佐伯 孝尚

The resurfacing process on Ryugu accompanying the artificial impact crater formation by Hayabusa2's Small Carry-on Impactor (SCI) was studied by comparing pre- and post-impact images of this region captured by an optical navigation camera. Three different aspects of the resurfacing process were examined: the crater rim profiles, the motion of boulders and the appearance of new boulders, and the motion vectors of Ryugu's surface around the SCI crater. The averaged crater rim height, h, was derived as follows: h = hr exp [−(r/Rrim − 1)/λrim], where Rrim is the SCI crater rim radius of 8.8 m, the fitted parameter, hr, is 0.475 m, and the λrim is 0.245. The ejecta blanket thickness of the SCI crater was thinner than that estimated from both the observation of natural craters and the crater formation theory. However, this discrepancy of the ejecta blanket thickness was resolved by taking into account the new boulders appearing in the post-impact images in the volume. The motion of the discovered boulders could be classified by its mechanisms as follows: a dragging motion created by excavation flow during the crater formation, a pushing motion created by falling-back ejecta, a dragging motion created by the slight motion of the Okamoto boulder, and a motion caused by seismic shaking induced by the SCI impact itself. The seismic shaking caused boulders to move farther than 3 cm from the original site in most of the region within 15 m distance from the SCI crater center, where the maximum acceleration of the impact induced seismic waves 7 times larger than the surface gravity of Ryugu based on the laboratory experiments (Matsue et al. [2020] Icarus, 338, 113520), and the evidence of the seismic shaking for boulders with a movement of >3 cm was detected in about 10% of the boulders in the region between 15 m and 30 m from the crater center, which region was inferred to experience acceleration larger than the Ryugu's surface gravity based on previous laboratory experiments (Matsue et al. [2020] Icarus, 338, 113520).

The near-Earth asteroid (162173) Ryugu displays a Cb-type average spectrum and a very low average normal albedo of 0.04. Although the majority of boulders on Ryugu have reflectance spectra and albedo similar to the Ryugu average, a small fraction of boulders exhibit anomalously high albedo and distinctively different spectra. A previous study (Tatsumi et al., 2021Nature Astronomy, 5, doi:https://doi.org/10.1038/s41550-020-1179-z) based on the 2.7-km observations and a series of low-altitude (down to 68 m) descent observations conducted prior to the first touchdown have shown that the spectra of these anomalous boulders can be classified into two distinct groups corresponding to S and C type asteroids. The former originate most likely from an impactor that collided with Ryugu's parent body, whereas the latter may be from portions of Ryugu's parent body that experienced a different temperature history than experienced by the majority of boulder materials. In this study, we analyzed images captured after the first touchdown to determine the quantitative properties of these bright boulders on Ryugu. We measured the sizes of more than a thousand bright boulders and characterized the morphologic properties of the largest ones. Analyses revealed many properties of bright boulders important for the evolution of Ryugu and its parent body. First, the size-frequency distributions of S-type and C-type bright boulders follow a power law with exponents of 1.6 +/- 1.3 and 3.0 +/- 0.7, respectively. Based on these size-frequency distributions, we obtained the ratios of the total volume and surface area of S-type bright boulders to those of average dark boulders on the Ryugu's surface, that is, 7.1(-5.0)(+6.3) x 10(-6) and 1.5(-1.2)(+3.2) x 10(-6), respectively, over the diameter range of 0.3 to 3 m. Similarly, the ratio of the total volume and surface area of C-type bright boulders to those of average dark boulders are 4.4(-2.2)(+14.0) x 10(-5) and 1.3(-1.1)(+9.8) x 10(-3), respectively, at a diameter range of 2 cm to 2 m. Second, the number density of bright boulders inside the artificial crater newly made by the Small Carry-on Impactor (SCI) experiment agrees with the outside number density within a factor of two. Third, many of the bright boulders are embedded in a larger substrate boulder, suggesting that they have experienced mixing and conglomeration with darker fragments on Ryugu's parent body, rather than gently landing on Ryugu during or after its formation by reaccumulation. This observation is consistent with the hypothesis that S-type bright boulders were likely mixed during and/or before a catastrophic disruption. C-type bright boulders embedded in substrate boulders suggests a brecciation process after thermal metamorphism. Furthermore, the embedding of S-type clasts in substrate boulders suggests that brecciation did indeed occur even after a large-scale impact on the parent body. If the brecciation on the Ryugu's parent body occurred over such a long period or over many stages of its evolution, breccias may end up being the dominant constituent materials on Ryugu's parent body. Moreover, the preponderance of breccias may contribute to the globally low thermal inertia of Ryugu.

A projectile accelerated by the Hayabusa2 Small Carry-on Impactor successfully produced an artificial impact crater with a final apparent diameter of 14.5 ± 0.8 m on the surface of the near-Earth asteroid 162173 Ryugu on April 5, 2019. At the time of cratering, Deployable Camera 3 took clear time-lapse images of the ejecta curtain, an assemblage of ejected particles forming a curtain-like structure emerging from the crater. Focusing on the optical depth of the ejecta curtain and comparing it with a theoretical model, we infer the size of the ejecta particles. As a result, the typical size of the ejecta particles is estimated to be several centimeters to decimeters, although it slightly depends on the assumed size distribution. Since the ejecta particles are expected to come from a depth down to ~1 m, our result suggests that the subsurface layer of Ryugu is composed of relatively small particles compared to the uppermost layer on which we observe many meter-sized boulders. Our result also suggests a deficit of particles of less than ~1 mm in the subsurface layer. These findings will play a key role in revealing the formation and surface evolution process of Ryugu and other small Solar System bodies.

