Hayabusa2 Asteroid Sample Return Mission: Technological Innovation and Advances

By Masatoshi Hirabayashi and Yuichi Tsuda

Hayabusa2 Asteroid Sample Return Mission: Technological Innovation and Advances covers the second Japanese asteroid sample return mission. The purpose of the mission is to survey the asteroid Ryugu’s surface features, touch down on the asteroid, form an artificial crater by shooting an impactor, and collect sample materials. This book covers these operations, along with everything known about key technologies, hardware and ground systems upon Hayabusa2’s return to Earth in 2020. This book is the definitive reference on the mission and provides space and planetary scientists with information on established technologies to further advance the knowledge and technologies in future space exploration missions.

• 2023 PROSE Awards - Winner: Finalist: Chemistry, Physics, Astronomy, and Cosmology: Association of American Publishers
• Broadly and comprehensively covers technologies necessary for space exploration missions
• Provides a unique focus on small body exploration missions
• Covers landing and impact experiments during the proximity operations of Hayabusa2

• Language: English
• Publisher: Elsevier Science
• Release date: Apr 14, 2022
• ISBN: 9780323997324

Masatoshi Hirabayashi

Masatoshi Hirabayashi is an assistant professor in the Department of Aerospace Engineering at Auburn University in the United States. He graduated from the undergraduate school of Mechanical and Aerospace Engineering at Nagoya University in 2007 and obtained an M.S. degree in Aerospace Engineering at the University of Tokyo in 2010. After moving to the U.S., he received an M.S. degree in 2012 and a Ph.D. degree in 2014 from Aerospace Engineering at the University of Colorado Boulder. After establishing a scientific research experience in the Planetary Sciences group at Purdue University, he joined Auburn in 2017. Over his career, he has been involved in international space exploration missions. During participating in the graduate school at the University of Tokyo, he was involved in system engineering development as an engineering team member of IKAROS led by JAXA, the first Japanese Solar Sail mission, to contribute to its success at ISAS/JAXA in Sagamihara, Japan. Currently, he is a Co-I of the Optical Navigation Camera team in the Hayabusa2 mission and has played a critical role in science investigations and international communications. He is also a member of the investigation team of NASA/DART. Furthermore, he is serving as a Co-I of the BepiColombo mission led by ESA/JAXA and a collaborator of the OSIRIS-REx mission led by NASA. Through these small-body mission experiences, he has accumulated experience in space mission design, development, and operations, as well as scientific investigations. The experience in these missions allows him to lead the development of this book.

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Chapter 1: Hayabusa2 as the beginning of deep space sample return

Yuichi Tsudaa; Makoto Yoshikawaa; Masatoshi Hirabayashib; Shota Kikuchia,*    a Japan Aerospace Exploration Agency, Kanagawa, Japan

b Auburn University, Alabama, United States

* Current address: Chiba Institute of Technology, Chiba, Japan.

Abstract

Hayabusa2 is the second Japanese small body sample return exploration mission, targeting the carbonaceous asteroid (162173) Ryugu. The spacecraft was launched with Japan's H2A launch vehicle from the Tanegashima Space Center in December 2014 and arrived at Ryugu in June 2018. After completing detailed remote sensing observations, two sampling operations, one kinetic impact experiment, and multiple deployments of robotic smaller probes, the spacecraft left the asteroid in November 2019. It was a six-year journey that the spacecraft traveled approximately 5.2 billion km. In December 2020, the spacecraft returned to the Earth with extraterrestrial materials. A special volume is developed as a primary reference to collect engineering efforts from mission planning through in-orbit operations that made Hayabusa2’s achievements. This chapter introduces a brief overview of this book.

Keywords

Hayabusa2; Sample return; Asteroid; Space exploration; Asteroid landing

Drawing a dazzling light trail in the night sky of the southern Earth hemisphere, the sample return capsule of Hayabusa2 re-entered the atmosphere, concluding its space flight with a successful return to Earth as the ultimate finale. It was the second time that Japan achieved an interplanetary round-trip flight with asteroid samples, following Hayabusa, the first mission that faced many critical issues. The result was more than perfect. Hayabusa2 overcame critical challenges in Hayabusa and the unexpectedly harsh asteroid environment on Ryugu and completed its planned operations perfectly.

Despite this successful mission, Hayabusa2 did not go through its path smoothly, and its entire success resulted from its long, tireless effort by engineers and scientists. The original mission concept design started in 2000 to pursue a post-Hayabusa mission. This activity was expanded as a working group in 2004. However, the Hayabusa spacecraft had critical issues while exploring the stony asteroid 25143 Itokawa. The issues were severe, delaying the post-Hayabusa2 mission and significantly reducing the possibility that it might happen (as discussed later, it never happened). In 2006, this incident was the starting point of the Hayabusa2 concept study, which was categorized at that time as a pre-project and not selected as an actual mission.

