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Orbit Determination of Hayabusa and Gravity Estimation of
Itokawa During Close Proximity Phase
H. Ikeda
Kyushu University, Fukuoka, Fukuoka, 819-0395, Japan
T. Kominato, M. Matsuoka
NEC Aerospace Systems Ltd., Fuchu, Tokyo, 226-0016, Japan
T. Ohnishi
Fujitsu Ltd., Chiba, Chiba, 261-8588, Japan
and
M. Yoshikawa
Japan Aerospace Exploration Agency, Sagamihara, Kanagawa, 229-8510, Japan
In September 2005, Hayabusa (MUSES-C) spacecraft successfully had a rendezvous with
asteroid 25143 Itokawa. After the arrival, Hayabusa made detailed observations of the
asteroid during its rendezvous period (about three months). As the results of various kinds
of scientific analysis, a variety of physical parameters of Itokawa (e.g. size, volume, mass,
and density) were derived. As to the orbit determination of Hayabusa spacecraft, during the
cruise phase, the radiometric (2-way X-band range and Doppler) data were used for analysis.
On the other hand, during the approach phase or rendezvous phase, we could obtain the
optical data by means of star tracker or optical navigation camera, thus both the
radiometric and the optical data were used for orbit determination. The present paper will
report on the results of the orbit determination of Hayabusa during the close proximity
phase. I will also mention about the mass estimation of Itokawa in this period. The data used
in this analysis are 2-way X-band Doppler data and the position data, which were calculated
from optical navigation camera’s data. As well as the large orbital maneuvers and the
gravitational acceleration of Itokawa, the effect of solar radiation pressure, and the effect of
attitude control are also taken into account for the calculation. As to the gravity model of
Itokawa, a spherical-harmonics gravity model or a polyhedron gravity model are used
depending on the situation.
イトカワ接近時における探査機「はやぶさ」の軌道決定と
イトカワの重力推定について
探査機「はやぶさ」は，2003 年5 月9 日に内之浦宇宙空間観測所からM-V ロケットによって
打上げられ，電気推進と地球スウィングバイによる軌道変更を経て，2005 年9 月12 日に小惑星イ
トカワに到着した．搭載カメラ，近赤外分光器，蛍光X 線分光器，レーザ高度計などの科学機器を
使用した各種科学観測により，イトカワに関するさまざまな物理パラメータが解析され，小惑星の
起源，形成過程を解明するための新しい知見をもたらした．イトカワ滞在時，リアクションホイー
ルに不具合が生じたため，化学スラスタを使用して姿勢制御を行なっていたが，スラスタ推力の微
小なアンバランスが積算され，軌道運動に影響を与える結果となった．本解析では，この姿勢制御
による影響を評価するために簡単なスラスタ推力差モデルを導入．また，イトカワの形状を考慮し
た重力場モデルとして，球面調和関数を用いた重力場モデルやイトカワの形状モデルを使用したポ
リヘドロン重力場モデルを組込み，光学航法データ，ドップラーデータを用いて小惑星接近時の探
査機の軌道，およびイトカワの重力場について解析を行った.
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