Seismic shaking has been regarded as an essential source of resurfacing on asteroids. The Small Carry-on Impactor (SCI) operation on Hayabusa2 has been expected to be a unique opportunity for testing in situ seismic shaking whose energy is sufficiently large to excite observable surface modification. However, no obvious regolith hopping was identified even immediately outside of the crater formed by the SCI impact. To understand this discrepancy from the expectation, we simulate seismic wave propagation on Ryugu with a wide range of surface material properties and evaluate maximum acceleration on the surface. Numerical results reveal that low-quality factor or low seismic efficiency is required to explain the lack of geomorphological change after the SCI experiment. Considering that scattering under anhydrous conditions cannot efficiently dissipate energy, such a low-quality factor is not plausible. The weak yield strength in porous materials can efficiently decrease seismic wave energies, making the apparent seismic efficiency extremely low. Based on this hypothesis, we propose a formulation of surface mobility on asteroids that considers the physical properties of regolith. We consistently estimate the occurrence of seismic shaking with the existence of unstable boulders on Ryugu.

Hayabusa2 is an asteroid sample return mission developed and operated by the Japan Aerospace Exploration Agency (JAXA). 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. Hayabusa2 succeeded in delivering three surface exploration robots to the asteroid surface, performing two landing and sample collection activities, generating one artificial crater impact, and deploying three small objects into orbit around the asteroid. The terrain of Ryugu was found to be unexpectedly harsh through the in-situ observations, and the operation strategy was obliged to be changed and aligned to the Ryugu environment. The project team overcame all the difficulties through tight and collaborative works between the team s scientists and engineers, and completed the planned missions perfectly. The total of 5.4 g of Ryugu sample was confirmed to contain in the returned capsule, which is now being analysed by specifically organized initial analysis teams, and will be delivered to international researchers through AO in 2022. This paper describes the entire flight result of the Hayabusa2 mission, and summarizes the engineering and scientific accomplishments of the mission.

The Hayabusa2 is an asteroid sample return mission. It returned to Earth on 5 Dec. 2020 and released the sample return capsule containing a confirmed 5.4 g sample. The AOCS (Attitude and Orbit Control System) then changed its trajectory and attitude for the capsule re-entry. This operation continued for about 24 hours and included six sequential attitude maneuvers. There was only one chance for this procedure to succeed, and it could not be aborted. The accurate estimation of this subsystem's operation was one of the most critical factors in the mission's success. The main difficulties were to realize several attitude maneuvers under control that had to be exceptionally strict because of the significant attitude disturbances Hayabusa2 was subjected to the sizable magnetic moment due to the Ion Engine System (IES) and the Earth's magnetic field, and the aerodynamic torque around the perigee point. The authors devised a precise timing control mode for switching and selection of RW unloading. This paper describes AOCS operation, the simulation model, and the results of an analysis of the re-entry operation. AOCS planning is described in detail, as are a comparison of actual operation results and planning.

Hayabusa2, a Japanese asteroid sample return probe, was launched on December 3, 2014, and arrived at Ryugu, a C-Type asteroid, on June 27, 2018. It left Ryugu on November 13, 2019, after completing its 1.5-year asteroid proximity phase operation, including sample collections and a kinetic impact experiment. The propulsive return cruise with the ion thrusters began on December 3, 2019, and in October 2020, the spacecraft entered the precise guidance phase, in which the trajectory corrections with the chemical thrusters were conducted. After several trajectory correction maneuvers, the spacecraft was precisely guided to the landing point of the sample return capsule, and its reentry capsule entered Earth s atmosphere and successfully landed on December 5, 2020. The capsule recovery team immediately found and retrieved the capsule. This paper describes the results of Hayabusa2 Earth return and capsule reentry, including the cruising and precise guidance phase and capsule release operation.

<p>小惑星探査機はやぶさ2は，2018年6月27日にリュウグウへ到着し，小惑星近傍フェーズへと入った．リュウグウの特性を調べるため，約1年半に渡り多様な科学観測が遂行された．また，ローバー/ランダーの展開，二度のタッチダウンによる表面/地下サンプル採取，人工クレーター生成実験など，多くの降下運用に成功した．本稿では，はやぶさ2ミッションにおける小惑星近傍フェーズの概要を，事前準備と実績に分けて紹介する．はやぶさ2の多様で複雑な小惑星近傍運用を成功させるため，往路巡航フェーズ期間を活用して事前に詳細な運用計画を検討し，多くの訓練を実施した．到着後は，明らかになるリュウグウの素性や運用の成否，探査機特性の変化へと柔軟に対応しつつ成果の最大化を図った．本稿では，小惑星近傍フェーズにおける降下運用の全体像に加え，各期間のハイライトや重要な判断の流れを報告する．</p>

We have analyzed the carbonaceous materials generated by the explosion of an High-melting explosive mixture in an Ar atmosphere in a laboratory simulation of the small carry-on impactor experiment. We used both non-destructive and destructive analytical techniques to identify the chemical nature of the materials. From SEM-EDS, we found the materials to be composed mainly of carbon, nitrogen, and oxygen, with a detectable amount of metals. Suitable parameters for identifying these materials are a FTIR peak at 1520 cm(-1), low reflectance and gentle red slope of FTIR spectrum compared with the Murchison CM2 chondrite, the Raman D and G bands, and the hydrogen, carbon, and nitrogen isotopic compositions and their spatial distributions. The scanning transmission X-ray microscopy (STXM)-XANES results provided information about the molecular nature of these highly aromatic materials, which was supported by results from TD-GC/MS. These results suggest that it is possible to distinguish Ryugu samples from SCI potential contaminants in a sample container by using proper combinations of analytical techniques. This assessment provides information that will be useful for the analysis of the Ryugu asteroidal samples.