In the meantime, studies of both Hayabusa2 and post-Hayabusa continued regardless of many facing challenges. For the post-Hayabusa mission, discussions started in 2006 to seek a possibility of collaborations with European scientists and engineers to develop a new joint small body exploration mission, Hayabusa Mk2, later renamed as Marco Polo. Given the tight budget situations at that time and Hayabusa's critical conditions, both Hayabusa2 and Marco Polo did not make significant progress in being selected as actual missions. However, the situation changed in 2010, when the Hayabusa spacecraft came back to the Earth after five years of tireless operation. This event allowed Hayabusa2 to receive strong support from the public. As a result, the Hayabusa2 mission concept study was finally selected as an actual mission in 2011, 3.5 years before the launch. On the other hand, Marco Polo eventually did not happen; however, further concept studies for post-Hayabusa are currently underway to explore future sample return missions from, e.g., Jupiter trojan asteroids.

The appearance of Ryugu was just astonishing. This was not for how it looked but also for the accomplishment over the decades of efforts, as discussed above. At the same time, it was also a starting point for many challenging operations. The days that the spacecraft started capturing Ryugu at high resolution were remarkably unique. There were two clear reactions: cheer and fear. Some were thrilled to look at Ryugu's geologic, geochemical, and geophysical features, while others showed a series of concerns about key operations, including sampling operations. Needless to say, the former were scientists, while the latter were engineers.

Rough terrains and numerous craters were considered to increase the risks of spacecraft landing. The spacecraft needs to land on rough terrain safely while there exist so many large boulders. Imagine how hard parallel parking is on a narrow downtown street in a big city without any scratches; having a tiny scratch means the mission's failure because tiny contacts with a boulder's surface, creating forces and torques, cause the spacecraft to roll over completely. Furthermore, as we, humans, cannot visit the asteroid, it is necessary to remotely control the spacecraft and use automated systems to let it complete critical tasks. This is the most challenging issue, which Hayabusa encountered and failed, while Hayabusa2 needed to overcome it to complete more complex operations than Hayabusa’s.

How could we solve this critical issue? This question was always the major discussion topic after the arrival even during lunchtime at the cafeteria at the Institute of Space and Astronautical Science/Japan Aerospace Exploration Agency (ISAS/JAXA), where Hayabusa2 was born. Certainly, there were many issues to be resolved timely, but this incident orchestrated the key challenge of small body exploration missions, we really do not know what conditions exist on our target until we arrive at it.

This special volume was edited as a primary reference to collect engineering aspects from mission planning through in-orbit operations. Over the entire mission operations since the launch, the Hayabusa2 team had numerous challenges and resolved each one of them; ignoring one small issue could lead to fatal issues. Resolving unexpected issues was not easy particularly when operations rapidly continued, and solutions needed to be provided promptly. A series of these experiences developed engineering of small body explorations. To emphasize this, we organize this book as follows.

Chapters 2–6 collect topics including mission design and overall operations: mission planning, spacecraft design, and orbit design and operation.

Chapters 7–16 deal with proximity operations at Ryugu. These chapters introduce major engineering activities including proximity operation planning and its results; spacecraft hovering, descending, landing, impact cratering operation, and rover delivery.

Chapters 17–24 detail in-depth insights into technologies that played significant roles in Hayabusa2. Such technologies include landing navigation and dynamics, gravity science, ion engine system, reaction control system, and sample return capsule technology.

Chapters 25 and 26 discuss operational training and outreach activities that contributed significantly to Hayabusa2’s success. One of the unique activities in Hayabusa2 was to actively distribute the spacecraft's up-to-date status and findings and actively communicate with the public. Such activities allowed Hayabusa2’s efforts to be informed to wider communities and promoted the Science, Technology, Engineering, and Mathematics (STEM) education of the next generation. Furthermore, this effort also contributed to promoting higher interests of the public in space science and engineering.

Finally, Chapter 27 briefly discusses the Hayabusa2 extended mission, following its nominal mission, targeting asteroids 2001 CC21 and 1998 KY26. It summarizes the currently planned cruise and proximity operations, although we note that further assessments will likely change the plans.

Interplanetary round-trip flights were considered one of the ultimate goals of space engineering 20 years ago. Now, this trend is rapidly shifting toward sample return, giving the capabilities of round-trip flights as a baseline technology. Hayabusa pioneered small body explorations rendezvousing with target bodies beyond the Moon and returning samples from them to the Earth, proving many enabling technologies such as efficient and accurate interplanetary cruise, landing on small bodies, sampling materials, and returning to Earth. Hayabusa2, followed by NASA's OSIRIS-REx, proves that such critical sample return technologies are now maturing and should move forward to the next goals of deep space sample return. Our deep space sample return explorations have just begun. Engineering acts as a baton of wisdom; each one may be a small step, but steady steps lead to a giant leap in the end. Hayabusa2 is just a small step but gives a steady step for further space explorations like the Apollo program did a half centuries ago.

