is always aware. The human problems on Earth could be diminished if everybody looked up to the stars and understood how small we are in this vast universe [5]. Fortunately, there are always people who believe this can happen, and then struggle to exceed our natural and societal limits to extend our understanding and our species into outer space. The future of humankind, in my belief, will be better based on how more we comprehend that our existence is dependent on a very fortunate event, and that the world we live in can be seriously affected at any time by external phenomena, like the radiation from the sun, or the collision of an asteroid. We are all vulnerable to these and many other factors, but the discussion in this paper is only partially related to one of these important events: asteroids. The author only wishes the reader to keep in mind the parallel importance to any scientific studies of these minor-bodies that are sometimes relatively close to Earth. There is a significant interest in studying these bodies from the very scientific perspective, as to increase our conception of their origin, as well as their relationship with our planet and its evolution. But it is as well advisable to allow for some awareness that these bodies could represent a real threat to our species as it is historically evident they have been such a threat in the past to other species. After the successful in-situ exploration mission to planetary bodies, such as the MER Missions to Mars, there is an increasing interest in robotic exploration of medium to low gravity environment bodies like asteroids, comets and small moons that represent the space debris that was left after the creation of our solar system. The study of asteroids has a great scientific importance to increase our knowledge about the birth of our solar system, as well as other reasons that will be covered in the following sections. Their study is currently limited by their small size and distance to Earth to make any proper observations about their composition. This analysis can most properly be done if we actually go there and make in-situ examinations of the soil and physical properties. However, engineering problems arise, as we want to send a robotic probe to touchdown on an asteroid in microgravity environment, without being expelled from it by the two-bodies dynamics in such conditions. In the recent past, various missions have tried to study these kind of minor-bodies more closely, especially comets and asteroids. Some of the last most successful missions include Deep Impact mission to collide and create a crater on comet 9P/Tempel 1, as well as the Hayabusa mission to asteroid Itokawa, which touched down on the asteroid and collected some samples to return them back to Earth. Further scientific study of such bodies requires in-situ detailed analysis in several specific locations, and this involves the necessity of some kind of mobility over the surface of the minor-body. Recently, there is an increasing interest in the robotics community to design the most proper rover to move in rough terrain under micro-gravity, providing not only accurate positioning on the surface, but also being able to remain attached to it. In this respect, engineers face a trade-off problem between designing a fine-positioning capable rover with high complexity, against a less complex one with little mobility control potential. Some previous designs, like Minerva,

7

offered little complexity and weight against little positioning control. Now, scientists request higher positioning accuracy for future missions to minor-bodies, although the challenge in engineering remains as how to design a rover that could provide this ability on rough and uncertain terrain under a microgravity environment. The present paper will focus on the design of a rover specific for a mission to an asteroid such as Itokawa. Currently, there has been some promising research work performed to solve the difficulties in rover design for moving over these challenging conditions. It is largely believed that multi-limbed walking offers better performance over rough terrain, as well as the capacity to grasp the surface, a property necessary to avoid being expelled from the asteroid by the dynamic forces of walking under microgravity. Such designs have been developed through bio-mimetics or inspiration on nature, such as on animals or insects with some kind of walking patterns that allow them to climb over rocky cliffs and move over rough terrain with high speed and fine positioning. Following this kind of research, scientists and engineers have been focusing on the design of a multi-limbed robot, such as a hexapod robot or �hexabot�. The advantages and disadvantages of such a design will be explained in Section 3.1. In this paper, the author will present the research work performed with such a hexabot rover meant to touchdown on the asteroid Itokawa and remain attached to its surface just after landing. After an overview of the needs in a feasible mission scenario, the author will explain in further detail the focus of the on-going research work to understand the behavior of such a rover in the most important part of the mission: the pre-landing phase. In this previous to landing phase, the contact forces interacting at the moment of touchdown constitute the most critical factor to control as a procedure to avoid the expulsion from the asteroid by reaction dynamic forces just after landing. Proper knowledge of the force cone interaction with the surface will play a significant role to develop proper control procedures that can allow the next generation rovers not even to land successfully, but to gain proper mobility in the microgravity environment. On this line of research lies the work of the author, using a microgravity emulator as a real test bed to study the forces interacting on the rover at the landing phase. As the experimental results description will show, the author proposes the thesis of combining a proper contact force and grasping force control, as a successful critical maneuver to allow a multi-limbed rover smoothly land and stay attached on the surface of an asteroid. The arrangement of such forces will not only provide successful touchdown under the low gravity environment, which can prevent the failure of a exploration mission, but will also allow proper mobility on the surface of the asteroid to gain the capacity necessary for its scientific in-situ studies. The author expects the reader to consider that this paper shows only a part of the current on-going research work of a cautious subsystem design for the development of the next generation of multi-limbed rovers intended for asteroid surface exploration.

8

2. ASTEROIDS: WHY DO WE WANT TO VISIT THEM? Orbiting in the asteroid belt between 2.1 to 3.2 AU from the sun, primitive asteroids represent key bodies to research on the early planetary system origin and evolution. They are small rocky bodies with no atmospheres and are believed to be remnants that date as old as the formation of our solar system. Beyond their size, shape and rotation (3 to 30 days) we know relatively little about these objects [3]. Probably more competent candidates than comets to provide clues about the birth and growth of our planetary system, asteroids are also closer to Earth and more accessible compared to the low frequency of comets close flybys with Earth. Meteoroids are believed to be tiny rocks coming from the asteroid belt after the result of some sort of collision between them. Becoming meteorites, after surviving the high temperature of Earth�s atmospheric entry, they have been collected and catalogued by scientists for decades. We know the meteorites� internal mineral compositions, but we do not know exactly from where in the asteroid belt they come from, as the only information on the composition of asteroids comes from their spectroscopic analysis. Scientists have catalogued asteroids in more than a dozen spectral classes, but without any in-situ observation, they are not able to link this catalog with a corresponding meteorite group. The in-situ study of asteroids can lead to important scientific findings, in the effort to map the asteroid belt with their corresponding meteorite group. Mapping the asteroid belt by spectral classes and also knowing from which region the meteorites on Earth come from, can provide key clues about the origin and evolution of our solar system, even including the geological history of our planet Earth [1]. Scientific study of these minor-bodies and their relationship with the history of our solar system and our planet requires, therefore, proper in-situ exploration. The best solution for this is sending spacecraft to orbit and closely observe an asteroid, but also to take samples back to Earth or conducting scientific measurements on its surface and interior. However, in a more challenging effort, scientists are looking forward to send some sort of robot to walk over the asteroid, and make in-situ studies at specific locations. In this manner, the scientific return of such studies can help us understand the composition of an asteroid at various regions of its surface and maybe provide answers about its formation and evolution. Another important reason to study asteroids is their frequency of close encounters with Earth. The geological history of our planet shows hard evidence suggesting the existence of significant environmental changes caused by collisions with asteroids, such as those shown by the K-T layers in high shock wave impact regions on Earth (> 4 G Pa) like the Chicxulub crater in the Yucatan peninsula, Mexico [13]. These collisions represent key events in the changes of the global climate that are believed to have caused major disturbances in the evolution of life in our planet, as is suggested by the global Iridium anomaly of meteorite background rain at the time period of this Apollo asteroid impact event. Therefore, we must also consider the importance of studying asteroids as a way to foresee any close encounter with any of them that could represent a threat to the continuity of life, as we know it, and the existence of our species.