The asteroid (162173) Ryugu and other rubble-pile asteroids are likely re-accumulated fragments of much larger parent bodies that were disrupted by impacts. However, the collisional and orbital pathways from the original parent bodies to subkilometre rubble-pile asteroids are not yet well understood1–3. Here we use Hayabusa2 observations to show that some of the bright boulders on the dark, carbonaceous (C-type) asteroid Ryugu4 are remnants of an impactor with a different composition as well as an anomalous portion of its parent body. The bright boulders on Ryugu can be classified into two spectral groups: most are featureless and similar to Ryugu’s average spectrum4,5, while others show distinct compositional signatures consistent with ordinary chondrites—a class of meteorites that originate from anhydrous silicate-rich asteroids6. The observed anhydrous silicate-like material is likely the result of collisional mixing between Ryugu’s parent body and one or multiple anhydrous silicate-rich asteroid(s) before and during Ryugu’s formation. In addition, the bright boulders with featureless spectra and less ultraviolet upturn are consistent with thermal metamorphism of carbonaceous meteorites7,8. They might sample different thermal-metamorphosed regions, which the returned sample will allow us to verify. Hence, the bright boulders on Ryugu provide new insights into the collisional evolution and accumulation of subkilometre rubble-pile asteroids.

<p>This paper presents an overview of a guidance, navigation and control method used in descent operations of Hayabusa2. The method consists of on-board and on-ground guidance systems to control the spacecraft, and an image-based navigation technique using a shape model and ground control points of the asteroid. Hayabusa2 has performed descent operations 11 times as of mid-April 2019. Flight results of the operations demonstrate that the guidance, navigation and control system has satisfied accuracy requirements and shown an overall good performance.</p>

<title>Abstract</title>In this study, we determined the alignment of the laser altimeter aboard Hayabusa2 with respect to the spacecraft using in-flight data. Since the laser altimeter data were used to estimate the trajectory of the Hayabusa2 spacecraft, the pointing direction of the altimeter needed to be accurately determined. The boresight direction of the receiving telescope was estimated by comparing elevations of the laser altimeter data and camera images, and was confirmed by identifying prominent terrains of other datasets. The estimated boresight direction obtained by the laser link experiment in the winter of 2015, during the Earth’s gravity assist operation period, differed from the direction estimated in this study, which fell on another part of the candidate direction; this was not selected in a previous study. Assuming that the uncertainty of alignment determination of the laser altimeter boresight was 4.6 pixels in the camera image, the trajectory error of the spacecraft in the cross- and/or along-track directions was determined to be 0.4, 2.1, or 8.6 m for altitudes of 1, 5, or 20 km, respectively.

<p>Hayabusa2 is a Japanese asteroid explorer launched by the H-IIA rocket on Dec. 3, 2014. After the long cruising with the ion engine system, the spacecraft approached its target body, Ryugu, using optical navigation. It arrived at Home Position (HP), 20 km above the sub-Earth point, on Jun. 27, 2018. The spacecraft is now on its way back to Earth. It will reach the Earth by the end of 2020. During the 1.5-year asteroid proximity phase, we performed many kinds of science observations using the remote sensing instruments. We performed several numbers of descent operations to deploy and lander/rovers, to collect the surface samples, and to create an artificial crater using a new small kinetic impactor. Detailed operation planning of the asteroid proximity phase started after the launch, and lots of training for the proximity phase were conducted from one year before the arrival. This paper summarizes the overview of the asteroid proximity operation plan of Hayabusa2 and its preliminary results.</p>

<title>Abstract</title>This paper describes the orbit design of the deployable payload Rover 2 of MINERVA-II, installed on the Hayabusa2 spacecraft. Because Rover 2 did not have surface exploration capabilities, the operation team decided to experiment with a new strategy for its deployment to the surface. The rover was ejected at a high altitude and made a semi-hard landing on the surface of the asteroid Ryugu after several orbits. Based on the orbital analysis around Ryugu, the expected collision speed was tolerable for the rover to function post-impact. Because the rover could not control its position, its motion was entirely governed by the initial conditions. Thus, the largest challenge was to insert the rover into a stable orbit (despite its large release uncertainty), and avoid its escape from Ryugu due to an environment strongly perturbed by solar radiation pressure and gravitational irregularities. This study investigates the solution space of the orbit around Ryugu and evaluates the orbit’s robustness by utilizing Monte Carlo simulations to determine the orbit insertion policy. Upon analyzing the flight data of the rover operation, we verified that the rover orbited Ryugu for more than one period and established the possibility of a novel method for estimating the gravity of an asteroid.

The precise orbit of the Hayabusa2 spacecraft with respect to asteroid Ryugu is dynamically determined using the data sets collected by the spacecraft's onboard laser altimeter (LIght Detection And Ranging, LIDAR) and automated image tracking (AIT). The LIDAR range data and the AIT angular data play complementary roles because LIDAR is sensitive to the line-of-sight direction from Hayabusa2 to Ryugu, while the AIT is sensitive to the directions perpendicular to it. Using LIDAR and AIT, all six components of the initial state vector can be derived stably, which is difficult to achieve using only LIDAR or AIT. The coefficient of solar radiation pressure (SRP) of the Hayabusa2 spacecraft and standard gravitational parameter (GM) of Ryugu can also be estimated in the orbit determination process, by combining multiple orbit arcs at various altitudes. In the process of orbit determination, the Ryugu-fixed coordinate of the center of the LIDAR spot is determined by fitting the range data geometrically to the topography of Ryugu using the Markov Chain Monte Carlo method. Such an approach is effective for realizing the rapid convergence of the solution. The root mean squares of the residuals of the observed minus computed values of the range and brightness-centroid direction of the image are 1.36 m and 0.0270 degrees, respectively. The estimated values of the GM of Ryugu and a correction factor to our initial SRP model are 29.8 +/- 0.3 m(3)/s(2) and 1.13 +/- 0.16, respectively.