This special volume is a fruit of reports written by authors engaged in Hayabusa2’s development, operation, and management. This book targets broader communities that have (undergraduate) college-level knowledge about engineering and science and above. However, this does not mean that the provided contents are too technical to understand without such knowledge. Each section focuses on one engineering topic in Hayabusa2 but avoids too technical discussions. In this way, the readers can obtain overviews of the Hayabusa2 engineering without going through detailed approaches and theories. For details, multiple references may be provided in each chapter.

Chapter 2: Mission objectives, planning, and achievements of Hayabusa2

Yuichi Tsudaa; Satoru Nakazawaa; Makoto Yoshikawaa; Takanao Saikia; Fuyuto Teruia,*; Masahiko Arakawab; Masanao Abea; Kohei Kitazatoc; Seiji Sugitad; Shogo Tachibanaa,d; Noriyuki Namikie; Satoshi Tanakaa; Tatsuaki Okadaa; Hitoshi Ikedaa; Sei-ichiro Watanabef    a Japan Aerospace Exploration Agency, Kanagawa, Japan

b Kobe University, Hyogo, Japan

c The University of Aizu, Fukushima, Japan

d University of Tokyo, Tokyo, Japan

e National Astronomical Observatory of Japan, Tokyo, Japan

f Nagoya University, Aichi, Japan

* Current address: Kanagawa Institute of Technology, Kanagawa, Japan.

Abstract

The Japan Aerospace Exploration Agency developed and performed an asteroid sample return mission: Hayabusa2. Hayabusa2 is a Japanese, second-in-the-world asteroid sample return mission. Hayabusa2 visited the C-type asteroid Ryugu in 2018, stayed in the proximity of the asteroid for 1.5 years, and returned to Earth in 2020. During the asteroid proximity operation, Hayabusa2 succeeded in delivering three mobile robots to the asteroid surface, performing two landing and sample collection activities, generating one artificial crater, and deploying three small objects into orbit around the asteroid. Although the terrain of Ryugu was found to be unexpectedly harsh for the Hayabusa2 spacecraft, the project successfully adjusted the operation strategy and improved the spacecraft's performance to finally complete the entire mission perfectly. Interplanetary operations, including launch, Earth swing-by, ion engine cruise, asteroid approach, and Earth reentry were also all successful. A total of 5.4 g of Ryugu samples were confirmed to contain in the returned reentry capsule. This paper introduces the mission design of Hayabusa2 and describes the flight results while focusing primarily on engineering achievements.

Keywords

Solar system exploration; Sample return technology; Asteroid mission

Acknowledgments

The authors would like to thank the entire Hayabusa2 team for their efforts in achieving a successful mission and for generating the data used in the present paper.

2.1: Introduction

Small-body exploration has come to occupy an indispensable and unique position in solar system exploration. The dawn of small-body exploration began with the Halley Armada in the 1980s, consisting of probes from the European Space Agency (ESA), the Soviet Union, France, and Japan conducting a flyby of the comet 1P/Halley. Since then, several spacecraft have conducted small-body flybys either as primary missions or as secondary missions, including Galileo, NEAR Shoemaker, Deep Space 1 (United States), and Rosetta (ESA) in the 1990s to 2000s, and more missions after the 2010s. The first one-way rendezvous mission to a small body was NEAR Shoemaker to 433 Eros in 2000, followed by Dawn (United States) to 4 Vesta and 1 Ceres, and Rosetta (ESA) to 67P/Churyumov-Gerasimenko.

As the next leap-step to these one-way missions, Hayabusa has opened the door to round-trip missions to small bodies [1]. The sample return mission provides new scientific value, i.e., laboratory-based direct sample analysis, to planetary science by landing on a small body, collecting samples, and returning the samples to Earth. At the same time, this demands higher level technology than one-way missions in areas such as efficient propulsion systems, landing systems, sampling systems, and Earth reentry from interplanetary space. In addition, in sample return missions, several critical operations are connected in series, and each must be successfully performed. After overcoming several hardships, Hayabusa demonstrated that such a seemingly risky enterprise could be planned and implemented under practical risk management. In fact, Hayabusa was followed by full-fledged sample-return missions developed in Japan (Hayabusa2 [2], launched in 2014) and in the United States (OSIRIS-REx [3], launched in 2016).

