Hayabusa2 is a Japanese sample return mission from the asteroid Ryugu. The Hayabusa2 spacecraft was launched on 3 December 2014 and arrived at Ryugu on 27 June 2018. It stayed there until December 2019 for in situ observation and soil sample collection, and will return to the Earth in November or December 2020. During the stay, the spacecraft performed the first touchdown operation on 22 February 2019 and the second touchdown on 11 July 2019, which were both completed successfully. Because the surface of Ryugu is rough and covered with boulders, it was not easy to find target areas for touchdown. There were several technical challenges to overcome, including demanding guidance, navigation, and control accuracy, to realize the touchdown operation. In this paper, strategies and technical details of the guidance, navigation, and control systems are presented. The flight results prove that the performance of the systems was satisfactory and largely contributed to the success of the operation.

The Japanese interplanetary probe Hayabusa2 was launched on December 3, 2014 and the probe arrived at the vicinity of asteroid 162173 Ryugu on June 27, 2018. During its 1.4 years of asteroid proximity phase, the probe successfully accomplished numbers of record-breaking achievements including two touchdowns and one artificial cratering experiment, which are highly expected to have secured surface and subsurface samples from the asteroid inside its sample container for the first time in history. The Hayabusa2 spacecraft was designed not to orbit but to hover above the asteroid along the sub-Earth line. This orbital and geometrical configuration allows the spacecraft to utilize its high-gain antennas for telecommunication with the ground station on Earth while pointing its scientific observation and navigation sensors at the asteroid. This paper focuses on the regular station-keeping operation of Hayabusa2, which is called "home position" (HP)-keeping operation. First, together with the spacecraft design, an operation scheme called HP navigation (HPNAV), which includes a daily trajectory control and scientific observations as regular activities, is introduced. Following the description on the guidance, navigation, and control design as well as the framework of optical and radiometric navigation, the results of the HP-keeping operation including trajectory estimation and delta- $V$ planning during the entire asteroid proximity phase are summarized and evaluated as a first report. Consequently, this paper states that the HP-keeping operation in the framework of HPNAV had succeeded without critical incidents, and the number of trajectory control delta- $V$ was planned efficiently throughout the period.