Faculty Profiles - WATANABE, Sei-ichiro

Papers - WATANABE, Sei-ichiro

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Characterization of the Ryugu surface by means of the variability of the near-infrared spectral slope in NIRS3 data Reviewed International coauthorship

Galiano A, Palomba E, D'Amore M, Zinzi A, Dirri F, Longobardo A, Kitazato K, Iwata T, Matsuoka M, Hiroi T, Takir D, Nakamura T, Abe M, Ohtake M, Matsuura S, Watanabe S, Yoshikawa M, Saiki T, Tanaka S, Okada T, Yamamoto Y, Takei Y, Shirai K, Hirata N, Matsumoto K, Tsuda Y

ICARUS Vol. 351 page： 113959 - 113959 2020.11

Author Correction: Shape of (101955) Bennu indicative of a rubble pile with internal stiffness (Nature Geoscience, (2019), 12, 4, (247-252), 10.1038/s41561-019-0330-x) Reviewed International coauthorship International journal Open Access

O. S. Barnouin, M. G. Daly, E. E. Palmer, R. W. Gaskell, J. R. Weirich, C. L. Johnson, M. M. Al Asad, J. H. Roberts, M. E. Perry, H. C.M. Susorney, R. T. Daly, E. B. Bierhaus, J. A. Seabrook, R. C. Espiritu, A. H. Nair, L. Nguyen, G. A. Neumann, C. M. Ernst, W. V. Boynton, M. C. Nolan, C. D. Adam, M. C. Moreau, B. Rizk, C. Y. Drouet D’Aubigny, E. R. Jawin, K. J. Walsh, P. Michel, S. R. Schwartz, R. L. Ballouz, E. M. Mazarico, D. J. Scheeres, J. W. McMahon, W. F. Bottke, S. Sugita, N. Hirata, N. Hirata, S. I. Watanabe, K. N. Burke, D. N. DellaGiustina, C. A. Bennett, D. S. Lauretta, D. E. Highsmith, J. Small, D. Vokrouhlický, N. E. Bowles, E. Brown, K. L. Donaldson Hanna, T. Warren, C. Brunet, R. A. Chicoine, S. Desjardins, D. Gaudreau, T. Haltigin, S. Millington-Veloza, A. Rubi, J. Aponte, N. Gorius, A. Lunsford, B. Allen, J. Grindlay, D. Guevel, D. Hoak, J. Hong, D. L. Schrader, J. Bayron, O. Golubov, P. Sánchez, J. Stromberg, M. Hirabayashi, C. M. Hartzell, S. Oliver, M. Rascon, A. Harch, J. Joseph, S. Squyres, D. Richardson, J. P. Emery, L. McGraw, R. Ghent, R. P. Binzel, M. M.Al Asad, C. L. Johnson, L. Philpott, H. C.M. Susorney, E. A. Cloutis, R. D. Hanna, H. C. Connolly, F. Ciceri, A. R. Hildebrand, E. M. Ibrahim, L. Breitenfeld, T. Glotch, A. D. Rogers, B. E. Clark, S. Ferrone, C. A. Thomas, H. Campins, Y. Fernandez, W. Chang, A. Cheuvront

Nature Geoscience Vol. 13 ( 11 ) page： 764 2020.11

Hayabusa2 Landing Site Selection: Surface Topography of Ryugu and Touchdown Safety Reviewed International coauthorship

Shota Kikuchi, Sei-ichiro Watanabe, Takanao Saiki, Hikaru Yabuta, Seiji Sugita, Tomokatsu Morota, Naru Hirata, Naoyuki Hirata, Tatsuhiro Michikami, Chikatoshi Honda, Yashuhiro Yokota, Rie Honda, Naoya Sakatani, Tatsuaki Okada, Yuri Shimaki, Koji Matsumoto, Rina Noguchi, Yuto Takei, Fuyuto Terui, Naoko Ogawa, Kent Yoshikawa, Go Ono, Yuya Mimasu, Hirotaka Sawada, Hitoshi Ikeda, Chikako Hirose, Tadateru Takahashi, Atsushi Fujii, Tomohiro Yamaguchi, Yoshiaki Ishihara, Tomoki Nakamura, Kohei Kitazato, Koji Wada, Shogo Tachibana, Eri Tatsumi, Moe Matsuoka, Hiroki Senshu, Shingo Kameda, Toru Kouyama, Manabu Yamada, Kei Shirai, Yuichiro Cho, Kazunori Ogawa, Yukio Yamamoto, Akira Miura, Takahiro Iwata, Noriyuki Namiki, Masahiko Hayakawa, Masanao Abe, Satoshi Tanaka, Makoto Yoshikawa, Satoru Nakazawa, Yuichi Tsuda