9

Some authors have also suggested using asteroids and NEOs (Near Earth Objects) to sustain space exploration missions, by extracting important resources from their surface, process them and supply a human base in space. The vital resources can even be such as oxygen and hydrogen, through processing the silicate minerals (pyroxene and olivine) in some close minor-bodies like asteroid 25143 Itokawa, considering they could serve as fuel to spacecraft flying close to the asteroid belt or any Lagrangian point nearby, as well as life support resources for human bases in space close to this region [14 & 15].

2.1 RECENT MISSIONS TO COMETS AND ASTEROIDS In the recent past, there have been various space missions to asteroids, comets and minor-bodies as it can be observed in Table 1-A, and Table 1-B [2].

Name of Mission

Institution Launch Date Flyby Dates Object Visited

Vega 1 and Vega 2

Soviet Academy of Sciences

December 15 and 21, 1984

March 6 and 9, 1986 Comet 1P/Halley

Sakigake (Pioneer)

Institute of Space and Aeronautical Science (ISAS)

January 8,1985 March 11, 1986 Comet 1P/Halley

Suisei

Institute of Space and Aeronautical Science (ISAS)

March 18, 1985 March 8, 1986 Comet 1P/Halley

Giotto ESA July 2, 1985Halley flyby: March 13, 1986Grigg-Skjellerup flyby: July 10, 1992

Comets 1P/Halley and 26P/Grigg-Skjellerup

Galileo NASA / JPL October 18, 1989

Gaspra flyby: October 29, 1991Ida/Dactyl flyby: August 28, 1993Witnessed Shoemaker-Levy crash: July 1994

Asteroids 951 Gaspra and 243 Ida

Near Earth Asteroid Rendezvous (NEAR)

NASA / JPL February 17, 1996Eros arrival: February 14, 2000Eros landing: February 12, 2001Shut down on February 28, 2001

Asteroid 433 Eros orbiter(eventually used as a lander!)

Stardust NASA / JPL February 7, 1999Wild 2 flyby: January 2, 2004Sample return: January 15, 2006

Flyby and coma samplereturn from cometP/Wild 2

Comet Nucleus Tour (CONTOUR)

NASA / JHU APL

July 3, 2002 Lost August 15, 2002

Intended to flyby comet2P/Encke in 2003,continuing with29P/Schwassmann-Wachmann in 2006,and 6P/d'Arrest in 2008

Table 1-A: History of space missions to asteroids, comets and minor-bodies.

10

Name of Mission

Institution Launch Date Flyby Dates Object Visited

Deep Impact NASA / JPL January 12, 2005Tempel 1 impactand flyby: July 4, 2005

Flyby and impact intocomet 9P/Tempel 1 andmaybe continue with aflyby of comet85P/Boethin in 2008

Hayabusa (MUSES-C)

JAXA / ISAS May 9, 2003

Itokawa arrival: September 2005Hopper 'Minerva' deployed:November 12, 2006Itokawa departure:scheduled for 2007Sample return: 2010

Orbiter and sample returnfrom asteroid Itokawa25143 (previously 1998 SF36)

RosettaESA / DASA / Rosetta

March 2, 2004

Churyumov-Gerasimenkoarrival: 2014

Flybys of asteroid2867 Steins onSeptember 5, 2008 and21 Lutetia onJuly 10, 2010, finally comet 67P/Churyumov-Gerasimenko orbiterand lander Philae.

Dawn NASA / JPLSummer of 2007 Orbit 4 Vesta in July 2011

and 1 Ceres in August 20144 Vesta and 1 Ceres

Table 1-B: Continued history of space missions to asteroids, comets and minor-bodies. In 2005, a couple of space robotic probes have constituted particular meaning in the study of minor bodies, such as comets and asteroids. Starting with the Deep Impact mission to comet 9P/Tempel 1, which made a flyby and released an impactor into a collision course with the comet, and then allowed the spacecraft and telescopes from Earth to observe the ejecta coming from the crater and quantify their volatile compositions. The second important mission is the formerly called MUSES-C mission (which stands for Mu Space Engineering Spacecraft, with �C� meaning third in the series) it was then renamed after launch as the Hayabusa mission, the Japanese meaning of Peregrine Falcon. The mission aimed to asteroid 25143 Itokawa (before 1998 SF36), was destined to first touchdown on the Muses Sea region of the asteroid, a region�s name that inspired the original name of the mission. Then, the Hayabusa spacecraft was designed to fire a bullet into the surface to create a crater and collect a sample of the crushed fragments produced by the impact right on the surface. Although the spacecraft had to make physical contact with the surface, samples are more efficiently collected in this strategy, given a fine design in terms of amount of sample collection and spacecraft safety in the touch-and-go maneuver [4]. The asteroid 25143 Itokawa, in Figure 1, was named after the famous aeronautical engineer, Mr. Hideo Itokawa (Dr. Rocket, as popularly known in Japan), who then became the popular Japanese rocket scientist, and a pioneer of the Japanese rocketry and space program. Mr. Itokawa, curiously, had previously worked for the Imperial Japanese Army Air Force, designing the Nakajima Ki-43 Hayabusa Army Type-1 Fighter. After World War II, he became popular in Japan because he traveled all the country teaching about space rocketry and space science, not only to persuade Japanese people about supporting the foundation of a Japanese space program, but to gain financial support to build his first successful �pencil rocket� prototype, and many others much larger in size later on [5].

