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The distance between Asteroid Itokawa and Hayabusa is obtained by the following method.
There are two distinct measurements. One is the optical observation by LIDAR (Light
Detection And Ranging), and the other is the radio observation (Range O-C). As shown in
Fig.1, the data by LIDAR are the distance between Hayabusa and the surface of the
asteroid Itokawa. On the other hand, the data by radio observation are the difference
between calculated distance to the asteroid Itokawa from the earth (C) and observed
distance of Hayabusa from the earth (O). Range O-C is the distance between Hayabusa
and the center of mass of the asteroid Itokawa. Therefore, the difference between these
two data is (1) in Fig.1. The data from September 20 to September 25 are shown and
plotted along with time in Fig.2. As shown in Fig.2, the difference is about 700m in this
period. It is no wonder that the distance offset (or bias) is not constant but varies with
time. In October, it is reduced half. Accumulation of the observation will tell us the
corrected and actual Itokawa orbit.



Fig.1 Geometry Description

Fig.2Range offset due to the Itokawa Ephemerisf Residual Error
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