Space Science Reviews Vol. 216 ( 7 ) page： 116 2020.10

Mission Status of Hayabusa2, Science Highlights, and Outlook for Sample Analyses

Seiji Sugita, Rie Honda, Tomokatsu Morota, Shingo Kameda, Eri Tatsumi, Shogo Tachibana, Kohei Kitazato, Tatsuaki Okada, Noriyuki Namiki, Masahiko Arakawa, Patrick Michel, Deborah Domingue, Satoshi Tanaka, Makoto Yoshikawa, Sei-ichiro Watanabe, Yuichi Tsuda

2020.10

Publisher：Copernicus GmbH

&lt;p&gt;JAXA&amp;#8217;s Hayabusa2 is a sample-return mission was launched on Dec. 3, 2014 for bringing back first samples from a C-complex asteroid [1,2]. It arrived at asteroid Ryugu on June 27, 2018 and left for Earth on Nov. 13, 2019 after conducting global remote-sensing observations, two touchdown sampling operations, rover deployments, and an artificial impact experiment. We review our science results and update the mission status of Hayabusa2 in this presentation.&amp;#160;&lt;/p&gt;
&lt;p&gt;The global observations revealed that Ryugu has a top-shaped body with very low density (1.19&amp;#177;0.02 g/cc) [3], spatially uniform Cb-type spectra without strong Fe-rich serpentine absorption at 0.7-um [4], and a weak but significant OH absorption at 2.7 um [5]. Based on these observations, we proposed that Ryugu materials may have experienced aqueous alteration and subsequent thermal metamorphism due to radiogenic heating [4]. However, other scenarios, such as impact-induced thermal metamorphism and extremely primitive carbonaceous materials before extensive alteration, were also considered because there were many new properties of Ryugu whose origins are unclear. Also, numerical calculations show that impact heating can raise the temperatures high enough to dehydrate serpentine at typical collision speed in the asteroid main belt [6]. &amp;#160;&lt;/p&gt;
&lt;p&gt;Further analysis using high-resolution data obtained at low-altitude descents for both rehearsal and actual touchdown operations as well as the artificial impact experiment by small carryon impactor (SCI) and landers observations the Ryugu surface on allowed us to find out what caused the properties of Ryugu. For example, subtle but distinct latitudinal variation of spectral slope in optical wavelengths found in the initial observations [4] turned out be caused by solar heating or space weathering during orbital excursion toward the Sun and subsequent erosion of the equatorial ridge owing to slowdown in Ryugu&amp;#8217;s spin rate [7]. The SCI impact created a very large (~17 m in crest diameter) crater consistent with gravity-controlled scaling showing that Ryugu surface has very low intra-boulder cohesion and the Ryugu surface is very young and well mixed [8].&lt;/p&gt;
&lt;p&gt;Furthermore, the MASCOT lander also showed that typical boulders on Ryugu is not covered with a layer of fine regolith [9] and yet possess very low thermal inertia (282+93/-35 MKS) consistent with highly porous structure [10]. This value is consistent with the global values or Ryugu [4, 11], suggesting that the vast majority of boulders on Ryugu are very porous. However, thermal infrared imager (TIR) also found that Ryugu has a number of &amp;#8220;dense boulders&amp;#8221; with high thermal inertia (&gt;600 MKS) consistent with typical carbonaceous chondrites, showing that Ryugu&amp;#8217;s parent body must have had a large enough gravity and pressure to compress the constituent materials [11]. This observation supports that Ryugu originated from a large parent body, such as proto-Polana and proto-Eulalia, which are estimated to be ~100 km in diameter.&lt;/p&gt;
&lt;p&gt;Some of the dense boulders were also covered by multi-band images of optical navigation camera (ONC-T) and turned out to have C-type spectra with albedos much higher than the Ryugu average [12]. These spectra and albedos are similar to carbonaceous chondrites heated at low temperatures. Although the total mass of these high-albedo boulders on Ryugu is estimated to be very small (&lt; 1%), the spectral and albedo varieties are much greater than the bulk Ryugu surface and approximately follow the dehydration track of carbonaceous chondrites [12]. These spectral match supports that Ryugu materials experienced aqueous alteration and subsequent thermal metamorphism. The dominance of a high-temperature component and scarcity of lower temperature components are consistent with radiogenic heating in a relatively large parent body because large bodies would have only thin low-temperature thermal skin and large volume of high-temperature interior.&amp;#160;&lt;/p&gt;
&lt;p&gt;If radiogenic heating is really responsible for Ryugu&amp;#8217;s moderate dehydration, this may place a very important constraint on the timing of the formation of Ryugu&amp;#8217;s parent body. Because the radiogenic heat source for most meteorite parent bodies are likely extinct species, such as 26Al, the peak temperature is chiefly controlled by the timing of accretion [13]. Thus, high metamorphism temperatures (several hundred degrees in Celsius) of Ryugu&amp;#8217;s bulk materials inferred from spectral comparison with laboratory heated CM and CI meteorites [4, 12] require Ryugu&amp;#8217;s parent body formed early in the Solar System. Because Ryugu&amp;#8217;s parent body contained substantial amount of water at the time of formation, it must have been formed outside the snowline. Thus, the birth place of Ryugu&amp;#8217;s parent body would be a high-accretion-rate location outside the snowline.&lt;/p&gt;
&lt;p&gt;Recent high-precision measurements of stable isotopes of meteorites have found that there is a major dichotomy between carbonaceous chondrites (CCs) and some iron meteorites, which formed outside Jupiter&amp;#8217;s orbit, and non-carbonaceous meteorites (NCs), which formed inside Jupiter&amp;#8217;s orbit [e.g., 14]. If Ryugu belongs to CCs, then Ryugu materials could be form near Jupiter, where accretion could occur early. Thus, measurements of stable isotopes of elements, such as Cr, Ti and Mo, of Ryugu samples to be returned to Earth by the end of 2020 would be highly valuable for constraining the original locations of Polana or Eulalia, among the largest C-complex asteroids in the inner main belt.&amp;#160;&lt;/p&gt;
&lt;p&gt;&lt;strong&gt;Acknowledgements:&lt;/strong&gt; This study was supported by JSPS Core-to-Core program &amp;#8220;International Network of Planetary Sciences&amp;#8221;, CNES, and Univ. Co?te d&amp;#8217;Azur.&amp;#160;&lt;/p&gt;
&lt;p&gt;&lt;strong&gt;References:&lt;/strong&gt;&amp;#160; [1] Watanabe et al., SSR, 208, 3-16, 2017. [2] Tsuda et at., Acta Astronaut. 91, 356-363, 2013. [3] Watanabe et al., Science, 364, 268-272, 2019. [4] Sugita et al., Science, 364, eaaw0422, 2019. [5] Kitazato et al., Science, 364, 272-275, 2019. [6] Michel et al., Nature Comm., 11, 5184, 2020. [7] Morota et al., Science, 368, 654-659, 2020. [8] Akarawa et al. Science, 368, 67-671, 2020. [9] Jaumann et al. Science, 365, 817-820, 2019.&amp;#160; [10] Grott et al., Nature Astron. 3, 971-976, 2019.&amp;#160; [11] Okada et al., Nature, 579, 518-522, 2020. [12] Sugimoto et al. 51st LPSC, #1770, 2020.&amp;#160; [13] Grimm and McSween, Science, 259, 653-655, 1993.&amp;#160; [14] Kruijer et al., PNAS, 114, 6712-6716, 2017.&amp;#160;&lt;/p&gt;

