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.

This paper describes the orbit design of the deployable payload Rover 2 of MINERVA-II, installed on the Hayabusa2 spacecraft. Because Rover 2 did not have surface exploration capabilities, the operation team decided to experiment with a new strategy for its deployment to the surface. The rover was ejected at a high altitude and made a semi-hard landing on the surface of the asteroid Ryugu after several orbits. Based on the orbital analysis around Ryugu, the expected collision speed was tolerable for the rover to function post-impact. Because the rover could not control its position, its motion was entirely governed by the initial conditions. Thus, the largest challenge was to insert the rover into a stable orbit (despite its large release uncertainty), and avoid its escape from Ryugu due to an environment strongly perturbed by solar radiation pressure and gravitational irregularities. This study investigates the solution space of the orbit around Ryugu and evaluates the orbit’s robustness by utilizing Monte Carlo simulations to determine the orbit insertion policy. Upon analyzing the flight data of the rover operation, we verified that the rover orbited Ryugu for more than one period and established the possibility of a novel method for estimating the gravity of an asteroid.

In late 2018, the asteroid Ryugu was in the Sun’s shadow during the superior solar conjunction phase. As the Sun-Earth-Ryugu angle decreased to below 3 ${}^{\circ }$, the Hayabusa2 spacecraft experienced 21 days of planned blackout in the Earth-probe communication link. This was the first time a spacecraft had experienced solar conjunction while hovering around a minor body. For the safety of the spacecraft, a low energy transfer trajectory named Ayu was designed in the Hill reference frame to increase its altitude from 20 to 110 km. The trajectory was planned with the newly developed optNEAR tool and validated with real time data. This article shows the results of the conjunction operation, from planning to flight data.

Subsurface exploration is one of the most ambitious scientific objectives of the Hayabusa2 mission. A small device called small carry-on impactor (SCI) was developed to create an artificial crater on the surface of asteroid Ryugu. This enables us to sample subsurface materials, which will provide a window to the past. The physical properties of the resulting crater are also useful for understanding the internal structure of Ryugu. Accurate understanding of the crater and ejecta properties, including the depth of excavation of subsurface materials, requires accurate information on impact conditions. In particular, the impact angle is a critical factor because it greatly influences the size and shape of the crater. On April 5, 2019, the Hayabusa2 spacecraft deployed the SCI at 500 m of altitude above the asteroid surface. The SCI gradually reduced its altitude, and it shot a 2 kg copper projectile into the asteroid 40 min after separation. Estimating the position of the released SCI is essential for determining the impact angle. This study describes the motion reconstruction of the SCI based on the actual operation data. The results indicate that the SCI was released with high accuracy.

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.

The Hayabusa2 asteroid explorer mission focuses principally on the touchdown and sampling on near-Earth asteroid 162173 Ryugu. Hayabusa2 successfully landed on its surface and ejected a projectile for sample collection on February 22, 2019. Hayabusa2 later landed near a crater formed by an impactor and executed the sampling sequence again on July 11, 2019. For a successful mission, a thorough understanding and evaluation of spacecraft dynamics during touchdown were crucial. The most challenging aspect of this study was the modeling of such spacecraft phenomena as the dynamics of landing on a surface with unknown properties. In particular, a Monte Carlo analysis was used to determine the parameters of the operational design for the final descent and touchdown sequence. This paper discusses the dynamical modeling of the simulation during the touchdown of Hayabusa2.