坂谷 尚哉

++ 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 Here we describe the novel, multi-point Comet Interceptor mission. It is dedicated to the exploration of a little-processed long-period comet, possibly entering the inner Solar System for the first time, or to encounter an interstellar object originating at another star. The objectives of the mission are to address the following questions: What are the surface composition, shape, morphology, and structure of the target object? What is the composition of the gas and dust in the coma, its connection to the nucleus, and the nature of its interaction with the solar wind? The mission was proposed to the European Space Agency in 2018, and formally adopted by the agency in June 2022, for launch in 2029 together with the Ariel mission. Comet Interceptor will take advantage of the opportunity presented by ESA’s F-Class call for fast, flexible, low-cost missions to which it was proposed. The call required a launch to a halo orbit around the Sun-Earth L2 point. The mission can take advantage of this placement to wait for the discovery of a suitable comet reachable with its minimum $\varDelta $V capability of $600\text{ ms}^{-1}$. Comet Interceptor will be unique in encountering and studying, at a nominal closest approach distance of 1000 km, a comet that represents a near-pristine sample of material from the formation of the Solar System. It will also add a capability that no previous cometary mission has had, which is to deploy two sub-probes – B1, provided by the Japanese space agency, JAXA, and B2 – that will follow different trajectories through the coma. While the main probe passes at a nominal 1000 km distance, probes B1 and B2 will follow different chords through the coma at distances of 850 km and 400 km, respectively. The result will be unique, simultaneous, spatially resolved information of the 3-dimensional properties of the target comet and its interaction with the space environment. We present the mission’s science background leading to these objectives, as well as an overview of the scientific instruments, mission design, and schedule.

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.

The zodiacal light (ZL) is sunlight scattered by interplanetary dust (IPD) in the optical wavelengths. The spatial distribution of IPD in the Solar system may hold an important key to understanding the evolution of the Solar system and material transportation within it. The IPD number density can be expressed as n(r)∼r^{−α}, and the result of α∼1.3 was obtained by the previous observations from the interplanetary space by Helios 1/2 and Pioneer 10/11 in the 1970s and 1980s. However, no direct measurements of α based on the ZL observation from the interplanetary space outside the Earth's orbit have been conducted since then. Here we introduce the initial result of the ZL radial profile at optical wavelengths observed at 0.76-1.06 au by ONC-T with Hayabusa2# mission in 2021-2022. The obtained ZL brightness is well reproduced by the model brightness, but there is a small excess of the observed ZL brightness over the model brightness at around 0.9 au. The obtained radial power-law index is α=1.30±0.08, which is consistent with the previous results based on the ZL observations. The dominant uncertainty source in α arises from the uncertainty in the Diffuse Galactic Light estimation.

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 degrees +/- 10 degrees C, about 5.2(-0.7)(+0.8) million (statistical) or 5.2(-2.1)(+1.6) 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 similar to 100 degrees C. The samples have a chemical composition that more closely resembles that of the Sun's photosphere than other natural samples do.

Abstract Japanese Hayabusa2 spacecraft has successfully carried out an impact experiment using a small carry-on impactor (SCI) on an asteroid (162173) Ryugu. We examine the size distribution of particles inside and outside an artificial impact crater (the SCI crater) based on the images taken by the optical navigation camera onboard the Hayabusa2 spacecraft. The circumferential variation in particle size distribution inside the SCI crater is recognized and we interpret that major circumferential variation is caused by the large boulders inside the SCI crater that existed prior to the impact. The size distribution inside the SCI crater also shows that the subsurface layer beneath the SCI impact site had a large number of particles with a characteristic size of – 9 cm, which is consistent with the previous evaluations. On the other hand, the size distribution outside the SCI crater exhibits the radial variation, implying that the deposition of ejecta from the SCI crater is involved. The slope of the size distribution outside the crater at small sizes differs from the slope of the size distribution on the surface of Ryugu by approximately 1 or slightly less. This is consistent with the claim that some particles are buried in fine particles of the subsurface origin included in ejecta from the SCI crater. Thus, the particle size distributions inside and outside the SCI crater reveal that the subsurface layer beneath the SCI impact site is rich in fine particles with – 9 cm in size while the particles on the surface have a size distribution of a power-law form with shallower slopes at small sizes due to the deposition of fine ejecta from the subsurface layer. Finally, we discuss a process responsible for this difference in particle size distribution between the surface and the subsurface layers. The occurrence of segregation in the gravitational flow of particles on the surface of Ryugu is plausible. Graphical Abstract

Context. After landing on C-type asteroid Ryugu, MASCOT imaged brightly colored, submillimeter-sized inclusions in a small rock. Hayabusa2 successfully returned a sample of small particles from the surface of Ryugu, but none of these appear to harbor such inclusions. The samples are considered representative of Ryugu. Aims. To understand the apparent discrepancy between MASCOT observations and Ryugu samples, we assess whether the MASCOT landing site, and the rock by implication, is perhaps atypical for Ryugu. Methods. We analyzed observations of the MASCOT landing area acquired by three instruments on board Hayabusa2: a camera (ONC), a near-infrared spectrometer (NIRS3), and a thermal infrared imager. We compared the landing area properties thus retrieved with those of the average Ryugu surface. Results. We selected several areas and landforms in the landing area for analysis: a small crater, a collection of smooth rocks, and the landing site itself. The crater is relatively blue and the rocks are relatively red. The spectral and thermophysical properties of the landing site are very close to those of the average Ryugu surface. The spectral properties of the MASCOT rock are probably close to average, but its thermal inertia may be somewhat higher. Conclusions. The MASCOT rock can also be considered representative of Ryugu. Some of the submillimeter-sized particles in the returned samples stand out because of their atypical spectral properties. Such particles may be present as inclusions in the MASCOT rock.

<title>Abstract</title>The near-Earth asteroid (162173) Ryugu, the target of Hayabusa2 space mission, was observed via both orbiter and the lander instruments. The infrared radiometer on the MASCOT lander (MARA) is the only instrument providing spectrally resolved mid-infrared (MIR) data, which is crucial for establishing a link between the asteroid material and meteorites found on Earth. Earlier studies revealed that the single boulder investigated by the lander belongs to the most common type found on Ryugu. Here we show the spectral variation of Ryugu’s emissivity using the complete set of in-situ MIR data and compare it to those of various carbonaceous chondritic meteorites, revealing similarities to the most aqueously altered ones, as well as to asteroid (101955) Bennu. The results show that Ryugu experienced strong aqueous alteration prior to any dehydration.