早川 雅彦

++ 1. IntroductionThe Hayabusa2 spacecraft is currently cruising through deep space for the extended mission Hayabusa2#. The spacecraft is scheduled to flyby asteroid 2002 CC21 in 2026 and rendezvous with asteroid 1998 KY26 in 2031. Hayabusa2's VIS cameras include the ONC-T (Onborad Navigation Camera - Telescopic) and the wide-angle ONC-W1 and ONC-W2 (Figure 1). ONC-T, with its high sensitivity and multi-band observation capability, is the primary scientific instrument [1]. During the long cruise, ecliptic light observations [2] and exoplanet observations [3] continue as ONC-T observations. On the other hand, we are exploring ways to further utilize ONC cameras during the cruisng phase, and in this study, we examine how to utilize ONC-W2 and plan to process the data. Figure 1. Schematic view of the configuration of ONC-T, W1, and W2 (after [4]). Blue line indicates the solar array paddle.++ 2. Characteristics of ONC-W2The disadvantages and advantages of using the ONC-W2 for distant objects are as follows.[Disadvantages] Low sensitivity and stray light- The sensitivity of the ONC-W2 is not sufficient to observe distant objects because it is designed to observe the surface of an asteroid with disk-resolved situation.- The stray light from the multi-layer insulation at the edge of ONC-W2's FOV is very large for long exposure observation.[Advantage] Wide range of observable direction- The ONC-W2 camera can observe a wide area, whereas the ONC-T camera can only point in a narrow directions due to the limitations of the solar array paddle. Since the W2 camera faces the side of the solar array paddle (in the +Z direction of the spacecraft), it can cover 48% of the entire sky by turning the spacecraft attitude around the +Z axis and pointing the camera in different directions without losing power.Due to its low sensitivity but wide field of view, W2 could be used, for example, to continuously observe bright new comets for several days or weeks. The most recent such possibility is the comet C/2023 A3 (Tsuchinshan-ATLAS). An example about the estimation of observable period is shown in Section 4.++ 3. Preparation of data processing methodsNew ONC-W2 applications will require additional tools different from those for Ryugu images. We are working on a list of necessary data processing methods and calibration tasks.+ Stray lightPrevious calibration studies have shown that the presence or absence of stray light in W2 depends on the attitude of the spacecraft [5]. When stray light does occur, the degree of stray light is significant (Figure 2). The primary countermeasure is to adopt an attitude that minimizes stray light, but it is also necessary to develop image processing methods to remove stray light. Figure 2. An example of ONC-W2 long exposure (44.6s) image with stray light. White dots are mainly hot pixels.+ Sensitivity checkThe sensitivity of ONC-W2 prior to Ryugu arrival has been confirmed by [5]. However, because of sensitivity changes due to the Ryugu touchdown and changes over time, it is necessary to confirm the current sensitivity. As a quick check tool, we have prepared a method to estimate the sensitivity statistically from multiple stars. Figure 3 below plots the relationship between the stars V mag and integrated DN from 43 frames observed in 2016, with stray light removed. These stars include variable stars, but the effect is expected to be smaller by using a large number of stars.  Figure 3. Relationship between the stars Vmag and integrated intensity (DN) of long exposure (44.6s) images.++ 4. Observation opportunitiesWe are also considering the preparation of methods and tools for narrowing down suitable observation opportunities for ONC-W2. The following is the case study of comet C/2023 A3.Figure 4 shows the timing of the comet's entry into the FOV of ONC-W2. The orbit of the comet was obtained from JPL Horizons Sytem [6]. In this figure, the entire space as seen from the spacecraft is projected in a simple cylindrical projection. The spacecraft is oriented with the solar array paddle (+Z) pointing toward the sun and the W2 camera side toward the lower ecliptic plane. The red dots are the direction of the comet calculated every other day. The comet was found to cross the FOV from August 20 to August 28, 2024. Further observation will be possible by changing the attitude of the spacecraft. Figure 5 shows the total magnitude of Comet C/2023 A3 as expected from the position of Hayabusa2, which is expected to be 2-3 magnitude at the end of August, bright enough to be observed by ONC-W2. At this time, the Earth is on the opposite side of the Sun, making it difficult to observe this comet. Therefore, observation of this comet by a spacecraft would be highly valuable as data. We plan to conduct an observational test with ONC-W2 during this period. We will present a preliminary report  in this presentation. Figure 4: Calculated timing of comet crossing in ONC-W2 field of view. Figure 5. Predicted total magnitude of Comet C/2023 A3 from the position of Hayabusa2.++6. ConclusionWe examine how to utilize Hayabusa2 ONC-W2 camera in the cruising phase. Due to its low sensitivity but wide field of view, ONC-W2 could be used to continuously observe bright new comets for several days or weeks. We plan to conduct an observational test of the the comet C/2023 A3 in August. We will present a preliminary report  in this presentation.++ Acknowledgement: We thank the Haybusa2# systems and science teams for discussing the feasibility of the operation.++ References: [1] Sugita et al. (2019) Science 364, eaaw0422. doi.org/10.1126/science.aaw0422 [2] Tsumura et al. (2023) Earth Planets Space 75, 121. doi.org/10.1186/s40623-023-01856-x [3] Yumoto et al. (2024) 55th LPSC, Abstract 1774.  [4] Kouyama et al. (2021) Icarus 360, 114353. doi.org/10.1016/j.icarus.2021.114353  [5] Tatsumi et al. (2019) Icarus 325,153-195. doi.org/10.1016/j.icarus.2019.01.015 [6] NASA JPL Horizons System. https://ssd.jpl.nasa.gov/horizons/app.html#/

