arXiv subjects

Seiji Sugita

Publications and source records attributed to Seiji Sugita.

At least 19 recordsLinked to original sources

arXiv2026

The Asteroid Framing Cameras on ESA's Hera mission

As the first asteroid deflection test, NASA's successfully hit asteroid Dimorphos (secondary of the binary asteroid 65803 Didymos) with the DART kinetic impactor on September 26, 2022. To fully characterise the physical properties of the objects, and measure precisely the effects of this impact in the context of planetary defence, ESA launched the Hera mission on 7 October 2024, with scheduled arrival at Didymos in fall 2026. Among the core payload of the mission, the Asteroid Framing Cameras are two identical imaging systems that will support navigation and scientific activities, by acquiring images from various distances and observing geometries during the course of the mission. Built by ena-Optronik (Germany), the cameras match the requirements designed by the science team and will provide data that supports a wide range of investigations: hazard detection, system dynamics, shape reconstruction, surface morphology and mapping, and surface photometry. Each instrument is a panchromatic camera equipped with a 5.5 x 5.5 degree field of view, and an angular resolution of 93.7 micro-radians per pixel. The cameras shall provide the necessary data to address the mission requirements through a global mapping of the two components of the binary system at spatial scales of 2-3 m/pixel in the Early Characterisation Phase, 1-2 m/pixel in the Detailed Characterisation Phase, and 0.5-2 m/pixel in the Close Operation Phase. Dedicated flybys will bring the resolution down to < 10 cm/pixel on specific areas of interest on Dimorphos, such as the DART impact site and the JUVENTAS cubesat landing site. Here, we present the technical specifications of the camera, as well as the status of the calibration. We then summarise the planned operations in cruise and at the asteroids. Finally, we provide examples of the scientific investigations and products that will make use of the data returned by the cameras

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arXiv2026

Characteristics of natural remanence records in fine-grained particles returned from asteroid Ryugu

Particles collected from the asteroid Ryugu by the Hayabusa2 spacecraft offer a unique opportunity to investigate the magnetic record of the primitive solar system, as any terrestrial magnetic contamination is minimal and can be accounted for. In previous studies, stepwise alternating field demagnetization (AFD) measurements of natural remanent magnetization (NRM) records have been conducted on seven Ryugu particles. However, due to the limited number of samples, there is no consensus regarding the interpretation of the results of these measurements. To address this problem, we performed stepwise AFD measurements of the NRM on 28 Ryugu particles. Twenty-three of the particles exhibited one or two stable NRM components, whereas the remaining five did not. Isothermal remanent magnetization-based paleointensity values derived from stable NRM components varied by more than one order of magnitude. These NRM characteristics were consistent with those observed in previous studies. Therefore, as a reflection of the original nature of the NRM record, some Ryugu particles exhibited stable NRM components, whereas others did not. The Ryugu particles investigated in this study and those from a previous study exhibited spatially inhomogeneous NRM directions within individual particles, constraining the NRM acquisition time to before the final solidification of the current Ryugu particles. A mechanism of remanence acquisition that can explain the observed NRM characteristics is a chemical remanent magnetization associated with the growth of framboidal magnetite during aqueous alteration in Ryugu's parent body.

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arXiv2025

Raman Spectroscopy of Salt Deposits from the Simulated Subsurface Ocean of Enceladus

Saturn's ice-covered moon Enceladus may host a subsurface ocean with biologically relevant chemistry. Plumes released from this ocean preserve information on its chemical state, and previous analyses suggest weakly to strongly alkaline pH (approximately pH 8--12). Constraining the pH requires identification of pH-sensitive minerals in plume deposits. Several analytical techniques could provide such mineralogical information, but few are practical for deployment on planetary missions. Raman spectrometers, which have recently advanced for \textit{in situ} exploration and have been incorporated into flight instruments, offer a feasible approach for mineral identification on icy moons. However, their applicability to pH estimation from plume-derived minerals has not been investigated. In this study, we evaluate whether Raman measurements of plume particles deposited on the surface of Enceladus can be used to distinguish between weakly and strongly alkaline subsurface ocean models. Fluids with pH values of 9 and 11 were frozen under vacuum conditions analogous to those on Enceladus. The resulting salt deposits were then analyzed using a flight-like Raman spectrometer. The Raman spectra show pH-dependent carbonate precipitation: NaHCO$_3$ and Na$_2$CO$_3$ peaks were detected at pH 9, whereas only Na$_2$CO$_3$ peaks were detected at pH 11. These findings demonstrate that Raman spectroscopy can distinguish pH-dependent carbonate phases. This capability allows us to constrain whether the pH of the subsurface ocean is weakly alkaline or strongly alkaline, which is a key parameter for assessing its chemical evolution and potential habitability.

