津田 雄一

The Hayabusa2 spacecraft investigated the C-type (carbonaceous) asteroid (162173) Ryugu. The mission performed two landing operations to collect samples of surface and subsurface material, the latter exposed by an artificial impact. We present images of the second touchdown site, finding that ejecta from the impact crater was present at the sample location. Surface pebbles at both landing sites show morphological variations ranging from rugged to smooth, similar to Ryugu’s boulders, and shapes from quasi-spherical to flattened. The samples were returned to Earth on 6 December 2020. We describe the morphology of >5 grams of returned pebbles and sand. Their diverse color, shape, and structure are consistent with the observed materials of Ryugu; we conclude that they are a representative sample of the asteroid....

C-type asteroids(1) are considered to be primitive small Solar System bodies enriched in water and organics, providing clues to the origin and evolution of the Solar System and the building blocks of life. C-type asteroid 162173 Ryugu has been characterized by remote sensing(2-7) and on-asteroid measurements(8,9) with Hayabusa2 (ref. (10)). However, the ground truth provided by laboratory analysis of returned samples is invaluable to determine the fine properties of asteroids and other planetary bodies. We report preliminary results of analyses on returned samples from Ryugu of the particle size distribution, density and porosity, spectral properties and textural properties, and the results of a search for Ca-Al-rich inclusions (CAIs) and chondrules. The bulk sample mainly consists of rugged and smooth particles of millimetre to submillimetre size, confirming that the physical and chemical properties were not altered during the return from the asteroid. The power index of its size distribution is shallower than that of the surface boulder observed on Ryugu(11), indicating differences in the returned Ryugu samples. The average of the estimated bulk densities of Ryugu sample particles is 1,282 +/- 231 kg m(-3), which is lower than that of meteorites(12), suggesting a high microporosity down to the millimetre scale, extending centimetre-scale estimates from thermal measurements(5,9). The extremely dark optical to near-infrared reflectance and spectral profile with weak absorptions at 2.7 and 3.4 mu m imply a carbonaceous composition with indigenous aqueous alteration, matching the global average of Ryugu(3,4) and confirming that the sample is representative of the asteroid. Together with the absence of submillimetre CAIs and chondrules, these features indicate that Ryugu is most similar to CI chondrites but has lower albedo, higher porosity and more fragile characteristics.

The Hayabusa2 extended mission is designed to rendezvous with the fast-rotating asteroid 1998 KY26 in 2031. The asteroid's diameter and rotation period are as small as 20-40 m and 10:7 min, respectively. Consequently, the low gravity and rapid rotation of the asteroid form distinctive dynamical environments. This paper rst characterizes orbital and surface environments about 1998 KY26. Then, possible orbital operations are investigated, particularly focusing on the hovering and descent operations. Our research, therefore, contributes to exploring one of the smallest members of the solar system.

Context. JAXA's Hayabusa2 mission rendezvoused the Ryugu asteroid for 1.5 years to clarify the carbonaceous asteroids' record for Solar System origin and evolution. Aims. We studied the photometric behavior of the spectral parameters characterizing the near-infrared (NIR) spectra of Ryugu provided by the Hayabusa2/NIRS3 instrument, that is to say 1.9 µm reflectance, 2.7 and 2.8 µm band depths (ascribed to phyllosilicates), and NIR slope. Methods. For each parameter, we applied the following empirical approach: (1) retrieval of the equigonal albedo by applying the Akimov disk function (this step was only performed for the reflectance photometric correction); (2) retrieval of the median spectral parameter value at each phase angle bin; and (3) retrieval of the phase function by a linear fit. Results. Ryugu's phase function shows a steepness similar to Ceres, according to the same taxonomy of the two asteroids. Band depths decrease with increasing phase angle: this trend is opposite to that observed on other asteroids explored by space missions and is ascribed to the very dark albedo. NIR and visible phase reddening are similar, contrary to other asteroids, where visible phase reddening is larger: this could be due to surface darkness or to particle smoothness. Albedo and band depths are globally uncorrelated, but locally anticorrelated. A correlation between darkening and reddening is observed.

Hayabusa2 was a mission with a series of challenging operations and a scientific goal that related to the origins of life. These attributes presented an opportunity to engage with a wide range of people beyond the scientific community who might be inspired by the difficulty of the engineering or relate to a search for how life on Earth began. The mission's outreach program aimed to share news throughout the mission in Japan and overseas, with regular updates on mission operations, real-time events for an immersive feel during major operations and campaigns to allow people to connect with the team.

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.