11

Figure 1: The 25143 Itokawa asteroid, © Courtesy of ISAS / JAXA

The Itokawa asteroid, in Figure 1, is an Apollo (Earth-crosser) and Mars-crosser type of asteroid, whose orbit semi-major axis is greater than that of Earth�s, with an eccentricity just enough to let its orbit cross the Earth�s orbit. The largest known Apollo asteroid is 1866 Sisyphus, with a diameter of about 10 km, approximately the size of the object that created the Chicxulub crater whose impact is presumed to have led to the extinction of the dinosaurs [6 & 13]. Asteroid 25143 Itokawa, discovered in 1998 by LINEAR (Lincoln Near-Earth Asteroid Research), is a S-type spectral class asteroid, with a bulk density of 1.9 g cm-3 and a porosity of ~41%, with lower density and higher porosity values than those determined for S-type asteroids, which normally show a ~2.6 g cm-3 density and ~30% porosity, Itokawa indicates to be a pile of loose soil or rubble of a non-monolithic asteroid possibly formed by two asteroids whose gravity slowly merged them together, and whose surface shows a slow erosion process caused by past impacts of interplanetary projectiles. Itokawa has a diameter of only 535 ± 1 m in the x-axis (largest axis), a mass of 3.51 ± 0.105 x 1010 Kg, and orbits around the sun at 1.3238 AU from it with an eccentricity of 0.2801 and a rotational period of 12.1324 ± 0.0001 hours, according to the discoveries of the Hayabusa mission [1]. The Hayabusa spacecraft, depicted in Figure 2, arrived to the Itokawa asteroid on September 12, 2005, hovering at a 20-km altitude with the primary mission of touching down on the Muses Sea region of the asteroid, sampling the surface and returning the samples back to Earth. After its arrival to Itokawa, Hayabusa spacecraft spent 6 weeks performing scientific observations and global remote sensing measurements at an altitude between 7 to ~20 km. Then based on scientific merits, it was programmed to touchdown on two sampling-site candidates: the Muses Sea region located in an approximately 60x100 m area adjacent to the �neck� of the asteroid, between the �head� and the �body,� and the Little Woomera region, the largest facet of rough terrain of the �body,� shown on

12

Figure 3. After further close observations of the Little Woomera region, it was decided not to land on it for safety reasons, considering that the meter-size boulders exceeded the dimensions of the Guidance-Navigation-and-Control (GNC) accuracy circle of 60-diameter to conduct a safe descent. Therefore, the two actual touchdown attempts on 20 and 26 November 2005 were conducted at the Muses Sea area [7].

Figure 2: Simulation rendering of the Hayabusa spacecraft touchdown on the 25143 Itokawa asteroid,

© Courtesy of SRL / Tohoku University The mission and technology demonstration has been successful so far, and although the spacecraft was severely affected by the touchdown phases, the samples, whatever they are, will hopefully arrive in a capsule that will detach from the spacecraft at a distance of ~400,000 km from the Earth and will reenter the Earth�s atmosphere in a ballistic trajectory, experiencing peak deceleration of about 25 G and heating rates approximately 30 times those experienced by the Apollo spacecraft, landing via parachute near Woomera, Australia on June 2010, postponed from the original June 2007 due to problems with the chemical propulsion system [6].

13

Figure 3: The 25143 Itokawa asteroid�s Muses Sea and Little Woomera

touchdown region candidates for Hayabusa spacecraft, © Courtesy of ISAS / JAXA

2.2 MOBILITY ON ASTEROIDS As part of the various technology demonstrations of the Hayabusa spacecraft, like the two Xenon ion electric propulsion engines, autonomous navigation, sampling of the asteroid�s surface, and high-speed reentry into the Earth�s atmosphere; was a more ambitious option, as Hayabusa carried as well a small robotic hopper called Minerva (stands for MIcro/Nano Experimental Robot Vehicle for Asteroid) depicted in Figure 4, weighting only 591 g. Minerva was intended to provide the mission with the capacity of mobility over the surface of the asteroid Itokawa, an important faculty to realize significant scientific in-situ observations and analysis. Scientists seek the study of specific locations on the asteroid�s surface, however engineering complexity of this task has not facilitated the design of a proper rover for stable locomotion under the microgravity environment of asteroids. Using an internal reaction wheel, the Minerva robot was capable to locomote on the surface of the Itokawa asteroid by a ballistic type of movement as the rover rolling locomotion interacts with the surface through large sticks attached to the body. However, the locomotion of Minerva had a small problem: it provided little positioning control after each hop. Despite of this drawback, the rover must have given impressive photographs at the asteroid�s surface. Unfortunately, Minerva did not reach the surface of the asteroid after problems in the synchronization of the pre-landing phase plan of Hayabusa on November 12th, 2005 [1, 4 & 8].

Little Woomera Muses

Sea

14

Figure 4: The Hayabusa mission Minerva microgravity robotic hopper, © Courtesy of ISAS / JAXA

3. DESIGNING ROVERS FOR MICROGRAVITY The robotics community is currently studying different technology candidates for robotic space probes to future asteroid missions. One specific characteristic that interests these future missions, is the ability to conduct stable mobility and more accurate locomotion on the rough terrain and microgravity condition of the surface of these bodies, and in this manner allow scientists to perform local in-situ scientific measurements of the physical characteristics of an asteroid [4]. Asteroids� physical characteristics provide a very hostile environment distinguished by the absence of almost any gravity. The effects of the microgravity environment can be approximated for convenience as those in the level of 10-6 G [10]. In such an environment, objects basically do not fall, but remain orbiting unless they reach the low escape velocity of the asteroid in the order of 0.0002 km/s, as in the case of the asteroid 25143 Itokawa. To attain stable mobility in these minor bodies, it is critical to consider the forces interaction between a rover and the asteroid�s surface in the microgravity environment. Therefore, the best method to achieve mobility in this environment still remains a subject of discussion [9]. Walking on asteroids requires proper stability control against the forces interacting between bodies in microgravity environment to increase any scientific return from the mission operating on the asteroid surface. In a feasible mission scenario, it is desired to touchdown a spacecraft on the surface of the asteroid, then execute any kind of surface sampling operation, and finally deploy a robotic probe on its surface to perform in-situ science at specific sites of the asteroid�s surface during the robot locomotion, such as boulders and craters exposed by the sampling maneuvers [9].