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arXiv2025

HyperScout-H: the hyperspectral imager for the ESA Hera mission

The HyperScout-H (HS-H) instrument is one of the payloads aboard ESA's Hera spacecraft. Hera is a planetary defence mission that aims to provide a detailed characterization of the near-Earth binary asteroid (65803) Didymos-Dimorphos after the NASA/DART mission impact. HS-H is a versatile dual-use payload, functioning as a hyperspectral imager that captures both images and spectral data within the 0.65--0.95 $μ$m wavelength range. The observations from this instrument will offer key insights regarding the composition of the two bodies Didymos and Dimorphos, space weathering effects, and the potential presence of exogenous material on these asteroids. Thanks to its wide field of view ($\approx 15.5^\circ \times 8.3^\circ$ in paraxial approximation), HS-H will be able to monitor the system's orbital dynamic and dust environment. At the same time, both components of this binary asteroid remain in the field of view for most of the asteroid phase of the mission. These results also complement the data obtained from other instruments in characterizing the geomorphological units. The data that will be obtained by HS-H will enable the creation of maps highlighting key spectral features, such as taxonomic classification, spectral slope, and band parameters. This article presents the pre-flight calibration of the instrument, outlines the science objectives, and discusses the expected investigations. The instrument's capabilities are demonstrated through laboratory observations of two meteorite samples and a dedicated software toolbox was developed specifically for processing the instrument's data.

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arXiv2025

Demonstrating Exoplanet Transit Photometry from Space with a 15-mm Aperture Optical Navigation Camera on Hayabusa2

Observations of exoplanet transits by small satellites have gained increasing attention for reducing detection biases. However, no unambiguous detection of an exoplanet has yet been demonstrated using optics with apertures smaller than 60 mm. Here, we investigated the detectability of exoplanet transits using the telescopic Optical Navigation Camera (ONC-T) onboard the Hayabusa2 spacecraft, which has an effective aperture of only 15 mm. We conducted transit observations of the hot Jupiters WASP-189 b and MASCARA-1 b, collecting data for ten and four events, respectively. The transit signal was detected with a signal-to-noise ratio (SNR) of 13 for WASP-189 b and 8 for MASCARA-1 b for each event. Stacking all events improved the SNR to 40 and 16, respectively. The transit mid-times of each event were measured with a precision of 6 minutes and were consistent with Transiting Exoplanet Survey Satellite (TESS) data to within 2 minutes. The planet-to-star radius ratio was determined with an absolute precision of 0.004 (6% relative) and agreed with TESS results to within 0.002 (3% relative). The recent ONC-T and TESS data enabled an update to the planetary ephemerides. We report a 4 sigma discrepancy between the updated orbital period of MASCARA-1 b and previously reported values. ONC-T sets a new record for the smallest-aperture instrument to detect an exoplanet transit from space, advancing the frontier of exoplanet science with miniature instrumentation. Our results suggest that optics as small as ONC-T may be capable of detecting transiting long-period Jupiters: a population that remains underrepresented in current surveys.

