Hayabusa arrived at what we call the Gate Position, about 20 km from Itokawa on
September 12th. Since then to 21st, Hayabusa had kept the relative position
control (Station Keeping) via the Optical Hybrid Navigation. A three dimensional
view of the trajectory as well as its time history are shown below.
The coordinate used here is, so called, HP (Home Position) frame, in which the Z axis is
taken to the Earth direction and Y-axis is southward in ecliptic frame and normal to the
plane that is spanned by Sun-to-Itokawa and Sun-to-Earth lines and X-axis should
constitute the right-hand coordinate. The station keeping maneuver maintains its
altitude (Z-position) at the same time controls the Itokawa direction at the center in
X-Y plane. The control box defined is a little loosened so that the number of corrections
can be minimized. Note even 1 cm/second velocity residual leads to the drift of up to
1 km per day, and this station keeping is performed very carefully.
As the following figures indicate, the station keeping has been carried out satisfactorily.
Next week, Hayabusa lowers its altitude to the Home Position, where this task will be
much harder due to the high sensitivity. The operation team will continue making an in
tensive effort to accomplish the proximity observation.