Asteroid Science 2019 (LPI Contrib. No. 2189) 2093.pdf

SHAPE RECONSTRUCTION OF THE ASTEROID RYUGU IN HAYABUSA2 MISSION. Naru Hirata1, Naoyuki Hirata2, S. Tanaka2, N. Nishikawa2, T. Sugiyama1, R. Noguchi3, Y. Shimaki3, R. Gaskell4, E. Palmer4, K. Matsumoto5, H. Senshu6, Y. Yamamoto3, S. Murakami3, Y. Ishihara3, 7, S. Sugita8, T. Morota8, R. Honda9, M. Arakawa2, K. Ogawa2, Y. Tsuda3, S. Watanabe10, 1ARC-Space, the University of Aizu (Aizu-Wakamatsu, Fukushima, 965-8580, Japan, naru@u-aizu.ac.jp), 2Kobe University, 3ISAS/JAXA, 4Planetary Science Institute, 5NAOJ, 6Chiba Institute of Technology, 7NIES, 8University of Tokyo, 9Kochi University, 10Nagoya University.

Introduction: The Hayabusa2 spacecraft arrived during initial observation campaigns from 07/2018 to at the asteroid Ryugu in 06/2018 and started its 08/2018. These campaigns include the Box-A (20 km proximity observation to select candidates of landing altitude), Box-B (6.5 km altitude), and Mid-Altitude sites for touchdown operations. The shape of the (5.1 km altitude) observations. Models were then asteroid is one of the most critical information not only revised with data from following various observations, for landing site selection but also for scientific including close-up imaging during decent operations, discussions. high- and low-phase angle observations, and high sub- We successfully reconstructed and updated the spacecraft latitude observations. Because of constraints shape models of Ryugu with images taken by the on the relative position of the spacecraft to the asteroid, optical navigation camera (ONC) onboard the the sub-spacecraft latitude is limited within ±30˚. This spacecraft and relating ancillary data acquired during limitation affects the performance of the shape model proximity observation of the asteroid [1]. Here we on the polar regions. The emission angle of the report the current status of our shape models and observation on the polar regions is restricted to be less introduce several scientific applications. than 60˚. Methods for Shape Modeling: Two different During the proximity operation, the spacecraft and methods are applied to model the shape; one is Ryugu experienced a solar conjunction in 12/2018. Structure-from-Motion (SfM), and another is The illumination condition of the asteroid viewing stereophotoclinometry (SPC). SfM is a method to be from the spacecraft was reversed after the conjunction. able to estimate the shape of the target object from Because SfM is based on image feature matching, it multiple images (e.g., multiple exposures of a moving has difficulty handling an image set including different camera) by stereogrammetry. It can solve the shape illumination conditions. Thus, shape model production from just the images themselves, without requiring with SfM is separated into two parts; the pre- and post- information on the spacecraft positions and attitudes. solar conjunction periods. SPC is safe from this issue The SfM technique is a popular shape modeling and is able to update the model with new images method in computer vision, and there are many open sequentially. source and commercial implementations. We adopted Because of a large number of images and high Agisoft Metashape version 1.5 (formerly released as spatial resolutions, ONC-T [2] images are primary PhotoScan until version 1.4), one of the commercial sources of shape modeling both with SfM and SPC. implementations of SfM. Even though images with the v-band filter consists of SPC is a method combining stereogrammetry and the majority of the image set, other color data are also photoclinometry and is adopted by many planetary used. Sequential color observations during decent missions including Hayabusa, NEAR Shoemaker, operations are important image sources covering local Dawn, Rosetta, and OSIRIS-REx both for shape regions with high spatial resolutions. Wide-angle modeling of a target body and optical navigation of a images obtained by ONC-W1 and ONC-W2 are also spacecraft. Because it was developed for space incorporated into an SPC data set to register on the missions, SPC can simultaneously determine the target shape model and to determine the spacecraft positions shape, spin state, and position and attitude of the and attitudes. spacecraft in the astronomical reference frame. Shape Models and Relating Byproducts: Although SfM models are scaled and aligned in the Currently, shape models and their byproducts are body-fixed frame defined by SPC, the topographic produced by both modeling methods. SPC products shape models are derived by two independent methods. include the global shape model in the Wavefront OBJ This gives us a unique opportunity for making a format and the SPICE DSK shape kernel format, comparison between the shape models of Ryugu by determined spacecraft position and attitude in the SPC two methods to evaluate the consistency of the models. native SUMFILES form and the SPICE SPK and CK Source Image Data: Early versions of shape kernels, the spin state information file in SPICE PCK models of Ryugu were produced from images obtained kernels. The latest model is constructed from ~ 5,000 Shape reconstruc.on of the asteroid Ryugu Asteroid Science 2019 (LPI Contrib. No. 2189) by Structure-from-Mo.on technique in Hayabusa2 mission 2093.pdf T. Sugiyama1 , N. Hirata1, N. Hirata2, S. Watanabe3, R. Noguchi4, Y. Shimaki4, S. Sugita5 ONC images, and the best spatial resolution1The University of Aizu, of profiles2 Kobe University, is another 3Nagoya University, evaluation of small4ISAS/JAXA, -scale 5The University of Tokyo. MAPLETs is 0.1 m. topographic features. Statistical analysis of the LIDAR SfM products also containSummary: the We reconstructed the shape of Ryugu from images taken by the Hayabusa2 with Structure-from-MoVon technique. global shape models data with the shape model gives a constraint on the AgiSo_ Photoscan is adopted for with the same data formatour reconstrucVon. Accuracy of the shape models are evaluated by comparison between observed images of Ryugu and synthesized images of the models. to the SPC ones. SfM also asteroid size. produces digital elevationObtained Models models (DEMs) on several Model Evalua.on: Limb profile Results local regions of interest such as landing sites and the SCI impact target region from very high-resolution 2 images. Those local DEMs are aligned to the global body-fixed frame. Observed image Synthesized image Diff. image