The deep-space multi-object orbit determination system (DMOODS) and its application in the asteroid proximity operation of the Hayabusa2 mission are described. DMOODS was developed by the Japan Aerospace Exploration Agency (JAXA) for the primary purpose of determining the trajectory of deep-space spacecraft for JAXA’s planetary missions. The weighted least-squares batch filter is used for the orbit estimator of DMOODS. The orbit estimator supports more than 10 data types, some of which are used for relative trajectory measurements between multiple space objects including natural satellites and small bodies. This system consists of a set of computer programs running on Linux-based consumer PCs on the ground, which are used for orbit determination and the generation of radiometric tracking data, such as delta differential one-way ranging and doppler tracking data. During the asteroid proximity phase of Hayabusa2, this system played an essential role in operations that had very strict navigation requirements or operations in which few optical data were obtained owing to special constraints on the spacecraft attitude or distance from the asteroid. One example is orbit determination during the solar conjunction phase, in which the navigation accuracy is degraded by the effect of the solar corona. The large range bias caused by the solar corona was accurately estimated with DMOODS by combining light detection and ranging (LIDAR) and ranging measurements in the superior solar conjunction phase of Hayabusa2. For the orbiting operations of target markers and the MINERVA-II2 rover, the simultaneous estimation of six trajectories of four artificial objects and a natural object was made by DMOODS. This type of simultaneous orbit determination of multi-artificial objects in deep-space has never been accomplished before.

<title>Abstract</title>In late 2018, the asteroid Ryugu was in the Sun’s shadow during the superior solar conjunction phase. As the Sun-Earth-Ryugu angle decreased to below 3°, the Hayabusa2 spacecraft experienced 21 days of planned blackout in the Earth-probe communication link. This was the first time a spacecraft had experienced solar conjunction while hovering around a minor body. For the safety of the spacecraft, a low energy transfer trajectory named Ayu was designed in the Hill reference frame to increase its altitude from 20 to 110 km. The trajectory was planned with the newly developed optNEAR tool and validated with real time data. This article shows the results of the conjunction operation, from planning to flight data.

The asteroid explorer Hayabusa2 carries multiple rovers and separates them to land on an asteroid surface. One of these rovers, called MASCOT, was developed under the international cooperation between the Deutsches Zentrum für Luft- und Raumfahrt and the Centre National d’Etudes Spatiales. This rover was designed to be separated to land and perform several missions on an asteroid surface. To support these missions, the mother ship Hayabusa2 must separate this rover at a low altitude of approximately 50 m and hover at approximately 3 km after separation to achieve are liable communication link with MASCOT. Because the on-board guidance, navigation, and control (GNC) does not have an autonomous hovering function, this hovering operation is performed by ground-based control. This paper introduces the GNC operation scheme for this hovering operation and reports on its flight results.

© 2020, Tsinghua University Press. Hayabusa2 is a Japanese sample return mission from the asteroid Ryugu. The Hayabusa2 spacecraft was launched on 3 December 2014 and arrived at Ryugu on 27 June 2018. It stayed there until December 2019 for in situ observation and soil sample collection, and will return to the Earth in November or December 2020. During the stay, the spacecraft performed the first touchdown operation on 22 February 2019 and the second touchdown on 11 July 2019, which were both completed successfully. Because the surface of Ryugu is rough and covered with boulders, it was not easy to find target areas for touchdown. There were several technical challenges to overcome, including demanding guidance, navigation, and control accuracy, to realize the touchdown operation. In this paper, strategies and technical details of the guidance, navigation, and control systems are presented. The flight results prove that the performance of the systems was satisfactory and largely contributed to the success of the operation.

The Japanese interplanetary probe Hayabusa2 was launched on December 3, 2014 and the probe arrived at the vicinity of asteroid 162173 Ryugu on June 27, 2018. During its 1.4 years of asteroid proximity phase, the probe successfully accomplished numbers of record-breaking achievements including two touchdowns and one artificial cratering experiment, which are highly expected to have secured surface and subsurface samples from the asteroid inside its sample container for the first time in history. The Hayabusa2 spacecraft was designed not to orbit but to hover above the asteroid along the sub-Earth line. This orbital and geometrical configuration allows the spacecraft to utilize its high-gain antennas for telecommunication with the ground station on Earth while pointing its scientific observation and navigation sensors at the asteroid. This paper focuses on the regular station-keeping operation of Hayabusa2, which is called “home position” (HP)-keeping operation. First, together with the spacecraft design, an operation scheme called HP navigation (HPNAV), which includes a daily trajectory control and scientific observations as regular activities, is introduced. Following the description on the guidance, navigation, and control design as well as the framework of optical and radiometric navigation, the results of the HP-keeping operation including trajectory estimation and delta-V planning during the entire asteroid proximity phase are summarized and evaluated as a first report. Consequently, this paper states that the HP-keeping operation in the framework of HPNAV had succeeded without critical incidents, and the number of trajectory control delta-V was planned efficiently throughout the period.