DOI： 10.5194/epsc2020-995

Hayabusa2's kinetic impact experiment: Operational planning and results Reviewed

Saiki Takanao, Takei Yuto, Mimasu Yuya, Sawada Hirotaka, Ogawa Naoko, Ono Go, Yoshikawa Kent, Terui Fuyuto, Arakawa Masahiko, Sugita Seiji, Watanabe Sei-ichiro, Yoshikawa Makoto, Nakazawa Satoru, Tsuda Yuichi

ACTA ASTRONAUTICA Vol. 175 page： 362 - 374 2020.10

Language：English Publishing type：Research paper (scientific journal) Publisher：PERGAMON-ELSEVIER SCIENCE LTD

Hayabusa2 is a Japanese interplanetary probe launched on December 3, 2014, which arrived at asteroid Ryugu on June 27, 2018. During its stay around Ryugu, it completed several challenging operations, including deploying two rovers and a lander, conducing two sample collections, and performing a kinetic impact experiment. The kinetic impact experiment was one of the biggest challenges of the Hayabusa2 mission. Investigating the physical and chemical properties of asteroid internal materials and structures is an important scientific objective for small body exploration. We developed a small kinetic impactor called the SCI (Small Carry-on Impactor) to achieve this objective. The SCI is a compact kinetic impactor designed to remove a small region of Ryugu's uppermost surface regolith layer and create an artificial crater. The spacecraft deployed the SCI on April 5, 2019, successfully creating an artificial crater with a diameter of 15 m. This paper describes the operational planning of the kinetic impact experiment and summarizes the operation results.

