Navigation and Guidance of Hayabusa around the Tiny Asteroid Itokawa
Yoshikawa, M.¹; Kawaguchi, J.¹; Hashimoto, T.¹; Kubota, T.¹; Terui, F.¹; Ogawa, N.¹; Ikeda, H.¹; Kominato, T.²; Matsuoka, M.²; Uo, M.^3
1JAXA; ²NEC Aerospace System; ³NEC Toshiba Space System

Hayabusa spacecraft, which was launched in May 2003, was arrived at Asteroid (25143) Itokawa in September 2005. It was the first time that man-made spacecraft arrived at such a small object like Itokawa, just 500m-sided object. We were surprised to see the strange nature of this small world. On the contrary to our assumption, its surface was covered not by craters but by numerous rocks.

It was also for the first experience in the world to control spacecraft around such a small object. Before arriving at Itokawa, we knew only limited information such as size, rotation period, rotation axis, and rough shape. When we saw the actual Itokawa, the first impression was there are no places for touching down, and we wondered how to operate spacecraft. We performed three descent operations as trial before actual touchdown was executed. This was not the original plan of operation. Another unexpected thing is the trouble of the reaction wheels. Two of three reaction wheels did not work at the touching down.

In this paper, we summarize how we managed the navigation and guidance of Hayabusa around Itokawa. Also we summarize how we obtained the mass of Itokawa. Such information will be useful not only for the next missions to small bodies but also for spaceguard.