Subsurface exploration is one of the most ambitious scientific objectives of the Hayabusa2 mission. A small device called small carry-on impactor (SCI) was developed to create an artificial crater on the surface of asteroid Ryugu. This enables us to sample subsurface materials, which will provide a window to the past. The physical properties of the resulting crater are also useful for understanding the internal structure of Ryugu. Accurate understanding of the crater and ejecta properties, including the depth of excavation of subsurface materials, requires accurate information on impact conditions. In particular, the impact angle is a critical factor because it greatly influences the size and shape of the crater. On April 5, 2019, the Hayabusa2 spacecraft deployed the SCI at 500 m of altitude above the asteroid surface. The SCI gradually reduced its altitude, and it shot a 2 kg copper projectile into the asteroid 40 min after separation. Estimating the position of the released SCI is essential for determining the impact angle. This study describes the motion reconstruction of the SCI based on the actual operation data. The results indicate that the SCI was released with high accuracy.

The Near-Earth Asteroid 162173 Ryugu (1999 JU3) was investigated by the JAXA Hayabusa2 mission from June 2018 to November 2019. The data acquired by NIRS3 spectrometer revealed a dark surface with a positive near-infrared spectral slope. In this work we investigated the spectral slope variations across the Ryugu surface, providing information about physical/chemical properties of the surface. We analysed the calibrated, thermally and photometrically corrected NIRS3 data, and we evaluated the spectral slope between 1.9 μm and 2.5 μm, whose values extend from 0.11 to 0.28 and the mean value corresponds to 0.163±0.022. Starting from the mean value of slope and moving in step of 1 standard deviation (0.022), we defined 9 “slope families”, the Low-Red-Slope families (LR1, LR2 and LR3) and the High-Red-Sloped families (HR1, HR2, HR3, HR4, HR5, HR6). The mean values of some spectral parameters were estimated for each family, such as the reflectance factor at 1.9 μm, the spectral slope, the depth of bands at 2.7 μm and at 2.8 μm. A progressive spectral reddening, darkening and weakening/narrowing of OH bands is observed moving from the LR families to the HR families. We concluded that the spectral variability observed among families is the result of the thermal metamorphism experienced by Ryugu after the catastrophic disruption of its parent body and space weathering processes that occurred on airless bodies as Ryugu, such as impact cratering and solar wind irradiation. As a consequence, the HR1, LR1, LR2 and LR3 families, corresponding to equatorial ridge and crater rims, are the less altered regions on Ryugu surface, which experienced the minor alteration and OH devolatilization; the HR2, HR3, HR4, HR5 families, coincident with floors and walls of impact craters, are the most altered areas, result of the three processes occurring on Ryugu. The strong reddening of the HR6 family (coincident with Ejima Saxum) is likely due to the fine-sized material covering the large boulder.

Hayabusa2 deployed two artificial landmarks called "target markers (TMs)" on the asteroid Ryugu for autonomous landing control. To achieve precise deployment on target landing sites, the TMs were designed to dissipate kinetic energy and released near the asteroid surface (with an altitude of less than 40 m). This study evaluates the performance of the ballistic deployment in the actual microgravity environment by reconstructing the trajectories of the TMs from optical, altimetric, and radiometric data. In addition, based on the reconstructed trajectories, low-velocity impacts of the TMs on the surface of Ryugu are characterized with dynamical parameters, such as dissipated energy and a coefficient of restitution. The physical implications of the impact analysis are discussed in comparison with on-ground experimental data. Furthermore, the gravitational environment is investigated using the reconstructed trajectory data and a shape model of Ryugu, providing information on the local gravity anomaly. Consequently, this paper demonstrates the usefulness of deployable artificial landmarks for small-body landings and further offers insight on surface conditions and internal structures near the Hayabusa2 landing sites where samples of Ryugu were collected.

One of the primary goals of Hayabusa2 is to land on the asteroid Ryugu to collect its surface materials. The key for a successful touchdown is to find a promising landing site that meets both scientific and engineering requirements. Due to the limited availability of pre-arrival information about Ryugu, the landing site selection (LSS) must be conducted based on proximity observations over a limited length of time. In addition, Ryugu was discovered to possess an unexpectedly high abundance of boulders with an absence of wide and flat areas, further complicating the LSS. To resolve these problems, we developed a systematic and stepwise LSS process with a focus on the surface topography of Ryugu and the associated touchdown safety. The proposed LSS scheme consists of two phases: Phase-I LSS, a comprehensive survey of potential landing areas at the 100-m scale based on the global mapping of Ryugu, and Phase-II LSS, a narrowing-down process of the candidate landing sites at the 10-m scale using high-resolution images and a local terrain model. To verify the feasibility of a precision landing at the target site, we also investigated the landing dispersion via a Monte Carlo simulation, which incorporates the effect of the irregular surface gravity field. One of the major characteristics of the Hayabusa2 LSS developed in this study is the iterative feedback between LSS analyses on the ground and actual spacecraft operations near the target asteroid. Using the newly developed method, we chose a landing site with a radius of 3 m, and Hayabusa2 successfully conducted its first touchdown on February 21, 2019. This paper reports the methodology and results of the stepwise iterative LSS for the first Hayabusa2 touchdown. The touchdown operation results reconstructed from flight data are also provided, demonstrating the validity of the adopted LSS strategy.

Hayabusa2 is a Japanese interplanetary probe launched on December 3, 2014, which arrived at asteroid Ryugu on June 27, 2018. During its stay around Ryugu, it completed several challenging operations, including deploying two rovers and a lander, conducing two sample collections, and performing a kinetic impact experiment. The kinetic impact experiment was one of the biggest challenges of the Hayabusa2 mission. Investigating the physical and chemical properties of asteroid internal materials and structures is an important scientific objective for small body exploration. We developed a small kinetic impactor called the SCI (Small Carry-on Impactor) to achieve this objective. The SCI is a compact kinetic impactor designed to remove a small region of Ryugu's uppermost surface regolith layer and create an artificial crater. The spacecraft deployed the SCI on April 5, 2019, successfully creating an artificial crater with a diameter of 15 m. This paper describes the operational planning of the kinetic impact experiment and summarizes the operation results.