15

When looking at the different kinds of mobility for microgravity environments, it is largely considered that wheel locomotion works improperly when no traction force is generated without any normal force to push the wheel on the surface, as the contact between the wheel and the surface happens during very short periods of time [9]. A clever design of a wheeled rover for microgravity environment is the Nanorover from NASA / JPL, shown in Figure 5. Using four wheels and a strut suspension system that holds them, the Nanorover was initially supposed to walk on the surface of the asteroid 25143 Itokawa, before it was decided to use Minerva. The wheels were not supposed to roll, but instead they were pretended to be used as legs end tips, stiffing the rear wheels while opening the rock suspension gate to advance in a kind of hopper walking motion, then touching the surface ahead with the front wheels, rolling and braking them to do the same movement with the rear wheels co-coordinately producing a kind of walking gate. The rover main task was to move around the surface taking high-resolution images of the crater made by a bullet fired from the main spacecraft, in the visible and non-visible spectrum and transmit them back to Earth relayed through the Hayabusa probe.

Figure 5: The Nanorover microgravity robotic rover, © Courtesy of JPL / NASA

As explained in section 2.2, Minerva, shown in Figure 4, is another design of a rover intended for microgravity environment that uses a simple mechanism to produce a hopping action. It consists of an internal reaction wheel inside the robot to produce the inertial reaction in a ballistic type of movement assisted by the interaction of large sticks from the body with the surface. Despite of its effectiveness, this design has a drawback, as the location of the robot when the bounds are finally damped out is very difficult to predict or control [11].

16

Comparing hopping locomotion with wheeled or tracked type of movement, legged locomotion is largely accepted as more capable for walking over rough and irregular terrains. The prototype design of legged robots impose a higher mechanical complexity, but they constitute an advisable type of locomotion systems for the exploration of planetary surfaces to accomplish traversing tasks through rough terrains, and more specifically that of minor bodies in microgravity environments, where wheeled locomotion is difficult to attain without a normal force from the surface against the wheels. Inspired on natural solutions exhibited by biological systems, such limbed robots represent the type of locomotion performed by large animals, such as bipod humans, quadruped animals, or hexapod insects, like spiders. Such robot prototypes provide a great advantage for microgravity environments, when a gripper is placed at their limbs� end-tips, allowing them to walk attached to the surface of the asteroid to avoid being pushed away from it by the dynamic damping forces of the locomotion. Such grippers can also permit the rover to walk up hills, cliffs and craters, just like a rock climber as depicted in Figure 6 [12], or a spider as depicted in Figure 7.

Figure 6: Simulation rendering of �The Rock Climber� microgravity multi-limbed robotic rover, © Courtesy of SRL / Tohoku University

17

Figure 7: Simulation rendering of �The Hexabot Spidy� microgravity hexapod robotic rover,

© Courtesy of SRL / Tohoku University

3.1 THE HEXABOT SPIDY During the time of internship at the Space Robotics Laboratory (SRL), in Tohoku University, the author had the opportunity to work closely with one robotic prototype of a microgravity environment intended hexapod robotic rover, unofficially nicknamed by the author as �The Hexabot Spidy�. Spidy is a multi-limbed ambulatory locomotion system developed through the observation and mimetics of clever solutions exhibited by biological systems. The rover is intended to use the natural features of the environment and the friction of the surface for omni directional walking having contact only in the limb end-tips. This type of locomotion has the possibility to provoke minimum reactions on the asteroid surface that could push the robot into space, or even to grasp the surface when some legs are controlled co-coordinately [9]. The rover went through several design stages before the author arrived to work with it. At the time the rover was in its final stage of development, the author was able to develop it further for simulation testing of its performance under microgravity environment. During its development, the limbed rover concept has been inspired from nature to obtain a platform complexity with a type of mechanism pretended to perform robustly in unstructured environments, through the replication of walking gates using six limbs, like arachnid insects.

18

The use of six legs was decided based on the needs of a mission to an asteroid, where the microgravity environment would impose on the rover the challenging task of walking with high accuracy and still performing science operations in a stable manner. The main purpose of this form of walking is to avoid get ejected from the surface. Therefore, six legs would be of better use than two or four, given the possibility as well of using at least three legs to grasp the surface to maintain the rover attached to the asteroid as a stable base while still using the remaining limbs for locomotion towards any direction or available for manipulation. The resulting system is a unit massing 2.7 kg. The hexapod rover is designed to land on an asteroid and have the ability of fine positioning over the surface to achieve science studies and mapping at several locations. As a result, the rover�s design has been developed in a modular basis for all of its components, facilitating any normal changes in the adaptation process to accomplish a robust design for the demanding activities imposed by a mission to an asteroid, starting from the most critical part of the mission: the landing phase. Furthermore, power, communication, intelligent control and other housekeeping functions must all be contained within the rover to permit for autonomous operation. For this purpose, at the moment of activities at SRL, the author found the microgravity six legged rover, shown in Figure 8, in a development phase just prior to modifying the limbs end-tips to prepare the rover for testing of contact forces over the surface at 1 G Earth gravity.

Figure 8: Design characteristics of �The Hexabot Spidy�, © Courtesy of SRL / Tohoku University

The rover is formed by a central body chassis that was designed for mass, strength and durability, with a hexagonal shape. The symmetrical body of the hexabot rover offers the special advantage of omni directional movement and reusability even after flipping over itself.

19

This concept of having limbs instead of arms or legs meant that the workspace and dexterity of the limb needed to be the union of those needed for walking and manipulation. Therefore, a 4 DOF limb, depicted in Figure 9, was designed consisting of 2 kinematic joints for the Hip and Knee located in the junction of the leg with the central body, another joint located at the Foot and a passive DOF for the gravity assisted gait movement on the end-tip of the limb. Six of these legs sum for a total of 24 DOF, and are symmetrically distributed around the body [9 & 16]. Actually, the author had to remove the passive DOF at the limbs� end-tips to replace them by force sensors for the contact force experiments, so the rover had after this change, only 18 DOF, see Table 4 for the rover specifications and Figure 10 for a Spidy prototype picture.