DOI： 10.1016/j.actaastro.2020.05.064

Web of Science

Thermophysical properties of the surface of asteroid 162173 Ryugu: Infrared observations and thermal inertia mapping Reviewed

Yuri Shimaki, Hiroki Senshu, Naoya Sakatani, Tatsuaki Okada, Tetsuya Fukuhara, Satoshi Tanaka, Makoto Taguchi, Takehiko Arai, Hirohide Demura, Yoshiko Ogawa, Kentaro Suko, Tomohiko Sekiguchi, Toru Kouyama, Sunao Hasegawa, Jun Takita, Tsuneo Matsunaga, Takeshi Imamura, Takehiko Wada, Kohei Kitazato, Naru Hirata, Naoyuki Hirata, Rina Noguchi, Seiji Sugita, Shota Kikuchi, Tomohiro Yamaguchi, Naoko Ogawa, Go Ono, Yuya Mimasu, Kent Yoshikawa, Tadateru Takahashi, Yuto Takei, Atsushi Fujii, Hiroshi Takeuchi, Yukio Yamamoto, Manabu Yamada, Kei Shirai, Yu-ichi Iijima, Kazunori Ogawa, Satoru Nakazawa, Fuyuto Terui, Takanao Saiki, Makoto Yoshikawa, Yuichi Tsuda, Sei-ichiro Watanabe

ICARUS Vol. 348 2020.9

Language：Japanese Publishing type：Research paper (scientific journal) Publisher：ACADEMIC PRESS INC ELSEVIER SCIENCE

TIR, the thermal infrared imager on Hayabusa2, acquired high-resolution thermal images of the asteroid 162173 Ryugu for one asteroid rotation period on August 1, 2018 to investigate the thermophysical properties of the asteroid. The surface temperatures of Ryugu suggest that the surface has a low thermal inertia, indicating the presence of porous materials. Thermophysical models that neglect or oversimplify surface roughness cannot reproduce the flat diurnal temperature profiles observed during daytime. We performed numerical simulations of a thermophysical model, including the effects of roughness on the diurnal brightness temperature, the predictions of which successfully reproduced the observed diurnal variation of temperature. The global thermal inertia was obtained with a standard deviation of 225 +/- 45 J m(-2) S-0.5 K-1 which is relatively low but still within the range of the value estimated in our previous study (Okada et al., Nature 579, 518-522, 2020), confirming that the boulders on Ryugu are more porous in nature than typical carbonaceous chondrites. The global surface roughness (the ratio of the variance of the height relative to a local horizontal surface length) was determined as 0.41 +/- 0.08, corresponding to a RMS surface slope of 47 +/- 5 degrees. We identified a slightly lower roughness distributed along the equatorial ridge, implying a mass movement of boulders from the equatorial ridge to the mid-latitudes.

DOI： 10.1016/j.icarus.2020.113835

Web of Science

Dynamic precise orbit determination of Hayabusa2 using laser altimeter (LIDAR) and image tracking data sets Reviewed Open Access

Yamamoto Keiko, Otsubo Toshimichi, Matsumoto Koji, Noda Hirotomo, Namiki Noriyuki, Takeuchi Hiroshi, Ikeda Hitoshi, Yoshikawa Makoto, Yamamoto Yukio, Senshu Hiroki, Mizuno Takahide, Hirata Naru, Yamada Ryuhei, Ishihara Yoshiaki, Araki Hiroshi, Abe Shinsuke, Yoshida Fumi, Higuchi Arika, Sasaki Sho, Oshigami Shoko, Tsuruta Seiitsu, Asari Kazuyoshi, Shizugami Makoto, Ogawa Naoko, Ono Go, Mimasu Yuya, Yoshikawa Kent, Takahashi Tadateru, Takei Yuto, Fujii Atsushi, Yamaguchi Tomohiro, Kikuchi Shota

EARTH PLANETS AND SPACE Vol. 72 ( 1 ) 2020.6

Language：English Publishing type：Research paper (scientific journal) Publisher：SPRINGER

The precise orbit of the Hayabusa2 spacecraft with respect to asteroid Ryugu is dynamically determined using the data sets collected by the spacecraft's onboard laser altimeter (LIght Detection And Ranging, LIDAR) and automated image tracking (AIT). The LIDAR range data and the AIT angular data play complementary roles because LIDAR is sensitive to the line-of-sight direction from Hayabusa2 to Ryugu, while the AIT is sensitive to the directions perpendicular to it. Using LIDAR and AIT, all six components of the initial state vector can be derived stably, which is difficult to achieve using only LIDAR or AIT. The coefficient of solar radiation pressure (SRP) of the Hayabusa2 spacecraft and standard gravitational parameter (GM) of Ryugu can also be estimated in the orbit determination process, by combining multiple orbit arcs at various altitudes. In the process of orbit determination, the Ryugu-fixed coordinate of the center of the LIDAR spot is determined by fitting the range data geometrically to the topography of Ryugu using the Markov Chain Monte Carlo method. Such an approach is effective for realizing the rapid convergence of the solution. The root mean squares of the residuals of the observed minus computed values of the range and brightness-centroid direction of the image are 1.36 m and 0.0270 degrees, respectively. The estimated values of the GM of Ryugu and a correction factor to our initial SRP model are 29.8 +/- 0.3 m(3)/s(2) and 1.13 +/- 0.16, respectively.