Hayabusa2 was planned as the Japanese, second-in-the-world asteroid sample return mission, as a successor to Hayabusa. Following the successful return of Hayabusa from asteroid 25143 Itokawa, the goal of Hayabusa2 was a round-trip mission to asteroid 162173 Ryugu. Hayabusa2 was developed by the Japan Aerospace Exploration Agency (JAXA), and was launched on December 3, 2014, using a Japanese H2A launch vehicle from Tanegashima Space Center. Ryugu is a near-Earth asteroid, the taxonomy of which is C-type (carbonaceous). The goal of the mission was to perform a variety of in-situ scientific experiments and bring samples back to Earth, in order to acquire fundamental information on solar system formation as well as clues to the delivery process for water and organics from the solar system to the early Earth.

In parallel with the scientific objectives, the Hayabusa2 project was designed to evolve interplanetary round-trip technology and small-body sample return technology based on the technical heritage of Hayabusa, such as a high-specific impulse ion engine, autonomous optical navigation, a sampling system, and interplanetary direct reentry technology.

In the following sections, the mission concept and planning aspects of the Hayabusa2 project are described, ending with a summary of the top-level achievements that have been completed up until, and including the successful return of the spacecraft and delivery of samples to the curation facility.

2.2: Mission design

2.2.1: Mission definition and success criteria

The objective of the Hayabusa2 mission was to visit a near-Earth C-type asteroid, conduct in-situ science experiments, collect surface and subsurface samples of the asteroid, and return the collected samples to Earth. The target asteroid, Ryugu, was selected from among known near-Earth asteroids (NEAs) that are reachable by a combination of a 600-kg class spacecraft system and a μ10 microwave discharge ion engine system, which was a readily available technology at the start of the development. The mission was planned as the second-in-the-world-second asteroid sample return attempt, following the first Hayabusa mission. In contrast with Hayabusa, which was planned as a technology demonstration mission, the Hayabusa2 mission plan was built as a full-fledged solar system exploration mission to pursue both engineering and scientific objectives. The mission definitions for Hayabusa2 are defined as follows:

•[Science 1] In-situ observation of a C-type asteroid at various scales

-By in-situ remote observation (resolution: 0.01–1 m), rover surface exploration (0.001–0.01 m), and analysis of the returned samples (< 1 μm).

-By investigating the interaction between minerals, water, and organic compounds.

•[Science 2] Revealing subsurface materials and the formation mechanism of the asteroid

-By exposing subsurface material by artificial crater generation.

-By testing crater generation dynamics and the re-accumulation process in a real environment.

•[Engineering 1] Increase robustness, reliability, and operationality of the sample return technology demonstrated by Hayabusa

-By evolving key technologies for small-body sample return missions, such as the ion engine, autonomous optical guidance and navigation, a sampling system for small bodies, and interplanetary direct reentry technologies.

•[Engineering 2] Perform artificial crater generation by kinetic impact

-For technology demonstration of a kinetic impact system.

For each of these four pillars, success criteria were defined in order to clarify the level of development, operation planning, and achievements required to fulfill the mission definitions. As shown in Table 2.1, there are three levels for each pillar: the minimum success level is the lowest border to achieve in order for the project to be considered successful, the full success level is the level of achievement that should be technically guaranteed by the project development activity, and the extended success level is the level to be pursued, although the feasibility of this level partially depends on the unknown asteroid environment.

Table 2.1

Note: Hatched items (i.e. items with green color shading) indicate the criteria have been fulfilled as of December 2020.

2.2.2: Sample return technology development

Despite many troubles experienced throughout the mission, Hayabusa finally returned to Earth in 2010 and brought back more than 1000 small particles of Itokawa, which corresponds to approximately 1 mg [4]. Following the success of Hayabusa, the Hayabusa2 project was authorized by the Japanese government in 2011. The spacecraft design was based extensively on Hayabusa, and many lessons learned were reflected in its design. The spacecraft's initial wet mass of 609 kg was approximately 100 kg heavier than that of Hayabusa, of which 50% was allocated for enhancing reliability based on the experience of Hayabusa. For example, Hayabusa had only three reaction wheels (i.e., no redundancy) due to strict mass constraints, whereas four reaction wheels were installed in Hayabusa2. Similarly, many single points of the electrical components of Hayabusa were upgraded to have hot, standby, or functional redundancy in Hayabusa2. The reaction control system (RCS) was another subsystem having a configuration that was drastically changed based on lessons learned from Hayabusa and other JAXA missions and is described in detail in a later chapter.

The remaining 50% of the mass was allocated for enhancing the capability of coping with the new mission objective to explore Ryugu, which is described in detail in the next subsection.

There are four key areas of technology to realize small-body sample return, which are described as follows together with the implementation of these technologies in Hayabusa2:

(1)Interplanetary round-trip flight technology.

Solar electric propulsion technology (i.e., ion engine system) has been developed and adopted in order to realize a compact spacecraft with large in-situ exploration capability.

(2)Autonomous optical navigation to rendezvous with, descend on, and land on a small body.