2021年6月のリュウグウ粒子の配分後、約１年にわたり８つの研究チーム(初期分析６チームとPhase2キュレーション２チーム)により系統的な分析が行われた。岩石・鉱物組織からみると、リュウグウは主として含水鉱物と有機物から構成され、微細な組織として複雑に共存することがわかった。また主要元素濃度、高精度酸素同位体などのバルク的分析の結果は、リュウグウ粒子は、太陽系の元素組成を代表する始原的なCIコンドライトと良い一致を示した。我々は、独自に開発した大気非曝露ナノ領域試料加工・分析システムを用い、有機物と含水鉱物の入り交ざった試料から物質科学的情報を得た。その結果、(１)含水鉱物集合体に特異的に濃集した脂肪属炭化水素に富む有機物の存在(２)太陽系で最も始原的な化学組成を持つ物質である証拠、そして(３)太陽系外縁部での形成が明らかになった。

MASCOT (‘Mobile Asteroid Surface Scout’) is a 10 kg mobile surface science package part of JAXA’s Hayabusa2 sample return mission. The mission was launched in December 2014 from Tanegashima Space Center, Japan. The Hayabusa2 spacecraft reached the target asteroid in summer 2018. After a mapping phase of the asteroid and a landing site selection process the MASCOT lander was deployed to the surface on the 3rd of October 2018. MASCOT operated successfully for about 17 h on the surface of Ryugu. It performed three relocation manoeuvres and one “Mini-Move” and returned 128 MBytes of data. MASCOT has been developed by the German Aerospace Center (DLR) in cooperation with the Centre National d’Etudes Spatiales (CNES). The main objectives were to perform in-situ investigations of the asteroid surface and to support the sampling site selection for the mother spacecraft. These objectives could be reached successfully. On 6th December 2020 Hayabusa2 successfully returned asteroid samples to the Earth.

Ryugu is a carbonaceous rubble-pile asteroid visited by the Hayabusa2 spacecraft. Small rubble pile asteroids record the thermal evolution of their much larger parent bodies. However, recent space weathering and/or solar heating create ambiguities between the uppermost layer observable by remote-sensing and the pristine material from the parent body. Hayabusa2 remote-sensing observations find that on the asteroid (162173) Ryugu both north and south pole regions preserve the material least processed by space weathering, which is spectrally blue carbonaceous chondritic material with a 0-3% deep 0.7-µm band absorption, indicative of Fe-bearing phyllosilicates. Here we report that spectrally blue Ryugu's parent body experienced intensive aqueous alteration and subsequent thermal metamorphism at 570-670 K (300-400 °C), suggesting that Ryugu's parent body was heated by radioactive decay of short-lived radionuclides possibly because of its early formation 2-2.5 Ma. The samples being brought to Earth by Hayabusa2 will give us our first insights into this epoch in solar system history.

Deployable payloads have been used to expand mission opportunities and enable autonomous navigation and guidance under uncertain terrain conditions. A spacecraft may deploy an artificial marker toward the surface of the target body. An estimation of the deployment trajectory of markers can provide important information for guidance and navigation. We propose a method estimating the deployment trajectory using only images and evaluate its accuracy and reliability. In addition, to determine the spacecraft's relative attitude with respect to the asteroid surface, we introduce an attitude estimation method using the spacecraft's own shadow. The results shown in this paper confirm that the estimated trajectory is sufficiently accurate and thus the proposed method is a viable option for future missions for determining the trajectory of deployable payloads using only images of a deployment.

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).

Planetesimals—the initial stage of the planetary formation process—are considered to be initially very porous aggregates of dusts1,2, and subsequent thermal and compaction processes reduce their porosity3. The Hayabusa2 spacecraft found that boulders on the surface of asteroid (162173) Ryugu have an average porosity of 30–50% (refs. 4–6), higher than meteorites but lower than cometary nuclei7, which are considered to be remnants of the original planetesimals8. Here, using high-resolution thermal and optical imaging of Ryugu’s surface, we discovered, on the floor of fresh small craters (<20 m in diameter), boulders with reflectance (~0.015) lower than the Ryugu average6 and porosity >70%, which is as high as in cometary bodies. The artificial crater formed by Hayabusa2’s impact experiment9 is similar to these craters in size but does not have such high-porosity boulders. Thus, we argue that the observed high porosity is intrinsic and not created by subsequent impact comminution and/or cracking. We propose that these boulders are the least processed material on Ryugu and represent remnants of porous planetesimals that did not undergo a high degree of heating and compaction3. Our multi-instrumental analysis suggests that fragments of the highly porous boulders are mixed within the surface regolith globally, implying that they might be captured within collected samples by touch-down operations10,11.