Figure 9: Limbs layout of �The Hexabot Spidy�, © Courtesy of SRL / Tohoku University

All the limbs are actuated with servomotor drives. Each è1 joint is actuated by HITEC HSR-5995TG servos and all the è2 and è3 joints have HITEC HS-5945MG servos as a temporary solution to the fast prototyping of the robot. The servomotors are controlled through a PWM signal fed by a microcontroller. The servomotors specifications can be seen in Table 2.

Servo Specification

Feature Description HITEC HSR-5995TG

Joint: Hip HITEC HS-5945MG Joints: Knee / Foot

Mass [g] 62 56 Torque [kg/cm] 24 11 Speed [Sec/60°] 0.15 0.16 Operation Voltage [V] 4.8-7.4 4.8-6 Current Consumption [mAh] 300 (No Load) 230 (No Load) Operation Angle [deg] 180° 180° Pulse Variation [s] 1500 ± 400 1500 ± 600 Pulse Cycle [ms] 12-26 20 Pulse Wave Voltage [V] 3.3-7.4 3-5

Table 2: Actuator Characteristics

20

As electronic systems, Spidy has two main electrical systems, as seen in Table 3, the main control board and the servo control module.

Element Jstamp+ Specification

CPU Board aJile aJ-80 Language RT-Java Runtime J2ME/CLDC Processor Speed [MHz] 74 RAM [KBytes] 512 Flash [MBytes] 2 Operation Voltage [V] 5 Current Consumption [mA] 37 Servo Control JSimm Module PWM Channels 18 Power External Source Operation Voltage [V] 5 Current Consumption [A] 40

Table 3: Electronic elements and specifications

Among the features are:

Direct Java Virtual Machine byte code execution Micro programmed real time Java thread manager Integrated power management, and Timers, interrupt controllers, memory controllers and a variety of communication

interfaces.

Description Specification

Mass [kg] 2.7

Limbs 6

Degrees of Freedom 18

Actuator count 18

Processor speed [MHz] 74

Table 4: Spidy global specifications

21

Figure 10: Prototype picture of �The Hexabot Spidy�, © Courtesy of SRL / Tohoku University

3.2 CONTACT AND GRASPING FORCES The hexabot rover �Spidy,� intended to improve the performance of the Minerva rover by implementing two mobility modes: �Large Stride� mode and �Crawling� mode; is aimed as well to land on the asteroid 25143 Itokawa. Its mobility through the Large Stride mode will be used to cover large distances in the same way as Minerva, by a hopping locomotion using an inner joints gimbals system as a reaction wheel for thrusting. In addition, it will have the capability of fine positioning in the surface using the Crawling mode, to move to specific locations in the asteroid with an accuracy of centimeters [16]. Clearly, an aspect of main concern for the next mission to asteroid 25143 Itokawa, after the unfortunate doom of Minerva trajectory passing the asteroid towards deep space, though not for design reasons but for problems during the synchronization of the landing maneuver; is to make sure the next generation rover aimed to touchdown Itokawa will successfully land and remain attached to its surface, surviving all forces against from any directions could compel the rover into space misfortunately once more away from the asteroid. In order to achieve this purpose, it is believed the rover will have to �grasp� the surface of the asteroid at the moment of landing. This implies the rover being able to remain attached to the surface while resisting forces (and torques) applied to it from any direction [18]. In order to calculate the interacting grasping forces of the surface of the asteroid, it is necessary to apply the algorithms for computing grasps described in [19 & 20]. Previous research work has already established a relationship between legged locomotion and fingered manipulators [17]. Based on this relationship, the behavior of any legged robot is equivalent to the effect of re-grasping an object by the position change of the fingers of a manipulator robot. Due to this effect, while a robot hand grasps an object, a legged robot �grasps� the surface [16].

Real-time Java

Processor

Servomotor

22

The surface of Itokawa is divided into rough terrain, mostly consisting of numerous boulders with a rubble-pile structure and smooth terrain (Muses Sea extending around the �neck� area between the �head� and �body� and Sagamihara around the north polar region, see Figure 11), found to be composed of regolith and fragmental debris with grain sizes of cm to mm scales from close-up images [1].

Figure 11: Western side of asteroid 25143 Itokawa, © Courtesy of ISAS / JAXA

The ground of an asteroid is modeled as a linear spring damper in the x and y directions and a non-linear spring damper in the z direction. Using a non-linear (hardening) spring in the z direction is a standard way to prevent ground chatter or bounce while still simulating a stiff ground [16]. In z direction, the non-linear force in the normal to the contact plane is defined by the following equation, Equation 1:

zBzzL

zzz

pnom

p

zz kF

(Eq. 1) Where, Fz : normal component of the end tip kz : spring elastic constant z : position in z of the end tip z : velocity in z of the end tip zp : penetration in z of the end tip Lnom: nominal length of the spring Bz: viscous constant

Sagamihara

Muses Sea

23

Previous research conducted on the computer simulation of the landing phase has shown that as the stiffness is lowered, greater penetration in the ground occurs. The lower the damping constants are, the longer the vibrations occur. However, a very high increment in stiffness or in damping constants can produce instabilities in the simulation due to numerical issues. Ground parameters can be tuned experimentally until an acceptable ground penetration and bounce is achieved [16].

4. EXPERIMENTS In preparation for a future mission to the asteroid 25143 Itokawa, the Space Robotics Laboratory of Tohoku University is developing a modular mission platform that includes the microgravity hexapod rover, telecommunications to ground control, simulation software of the current status of the mission, and a real test bed platform for simulation purposes of the real physical scenario of the mission with decision making support. As part of the development phase of the hexapod rover �Spidy�, the author designed the necessary hardware for conducting several experiments to study the nature of contact and grasping forces involved in the locomotion of the robot at the moment of landing over an asteroid. The purpose of this research is to define the best strategy that would fulfill a previous to landing phase maneuver of a real mission scenario to ensure proper touchdown on the asteroid and remain attached to the surface against all the reaction forces resulting from the dynamics of the arrival. In order to implement the correct preliminary experiments to simulate the effects of a landing maneuver on the asteroid, the rover has to operate in similar conditions to those in the microgravity environment. As pointed out in section 3, the conditions on an asteroid are similar to those on Earth by an approximation in the gravity factor [10]. For this objective, it was necessary to attach the rover to some sort of counterweight balance system, like the one depicted in Figure 12.

Figure 12: Preliminary testing counterweight balance pulley system,

consisting of a tripod base supporting a counterweight on one side and the hexabot on the other.