DOI： 10.1186/s40623-020-01213-2

Open Access

Web of Science

Hayabusa2 mission status: Landing, roving and cratering on asteroid Ryugu Reviewed

Tsuda Yuichi, Saiki Takanao, Terui Fuyuto, Nakazawa Satoru, Yoshikawa Makoto, Watanabe Sei-ichiro

ACTA ASTRONAUTICA Vol. 171 page： 42 - 54 2020.6

Collisional formation of top-shaped asteroids and implications for the origins of Ryugu and Bennu Reviewed International coauthorship Open Access

Michel P, Ballouz R-L, Barnouin O. S, Jutzi M, Walsh K. J, May B. H, Manzoni C, Richardson D. C, Schwartz S. R, Sugita S, Watanabe S, Miyamoto H, Hirabayashi M, Bottke W. F, Connolly H. C, Yoshikawa M, Lauretta D. S

NATURE COMMUNICATIONS Vol. 11 ( 1 ) 2020.5

Language：English Publishing type：Research paper (scientific journal)

Asteroid shapes and hydration levels can serve as tracers of their history and origin. For instance, the asteroids (162173) Ryugu and (101955) Bennu have an oblate spheroidal shape with a pronounced equator, but contain different surface hydration levels. Here we show, through numerical simulations of large asteroid disruptions, that oblate spheroids, some of which have a pronounced equator defining a spinning top shape, can form directly through gravitational reaccumulation. We further show that rubble piles formed in a single disruption can have similar porosities but variable degrees of hydration. The direct formation of top shapes from single disruption alone can explain the relatively old crater-retention ages of the equatorial features of Ryugu and Bennu. Two separate parent-body disruptions are not necessarily required to explain their different hydration levels.

DOI： 10.1038/s41467-020-16433-z

Improving Hayabusa2 trajectory by combining LIDAR data and a shape model Reviewed

Matsumoto Koji, Noda Hirotomo, Ishihara Yoshiaki, Senshu Hiroki, Yamamoto Keiko, Hirata Naru, Hirata Naoyuki, Namiki Noriyuki, Otsubo Toshimichi, Higuchi Arika, Watanabe Sei-ichiro, Ikeda Hitoshi, Mizuno Takahide, Yamada Ryuhei, Araki Hiroshi, Abe Shinsuke, Yoshida Fumi, Sasaki Sho, Oshigami Shoko, Tsuruta Seiitsu, Asari Kazuyoshi, Shizugami Makoto, Yamamoto Yukio, Ogawa Naoko, Kikuchi Shota, Saiki Takanao, Tsuda Yuichi, Yoshikawa Makoto, Tanaka Satoshi, Terui Fuyuto, Nakazawa Satoru, Yamaguchi Tomohiro, Takei Yuto, Takeuchi Hiroshi, Okada Tatsuaki, Yamada Manabu, Shimaki Yuri, Shirai Kei, Ogawa Kazunori, Iijima Yu-ichi

ICARUS Vol. 338 2020.3

The spatial distribution of impact craters on Ryugu Reviewed Open Access

Hirata Naoyuki, Morota Tomokatsu, Cho Yuichiro, Kanamaru Masanori, Watanabe Sei-ichiro, Sugita Seiji, Hirata Naru, Yamamoto Yukio, Noguchi Rina, Shimaki Yuri, Tatsumi Eri, Yoshioka Kazuo, Sawada Hirotaka, Yokota Yasuhiro, Sakatani Naoya, Hayakawa Masahiko, Matsuoka Moe, Honda Rie, Kameda Shingo, Yamada Manabu, Kouyama Toru, Suzuki Hidehiko, Honda Chikatoshi, Ogawa Kazunori, Tsuda Yuichi, Yoshikawa Makoto, Saiki Takanao, Tanaka Satoshi, Terui Fuyuto, Nakazawa Satoru, Kikuchi Shota, Yamaguchi Tomohiro, Ogawa Naoko, Ono Go, Mimasu Yuya, Yoshikawa Kent, Takahashi Tadateru, Takei Yuto, Fujii Atsushi, Takeuchi Hiroshi, Okada Tatsuaki, Shirai Kei, Iijima Yu-ichi