Radiometric-optical hybrid navigation, landmark-based asteroid-relative navigation, autonomous landing using artificial navigation landmarks called target markers (TMs), have been developed and adopted.

(3)Planetary surface sample collection in a low-gravity environment.

The mission requirement for the total yield was 0.1 g at minimum, and metal sealing was adopted to collect and store the gas-state sample of the asteroid [5,6].

(4)Direct Earth reentry technology from interplanetary orbit.

The reentry capsule and entry, descent, and landing (EDL) technology are indispensable to the completion of the sample return mission design.

These four technologies are all derived from the technical heritage of Hayabusa with enhanced performance and higher reliability.

In addition to these Hayabusa-derived technologies, Hayabusa2 adopted some new technologies. For example, a Ka-band downlink communication system was used in order to increase the telemetry bandwidth and thus contribute to more efficient scientific activity. An experimental X/Ka multi-band delta differential one-way ranging (DDOR) system was adopted in order to contribute to the interplanetary navigation experiment. The small carry-on impactor (SCI) is a novel kinetic impact device with numerous technical challenges, both in its development and operation. The pinpoint touchdown technology developed for Hayabusa2 used the same equipment as the Hayabusa-derived normal touchdown but is a drastically different technology. The normal touchdown is a technology that aims for the landing accuracy of approximately 50 m with 99% (≈ 2.5σ) confidence level, whereas the pinpoint touchdown aims for realizing 1-m-level precision landing. Hayabusa2 was equipped with five TMs so that it could perform two normal touchdowns using one TM each, followed by a pinpoint touchdown using three TMs. (In reality, in an effort to adapt to the severe environment of Ryugu, two modified pinpoint touchdowns were carried out using a single TM each, as described in detail in later sections and chapters.)

Thus, the design of the Hayabusa2 spacecraft effectively fulfilled the mission requirements through a combination of heritage-based and newly developed technologies. The resulting spacecraft design is shown in Fig. 2.1.

Fig. 2.1

Fig. 2.1 Hayabusa2 spacecraft and its external components.

2.2.3: Scientific instruments aboard the Hayabusa2 spacecraft

Hayabusa2 was equipped with various types of scientific payloads, to be used for in-situ observations, roving, sampling, and impacting. The 14 scientific payloads aboard Hayabusa2 are summarized in Table 2.2.

Table 2.2

Three optical navigation cameras (two wide field-of-view cameras ONC-W1 and ONC-W2, one telescopic camera ONC-T) [7,8] were used for imaging observations at visible wavelengths. The laser altimeter (LIDAR) [9] was used for precise surface topography measurement as well as terrain relative navigation. The near-infrared spectrometer (NIRS3) [10] measured water absorption in the near-infrared band in order to characterize the surface composition of Ryugu. The thermal infrared imager (TIR) [11] was used to observe the surface thermal environment of Ryugu and to investigate the derived thermophysical properties.

Hayabusa2 carried one lander and three rovers. MASCOT [12] was a 10-kg lander developed and provided by DLR and CNES, which performed surface science using MicroOmega (infrared hyperspectral microscope), MAG (magnetometer), MARA (radiometer), and CAM (visible camera). The three rovers were MINERVA-II-1A, MINERVA-II-1B, MINERVA-II-2 [13,14], where rovers 1A and 1B were developed based on the MINERVA rover aboard Hayabusa, and rover 2 was developed by a consortium of Japanese universities led by Tohoku University. Each rover weighed approximately 1 kg and was equipped with cameras, temperature sensors, and a mobility system.

The sampling operation was realized by a sampler horn (SMP) [15]. At the instance of touchdown, a projectile was fired and impacted the asteroid surface. The ejected fragments were then collected by the SMP and guided into one of three chambers of the sample catcher inside the sampling container. Three projectiles were equipped for, at maximum, three sampling operations. The target sample yield was 0.1 g in total. The container was finally transported to the re-entry capsule and metal-sealed (to stow a gas sample) using a series of mechanical actions realized by pyrotechnic devices and non-explosive actuators. The sampler horn monitor camera (CAM-H) [16] observed the tip of the SMP.

The cratering operation was performed by an SCI [17], which was to be deployed at an altitude of approximately 500 m. After a preset wait time, the SCI was detonated, and a 2-kg copper bullet was instantaneously accelerated to 2 km/s to impact Ryugu's surface. Prior to the impact, the spacecraft escaped to a safe zone on the opposite side of the asteroid from the impact point so that detonation debris and impact ejecta did not damage the spacecraft. A deployable remote camera (DCAM3) [16] was deployed just before the spacecraft completely escaped into the safe zone to observe the impact process.