<title>Abstract</title> C-type asteroids are considered to be primitive small Solar-System bodies enriched in water and organics, providing clues for understanding the origin and evolution of the Solar System and the building blocks of life. C-type asteroid 162173 Ryugu has been characterized by remote sensing and on-asteroid measurements with Hayabusa2, but further studies are expected by direct analyses of returned samples. Here we describe the bulk sample mainly consisting of rugged and smooth particles of millimeter to submillimeter size, preserving physical and chemical properties as they were on the asteroid. The particle size distribution is found steeper than that of surface boulders11. Estimated grain densities of the samples have a peak around 1350 kg m-3, which is lower than that of meteorites suggests a high micro-porosity down to millimeter-scale, as estimated at centimeter-scale by thermal measurements. The extremely dark optical to near-infrared reflectance and the spectral profile with weak absorptions at 2.7 and 3.4 microns implying carbonaceous composition with indigenous aqueous alteration, respectively, match the global average of Ryugu, confirming the sample’s representativeness. Together with the absence of chondrule and Ca-Al-rich inclusion of larger than sub-mm, these features indicate Ryugu is most similar to CI chondrites but with darker, more porous and fragile characteristics.

The article “Orbit insertion strategy of Hayabusa2’s rover with large release uncertainty around the asteroid Ryugu” written by Yusuke Oki, Kent Yoshikawa, Hiroshi Takeuchi et al., was originally published electronically on the publisher’s internet portal (currently SpringerLink) on 05 November 2020 without open access. After publication in Volume 4, Issue 4, page 309–329, the author(s) decided to opt for Open Choice and to make the article an open access publication. Therefore, the copyright of the article has been changed to © The Author(s) 2020 and the article is forthwith distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits use, duplication, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.

© 2020 The Authors 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.

© 2020. American Geophysical Union. All Rights Reserved. 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.

© 1963-2012 IEEE. An improved weather-forecast-based link-budget design technique for space-to-Earth links is described. The aim is the stochastic optimization of both transmission symbol rate and received signal-to-noise ratio. The proposed radiometeorological operations prediction (RadioMetOP) model takes into account the forecast uncertainty by a space-time ensemble method exploiting the temporal evolution of the predicted radiometeorological variables over the weather-forecast spatial grid. The unique possibility of testing and validating the RadioMetOP model is presented, thanks to the Ka -band downlink measurements available from the support of the European Space Agency's antenna tracking network to deep-space Hayabusa-2 (HB2) mission, operated by the Japan Aerospace Exploration Agency. First, the RadioMetOP model accuracy is tested by comparing the signal-to-noise ratio, measured during the transmission periods, with the simulated one, properly scaled to the symbol rate operated by HB2, finding correlation values of 0.9 that confirm the effectiveness of the proposed approach. Second, the a posteriori analysis of the optimization process is accomplished, showing that depending on the considered criteria for the link-budget optimization, the use of the RadioMetOP model would have allowed a transmitted data volume more than doubled and an average signal-to-noise ratio gain between 2.1 and 3.8 dB.

© 2020 Elsevier Inc. Asteroid 162,173 Ryugu is a rubble-pile asteroid, whose top-shape is compatible with models of deformation by spin up. Rims of major craters on Ryugu have an east–west asymmetric profile; their western crater rims are sharp and tall, while their eastern crater rims are rounded and low. Although there are various possible explanations, we theoretically assess the effect of asteroid rotation as the possible reason for this east–west asymmetry. It is known that the trajectories and fates of ejecta are affected by the rotation. The Coriolis force and the inertial speed of the rotating surface are the factors altering the ejecta trajectories. Consequently, we found that the east–west asymmetric crater rims might be formed as a result of rotation, when the inertial speed of the rotating surface is nearly equal to the first cosmic velocity of the body. In other words, it is possible that the observed east–west asymmetric rims of the Urashima, Cendrillon, and Kolobok craters were formed when Ryugu's rotation period was ~3.6 h.

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.

Hayabusa2 is the second asteroid sample return mission in the world following Hayabusa. The target asteroid was (162173) Ryugu, a C-Type near-Earth asteroid. The principal science purpose of the mission is to study the organic matter and water in the early stages of the Solar System, with the aim to understand the origin of the Earth s water and that of the substances that began life, as well as the origin and evolution of Solar System bodies. The mission successfully returned samples from Ryugu and collected a large amount of data on the asteroid through the onboard instruments, rovers, and lander. The mission completed several challenges such as two touchdowns, an impact experiment, and artificial satellite experiments. All of these challenges were important from the scientific point of view. The results revealed a range of the physical properties of Ryugu. With the samples of Ryugu now back on Earth, the sample analysis is currently underway to understand the materials from the early era of our Solar System. In addition to the scientific research, the mission also focused on outreach. We carried out a number of special campaigns, such as observations of the asteroid and spacecraft from Earth and an art contest, many talk events, web and twitter releases among other activities. We tried to inform people about our mission in real time and also tried to publish information both in Japanese and English simultaneously. Through these activities, we think we were able to make people feel connected with the Hayabusa2 mission and we hope that many more people have become interested in space activities. In this paper, we summarize the results of science and outreach for the Hayabusa2 mission.