Counterweight Hexabot �Spidy�

24

As a first stage of the preliminary tests, the counterbalance system shown in Figure 12 served to test the mechanical capabilities of �Spidy� in the 1 G Earth�s gravity environment, to discover basic walking gaits that might be feasible on Earth-like gravity. The importance of these gaits is founded on the fact that a suitable walking on the asteroid is necessary to avoid the likelihood of hitting the surface caused by the weaker gravity field. On this respect, the rover will need to use a walking gate that does not imply creating repulsive forces, but rather a crawling mode that should stick the limbs end-tips into the surface at every step while combining some kind of grasping force with at least three limbs as a stable base for the rest of the limbs to move co-coordinately to perform the next step. In preparation for a second stage of the preliminary tests, the hexabot required full integration of F/T (force and torque) sensors at the level of the limbs end-tips. These sensors needed to be very low in weight and very sensitive to any external forces applied. Furthermore, the sensors had to be read by some sort of computing device. For this purpose, the author manufactured an electronic connection card to an ADC (analog to digital converter) board that would serve as an interface to read the sensors from a personal computer. To perform this integration, the author manufactured as well the mechanical interfaces necessary to attach three F/T sensors to three of the limbs end-tips of the rover. In this way, the rover became ready for testing the contact and grasping forces of its mechanical locomotion. The author used for this experimental setup, three Nitta F/T sensors massing 13 g with an accuracy of ~2.5 mV per bit (or per Newton of force) in each three dimensional axis, giving a dynamic range of ~400 mV. In addition to this configuration, the author used a Hitachi SR7046 embedded microcontroller with a 32-bit RISC processor and twelve 10-bit analog channels, as an interface board to read data from the mounted F/T sensors into a data logger running on a personal computer, as shown in Figure 13.

Figure 13: F/T sensors on the hexabot limbs end-tips

and the ADC interface card for data logging.

Limbs F/T sensors ADC

Limbs F/T sensor

25

Once the rover construction was finalized and the F/T sensors were installed on the rover and connected to the personal computer, the groundwork platform was ready to evaluate the basic gait theory to develop advanced gait time-lines using static locomotion, assuming that a grasping force to the surface has been achieved [9]. The limbs end-tips of the rover, as it has been stated in section 3.2, are intended to aid the rover generate some sort of �gripping� force as a result of the force vector created by the intersection of the force cones produced in the moment the end-tips touch the asteroid surface and perform a grasping mode locomotion, as depicted in Figure 14.

Figure 14: Limbs used as �grippers� create Force Cones under the asteroid surface that intersect producing a

resultant gripping force vector. In this manner, the author was able to conduct several preliminary tests of the �Large Stride� and �Crawling� modes explained in section 3.2, and in combination with the preliminary counterweight balance system of Figure 12, to discover the end effect theory for the hexabot limbs that would allow choosing the best strategy for the system implementation in a real asteroid exploration mission scenario. Once the right locomotion modes were discovered, the author had to move to the next level experimental platform. To simulate the real dynamic conditions of landing phase over an asteroid, it was necessary to use a robust control system that could counterbalance the Earth�s gravity and leave the rover in an emulated state of microgravity. For this reason, the hexabot rover �Spidy� was mounted on the tip of a Mitsubishi PA10 manipulator arm controlled by dynamics model simulation running on a personal computer. With all these systems in place, the correct configuration was setup to perform further experiments with the hexabot to observe and analyze the behavior of Spidy, as it would approach the surface of an asteroid prior to touchdown. These systems constituted the real test bed setup of the mission platform�s decision module developed at the Space Robotics Laboratory, as explained at the beginning of this section.

Hexabot

FC

Force Cones

FC

FR

Force Cones Intersection

26

4.1 TEST BED SETUP

As previously overviewed, the decision module of the mission platform consisted of the hexapod robotic rover Spidy in its full mechanical functionality and the F/T sensors mounted at its limbs end-tips. The sensors were plugged to an electronic connection card to nine analog channels of the ADC microcontroller board, one channel per each x, y and z force axis per each of the three limbs, which served as an analog-to-digital interface with a data logger running on a personal computer through a serial port telecommunication connection using the USB protocol. The rover was securely mounted at the end-tip of a Mitsubishi PA10 manipulator arm, equipped with a sensitive F/T sensor just before the rover. The F/T sensor would allow the appropriate control of the manipulator arm to emulate the microgravity condition of the asteroid 25143 Itokawa imposed on the rover by means of calculating its position, force, acceleration and velocity with high accuracy using real-time impedance and dynamic compliance control. The test bed described above was setup connecting the PA10 manipulator arm to the personal computer through the PA10 electronic real-time controller box. In addition, the F/T sensor of the robotic manipulator was also connected to the computer through the F/T sensor microcontroller using a low-delay �arcnet� network protocol connection. One personal computer would control the PA10 by running a dynamics model simulation, and a second personal computer would register the data provided by the F/T sensors mounted at the hexabot rover�s limbs end-tips by running a data logger, as depicted in Figure 15.

Figure 15: Real microgravity emulation test bed setup: Mitsubishi PA10 manipulator arm with the hexabot,

F/T sensors, and control and data logging PCs

Limbs F/T sensors ADC interface

PA10 F/T sensor

Hexabot limbs F/T sensor

PA10 F/T Sensor Controller

PA10 Dynamics Model Simulation

ArcNet Ntwrk

PA10 Control

Box

Hexabot F/T Sensors Data

Logger

27

After a long tuning procedure, the described system configuration was correctly setup to be used as a real test bed. For the different experiments, the hexabot was set on a microgravity condition and a landing course towards the surface of a flat foam plaque. The author and team of experts conducted more than 25 experiments of which at least 10 were structured in the combination of five conditions: Hard, soft and offset landing with either stiffened or passive legs, as shown in Figure 16.

Figure 16: Hexabot Spidy landing strategies tested on a flat foam plaque.

Hard/soft landing with stiffened legs

Hard/soft landing with passive legs

Offset landing with stiffened/passive legs

28

After experimenting with a flat foam plaque and having learned the lessons of these tests; the author and team proceeded to experiment the landing strategies of the hexabot Spidy approaching a more appropriate surface: an asteroid mockup, as shown in Figure 17.

Figure 17: Hexabot Spidy landing strategies tested on an asteroid mockup.