2.2.4: Mission planning and spacecraft development

The development of Hayabusa2 was approved by the Japanese government in 2010. The Hayabusa2 project team was organized under JAXA in May 2011. The Hayabusa2 project passed the critical design review (CDR) in March 2012. System-level assembly, integration, and testing continued until August 2014, and the final assembly at the launch site was conducted from September through December 2014.

Ryugu has an orbital period of 1.3 years. The synodic period between Earth and Ryugu is approximately 4.3 years. Due to the difference in orbital inclinations and eccentricities of the two celestial bodies, the year 2014–2015 was the only practical departure opportunity for the round-trip flight in the 2010s. Through an in-depth continuous thrust trajectory design, the original schedule was set such that the launch was in November–December 2014, the Earth swing-by occurred one year later in December 2015, followed by the arrival at Ryugu in June 2018. After 1.5 years of asteroid proximity operations, Ryugu departure was in November–December 2019, and Earth return was in November–December 2020. The mission scenario for Hayabusa2 is shown in Fig. 2.2 together with the actual date of each event.

Fig. 2.2

Fig. 2.2 Mission scenario and actual operation history.

2.3: Mission operation results

This section describes the operation results for the mission. The history of the operation from launch to Earth return is shown in Table 2.3.

Table 2.3

All dates in UTC.

2.3.1: Launch and forward trip cruise

Hayabusa2 was launched using the Japanese H2A launch vehicle at 04:22:04 UTC on December 3, 2014 [18]. After three months of the commissioning operation phase, Hayabusa2 started its propulsive cruise using the ion engine system (IES) on March 3, 2015. Hayabusa2 performed an Earth gravity assist (EGA) operation on December 3, 2015. The orbit of Hayabusa2 was shifted to a Ryugu-transfer orbit by this EGA. After the EGA, three long-term IES operations were conducted. A total velocity increment of 1015 m/s was achieved by IES operation from launch to Ryugu arrival. During the forward trip, IES operation was terminated on June 3, 2018, when the spacecraft was 2700 km from Ryugu. The final approach to Ryugu was performed using the RCS. An optical-radiometric hybrid navigation was utilized to fine-guide the spacecraft to Ryugu [19]. Hayabusa2 arrived at the home position (HP) of Ryugu at 00:51 UTC on June 27, 2018 as planned. The HP is defined as the point 20 km from the center of Ryugu toward the asteroid-to-Earth direction.

2.3.2: Asteroid proximity operations

Hayabusa2 established a hovering state at the HP on June 27, 2018 and started asteroid proximity operations [20] by in-situ remote observations of Ryugu using remote science instruments, such as ONC-T, ONC-W1, TIR, NIRS3, and LIDAR.

The project conducted the landing site selection (LSS) activity for the first 1.5 months, in which the observation data were systematically acquired, analyzed, and integrated in order to derive three sets of landing sites (i.e., one primary and two backups) for two MINERVA-II1s, MASCOT, and the first spacecraft touchdown. The derivation of landing sites considered scientific value and landing-safety criteria. The LSS activity was concluded at the LSS decision meeting held on August 17, 2018. However, since Ryugu's surface was found to be full of boulders and craters (Fig. 2.3), and none of the candidate sites were safe enough to meet the original landing safety requirements of 50-m (2.5σ)-radius flat terrain, the project decided to add some guidance-navigation-control practice operations and modified the touchdown sequence in order to improve the landing accuracy and the safety measures of the touchdown sequence.

Fig. 2.3

Fig. 2.3 Terrain of Ryugu at three different scales. The resolution of each image is (upper left)  < 1 mm at the bottom of the image, (lower left) approximately 7 mm, and (right) approximately 0.7 m. The three boxes in the right image indicate 100 m × 100 m regions considered as landing sites for the first touchdown.

Two MINERVA-II1 rovers (Rover-1A, nicknamed HIBOU, and Rover-1B, nicknamed OWL) were deployed to the N6 region (Fig. 2.4) on September 21, 2018. The two rovers safely landed on and autonomously hopped around the ground, sending a few hundred close-up images of Ryugu, as well as asteroid surface temperature data, to the spacecraft. Rover 1A was confirmed to be active at least until November 2, 2018, and Rover-1B was confirmed to be active at least until August 2, 2019.

Fig. 2.4

Fig. 2.4 International Astronomical Union-authorized terrain names for Ryugu (yellow letters) and landing site nicknames (orange letters) . [N6] is the landing site for MINERVA-II1-A and -B, [MA-9] for MASCOT, [L08-E1] for the first touchdown, [SCI] for the kinetic impact point, and [C01-Cb] for the second touchdown.

The MASCOT deployment operation was then conducted on October 3, 2018. The MASCOT lander, developed by DLR and CNES, successfully landed on the MA-9 region (Fig. 2.4) and was active for approximately 17 h powered by the built-in primary battery. MASCOT acquired precious close-up surface features and the thermal properties of Ryugu using its MAG (magnetometer), MARA (radiometer), and CAM (visible wavelength camera).