TIR, the thermal infrared imager on Hayabusa2, acquired high-resolution thermal images of the asteroid 162173 Ryugu for one asteroid rotation period on August 1, 2018 to investigate the thermophysical properties of the asteroid. The surface temperatures of Ryugu suggest that the surface has a low thermal inertia, indicating the presence of porous materials. Thermophysical models that neglect or oversimplify surface roughness cannot reproduce the flat diurnal temperature profiles observed during daytime. We performed numerical simulations of a thermophysical model, including the effects of roughness on the diurnal brightness temperature, the predictions of which successfully reproduced the observed diurnal variation of temperature. The global thermal inertia was obtained with a standard deviation of 225 ± 45 J m s K , which is relatively low but still within the range of the value estimated in our previous study (Okada et al., Nature 579, 518–522, 2020), confirming that the boulders on Ryugu are more porous in nature than typical carbonaceous chondrites. The global surface roughness (the ratio of the variance of the height relative to a local horizontal surface length) was determined as 0.41 ± 0.08, corresponding to a RMS surface slope of 47 ± 5°. We identified a slightly lower roughness distributed along the equatorial ridge, implying a mass movement of boulders from the equatorial ridge to the mid-latitudes. −2 −0.5 −1

Hayabusa2 is a Japanese sample return mission from the near-Earth asteroid Ryugu. The Hayabusa2 spacecraft was launched on December 3, 2014, and arrived at Ryugu on June 27, 2018. It stayed there until November 13, 2019 for in situ observation and soil sample collection, and will return to the Earth in November or December 2020. During the stay at the asteroid, the spacecraft performed a touchdown operation successfully for the first time in February 2019. Since the surface of Ryugu was rough and full of boulders, a targeted area finally found had a radius of only 3 m. There were several technical challenges to overcome including demanding guidance, navigation and control accuracy to realise the touchdown operation. In this paper, strategies and technical details of the guidance, navigation and control systems are presented. The flight results prove that the performance of the systems was satisfactory and largely contributed to the success of the operation.

<title>Abstract</title> Hayabusa2 is the ongoing JAXA’s sample and return mission to the asteroid Ryugu. In late 2018, Ryugu was in superior solar conjunction with the Earth. It is the first time that a spacecraft experiences the blackouts in the communication link with the Earth while hovering around a small celestial body. In this article, the design of the nominal conjunction trajectory flown by the Hayabusa2’s spacecraft is presented. The requirements for the conjunction trajectory were (1) to guarantee a low fuel consumption, (2) to ensure the visibility of the asteroid by the spacecraft’s wide angle camera (<inline-formula><alternatives><tex-math>$60^{\circ }$</tex-math><mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msup> <mml:mn>60</mml:mn> <mml:mo>∘</mml:mo> </mml:msup> </mml:math></alternatives></inline-formula> FoV), and (3) to increase the spacecraft altitude to a safety location (<inline-formula><alternatives><tex-math>$\sim109~\mbox{km}$</tex-math><mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mo>∼</mml:mo> <mml:mn>109</mml:mn> <mml:mspace /> <mml:mtext>km</mml:mtext> </mml:math></alternatives></inline-formula>) from the nominal BOX-A operation of 20 km (Home Position - HP). Finally, (4) to return at BOX-A after the conjunction phase. Given the mission constraints, the designed conjunction trajectory appears to have a fish-shape in the Hill coordinates therefore we renamed it as “ayu” (sweetfish in Japanese) trajectory. The optNEAR tool was developed for the guidance (<inline-formula><alternatives><tex-math>$\Delta V\mbox{s}$</tex-math><mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mi>Δ</mml:mi> <mml:mi>V</mml:mi> <mml:mtext>s</mml:mtext> </mml:math></alternatives></inline-formula> planning) and navigation design of the Hayabusa2’s conjunction mission phase. A preliminary sensitivity analysis in the Hill reference frame proved that the ayu trajectory is a good candidate for the conjunction operation of hovering satellite. The solution in the Hill coordinates is refined in the full-body planetary dynamics (optNEAR Tool) before flight. The ayu conjunction trajectory requires (a) two deterministic <inline-formula><alternatives><tex-math>$\Delta V\mbox{s}$</tex-math><mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mi>Δ</mml:mi> <mml:mi>V</mml:mi> <mml:mtext>s</mml:mtext> </mml:math></alternatives></inline-formula> at the Conjunction Orbit Insertion (COI) point and at the Home-position Recovery Maneuver (HRM) point respectively. (b) Two stochastic <inline-formula><alternatives><tex-math>$\Delta V\mbox{s}$</tex-math><mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mi>Δ</mml:mi> <mml:mi>V</mml:mi> <mml:mtext>s</mml:mtext> </mml:math></alternatives></inline-formula>, known as Trajectory Correction Manoeuvres (TCMs), before and after the deep conjunction phase are also required. The constraint linear covariance analysis in the full-body dynamics is here derived and used for the preliminary guidance and navigation planning. The results of the covariance analysis were validated in a nonlinear sense with a Monte Carlo approach which proved the validity of the semi-analytic method for the stochastic <inline-formula><alternatives><tex-math>$\Delta V\mbox{s}$</tex-math><mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mi>Δ</mml:mi> <mml:mi>V</mml:mi> <mml:mtext>s</mml:mtext> </mml:math></alternatives></inline-formula> planning derived in this paper.