Clearly, the idea of using an asteroid mockup was to test the grasping mode of the hexapod rover locomotion in the emulated microgravity environment for the landing strategies developed, with the objective of attaining a successful touchdown on the asteroid with the appropriate scale of the limb motion and control forces to grip the surface and remain affixed against all dynamic reaction forces coming in all directions. The author and team spent long time trying to synchronize and combine together the best strategy in a pre-landing phase to achieve a successful result. The following section 4.2 will describe the results and analysis of these experiments as a base to support the conclusions of this report.

Asteroid Mockup

29

4.2 RESULTS The first experiments conducted, as described in section 4.1, were aimed to study the behavior of the hexapod rover at the moment of landing over a flat surface under microgravity environment, when performing a hard landing with some force on the limbs actuators. When making this experiment, the 2.7 kg rover arrived to the surface with relatively high speed, where the reading from one of the F/T sensors peaked a 41 N shock component force at the 7.6 second of landing from a slightly high distance, as shown in Figure 18.

Figure 18: Picture and plot showing the Hexabot Spidy surface contact forces when �hard landing with stiffened legs�.

Hard Landing with Stiffened Legs

01020304050

3.5

3.8

4.2

4.6

4.9

5.3

5.7

6.0

6.4

6.7

7.1

7.5

7.8

8.2

8.6

Time (s)

Com

pone

nt F

orce

(N)

CF

Contact

Reaction Energy

Peak = 41 N

30

As the graph in Figure 18 shows, in less than one second of time, the shock against the surface produces a relatively high energetic reaction that rejects the rover into space with a reactive force of 41 N. The results of this experiment suggested that to avoid being rejected from the surface of the asteroid, it was necessary to touchdown with less velocity and with little stiffness in the limbs actuators, in order to absorb most of the reaction forces after the contact shock. The following experiments performed, were aimed to analyze the behavior of the hexapod rover at the moment of landing over a flat surface under microgravity environment, when performing a soft landing with no force on the limbs actuators. When making this experiment, the 2.7 kg rover touched down the surface with relatively low speed, where the readings from one of the F/T sensors peaked up to a 23 N shock component force at the 2.4 second of landing from a low distance, as shown in Figure 19.

Figure 19: Picture and plot showing the Hexabot Spidy surface contact forces when �soft landing with passive legs�.

Soft Landing with Passive Legs

0510152025

0.0

0.3

0.6

0.9

1.2

1.4

1.7

2.0

2.3

2.6

2.9

3.2

3.5

3.8

Time (s)

Com

pone

nt F

orce

(N)

CF

Contact

Energy Absorption

Peak = 23 N

31

As the graph in Figure 19 shows, in more than one second of time, the shock against the surface produces a low energetic reaction that rejects the rover into space with a low reactive force of only 23 N. The results of this experiment suggested that when the rover performed a soft landing with no force on the limbs as some sort of suspension, almost half of the shock energy was absorbed in the movement of the legs. Therefore, to test another way of absorbing the energy of the landing shock, the following experiments performed, were aimed to analyze the behavior of the hexapod rover at the moment of landing when performing an offset landing with passive limbs actuators. An offset landing at a certain angle would produce some rotational movement on the body of the rover and that was presumed to serve as an increased factor of suspension or energy absorption. When making this experiment, the 2.7 kg rover touched down the surface at an inclination of 30 degrees and relatively low speed, where the readings from one of the F/T sensors peaked up to a 37 N shock component force at the 2.9 second of landing from a low distance. A picture of this test can be observed in Figure 20, where the rover is being rejected from the surface with a certain inclination, just after touching down. The picture demonstrates once more that the microgravity emulation test bed platform works correctly as expected.

Figure 20: The Hexabot Spidy performing an �offset landing with passive legs�.

32

The results of the offset landing experiments showed that there was lower suspension when landing with a certain inclination, since any small touch of half of the limbs on the surface would start the rejection energy to be released and would expel the rover with rotational movement but long before the rest of the limbs had even touched the ground. Therefore, as the results of these primary experiments suggest, it was necessary to choose as the best candidate for an appropriate landing strategy, the following: the soft landing with passive limbs actuators of some sort of suspension movement. This strategy would have satisfied the first requisite: reduce the rejection forces but ~50%. Then, to complete the second requisite of touching down on the surface and remain attached to it against all the reaction forces coming from many directions; the experimental results implied that a combination of soft landing and some sort of grasping force on the surface at the moment of touchdown would ensure that the rover landed and remained fixed to the ground, as can be observed in Figure 21. Such a strategy was interesting and difficult to setup and synchronize, but the author and experts team worked hard to accomplish successful results, as demonstrated in Figure 22.

Figure 21: Picture of the successful experiment with the Hexabot Spidy performing a combination of �soft landing and grasping mode� to achieve remain attached to the surface just after touchdown.

33

Figure 22: Plot showing the Hexabot Spidy surface contact forces when �soft landing with grasping control of the legs�.

As Figure 21 and Figure 22 demonstrate, in this successful experiment, the 2.7 kg rover touched down the surface at 0 degrees inclination and relatively low speed, where the readings from one of the F/T sensors peaked up to a shock component force of only 15 N at the 2.9 second of landing from a low distance. The rejection energy resulting from the force shock of landing is basically distributed along more than 3 seconds after the surface contact having a shock rejection force absorbed to a minimum of more than half the peak reaction force. This results suggest that the best strategy to successfully land on the asteroid surface under the microgravity condition in to combine a soft landing strategy with a grasping mode at the moment of touchdown, accomplishing a safe surface contact and remain affixed to it against the dynamic reaction forces coming from all directions to avoid being rejected from the asteroid and into deep space.

