arXiv subjects

Yasuhiro Yokota

Publications and source records attributed to Yasuhiro Yokota.

11 recordsLinked to original sources

arXiv2026

Updated in-flight calibration of the Hayabusa2/NIRS3 spectrometer: new global near-infrared photometric properties of asteroid (162173) Ryugu

The Near InfraRed Spectrometer (NIRS3) onboard Hayabusa2 observed asteroid (162173) Ryugu, searching especially for signatures of water-bearing minerals. However, during the mission, absolute reflectance measured by the Telescopic Optical Navigation Camera (ONC-T) and NIRS3 showed a systematic offset. Additionally, touchdown operations have modified the instrumental response, which has made post-second-touchdown data less reliable. Correction of these issues is crucial to interpret the subtle spectroscopic differences across the surface, better understand the origin and evolution of carbonaceous asteroids, and support the Hayabusa2$\sharp$ mission's future observations of the asteroids (98943) Torifune and 1998 KY26. We used Ryugu observations at different mission phases for relative calibrations and lunar observations for the absolute calibration. Moon spectra acquired during the first Earth swing-by were compared to simulated ones from photometric models based on lunar orbiter and ground-based data. We then applied the updated calibration to the full Hayabusa2 proximity phase dataset, enabling disk-resolved global and regional photometric analyses. We showed that the data obtained following the first and second touchdowns required downscaled corrections of $5.4 \pm 1.8 $% and $10.3 \pm 0.4 $% at 2 $\mu m$, respectively, while the absolute calibration revealed that NIRS3 reflectance was underestimated by about 8% compared with the original calibration. Our new photometric parameters are consistent with previous studies and indicate a weak east-west dichotomy in phase ratio and phase reddening, pointing to higher surface roughness and higher abundance of fine-grained regolith in Ryugu eastern hemisphere.

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arXiv2026

Overview of Hayabusa2 extended mission's flyby of Near-Earth Asteroid (98943) Torifune

The Hayabusa2 extended mission, nicknamed Hayabusa2# (# is pronounced SHARP, which stands for the Small Hazardous Asteroid Reconnaissance Probe), is JAXA's small body explorer to conduct science and engineering investigations in space. After the successful return to the Earth with the samples from the carbonaceous asteroid (162173) Ryugu on December 6, 2020, Hayabusa2 diverted away from Earth to start its decade-long extended mission. The major scope includes engineering demonstration of long-term maintenance strategies for spacecraft and operation systems and scientific investigations during various mission phases. Major scientific investigations include spacecraft-based telescopic observations of exoplanets and zodiacal dust observations during the cruise phase, flyby observations of the near-Earth asteroid (98943) Torifune in July 2026, and rendezvous observations of near-Earth asteroid 1998 KY26 in 2031. This study overviews Hayabusa2#'s flyby and the physical properties of Torifune. Although the flyby operation planning is still ongoing, the mission will attempt to fly by the target at a distance (from the asteroid's center) of ~1-10 km. The flyby speed is planned to be 5.25 km/s, while the encounter location is 0.81 au from the sun. The mission plans to fix the spacecraft's orientation during the flyby, only allowing for a very limited pointing change to attain higher resolution imaging. The mission will attempt to obtain science and engineering returns during the flyby. The planned investigations will offer stronger insights into material transport mechanisms in the inner solar system and a demonstration of planetary defense technologies.

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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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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

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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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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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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arXiv2018

Updated Inflight Calibration of Hayabusa2's Optical Navigation Camera (ONC) for Scientific Observations during the Cruise Phase

The Optical Navigation Camera (ONC-T, ONC-W1, ONC-W2) onboard Hayabusa2 are also being used for scientific observations of the mission target, C-complex asteroid 162173 Ryugu. Science observations and analyses require rigorous instrument calibration. In order to meet this requirement, we have conducted extensive inflight observations during the 3.5 years of cruise after the launch of Hayabusa2 on 3 December 2014. In addition to the first inflight calibrations by Suzuki et al. (2018), we conducted an additional series of calibrations, including read-out smear, electronic-interference noise, bias, dark current, hot pixels, sensitivity, linearity, flat-field, and stray light measurements for the ONC. Moreover, the calibrations, especially flat-fields and sensitivities, of ONC-W1 and -W2 are updated for the analysis of the low-altitude (i.e., high-resolution) observations, such as the gravity measurement, touchdowns, and the descents for MASCOT and MINERVA-II payload releases. The radiometric calibration for ONC-T is also updated in this study based on star and Moon observations. Our updated inflight sensitivity measurements suggest the accuracy of the absolute radiometric calibration contains less than 1.8% error for the ul-, b-, v-, Na-, w-, and x-bands based on star calibration observations and ~5% for the p-band based on lunar calibration observations. The radiance spectra of the Moon, Jupiter, and Saturn from the ONC-T show good agreement with the spacecraft-based observations of the Moon from SP/SELENE and WAC/LROC and with ground-based telescopic observations for Jupiter and Saturn.

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arXiv2009

The Hayabusa Spacecraft Asteroid Multi-Band Imaging Camera: AMICA

The Hayabusa Spacecraft Asteroid Multiband Imaging Camera (AMICA) has acquired more than 1400 multispectral and high-resolution images of its target asteroid, 25143 Itokawa, since late August 2005. In this paper, we summarize the design and performance of AMICA. In addition, we describe the calibration methods, assumptions, and models, based on measurements. Major calibration steps include corrections for linearity and modeling and subtraction of bias, dark current, read-out smear, and pixel-to-pixel responsivity variations. AMICA v-band data were calibrated to radiance using in-flight stellar observations. The other band data were calibrated to reflectance by comparing them to ground-based observations to avoid the uncertainty of the solar irradiation in those bands. We found that the AMICA signal was linear with respect to the input signal to an accuracy of << 1% when the signal level was < 3800 DN. We verified that the absolute radiance calibration of the AMICA v-band (0.55 micron) was accurate to 4% or less, the accuracy of the disk-integrated spectra with respect to the AMICA v-band was about 1%, and the pixel-to-pixel responsivity (flatfield) variation was 3% or less. The uncertainty in background zero-level was 5 DN. From wide-band observations of star clusters, we found that the AMICA optics have an effective focal length of 120.80 \pm 0.03 mm, yielding a field-of-view (FOV) of 5.83 deg x 5.69 deg. The resulting geometric distortion model was accurate to within a third of a pixel. We demonstrated an image-restoration technique using the point-spread functions of stars, and confirmed that the technique functions well in all loss-less images. An artifact not corrected by this calibration is scattered light associated with bright disks in the FOV.

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