Masahiko HAYAKAWA

Abstract Diffuse Galactic light (DGL) is starlight scattered by interstellar dust. In visible wavelengths, earlier studies observed DGL toward regions of low optical depth in high Galactic latitude, and show marginal consistency with a theoretical model assuming single scattering by dust grains. However, a model for DGL in regions of high optical depth has not been established. In this study, we analyze wide-field imaging data toward a region of high optical depth near the Galactic center, which was obtained with the Optical Navigation Camera on board the Hayabusa2 spacecraft. The data are reduced by dark-current and stray-light subtraction, flat-field correction, and sensitivity calibration for the DGL measurement. In the image, we select dark low-intensity areas where background starlight is highly absorbed by interstellar dust, and extract the DGL component by masking pixels contaminated by stars. As a result, we find that the DGL intensity decreases toward high optical depth, and this trend is reversed from the previous measurements in optically thin regions. To explain the observed trend, we introduce DGL models inferred from a radiative transfer equation in a plane-parallel dusty slab. By assuming literature values for the albedo and scattering asymmetry factor of interstellar dust, the measured DGL intensity can be fitted by a model in which a dust slab without internal emitters is illuminated by backside stars.

++ 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.