The Hayabusa2 asteroid explorer mission focuses principally on the touchdown and sampling on near-Earth asteroid 162173 Ryugu. Hayabusa2 successfully landed on its surface and ejected a projectile for sample collection on February 22, 2019. Hayabusa2 later landed near a crater formed by an impactor and executed the sampling sequence again on July 11, 2019. For a successful mission, a thorough understanding and evaluation of spacecraft dynamics during touchdown were crucial. The most challenging aspect of this study was the modeling of such spacecraft phenomena as the dynamics of landing on a surface with unknown properties. In particular, a Monte Carlo analysis was used to determine the parameters of the operational design for the final descent and touchdown sequence. This paper discusses the dynamical modeling of the simulation during the touchdown of Hayabusa2.

Hayabusa2 is an asteroid sample return mission carried out by the Japan Aerospace Exploration Agency. The spacecraft was launched in 2014 and arrived at the target asteroid Ryugu on June 27, 2018. During the 1.5-year proximity phase, several critical operations (including two landing/sampling operations) were successfully performed. They were based on autonomous image-based descent and landing techniques. This paper describes an imagebased autonomous navigation scheme of the Hayabusa2 mission using artificial landmarks named target markers (TMs). Its basic algorithm, and the in-flight results of the first touchdown and its rehearsal, are shown.

Hayabusa2 is a Japanese sample return mission from the near-Earth asteroid Ryugu. The Hayabusa2 spacecraft was launched on December 3, 2014, and reached the asteroid on June 27, 2018. It remained there until November 13, 2019 for in situ observation and soil sample collection and will return to the Earth in November or December 2020. During its stay at the asteroid, Hayabusa2 performed descent operations 16 times. This paper presents an overview of a guidance, navigation, and control method used in such descent operations. The method consists of on-board and on-ground guidance systems to control the spacecraft and an image-based navigation technique that uses a shape model and ground control points of the asteroid. Flight results in the first touchdown operation are shown as an example, which demonstrate that the method showed a good performance overall and contributed to the success of the mission.

Hayabusa2 arrived at the C-type asteroid Ryugu in June 2018. During one and a half year of the Ryuguproximity operation, we succeeded in two rovers landing, one lander landing, two spacecraft touchdown/sample collection, one kinetic impact operation and two tiny reflective balls and one rover orbiting. Among the two successful touchdowns, the second one succeeded in collecting subsurface material exposed by the kinetic impact operation. This paper describes the asteroid proximity operation activity of the Hayabusa2 mission, and gives an overview of the achievements done so far. Some important engineering and scientific activities, which have been done in synchronous with the spacecraft operations to tackle with unexpected Ryugu environment, are also described.

The near-Earth asteroid (162173) Ryugu is thought to be a primitive carbonaceous object that contains hydrated minerals and organic molecules. We report sample collection from Ryugu’s surface by the Hayabusa2 spacecraft on 21 February 2019. Touchdown images and global observations of surface colors are used to investigate the stratigraphy of the surface around the sample location and across Ryugu. Latitudinal color variations suggest the reddening of exposed surface material by solar heating and/or space weathering. Immediately after touchdown, Hayabusa2’s thrusters disturbed dark, fine grains that originate from the redder materials. The stratigraphic relationship between identified craters and the redder material indicates that surface reddening occurred over a short period of time. We suggest that Ryugu previously experienced an orbital excursion near the Sun.

The Hayabusa2 spacecraft investigated the small asteroid Ryugu, which has a rubble-pile structure. We describe an impact experiment on Ryugu using Hayabusa2’s Small Carry-on Impactor. The impact produced an artificial crater with a diameter &gt;10 meters, which has a semicircular shape, an elevated rim, and a central pit. Images of the impact and resulting ejecta were recorded by the Deployable CAMera 3 for &gt;8 minutes, showing the growth of an ejecta curtain (the outer edge of the ejecta) and deposition of ejecta onto the surface. The ejecta curtain was asymmetric and heterogeneous and it never fully detached from the surface. The crater formed in the gravity-dominated regime; in other words, crater growth was limited by gravity not surface strength. We discuss implications for Ryugu’s surface age.

Precise information of spacecraft position with respect to target body is of importance in terms of scientific interpretation of remote sensing data. In case of Hayabusa2, a sample return mission from asteroid Ryugu, such information is also necessary for landing site selection activity. We propose a quick method to improve the spacecraft trajectory when laser altimeter range measurements and a shape model are provided together with crude initial trajectory, spacecraft attitude information, and asteroid spin information. We compared topographic features contained in the altimeter data with those expressed by the reference shape model, and estimated long-period trajectory correction so that discrepancy between the two topographic profiles was minimized. The improved spacecraft positions are consistent with those determined by image-based stereophotoclinometry method within a few tens of meters. With such improved trajectory, the altimeter ranges can be converted to Ryugu's topographic profiles that are appropriate for geophysical interpretation. We present a geophysical application that invokes possibility of impact-induced formation of the Ryugu's western bulge.

Asteroid 162173 Ryugu has numerous craters. The initial measurement of impact craters on Ryugu, by Sugita et al. (2019), is based on Hayabusa2 ONC images obtained during the first month after the arrival of Hayabusa2 in June 2018. Utilizing new images taken until February 2019, we constructed a global impact crater catalogue of Ryugu, which includes all craters larger than 20 m in diameter on the surface of Ryugu. As a result, we identified 77 craters on the surface of Ryugu. Ryugu shows variation in crater density which cannot be explained by the randomness of cratering; there are more craters at lower latitudes and fewer at higher latitudes, and fewer craters in the western bulge (160 degrees E - 290 degrees E) than in the region around the meridian (300 degrees E - 30 degrees E). This variation implies a complicated geologic history for Ryugu. It seems that the variation in crater density indicates that the equatorial ridge located in the western hemisphere is relatively young, while that located in the eastern hemisphere is a fossil structure formed during the short rotational period in the distant past.

