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The Forefront of Space Science

Mass Estimation of Very Small Asteroid
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On September 12, 2005, two years and four months after its launch, the asteroid explorer HAYABUSA finally arrived at its destination, the asteroid Itokawa. The explorer reached a point about 20km away from Itokawa. Considering the immense scale of the universe, it was a pinpoint approach. For about three months after its arrival, HAYABUSA (FALCON in English) flew around Itokawa and repeated approaches to get its prey (as its name suggests). Of course, this was the first time for an explorer to fly freely around a small celestial body 540m in diameter. In this article, I would like to introduce HAYABUSA’s orbit and the estimation of the mass of Itokawa.

HAYABUSA’s orbit around Itokawa

Figure 1
Figure 1. Relative positions of HAYABUSA, Itokawa, Earth and Sun
(Figure drawn by Takashi Kominato)

Fig. 1 shows the relative positions of HAYABUSA, Itokawa, the Earth and the Sun. HAYABUSA was always positioned close by a line connecting Itokawa and the Earth. From HAYABUSA’s point of view, the Earth and Sun were on the opposite side of Itokawa. HAYABUSA did not circle around Itokawa but moved up and down and also flew in the direction crossing the line of sight. Fig. 2 plots HAYABUSA’s actual flight path from its arrival through the end of October. It shows that HAYABUSA followed almost a straight line in September, but from October it started to fly in a lateral direction.

Figure 2
Figure 2. Movement of HAYABUSA from Sep. 12 to end Oct., 2005
Itokawa is located at the origin and Earth is in plus direction of the Z axis.
(Figure drawn by Takashi Kominato.)

As described in Fig. 1, the mission team decided to call the area around 20km from Itokawa “the gate position” and 7km area “the home position.” The gate position was the location HAYABUSA reached on September 12. The explorer stayed in this location for about two weeks after its arrival. At the end of September, it lowered its altitude to the home position. At the end of October, it further descended lower than 4km. Since this was the first time that we execute such operation, we operated HAYABUSA carefully, not lowering its altitude immediately after arrival.

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