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arXiv2024

Analytical capability of K-Ar isochron dating on Mars: assessment from mineral compositions of Martian meteorites

Dating rocks with a precision of $\pm$200 Myr (2$σ$) has been required to understand the Martian habitability and volcanic history around 4000 Myr ago. In situ potassium-argon (K-Ar) dating techniques employing spot-by-spot laser ablation have been developed for isochron dating on Mars. The precision of isochron ages is predominantly determined by the relationship between the laser spot diameter and the mineral grain size. However, the achievable age precisions for the realistic mineralogy of Martian rocks were not investigated yet. This study simulates isochrons under various conditions, including laser spot size, measurement errors for K and Ar, the number of isochron points, and the mineral compositions of representative Martian meteorites (NWA 817, Zagami, and NWA 1068) analyzed with an electron microprobe. We find that attaining the 200 Myr precision necessitates an isochron data range wider than 6, a laser spot diameter of 250 $μ$m, and measurement errors of 10% for both K and Ar. Furthermore, minimizing the variance in measurement errors between K and Ar is essential for increasing age accuracy. This investigation demonstrates that the required precision for Mars missions is achievable with realistic instrument settings, thus supporting the viability of in situ K-Ar geochronology on Mars.

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arXiv2023

Heliocentric Distance Dependence of Zodiacal Light Observed by Hayabusa2#

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 $n(r) \sim r^{-α}$, and the exponent $α\sim 1.3$ 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 $α$ 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 $α= 1.30 \pm 0.08$, 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).

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arXiv2022

Characterization of the MASCOT landing area by Hayabusa2

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 (TIR). 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.

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arXiv2022

In-situ wavelength calibration without a calibration target: use of Fraunhofer lines after landing on planetary surface

Accurate wavelength calibration is critical for qualitative and quantitative spectroscopic measurements. Many spectrometers for planetary exploration are equipped with onboard calibration sources. However, such calibration sources are not always available because planetary lander missions often have strong limitations in size and mass. In this study, we propose and validate a wavelength calibration method using solar Fraunhofer lines observed in reflective spectra. As a result, for a visible Raman spectrometer, the accuracy is better than 0.6 cm-1 in 0-4000 cm-1 range, and the magnesium abundance of olivine is estimated more accurately than 2%.

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arXiv2022

The spatial distribution of impact craters on Ryugu

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 E - 290 E) than in the region around the meridian (300 E - 30 E). This variation implies a complicated geologic history for Ryugu. It seems that the longitudinal variation in crater density simply indicates variation in the crater ages; the cratered terrain around the meridian seems to be geologically old while the western bulge is relatively young. The latitudinal variation in crater density suggests that the equatorial ridge of Ryugu is a geologically old structure; however, this could be alternatively explained by a collision with many fission fragments during a short rotational period of Ryugu in the past.

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arXiv2022

Development of a multispectral stereo-camera system comparable to Hayabusa2 Optical Navigation Camera (ONC-T) for observing samples returned from asteroid (162173) Ryugu

Hayabusa2 collected 5.4 g of samples from the asteroid (162173) Ryugu and brought them back to Earth. Obtaining multiband images of these samples with the spectral bands comparable to those used for remote-sensing observations is important for characterizing the collected samples and examining how representative the sample is compared with spacecraft observations of Ryugu as a whole. In this study, we constructed a multiband microscopic camera system that enables both visual multispectral imaging at 390 (ul), 475 (b), 550 (v), 590 (Na), 700 (w), and 850 nm (x), and three-dimensional (3D) shape reconstruction of individual grain samples based on stereo imaging. The imaging system yields the images of 4096 x 2160 pixels with the pixel resolution of 1.93 μm/pix and field of view of 7.9 x 4.2 mm. Our validation measurements demonstrate that our multispectral imaging system, which observes the samples with the spectral bands comparable to those on the telescopic optical navigation camera (ONC-T) on Hayabusa2, yields reflectance spectra with a relative error of 3% and a 3D model with an error of 5%. These results indicate that the multiband imaging system with a 3D shape reconstruction capability yields accurate spectral and shape data on the returned samples. Using this instrument, we conducted multispectral measurements of two Ryugu samples (grains in the dishes A3 and C1) acquired from two locations on the asteroid. The average spectra of the measured Ryugu samples were flat and consistent with the global averaged spectrum of Ryugu. The 550-nm band (v-band) reflectance of the returned grains in the dishes was 2.4% on average, higher than that of the global averaged spectrum of Ryugu observed with ONC-T. This apparent difference could be because the returned grains have greater specular reflectance.

