Trajectories and attitudes of an asteroid sample return spacecraft "Hayabusa" during touchdowns on Itokawa

On May 9th of 2003, the Institute of Space and Astronautical Science (ISAS/JAXA) launched an asteroid sample return spacecraft called "Hayabusa" (named after falcon). Hayabusa aims for the world's first sample return from an asteroid along with demonstrations of several space engineering technologies for sample-return. Samples from an asteroid provide us clues as to origins of planets and asteroids. After more than two years' cruising phase by its ion engines, it arrived at Itokawa (Asteroid 25143) on September 12th of 2005. In the vicinity of the asteroid, Hayabusa succeeded in detail asteroid observations and performed five times' descent flights including twice landing operation on Itokawa to obtain asteroid samples. Its Earth return is expected around 2010. In this research, the Hayabusa trajectory and attitude during its touchdowns are discussed in detail. In the initial descent operation, above the altitude of 250 m, the guidance and navigation was conducted by determining the position using Itokawa images. After that, Hayabusa descended to Itokawa autonomously. The trajectory at this phase has not been revealed yet. It is because Itokawa images and navigation sensor data were not fully obtained. However, as to the first landing, several kinds of useful sensor data exist, though they are fragmentary. The trajectory and attitude of the final descent phase during the first landing operation was estimated by combining information from these sensor data. The Itokawa 3D shape model and the T/M image on the Itokawa ground surface were also used. Then, the velocity variation due to the touchdown were estimated.