Abstract Returned samples from Cb-type asteroid (162173) Ryugu exhibit very dark spectra in visible and near-infrared ranges, generally consistent with the Hayabusa2 observations. A critical difference is that a structural water absorption of hydrous silicates is around twice as deep in the returned samples compared with those of Ryugu’s surface, suggesting Ryugu surface is more dehydrated. Here we use laboratory experiments data to indicate the spectral differences between returned samples and asteroid surface are best explained if Ryugu surface has (1) higher porosity, (2) larger particle size, and (3) more space-weathered condition, with the last being the most effective. On Ryugu, space weathering by micrometeoroid bombardments promoting dehydration seem to be more effective than that by solar-wind implantation. Extremely homogeneous spectra of the Ryugu’s global surface is in contrast with the heterogeneous S-type asteroid (25143) Itokawa’s spectra, which suggests space weathering has proceeded more rapidly on Cb-type asteroids than S-type asteroids.

Abstract Zodiacal light (ZL) is sunlight scattered by interplanetary dust particles (IDPs) at optical wavelengths. The spatial distribution of IDPs in the Solar System may hold an important key to understanding the evolution of the Solar System and material transportation within it. The number density of IDPs can be expressed as $$n(r) \sim r^{-\alpha }$$, and the exponent $$\alpha \sim 1.3$$ was obtained by previous observations from interplanetary space by Helios 1/2 and Pioneer 10/11 in the 1970s and 1980s. However, no direct measurements of $$\alpha $$ based on ZL observations from interplanetary space outside Earth’s orbit have been performed since then. Here, we introduce initial results for the radial profile of the ZL at optical wavelengths observed over the range 0.76$$-$$1.06 au by ONC-T aboard the Hayabusa2# mission in 2021-2022. The ZL brightness we obtained is well reproduced by a model brightness, although there is a small excess of the observed ZL brightness over the model brightness at around 0.9 au. The radial power-law index we obtained is $$\alpha = 1.30 \pm 0.08$$, which is consistent with previous results based on ZL observations. The dominant source of uncertainty arises from the uncertainty in estimating the diffuse Galactic light (DGL). Graphical Abstract

Samples of the carbonaceous asteroid (162173) Ryugu were collected and brought to Earth by the Hayabusa2 spacecraft. We investigated the macromolecular organic matter in Ryugu samples and found that it contains aromatic and aliphatic carbon, ketone, and carboxyl functional groups. The spectroscopic features of the organic matter are consistent with those in chemically primitive carbonaceous chondrite meteorites that experienced parent-body aqueous alteration (reactions with liquid water). The morphology of the organic carbon includes nanoglobules and diffuse carbon associated with phyllosilicate and carbonate minerals. Deuterium and/or nitrogen-15 enrichments indicate that the organic matter formed in a cold molecular cloud or the presolar nebula. The diversity of the organic matter indicates variable levels of aqueous alteration on Ryugu’s parent body.

The Hayabusa2 spacecraft collected samples from the surface of the carbonaceous near-Earth asteroid (162173) Ryugu and brought them to Earth. The samples were expected to contain organic molecules, which record processes that occurred in the early Solar System. We analyzed organic molecules extracted from the Ryugu surface samples. We identified a variety of molecules containing the atoms CHNOS, formed by methylation, hydration, hydroxylation, and sulfurization reactions. Amino acids, aliphatic amines, carboxylic acids, polycyclic aromatic hydrocarbons, and nitrogen-heterocyclic compounds were detected, which had properties consistent with an abiotic origin. These compounds likely arose from an aqueous reaction on Ryugu’s parent body and are similar to the organics in Ivuna-type meteorites. These molecules can survive on the surfaces of asteroids and be transported throughout the Solar System.

The near-Earth carbonaceous asteroid (162173) Ryugu is expected to contain volatile chemical species that could provide information on the origin of Earth’s volatiles. Samples of Ryugu were retrieved by the Hayabusa2 spacecraft. We measured noble gas and nitrogen isotopes in Ryugu samples and found that they are dominated by presolar and primordial components, incorporated during Solar System formation. Noble gas concentrations are higher than those in Ivuna-type carbonaceous (CI) chondrite meteorites. Several host phases of isotopically distinct nitrogen have different abundances among the samples. Our measurements support a close relationship between Ryugu and CI chondrites. Noble gases produced by galactic cosmic rays, indicating a ~5 million year exposure, and from implanted solar wind record the recent irradiation history of Ryugu after it migrated to its current orbit.