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arXiv2021

Resurfacing processes constrained by crater distribution on Ryugu

Understanding the geological modification processes on asteroids can provide information concerning their surface history. Images of small asteroids from spacecraft show a depletion in terms of smaller craters. Seismic shaking was considered to be responsible for crater erasure and the main driver modifying the geology of asteroids via regolith convection or the Brazil nut effect. However, a recent artificial impact experiment on the asteroid Ryugu by the Japanese Hayabusa2 mission revealed minimal seismic activity. To investigate whether a seismic shaking model can reproduce the observed crater record, the crater distribution on Ryugu was analyzed using crater production functions under cohesionless conditions. Crater retention ages were estimated as a function of crater diameter for Ryugu, Itokawa, Eros, and Bennu using the crater size-frequency distribution and crater production function estimated for those asteroids. We found that the power-law indices "a" are inconsistent with diffusion processes (e.g., seismic shaking, a=2). This result suggests that seismic shaking models based on diffusion equations cannot explain the crater distribution on small asteroids. Alternative processes include surface flows, possibly at the origin of geomorphological and spectral features of Ryugu. We demonstrate that the vertical mixing of material at depths shallower than 1 m occurs over 10^3-10^5 yr by cratering and obliteration. The young surface age of Ryugu is consistent with the slow space weathering that results from cratering, as suggested in previous studies. The timescale (10^4-10^6 yr) required for resurfacing at depths of 2-4 m can be compared with the cosmic-ray exposure ages of returned samples to constrain the distribution of impactors that collide with Ryugu.

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arXiv2021

Image registration for multi-band images taken by ONC-T onboard Hayabusa2

Hayabusa2, a Japanese sample-return mission to a C-type asteroid, arrived at its target 162173 Ryugu in June 2018. The optical navigation cameras (ONC-T, ONC-W1, ONC-W2) successfully obtained numerous images of Ryugu. ONC-T is a telescopic framing camera with a charge-coupled device (CCD), has seven filter bands in ultraviolet, visible and near infrared wavelength ranges, and were used to map the spectral distribution of the Ryugu surface. Since the locations of a target seen in ONC-T images are slightly different among different wavelength images in one multi-band observation sequence due to changes in spacecraft positions and attitudes during the filter-changing sequence, one of the image processing issues is image co-registration among images for different wavelength bands. To quickly complete the image co-registration to meet a limited mission schedule, we combined conventional image co-registration techniques with several improvements based on previous planetary missions. The results of our analysis using actual ONC-T images indicate that image co-registration can reach accuracy on the order of 0.1 pixels, which is sufficient for many spectral mapping applications for Ryugu analyses.

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arXiv2021

High-Resolution Observations of Bright Boulders on Asteroid Ryugu: 1. Size Frequency Distribution and Morphology

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., 2021) 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. The size-frequency distributions of S- and C-type bright boulders follow power laws. 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 surface. Also, many of the bright boulders are embedded in a larger substrate boulder, suggesting that they have experienced mixing and conglomeration with darker fragments on the parent body, rather than gently landing on Ryugu during or after its formation by reaccumulation. This is consistent with the hypothesis that S-type bright boulders were likely mixed during and/or before a catastrophic disruption.

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arXiv2021

High-Resolution Observations of Bright Boulders on Asteroid Ryugu: 2. Spectral Properties

Many small boulders with reflectance values higher than 1.5 times the average reflectance have been found on the near-Earth asteroid 162173 Ryugu. Based on their visible wavelength spectral differences, Tatsumi et al. (2021) defined two bright boulder classes: C-type and S-type. These two classifications of bright boulders have different size distributions and spectral trends. In this study, we measured the spectra of 79 bright boulders and investigated their detailed spectral properties. Analyses obtained a number of important results. First, S-type bright boulders on Ryugu have spectra that are similar to those found for two different ordinary chondrites with different initial spectra that have been experimentally space weathered the same way. This suggests that there may be two populations of S-type bright boulders on Ryugu, perhaps originating from two different impactors that hit its parent body. Second, the model space-weathering ages of meter-size S-type bright boulders, based on spectral change rates derived in previous experimentally irradiated ordinary chondrites, are 0.1-1 Myr, which is consistent with the crater retention age (<Myr) of the ~1-m deep surface layer on Ryugu. This agreement strongly suggests that the Ryugu surface is extremely young, implying that the samples acquired from the Ryugu surface should be fresh. Third, the lack of a serpentine absorption in the S-type clast embedded in one of the large brecciated boulders indicates that fragmentation and cementation that created the breccias occurred after the termination of aqueous alteration. Fourth, C-type bright boulders exhibit a continuous spectral trend similar to the heating track of low-albedo carbonaceous chondrites, such as CM and CI. Other processes, such as space weathering and grain size effects, cannot primarily account for their spectral variation.

