TERRAPUB Earth, Planets and Space

Earth Planets Space, Vol. 60 (No. 1), pp. 33-37, 2008

Searching satellites of asteroid Itokawa by imaging observation with Hayabusa spacecraft

Tetsuharu Fuse1, Fumi Yoshida2, David Tholen3, Masateru Ishiguro4, and Jun Saito5

1Subaru Telescope, National Astronomical Observatory of Japan, Hilo, HI 96720, USA
2National Astronomical Observatory of Japan, Mitaka, Tokyo 181-8588, Japan
3Institute for Astronomy, University of Hawaii, Honolulu, HI 96822, USA
4Department of Astronomy, Seoul National University, Kwanak-gu, Seoul 151-747, Korea
5Satellite Business Division, PASCO Corporation, Meguro, Tokyo 153-0043, Japan

(Received November 28, 2006; Revised January 25, 2008; Accepted January 29, 2008; Online published February 12, 2008)

Abstract: We carried out a search for satellites around Itokawa, the target asteroid of the Japanese Hayabusa mission. An imaging instrument, AMICA, on the spacecraft was used to take four images of Itokawa and its vicinity on September 1, 2005. The distance of the spacecraft from Itokawa was approximately 1,700 km, somewhat longer than the original plan (1,000 km) due to the mission schedule. The field-of-view of AMICA corresponded to 170 km times 170 km around Itokawa. Since the Hill sphere of Itokawa was estimated to be 33 km at the time of the observations, the images completely covered the detectable area of satellites. However, we cannot find the motion of Itokawa satellites between the four images, because the observation period was set to 2 hr due to the mission schedule and the region where satellites would be observable was only within 8 km of Itokawa, which is inside the extensive glare of Itokawa on the images. In addition, high-energy protons produced by a huge solar flare impacted the spacecraft during the observations, and hence the four images suffered from many random spots. Comparing the positions of the spots with catalogued stars, we managed to identify stellar images. No evidence of satellites was found. Additionally, we evaluated the 1-m detection limit of the images from the limiting magnitude of 9.5. The absence of satellites of Itokawa is consistent with past optical and radar observations as well as other results by the Hayabusa spacecraft.
Key words: Asteroid, satellite, space mission.

Corresponding author E-mail: tetsu@subaru.naoj.org

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