ICARUS Vol. 338 2020.3

Initial Achievements of Hayabusa2 in Asteroid Proximity Phase Reviewed Open Access

Yuichi TSUDA, Takanao SAIKI, Fuyuto TERUI, Satoru NAKAZAWA, Makoto YOSHIKAWA, Seiichiro WATANABE

TRANSACTIONS OF THE JAPAN SOCIETY FOR AERONAUTICAL AND SPACE SCIENCES Vol. 63 ( 4 ) page： 115 - 123 2020

Design and Reconstruction of the Hayabusa2 Precision Landing on Ryugu Reviewed International coauthorship

Shota Kikuchi, Fuyuto Terui, Naoko Ogawa, Takanao Saiki, Go Ono, Kent Yoshikawa, Yuto Takei, Yuya Mimasu, Hitoshi Ikeda, Hirotaka Sawada, Stefaan Van Wal, Seiji Sugita, Sei-ichiro Watanabe, Yuichi Tsuda

Journal of Spacecraft and Rockets Vol. 57 ( 5 ) page： 1033 - 1060 2020

Improving Hayabusa2 trajectory by combining LIDAR data and a shape model Reviewed

Koji Matsumoto, Hirotomo Noda, Yoshiaki Ishihara, Hiroki Senshu, Keiko Yamamoto, Naru Hirata, Naoyuki Hirata, Noriyuki Namiki, Toshimichi Otsubo, Arika Higuchi, Sei-ichiro Watanabe, Hitoshi Ikeda, Takahide Mizuno, Ryuhei Yamada, Hiroshi Araki, Shinsuke Abe, Fumi Yoshida, Sho Sasaki, Shoko Oshigami, Seiitsu Tsuruta, Kazuyoshi Asari, Makoto Shizugami, Yukio Yamamoto, Naoko Ogawa, Shota Kikuchi, Takanao Saiki, Yuichi Tsuda, Makoto Yoshikawa, Satoshi Tanaka, Fuyuto Terui, Satoru Nakazawa, Tomohiro Yamaguchi, Yuto Takei, Hiroshi Takeuchi, Tatsuaki Okada, Manabu Yamada, Yuri Shimaki, Kei Shirai, Kazunori Ogawa

Icarus Vol. Accepted 2019.11

Boulder size and shape distributions on asteroid Ryugu Reviewed International coauthorship Open Access

Tatsuhiro Michikami, Chikatoshi Honda, Hideaki Miyamoto, Masatoshi Hirabayashi, Axel Hagermann, Terunori Irie, Keita Nomura, Carolyn M. Ernst, Masaki Kawamura, Kiichi Sugimoto, Eri Tatsumi, Tomokatsu Morota, Naru Hirata, Takaaki Noguchi, Yuichiro Cho, Shingo Kameda, Toru Kouyama, Yasuhiro Yokota, Rina Noguchi, Masahiko Hayakawa, Naoyuki Hirata, Rie Honda, Moe Matsuoka, Naoya Sakatani, Hidehiko Suzuki, Manabu Yamada, Kazuo Yoshioka, Hirotaka Sawada, Ryodo Hemmi, Hiroshi Kikuchi, Kazunori Ogawa, Sei-ichiro Watanabe, Satoshi Tanaka, Makoto Yoshikawa, Yuichi Tsuda, Seiji Sugita

Icarus Vol. 331 page： 179 - 191 2019.10

Authorship：Corresponding author Language：English Publishing type：Research paper (scientific journal)

© 2019 The Authors In 2018, the Japanese spacecraft Hayabusa2, arrived at the small asteroid Ryugu. The surface of this C-type asteroid is covered with numerous boulders whose size and shape distributions are investigated in this study. Using a few hundred Optical Navigation Camera (ONC) images with a pixel scale of approximately 0.65 m, we focus on boulders greater than 5 m in diameter. Smaller boulders are also considered using five arbitrarily chosen ONC close-up images with pixel scales ranging from 0.7 to 6 cm. Across the entire surface area (~2.7 km2) of Ryugu, nearly 4400 boulders larger than 5 m were identified. Boulders appear to be uniformly distributed across the entire surface, with some slight differences in latitude and longitude. At ~50 km−2, the number density of boulders larger than 20 m is twice as large as on asteroid Itokawa (or Bennu). The apparent shapes of Ryugu's boulders resemble laboratory impact fragments, with larger boulders being more elongated. The ratio of the total volume of boulders larger than 5 m to the total excavated volume of craters larger than 20 m on Ryugu can be estimated to be ~94%, which is comparatively high. These observations strongly support the hypothesis that most boulders found on Ryugu resulted from the catastrophic disruption of Ryugu's larger parent body, as described in previous papers (Watanabe et al., 2019; Sugita et al., 2019). The cumulative size distribution of boulders larger than 5 m has a power-index of −2.65 ± 0.05, which is comparatively shallow compared with other asteroids visited by spacecraft. For boulders smaller than 4 m, the power-index is even shallower and ranges from −1.65 ± 0.05 to −2.01 ± 0.06. This particularly shallow power-index implies that some boulders are buried in Ryugu's regolith. Based on our observations, we suggest that boulders near the equator might have been buried by the migration of finer material and, as a result, the number density of boulders larger than 5 m in the equatorial region is lower than at higher latitudes.