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arXiv2020

Spectrophotometric analysis of the Ryugu rock seen by MASCOT: Searching for a carbonaceous chondrite analog

We analyze images of a rock on Ryugu acquired in situ by MASCam, camera of the MASCOT lander, with the aim of identifying possible carbonaceous chondrite (CC) analogs. The rock's reflectance ($r_{\rm F} = 0.034 \pm 0.003$ at phase angle $4.5^\circ \pm 0.1^\circ$) is consistent with Ryugu's average reflectance, suggesting that the rock is typical for this asteroid. A spectrophotometric analysis of the rock's inclusions provides clues to CC group membership. Inclusions are generally brighter than the matrix. The dominant variation in their color is a change of the visible spectral slope, with many inclusions being either red or blue. Spectral variation in the red channel hints at the presence of the 0.7~$μ$m absorption band linked to hydrated phyllosilicates. The inclusions are unusually large for a CC; we find that their size distribution may best match that of the Renazzo (CR2) and Leoville (CV3) meteorites. The Ryugu rock does not easily fit into any of the CC groups, consistent with the idea that typical Ryugu-type meteorites are too fragile to survive atmospheric entry.

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arXiv2020

Spin-driven evolution of asteroids' top-shapes at fast and slow spins seen from (101955) Bennu and (162173) Ryugu

Proximity observations by OSIRIS-REx and Hayabusa2 provided clues on the shape evolution processes of the target asteroids, (101955) Bennu and (162173) Ryugu. Their oblate shapes with equatorial ridges, or the so-called top shapes, may have evolved due to their rotational conditions at present and in the past. Different shape evolution scenarios were previously proposed; Bennu's top shape may have been driven by surface processing, while Ryugu's may have been developed due to large deformation. These two scenarios seem to be inconsistent. Here, we revisit the structural analyses in earlier works and fill a gap to connect these explanations. We also apply a semi-analytical technique for computing the cohesive strength distribution in a uniformly rotating triaxial ellipsoid to characterize the global failure of top-shaped bodies. Assuming that the structure is uniform, our semi-analytical approach describes the spatial variations in failed regions at different spin periods; surface regions are the most sensitive at longer spin periods, while interiors fail structurally at shorter spin periods. This finding suggests that the shape evolution of a top shape may vary due to rotation and internal structure, which can explain the different evolution scenarios of Bennu's and Ryugu's top shapes. We interpret our results as the indications of top shapes' various evolution processes.

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arXiv2019

The Western Bulge of 162173 Ryugu Formed as a Result of a Rotationally Driven Deformation Process

162173 Ryugu, the target of Hayabusa2, has a round shape with an equatorial ridge, which is known as a spinning top-shape. A strong centrifugal force is a likely contributor to Ryugu's top-shaped features. Observations by Optical Navigation Camera onboard Hayabusa2 show a unique longitudinal variation in geomorphology; the western side of this asteroid, later called the western bulge, has a smooth surface and a sharp equatorial ridge, compared to the other side. Here, we propose a structural deformation process that generated the western bulge. Applying the mission-derived shape model, we employ a finite element model technique to analyze the locations that experience structural failure within the present shape. Assuming that materials are uniformly distributed, our model shows the longitudinal variation in structurally failed regions when the spin period is shorter than ~3.75 h. Ryugu is structurally intact in the subsurface region of the western bulge while other regions are sensitive to structural failure. We infer that this variation is indicative of the deformation process that occurred in the past, and the western bulge is more relaxed structurally than the other region. Our analysis also shows that this deformation process might occur at a spin period between ~3.5 h and ~3.0 h, providing the cohesive strength ranging between ~4 Pa and ~10 Pa.

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