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.

The asteroid explorer Hayabusa2 carries multiple rovers and separates them to land on an asteroid surface. One of these rovers, called MASCOT, was developed under the international cooperation between the Deutsches Zentrum für Luft- und Raumfahrt and the Centre National d’Etudes Spatiales. This rover was designed to be separated to land and perform several missions on an asteroid surface. To support these missions, the mother ship Hayabusa2 must separate this rover at a low altitude of approximately 50 m and hover at approximately 3 km after separation to achieve are liable communication link with MASCOT. Because the on-board guidance, navigation, and control (GNC) does not have an autonomous hovering function, this hovering operation is performed by ground-based control. This paper introduces the GNC operation scheme for this hovering operation and reports on its flight results.

The deep-space multi-object orbit determination system (DMOODS) and its application in the asteroid proximity operation of the Hayabusa2 mission are described. DMOODS was developed by the Japan Aerospace Exploration Agency (JAXA) for the primary purpose of determining the trajectory of deep-space spacecraft for JAXA’s planetary missions. The weighted least-squares batch filter is used for the orbit estimator of DMOODS. The orbit estimator supports more than 10 data types, some of which are used for relative trajectory measurements between multiple space objects including natural satellites and small bodies. This system consists of a set of computer programs running on Linux-based consumer PCs on the ground, which are used for orbit determination and the generation of radiometric tracking data, such as delta differential one-way ranging and doppler tracking data. During the asteroid proximity phase of Hayabusa2, this system played an essential role in operations that had very strict navigation requirements or operations in which few optical data were obtained owing to special constraints on the spacecraft attitude or distance from the asteroid. One example is orbit determination during the solar conjunction phase, in which the navigation accuracy is degraded by the effect of the solar corona. The large range bias caused by the solar corona was accurately estimated with DMOODS by combining light detection and ranging (LIDAR) and ranging measurements in the superior solar conjunction phase of Hayabusa2. For the orbiting operations of target markers and the MINERVA-II2 rover, the simultaneous estimation of six trajectories of four artificial objects and a natural object was made by DMOODS. This type of simultaneous orbit determination of multi-artificial objects in deep-space has never been accomplished before.