Samples of the carbonaceous asteroid Ryugu were brought to Earth by the Hayabusa2 spacecraft. We analyzed 17 Ryugu samples measuring 1 to 8 millimeters. Carbon dioxide–bearing water inclusions are present within a pyrrhotite crystal, indicating that Ryugu’s parent asteroid formed in the outer Solar System. The samples contain low abundances of materials that formed at high temperatures, such as chondrules and calcium- and aluminum-rich inclusions. The samples are rich in phyllosilicates and carbonates, which formed through aqueous alteration reactions at low temperature, high pH, and water/rock ratios of <1 (by mass). Less altered fragments contain olivine, pyroxene, amorphous silicates, calcite, and phosphide. Numerical simulations, based on the mineralogical and physical properties of the samples, indicate that Ryugu’s parent body formed ~2 million years after the beginning of Solar System formation.

Carbonaceous meteorites are thought to be fragments of C-type (carbonaceous) asteroids. Samples of the C-type asteroid (162173) Ryugu were retrieved by the Hayabusa2 spacecraft. We measured the mineralogy and bulk chemical and isotopic compositions of Ryugu samples. The samples are mainly composed of materials similar to those of carbonaceous chondrite meteorites, particularly the CI (Ivuna-type) group. The samples consist predominantly of minerals formed in aqueous fluid on a parent planetesimal. The primary minerals were altered by fluids at a temperature of 37° ± 10°C, about million (statistical) or million (systematic) years after the formation of the first solids in the Solar System. After aqueous alteration, the Ryugu samples were likely never heated above ~100°C. The samples have a chemical composition that more closely resembles that of the Sun’s photosphere than other natural samples do.

Abstract Without a protective atmosphere, space-exposed surfaces of airless Solar System bodies gradually experience an alteration in composition, structure and optical properties through a collective process called space weathering. The return of samples from near-Earth asteroid (162173) Ryugu by Hayabusa2 provides the first opportunity for laboratory study of space-weathering signatures on the most abundant type of inner solar system body: a C-type asteroid, composed of materials largely unchanged since the formation of the Solar System. Weathered Ryugu grains show areas of surface amorphization and partial melting of phyllosilicates, in which reduction from Fe³⁺ to Fe²⁺ and dehydration developed. Space weathering probably contributed to dehydration by dehydroxylation of Ryugu surface phyllosilicates that had already lost interlayer water molecules and to weakening of the 2.7 µm hydroxyl (–OH) band in reflectance spectra. For C-type asteroids in general, this indicates that a weak 2.7 µm band can signify space-weathering-induced surface dehydration, rather than bulk volatile loss.

The Hayabusa2 spacecraft returned to Earth from the asteroid 162173 Ryugu on 6 December 2020. One day after the recovery, the gas species retained in the sample container were extracted and measured on-site and stored in gas collection bottles. The container gas consists of helium and neon with an extraterrestrial ³ He/ ⁴ He and ²⁰ Ne/ ²² Ne ratios, along with some contaminant terrestrial atmospheric gases. A mixture of solar and Earth’s atmospheric gas is the best explanation for the container gas composition. Fragmentation of Ryugu grains within the sample container is discussed on the basis of the estimated amount of indigenous He and the size distribution of the recovered Ryugu grains. This is the first successful return of gas species from a near-Earth asteroid.

Abstract Volatile and organic-rich C-type asteroids may have been one of the main sources of Earth’s water. Our best insight into their chemistry is currently provided by carbonaceous chondritic meteorites, but the meteorite record is biased: only the strongest types survive atmospheric entry and are then modified by interaction with the terrestrial environment. Here we present the results of a detailed bulk and microanalytical study of pristine Ryugu particles, brought to Earth by the Hayabusa2 spacecraft. Ryugu particles display a close compositional match with the chemically unfractionated, but aqueously altered, CI (Ivuna-type) chondrites, which are widely used as a proxy for the bulk Solar System composition. The sample shows an intricate spatial relationship between aliphatic-rich organics and phyllosilicates and indicates maximum temperatures of ~30 °C during aqueous alteration. We find that heavy hydrogen and nitrogen abundances are consistent with an outer Solar System origin. Ryugu particles are the most uncontaminated and unfractionated extraterrestrial materials studied so far, and provide the best available match to the bulk Solar System composition.

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.

<title>Abstract</title>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.

ONC (Optical Navigation Camera; 光学航法カメラ) は探査機はやぶさ２の目であり， リュウグウを訪れた際には科学的にも工学的にも広報的にも幅広く活⽤された．ONCは広域撮像⽤のONC-W1，ONC-W2，望遠カメラ且つ7色のバンドパスフィルターをもつONC-Tで構成されている．ONC-Tは科学観測において特に重要で，フィルターを活⽤し⼩惑星表⾯の色の変化を記載することや解像度の高い画像から詳細な地形の観測を目的としている．試料採取地点の選定にも，粒径や風化作⽤の推定といった核となる情報を得て貢献してきた．本稿では，今後のサンプル分析を見据えて，主にONCチームのONC-Tを⽤いた分光観測における活動とその結果として得られた“仮説”を振り返りたいと思う．

<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.

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.