Carbonaceous (C-type) asteroids1 are relics of the early Solar System that have preserved primitive materials since their formation approximately 4.6 billion years ago. They are probably analogues of carbonaceous chondrites2,3 and are essential for understanding planetary formation processes. However, their physical properties remain poorly known because carbonaceous chondrite meteoroids tend not to survive entry to Earth's atmosphere. Here we report on global one-rotation thermographic images of the C-type asteroid 162173 Ryugu, taken by the thermal infrared imager (TIR)4 onboard the spacecraft Hayabusa25, indicating that the asteroid's boulders and their surroundings have similar temperatures, with a derived thermal inertia of about 300 J m-2 s-0.5 K-1 (300 tiu). Contrary to predictions that the surface consists of regolith and dense boulders, this low thermal inertia suggests that the boulders are more porous than typical carbonaceous chondrites6 and that their surroundings are covered with porous fragments more than 10 centimetres in diameter. Close-up thermal images confirm the presence of such porous fragments and the flat diurnal temperature profiles suggest a strong surface roughness effect7,8. We also observed in the close-up thermal images boulders that are colder during the day, with thermal inertia exceeding 600 tiu, corresponding to dense boulders similar to typical carbonaceous chondrites6. These results constrain the formation history of Ryugu: the asteroid must be a rubble pile formed from impact fragments of a parent body with microporosity9 of approximately 30 to 50 per cent that experienced a low degree of consolidation. The dense boulders might have originated from the consolidated innermost region or they may have an exogenic origin. This high-porosity asteroid may link cosmic fluffy dust to dense celestial bodies10.

An analytic construction of 1:1 resonances around irregular bodies is here investigated. A SPH-Mas based gravity model allows a semi-analytic expression of the linearised equations around the equilibrium points. Depending on the sphere packing distribution, the SPH-Mas model can retrieve the same dynamical objects common to others gravity models (i.e. spherical harmonics and polyhedron) or for non uniform density objects. This model has the advantage to define the same particles mesh distribution for both astrophysical and astrodynamics tools and it is computationally optimised for Matlab. The Hayabusa2's Small Carry-on Impactor operation is used as a scenario to study the ejecta particle dynamics around an irregular body. The goNEAR tool was used to simulate the impact operation in a non-linear sense when the effect of the solar radiation pressure perturbation is taken into account for particles size of 10 cm, 5 cm, 1 cm and 1 mm in diameter.

Hayabusa2 arrived at the asteroid Ryugu in June 2018, and as of April 2019, the mission succeeded in conducting two rovers landing, one lander landing, one spacecraft touchdown/sample collection and one kinetic impact operation. This paper describes the initial nine months of the asteroid proximity operation activity of the Hayabusa2 mission, and gives an overview of the achievements thus far. Some important engineering and scientific activities conducted synchronously with spacecraft operations in order to complete all planned operations in time against unexpectedly harsh environment of Ryugu are also described.

<p>Hayabusa2 is a Japanese sample return mission from the asteroid Ryugu. The Hayabusa2 spacecraft was launched on 3 December 2014, arrived at Ryugu on 27 June 2018. It will stay in proximity of Ryugu until December 2019 for in situ observation and soil sample collection aiming to return to the Earth in December 2020 with the collected sample. During the 1.5-year asteroid proximity operation, the spacecraft performs several numbers of descent operation to deploy and land rovers, and to touchdown collecting soil sample. To obtain large amount of various data during asteroid proximity operation, data downlink from Hayabusa2 is realized using two high gain antennas (HGAs) which are designated to X-band (8GHz) downlink and Ka-band (32GHz) downlink respectively. This paper summarizes Ka-band capability of Hayabusa2 and its contribution to asteroid proximity operation including the Landing Site Selection (LSS) activity. Also, feedbacks for Ka-band equipped future deep space mission are extracted based on the comparison between X/Ka-band operation results.</p>

The near-Earth asteroid (162173) Ryugu is a 900-m-diameter dark object expected to contain primordial material from the solar nebula. The Mobile Asteroid Surface Scout (MASCOT) landed on Ryugu’s surface on 3 October 2018. We present images from the MASCOT camera (MASCam) taken during the descent and while on the surface. The surface is covered by decimeter- to meter-sized rocks, with no deposits of fine-grained material. Rocks appear either bright, with smooth faces and sharp edges, or dark, with a cauliflower-like, crumbly surface. Close-up images of a rock of the latter type reveal a dark matrix with small, bright, spectrally different inclusions, implying that it did not experience extensive aqueous alteration. The inclusions appear similar to those in carbonaceous chondrite meteorites.

The near-Earth asteroid 162173 Ryugu, the target of Hayabusa2 sample return mission, is thought to be a primitive carbonaceous object. We report reflectance spectra of Ryugu’s surface acquired with the Near Infrared Spectrometer (NIRS3) on Hayabusa2, to provide direct measurements of the surface composition and geological context for the returned samples. A weak, narrow absorption feature centered at 2.72 μm was detected across the entire observed surface, indicating that hydroxyl (OH)-bearing minerals are ubiquitous there. The intensity of the OH feature and low albedo are similar to thermally- and/or shock-metamorphosed carbonaceous chondrite meteorites. There are few variations in the OH-band position, consistent with Ryugu being a compositionally homogeneous rubble-pile object, generated from impact fragments of an undifferentiated aqueously altered parent body.