DOI： 10.1016/j.icarus.2019.05.019

Open Access

Web of Science

Scopus

Images from the surface of asteroid Ryugu show rocks similar to carbonaceous chondrite meteorites Reviewed International coauthorship Open Access

R. Jaumann, N. Schmitz, T. M. Ho, S. E. Schröder, K. A. Otto, K. Stephan, S. Elgner, K. Krohn, F. Preusker, F. Scholten, J. Biele, S. Ulamec, C. Krause, S. Sugita, K. D. Matz, T. Roatsch, R. Parekh, S. Mottola, M. Grott, P. Michel, F. Trauthan, A. Koncz, H. Michaelis, C. Lange, J. T. Grundmann, M. Maibaum, K. Sasaki, F. Wolff, J. Reill, A. Moussi-Soffys, L. Lorda, W. Neumann, J. B. Vincent, R. Wagner, J. P. Bibring, S. Kameda, H. Yano, S. Watanabe, M. Yoshikawa, Y. Tsuda, T. Okada, T. Yoshimitsu, Y. Mimasu, T. Saiki, H. Yabuta, H. Rauer, R. Honda, T. Morota, Y. Yokota, T. Kouyama

Science Vol. 365 ( 6455 ) page： 817 - + 2019.8

Language：English Publishing type：Research paper (scientific journal)

© 2017 The Authors. The near-Earth asteroid (162173) Ryugu is a 900-m-diameter dark object expected to contain primordial material from the solar nebula. The Mobile Asteroid Surface Scout (MASCOT) landed on Ryugu’s surface on 3 October 2018. We present images from the MASCOT camera (MASCam) taken during the descent and while on the surface. The surface is covered by decimeter- to meter-sized rocks, with no deposits of fine-grained material. Rocks appear either bright, with smooth faces and sharp edges, or dark, with a cauliflower-like, crumbly surface. Close-up images of a rock of the latter type reveal a dark matrix with small, bright, spectrally different inclusions, implying that it did not experience extensive aqueous alteration. The inclusions appear similar to those in carbonaceous chondrite meteorites.

DOI： 10.1126/science.aaw8627

Web of Science

Scopus

PubMed

Multivariable statistical analysis of spectrophotometry and spectra of (162173) Ryugu as observed by JAXA Hayabusa2 mission Reviewed International coauthorship Open Access

Barucci M. A, Hasselmann P. H, Fulchignoni M, Honda R, Yokota Y, Sugita S, Kitazato K, Deshapriya J. D. P, Perna D, Tatsumi E, Domingue D, Morota T, Kameda S, Iwata T, Abe M, Ohtake M, Matsuura S, Matsuoka M, Hiroi T, Nakamura T, Kouyama T, Suzuki H, Yamada M, Sakatani N, Honda C, Ogawa K, Hayakawa M, Yoshioka K, Cho Y, Sawada H, Takir D, Vilas F, Hirata N, Hirata N, Tanaka S, Yamamoto Y, Yoshikawa M, Watanabe S, Tsuda Y

ASTRONOMY & ASTROPHYSICS Vol. 629 page： A13 - A13 2019.8

Language：English Publishing type：Research paper (scientific journal) Publisher：EDP Sciences

<italic>Context.</italic> Starting from late June 2018, the JAXA asteroid sample return mission Hayabusa2 acquired a large quantity of resolved images and spectra of the surface of the asteroid (162173) Ryugu.

<italic>Aims.</italic> By studying the visible and near-infrared spectral behavior across the surface of Ryugu using a statistical analysis, we aim to distinguish spectral homogeneous groups and to detect the small heterogeneities. This allows us to better constrain the surface composition variations.

<italic>Methods.</italic> In order to isolate and interpret the difference in the asteroid surface spectral behavior, we applied the <italic>G</italic>-mode multivariate statistical analysis to a set of pixels containing information of (i) the visible ONC-T spectrophotometry, and (ii) the near-infrared NIRS3 spectra thereby obtaining automatic statistical clustering at different confidence levels.