The first touchdown operation was conducted on February 22, 2019 after a delay of four months from the original plan in order to account for the severe terrain of Ryugu. The improved touchdown sequence was uploaded to the spacecraft, which made full use of the TM to achieve a landing accuracy of less than 3 m (2.5σ). The TM is a 10-cm-diameter ball-shaped artificial landmark dropped to the ground prior to the touchdown operation. Hayabusa2 succeeded in touching down and performing sample collection from point L08-E1 of Ryugu, which is a 6-m-diameter area within the L08 region (Fig. 2.4). The post-flight analysis revealed that the resulting landing error was only 1 m. The entire sampling sequence functioned properly, and the collected sample was stowed and secured in sample chamber A, one of the three chambers inside the sample container.

On April 5, 2019, the kinetic impact operation was conducted using the SCI. The operation was a perfect success. The impact process was successfully recorded by a remote deployable camera (DCAM3), and a large amount of impact ejecta was observed. The terrain deformation due to the impact was observed by the spacecraft instruments, showing that an artificial crater with a rim-to-rim diameter of 17.6 m and a depth of 2.7 m was generated [21]. We observed that the ejecta deposited by the kinetic impact covered an area of more than 30 m in radius from the impact center.

The goal of the second touchdown operation was to collect the subsurface material exposed and scattered by the kinetic impact operation. Point C01-Cb (Fig. 2.4), 20 m north of the impact center, was selected as the target touchdown site. The touchdown operation was conducted on July 11, 2019 and was successful, with a landing accuracy of 0.6 m. The collected sample was stowed and secured in sample chamber C.

The last critical operations on Ryugu were the TM and MINERVA-II2 orbiting operations. Two TMs (afterward nicknamed Sputnik and Explorer) were inserted into polar and equatorial orbits, respectively, around Ryugu on September 22, 2019, and the MINERVA-II2 rover (afterward nicknamed ULULA) was inserted into another equatorial orbit on October 8, 2019. The trajectories of these objects were tracked by onboard cameras (ONC-T and ONC-W1) on the spacecraft, which were used for gravity science. The three objects eventually landed and settled on the asteroid's surface after several orbits.

At this point, the project confirmed that all of the planned scientific objectives were fulfilled, and the asteroid proximity operations were completed. Hayabusa2 left Ryugu on November 13, 2019 to begin its return to Earth.

2.3.3: Return trip cruise and earth return

Hayabusa2 started its propulsive return cruise using the IES on December 3, 2019. Two long-term IES operations were conducted to intercept Earth orbit. During the return trip cruise, a velocity of 270 m/s was achieved by the IES. The IES successfully completed its duty for the round-trip orbital maneuver on September 17, 2020.

The operation was then followed by the reentry terminal guidance phase. The spacecraft was accurately guided to Earth by four trajectory correction maneuvers (TCMs) executed from October 22 through December 1. The reentry capsule separated 12 h before reentry and successfully landed at Woomera Prohibited Area (WPA) in Australia at 17:24 UTC on December 5, 2020.

The capsule recovery team, consisting of approximately 80 people, was deployed in the WPA. The reentry capsule was found and retrieved immediately after landing, and the Ryugu-derived gas sample was collected at the on-site quick-look facility. The capsule was then transported to Japan and brought into the curation facility at the Institute of Space and Astronautical Science (ISAS), JAXA, on December 8. The time between landing and transporting the capsule into the quick-look facility was approximately 5.2 h, and the time between atmospheric reentry and carrying the sample container to the curation facility was 57 h. This short amount of time from capsule landing to its placement inside the curation facility reduced the risk of sample contamination, and demonstrated that both the spacecraft reentry operation and the recovery operation were performed perfectly.

The capsule was opened in the curation facility, and a considerable amount of black sand and gravel-like samples were found in all of the three sample chambers inside the capsule. Chamber A contained samples from the first touchdown. Chamber B was open between the first and second touchdowns, and many black powdery particles were found. These may have been captured in space around Ryugu during several low-altitude descent operations, or maybe residue left in the sample collection path during the first touchdown which moved into Chamber B at this time. Chamber C contained samples from the second touchdown. The total amount of sample weighed 5.4 g, which was far larger than the mission requirement of 0.1 g.

2.3.4: Extended mission

Hayabusa2 performed a diversion maneuver by long burns of the chemical thrusters 1 h after capsule separation and made an Earth flyby to continue to the extended mission phase.