Diff. on the 20.m Limb ti8o884 10 6 Limb difference on the Northern hemisphere Average ewAP 8 4 Standard deviaVon 6 4 2

Diff [pixel] Diff. profile 2 on the Limb 0 0 6 Limb difference on the Southern hemisphere 1 101 201 301 401 501 Diff [pixel] Distance along the limb [pixel] 4 Figure 2. Schematic diagram of shape model 2 Model Evalua.on: image correla.on 0 0.6 evaluation by comparison between synthetic images CorrelaVon coefficient nwc 0.55 Observa.on phases 1 from the shape models with ONC-T images. 4r Approach Box-A Box-C Mid-alt Model name Alt. ~ 40 km Alt. ~ 20 km Alt. 5-7 km Alt. ~ 5 km 0.5 Res. ~ 4 m/pix Res. ~ 2 m/pix Res. 0.5-0.7 m/pix Res. 0.5 m/pix 9 Approach x 0.45 Pre Box-A x x Acknowledgement: This work is supported by 0 60 120 180 240 300 360 Box-A x longitude (on the limb/at the center) Pre Box-C x KAKENHI from the Japanese Society for Promotion • We successfully reconstructed the shape model of Ryugu Box-C x x of Science (JSPS, Grant No. JP17K05639) and JSPS Mid-alt x x x with Structure-from-MoVon technique Core-to-Core Program "International Network of • Limb profile analysis shows that the surface topography Models are sequenVally updated with newer is reproduced with error of < 2 pixel (1.0 - 1.5 m) images with beXer resoluVons. The Mid.-alt Planetary Sciences". - Scaling (the SfM model is scaled to the SPC model) model is the best model in this study. References: [1] Watanabe et al. (2019) Science, should be improved 364, 268. [2] Sugita et al. (2019) Science, 364, 252. • Trend in image correlaVon analysis is not consistent with that in limb profile analysis

Figure 1. 6-sided views of the shape model reconstructed by SPC (top panel) and SfM (bottom panel).

Model Evaluation: Performance of the shape models was evaluated by several different ways: 1) errors or residuals computed during the shape reconstruction, 2) consistency among obtained shape models, and 3) comparison between synthetic images from the shape models with ONC-T images (Fig. 2), 4) evaluation with LIDAR ranging data. Residuals of shape reconstruction give a quality of image matching and overall self-consistency of the model. Consistency among models with different methods shows a bottom- line of the model quality. Comparison between synthetic images and observed images evaluates the reproducibility of the models in small scales. Comparison with the individual LIDAR altitude