Grasping the Asteroid Surface

0246810121416

t

0.4

0.9

1.4

2.0

2.5

3.0

3.5

4.0

4.5

5.0

5.5

6.0

6.5

Time (s)

Com

pone

nt F

orce

(N)

CF

Contact

Landing Stabilization

Peak = 15 N

34

5. CONCLUSIONS The study of asteroids and minor NEOs that cross Earth�s orbit is extremely important from the scientific perspective as well as from the survival insight of humankind as a species. However, future missions to asteroids will intrinsically need robotic mobility over the surface to perform scientific measurements and in-situ studies of the soil and other physical properties at specific locations defined by scientists. Still there are many engineering problems to solve, as we want to send a robotic probe to touchdown on an asteroid in microgravity environment, without being expelled from it by the two-bodies dynamics in such conditions. The recently increasing interest in the robotics community to design the most proper rover to move in rough terrain under microgravity, providing not only accurate positioning on the surface, but also being able to remain attached to it, has led engineers face a trade-off problem between designing a fine-positioning capable rover with high complexity, against a less complex one with little mobility control potential. Coming from various designs, the hexapod rover developed through inspiration from solutions in nature such as those of arachnid insects, offers a highly robust system that is ideal to the rough and uncertain terrain under a microgravity environment. It is largely believed that multi-limbed walking offers better performance over rough terrain, as well as the capacity to grasp the surface, a property necessary to avoid being expelled from the asteroid by the dynamic forces of walking under microgravity. The proper testing of this hexabot rover design has been implemented, as the author setup a test bed platform developed for analyzing control strategies for a rover landing and grasping the asteroid surface under microgravity environment, using a microgravity emulation system as well as the exploration rover. Therefore, as the author performed the right experiments using the highly efficient microgravity emulator, it has been demonstrated the need of soft landing in combination with a grasping mode to grip the surface of the asteroid during the touchdown phase. Further research base on the control of the rover has been set. Research will have to continue concerning the rover locomotion and the surroundings modeling with the limbs F/T sensors in microgravity. As a recommendation to the Space Robotics Laboratory, the author believes the rover actuators need to be changed for more accurate force and speed control, to prepare the development of compliance control for the rover limbs locomotion. Proper knowledge of the force cone interaction with the surface will play a significant role in the future to develop proper control procedures that can allow the next generation rovers not even to land successfully, but to gain proper mobility in the microgravity environment. The arrangement of such forces will not only provide successful touchdown under the low gravity environment, which can prevent the failure of an exploration mission, but will also allow proper mobility on the surface of the asteroid to gain the capacity necessary for its scientific in-situ studies.

35

Finally, as a remark for future research, subsystem design for the development of the next generation of multi-limbed rovers intended for asteroid surface exploration shall aim to integrate closed loop control algorithms embedded inside the rovers electronic hardware to allow it read data from the F/T sensors mounted on its limbs end-tips, interpret it and make compliance control decisions on the most proper locomotion strategy. This will function to achieve the autonomous operation required for its scientific return under such a hostile, yet challenging environment as can only be imposed by a minor body as threatening and scientifically interesting as what the near Earth asteroid 25143 Itokawa can still surprise us!!

36

REFERENCES [1] Fujiwara A., Kawaguchi J., Yeomans D. K. et al (2006). The Rubble Pile Asteroid Itokawa as observed by

Hayabusa. Report: Hayabusa at asteroid Itokawa, vol. 312, Science [2] The Planetary Society (2006). Space Topics: Missions to Asteroids and Comets. Retrieved 08/05/2006

from: http://www.planetary.org/explore/topics/asteroids_and_comets/missions.html [3] Chacín M., Yoshida K. (2006). Evolving Legged Rovers for Minor Body Exploration Missions. Tohoku

University, Japan [4] Yoshida K. (2006). Microgravity Robotics for Sampling and In-Situ Science Missions. Tohoku University,

Japan [5] Manabe S. (2006/08/01). Private talk at the 16th Workshop on Astrodynamics and Flight Mechanics.

ISAS/JAXA, Sagamihara Campus, Fuchinobe, Japan [6] Wikipedia (2006). 25143 Itokawa � Wikipedia the free encyclopedia. Retrieved 08/05/2006 from:

http://en.wikipedia.org/wiki/25143_Itokawa [7] Yano H., Kubota T., Miyamoto H., Yoshida K. et al (2006). Touchdown of the Hayabusa Spacecraft at the

Muses Sea on Itokawa. Report: Hayabusa at asteroid Itokawa, vol. 312, Science [8] Wikipedia (2006). Hayabusa� Wikipedia the free encyclopedia. Retrieved 08/05/2006 from:

http://en.wikipedia.org/wiki/Hayabusa#Minerva_mini-lander [9] Chacín M., Nagatani K., Yoshida K. (2006). Next-Generation Rover Development for Asteroid Surface

Exploration: System Description. ISTS 2006-d-101p, JSASS / ISTS, Japan [10] Scheeres D.J., Broschart S., Ostro S.J. and Benner L.A. (2004). �The Dynamical Environment About

Asteroid 25143 Itokawa�. In Proc. of 24th International Symposium on Space Technology and Science, Miyazaki, Japan

[11] Chacín M., Yoshida K. (2006). Stability and Adaptability Analysis for Legged Robots Intended for Asteroid

Exploration. Tohoku University, Japan [12] Yoshida K., Maruki T., Yano H. (2002). �A Novel Strategy for Asteroid Exploration with a Surface

Robot�. In Proc. of the 3rd International Conference on Field and Service Robotics, pp. 281-286, Finland [13] Ryder G., Fastovsky D., Gartner S. (1996). �The Cretaceous-Tertiary Event and Other Catastrophes in

Earth History�. Special Paper 307, pp. 151-180, Geological Society of America, USA [14] ISU (2005). In-Situ Resources Utilization on Moon and Mars. Unpublished report. International Space

University, Strasbourg, France [15] ISU (2002). Beyond the International Space Station: The Future of Human Spaceflight. Proc. of an

International Symposium, pp. 221, ed. by Rycroft M., Springer-Klumer Academic Publishers. International Space University, Strasbourg, France

[16] Chacín M., Yoshida K. (2005). �Multi-limbed Rover for Asteroid Surface Exploration Using Static

Locomotion.� In Proc. of ISAIRAS 2005 Conference, ESA SP-603, Munich, Germany [17] Rimon E., Shoval S., and Shapiro A. (2001). �Design of a quadruped robot for motion with quasistatic

force constraints.� Autonomous Robots, pp. 10:279-296, USA [18] Bicchi A. and Kumar V. (2000). �Robotic grasping and contact: A review.� In IEEE Int. Conf. on

Robotics and Automation, pp. 348-353, San Francisco, CA

37

[19] Der Stappen A. V., Wentink C., and Overmars M. (1999). �Computing form-closure configurations.� In IEEE Int. Conf. on Robotics and Automation, pp. 1837-1842, USA

[20] Ponce J., Burdick J., and Rimon E. (1995). �Computing the immobilizing three-finger grasps of planar

objects.� In Workshop on Computational Kinematics, USA