JAXA authorized the Hayabusa2 extended mission in 2020, after Hayabusa2 successfully completed the Ryugu proximity operation. In the extended mission plan, Hayabusa2 will demonstrate a very long ion-engine-assisted, multi-swing-by interplanetary flight. The final destination is to rendezvous with a 30-m-diameter fast-rotator near-Earth asteroid 1998 KY26 in 2031. At the midway point, Hayabusa2 is to fly by the 700-m-diameter L-type near-Earth asteroid 2001 CC21. The mission will contribute to planetary defense technology and science by a very close high-speed flyby of a small body and by the first exploration of a fast rotating near-Earth asteroid in human history.

2.4: Mission achievements

2.4.1: Engineering achievements

As described in the previous section, Hayabusa2 succeeded in the round-trip mission to the asteroid Ryugu and returned a significant amount of solid and gas samples from Ryugu.

The microwave discharged ion engine, developed for Hayabusa and upgraded for Hayabusa2, worked perfectly, and low- and continuous-thrust trajectory control combined with an Earth gravity assist was successfully applied to the mission. The optical guidance and navigation for approaching, descending, and landing on Ryugu worked perfectly, contributing to timely mission progress and highly accurate in-situ mission activities during proximity operations. High-speed direct reentry from the interplanetary field was also successful and the entry-descent-landing sequence proceeded as planned. This marked the second Earth return from deep space with extraterrestrial samples among Japanese space missions.

Consequently, the engineering success criteria defined in Table 2.1 were all satisfied ([E1-Min] through [E2-Ext]).

From the viewpoint of contributions to space engineering, the Hayabusa2 mission accomplished the following nine engineering world-firsts (see Fig. 2.5 for an illustrative explanation):

(1)Mobile activity of exploration robots on a small body

(2)Multiple robot deployment on a small body

(3)60-cm-accuracy landing and sampling on a celestial body

(4)Multiple landings and samplings from a celestial body

(5)Access to subsurface material

(6)Artificial crater formation and detailed observation of the impact process

(7)Smallest-object constellation around a celestial body

(8)Sample return of gas-state material from a celestial body beyond the Moon

(9)Sample return of material from a C-type asteroid

Fig. 2.5

Fig. 2.5 Summary of nine world-first achievements of Hayabusa2.

where (1, 2) are categorized as small body-surface exploration technology, (3–5) as small body-landing and sample collection technology, (6) as subsurface exploration technology, (7) as small-body orbiting technology, and (8, 9) as sample return technology.

In addition to the above major accomplishments associated directly with the mission definitions, the Hayabusa2 mission accomplished several engineering achievements by taking advantage of the long interplanetary cruise environment and/or mission-specific spacecraft capabilities, which are summarized in Table 2.4.

Table 2.4

2.4.2: Scientific achievements

Ryugu has been characterized in detail as a result of the asteroid proximity operation of Hayabusa2. Ryugu was found to have a spinning-top shape with an equatorial radius of 502 m and a polar-to-equatorial axis ratio of 0.872. The asteroid spin state is upright and retrograde, having an obliquity of 171.64° with a period of 7.63262 h [26]. Surface slope analysis shows that the shape of Ryugu may have resulted from having once spun at twice the current rotation rate [26]. The visible and near-infrared spectral data obtained by ONC-T and NIRS3 indicate that Ryugu is a Cb-type (carbon-rich) asteroid with a very low geometric albedo of 4.5% and contains hydroxyl (OH)-bearing minerals over all of its surface [27,28]. One of the extraordinary features of Ryugu is that the number density of boulders is uniformly high across the surface, specifically, twice as high as that of Itokawa for boulders having a diameter larger than 20 m [28]. The boulder surface photographed at night by the MASCOT camera showed millimeter-sized inclusions [29]. The gravity of Ryugu was measured to be GM = 30.0 m³/s², which corresponds to a bulk density of 1.19 g/cm³, showing that Ryugu is a rubble pile with high porosity.

As well as these basic characteristics of Ryugu, an increasing number of inter-disciplinary scientific discoveries are being made from the results of in-situ observations and low-altitude descent/landing/impact operations. The low thermal inertia of surface boulders obtained from TIR observations indicates their high porosity of approximately 30%–50% [30,31]. The SCI impact experiment reveals that the crater-to-impactor size ratio is more than 100 [21], which is in contrast to the lower value of 10 estimated from the size-frequency distributions of main-belt asteroids and craters on asteroids. Based on the SCI results, the surface age of Ryugu is estimated to be approximately 16 Ma [21,28], which is much shorter than the estimated ages (100 Ma to 1 Ga) of the candidate source collisional families in the inner main belt based on spectral analysis [28]. Some of the bright boulders on Ryugu were found to be remnants of impactor(s) originating from anhydrous S-type asteroids [32]. Visible color variations on Ryugu suggest the reddening of surface material by solar heating and/or space weathering under a temporal excursion of Ryugu near the Sun [33]. On the other hand, SCI crater observations by NIRS3 show that 2.72-μm OH absorption is little affected by solar radiation or space weathering