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Posted by baalke on September 13, 2005, 9:23 am
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http://www.isas.ac.jp/e/snews/2005/0913.shtml
The doppler and altimeter data at Hayabusa arrival
JAXA
September 13, 2005
Figure-1 below shows the Doppler velocity difference between Itokawa
and
Hayabusa. The velocity dropped down to zero around 01:17 UTC. It
indicates that Hayabusa fired its chemical propulsion thruster so that
the relative velocity was canceled. Vertically approaching speed is
taken in km/sec and is shown the doubled speed corresponding to a round
trip measurement. Horizontally is shown the Universal Time (World Time:
UTC) when the Doppler information was collected. Since radio travels
for
about 17 minutes from Hayabusa, the last firing occurred around 01:17
UTC. The resulted relative velocity was confirmed down to about 0.25
mm/sec.
Doppler Data
Fig. 1 Doppler measurement at Stop Maneuver. (Displayed two times)
Figure-2 draws the angular separation history of Hayabusa seen from
Itokawa with respect to Earth direction. If Itokawa is purely spherical
and Hayabusa is completely still to Itokawa, this separation angle
stays
zero. The motion perpendicular to Itokawa-Earth direction is hardly
known from Doppler information. However, taking the advantage of this
Navigation Camera plot as well as the Laser altimeter measurement,
Hayabusa obtains the full motion information by itself aboard. This
illumination tracking is affected by the shape and the rotation motion.
After the stop maneuver, the Figure suggests that Hayabusa still moves
about 0.1degree/hour, which corresponds to the lateral speed of 1cm/sec
normal to Earth direction, with the illumination center drift
incorporated.
Optical Navigation : Illumination Center motion observed aboard
Fig. 2 Optical Navigation : Illumination Center motion observed aboard.
Figure-3 draws the Laser Altimeter measurement about two hours after
the
stop maneuver. Since Hayabusa is almost still to Itokawa, the range
fluctuation comes from the terrain of Itokawa. The stop distance
intended was about 20km relying on the Itokawa's ephemeris estimated
based on the ground observation. However, the ephemeris contains some
uncertainty and it affects the range measurement off-set. Details need
to be analyzed.
Fig. 3 Laser Altimeter data after arrival
Whether Hayabusa is still may be interpreted by looking at the relative
speed that shall be within escape velocity. Current estimation says the
escape velocity from Itokawa is about several centi-meters per second
at
the distance of 20km. In view of the above mentioned residual relative
speed to Itokawa, it is satisfactorily interpreted that Hayabusa stays
within the sphere of influence of Itokawa. Note the mass of Itokawa is
one of the most important science observations in this Hayabusa
mission.
With the residual speed of 1cm/sec, Hayabusa motion accompanies the
angular separation shift of 3 degrees per day. Since the narrow angle
camera field of view is +/- 3 degrees, this implies Hayabusa needs to
correct its orbit. Hayabusa spacecraft is designed to perform this
control automatically and the function will go into use soon.
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