One of the primary goals of Hayabusa2 is to land on the asteroid Ryugu to collect its surface materials. The key for a successful touchdown is to find a promising landing site that meets both scientific and engineering requirements. Due to the limited availability of pre-arrival information about Ryugu, the landing site selection (LSS) must be conducted based on proximity observations over a limited length of time. In addition, Ryugu was discovered to possess an unexpectedly high abundance of boulders with an absence of wide and flat areas, further complicating the LSS. To resolve these problems, we developed a systematic and stepwise LSS process with a focus on the surface topography of Ryugu and the associated touchdown safety. The proposed LSS scheme consists of two phases: Phase-I LSS, a comprehensive survey of potential landing areas at the 100-m scale based on the global mapping of Ryugu, and Phase-II LSS, a narrowing-down process of the candidate landing sites at the 10-m scale using high-resolution images and a local terrain model. To verify the feasibility of a precision landing at the target site, we also investigated the landing dispersion via a Monte Carlo simulation, which incorporates the effect of the irregular surface gravity field. One of the major characteristics of the Hayabusa2 LSS developed in this study is the iterative feedback between LSS analyses on the ground and actual spacecraft operations near the target asteroid. Using the newly developed method, we chose a landing site with a radius of 3 m, and Hayabusa2 successfully conducted its first touchdown on February 21, 2019. This paper reports the methodology and results of the stepwise iterative LSS for the first Hayabusa2 touchdown. The touchdown operation results reconstructed from flight data are also provided, demonstrating the validity of the adopted LSS strategy.

中文翻译：

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Hayabusa2 着陆点选择：Ryugu 的地表地形和着陆安全

Hayabusa2 的主要目标之一是降落在小行星 Ryugu 上以收集其表面材料。成功着陆的关键是找到一个符合科学和工程要求的有前途的着陆点。由于有关 Ryugu 的抵达前信息的可用性有限，着陆点选择 (LSS) 必须基于有限时间段内的近距离观察进行。此外，Ryugu 被发现拥有出乎意料的大量巨石，但没有宽阔平坦的区域，这进一步使 LSS 复杂化。为了解决这些问题，我们开发了一个系统的、逐步的 LSS 过程，重点是 Ryugu 的表面地形和相关的触地安全。提议的 LSS 方案包括两个阶段：阶段 I LSS，基于 Ryugu 全球测绘的 100 米尺度潜在着陆区的综合调查，以及第二阶段 LSS，使用高分辨率图像和 10 米尺度候选着陆点的缩小过程局部地形模型。为了验证在目标地点精确着陆的可行性，我们还通过蒙特卡罗模拟研究了着陆分散，该模拟结合了不规则表面重力场的影响。本研究开发的Hayabusa2 LSS的主要特征之一是地面LSS分析与目标小行星附近实际航天器操作之间的迭代反馈。使用新开发的方法，我们选择了一个半径为 3 m 的着陆点，隼鸟 2 号于 2019 年 2 月 21 日成功进行了首次着陆。本文报告了 Hayabusa2 首次着陆的逐步迭代 LSS 的方法和结果。还提供了从飞行数据重建的着陆操作结果，证明了所采用的 LSS 策略的有效性。