<italic>Results.</italic> The analysis of both ONC-T and NIRS3 data allows us to highlight small spectral variations on the Ryugu surface. At a 3<italic>σ</italic> confidence level, only two groups are evident, while going down to 2<italic>σ</italic> more groups are obtained with differences in spectral slope and band depth.

<italic>Conclusions.</italic> The identified groups have been associated with main morphological surface features. The spectral slope variations that characterize the small groups obtained by ONC-T data analysis, are interpreted as a consequence of space weathering with the presence of more or less fresh material and/or the different grain sizes of the regolith. The variations found analyzing the NIRS3 data are attributed to slightly different contents of hydrated material and different regolith sizes. The distribution on the Ryugu surface of the groups obtained by the analysis of the two instruments indicates a clear spectral dichotomy both between the east and west, and the north and south hemispheres. Small sized regolith grains associated to the redder spectra seem concentrated in the southwestern part of the body.

DOI： 10.1051/0004-6361/201935851

Open Access

Web of Science

HAYABUSA2: CURRENT SUMMARY. Reviewed International coauthorship

Nakamura T., Watanabe S., Hirabayashi M., Hirata N., Hirata N., Noguchi R., Shimaki Y., Ikeda H., Tatsumi E., Yoshikawa M., Kikuchi S., Yabuta H., Tachibana S., Ishihara Y., Morota T., Kitazato K., Sakatani N., Matsumoto K., Wada K., Senshu H., Honda C., Michikami T., Takeuchi H., Kouyama T., Honda R., Kameda S., Fuse T., Miyamoto H., Komatsu G., Sugita S., Okada T., Namiki N., Arakawa M., Ishiguro M., Abe M., Gaskell R., Palmer E., Barnouin O. S., Michel P., French A. S., McMahon J. W., Scheeres D. J., Abell P. A., Yamamoto Y., Tanaka S., Shirai K., Matsuoka M., Yamada M., Yokota Y., Suzuki H., Yoshioka K., Cho Y., Tanaka S., Nishikawa N., Sugiyama T., Kikuchi H., Hemmi R., Yamaguchi T., Ogawa N., Ono G., Mimasu Y., Yoshikawa K., Takahashi T., Takei Y., Fujii A., Hirose C., Iwata T., Hayakawa M., Hosoda S., Mori O., Sawada H., Shimada T., Soldini S., Yano H., Tsukizaki R., Ozaki M., Iijima Y., Ogawa K., Fujimoto M., Ho T. -M., Moussi A., Jaumann R., Bibring J. -P., Krause C., Terui F., Saiki T., Nakazawa S., Tsuda Y.

METEORITICS & PLANETARY SCIENCE Vol. 54 page： . 2019.8

SPECTRAL CHARACTERISTICS OF ASTEROID (162173) RYUGU WITH HAYABUSA2 NIRS3 Reviewed International coauthorship

Takir D, Kitazato K, Milliken R. E, Iwata T, Abe M, Ohtake M, Matsuura S, Arai T, Nakauchi Y, Nakamura T, Matsuoka M, Senshu H, Hirata N, Hiroi T, Pilorget C, Brnnetto R, Poulet F, Riu L, Bibring J. -P, Domingue D. L, Vilas F, Barucci M. A, Perna D, Palomba E, Galiano A, Tsumura K, Osawa T, Komatsu M, Nakato A, Arai T, Takato N, Matsunaga Y, Takagi Y, Matsumoto K, Kouyama T, Yokota Y, Tatsumi E, Sakatani N, Yamamoto Y, Okada T, Sugita S, Honda R, Morota

METEORITICS & PLANETARY SCIENCE Vol. 54 page： . 2019.8

RYUGU'S PARENT-BODY PROCESSES ESTIMATED FROM HAYABUSA2 MULTI-BAND OPTICAL OBSERVATIONS Reviewed International coauthorship

Sugita S, Honda R, Morota T, Kameda S, Tatsumi E, Honda C, Yokota Y, Yamada M, Kouyama T, Sakatani N, Suzuki H, Yoshioka K, Cho Y, Matsuoka M, Ogawa K, Domingue D, Miyamoto H, Barnouin O. S, Michel P, Ernst C. M, Hiroi T, Nakamura T, Sawada H, Hayakawa M, Hirata N, Hirata N, Kikuchi H, Hemmi R, Michikami T, Palmer Eric, Gaskell R, Hirabayashi M, Jaumann R, Otto K, Schmitz N, Schroder S. E, Komatsu G, Tanaka S, Shirai K, Yoshikawa M, Watanabe S, Tsuda Y

METEORITICS & PLANETARY SCIENCE Vol. 54 page： . 2019.8

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