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Last November, Hayabusa suffered from a serious fuel leak immediately following its
successful second touching down to the surface of Itokawa, a near Earth asteroid.
Since the chemical engines were not available, the strong attitude disturbance
occurred on December 8th caused the communication lost since then. According
to the analysis, the chance of having the spacecraft communication resumed was
found 60 to 70 percent high during a year ahead, while the spacecraft is captured
well within the ground stationfs antenna beam width. JAXA decided to take an
alternative flight plan that makes Hayabusa return in June of 2010, three years
behind the nominal schedule, assuming the spacecraft starts driving its ion engines
from early 2007. In this context, the Hayabusa project team had started the rescue
operation from the middle of December, 2005. (JAXA Press Release on December
14th, 2005)



Resumption History

On January 23rd of 2006, the beacon, un-modulated radio signal was received during
while the piecewise euplink sweepf plus ecommands transmissionf operation.
The spin axis attitude shifted almost 90 degrees and the spacecraft was found with
its high gain antenna axis offset about 70 degrees from the Earth direction, when
discovered. In addition, the spacecraft spin rate was direct and about 1 degree per
second on December 8th. However, when the signal was recaptured, the spin got
retrograde at the speed of about 7 degrees per second.

The communication, especially uplink commanding, first did not go through easily.
But from January 26th, the autonomy function aboard Hayabusa started responding
to the inquiries from the ground, and the spacecraft status had been revealed one
by one till early February.

The information obtained indicated the Hayabusa spacecraft power was completely
lost once, after the spacecraft lost its attitude, and short circuitry phenomena are
observed for the Li-ion battery cells aboard. This means the battery may not be
used any more. While the chemical fuel was lost last December, this time the oxidizer
seemed lost completely, as the instrumentation reads zero pressure. The Xenon gas
amount aboard remains unchanged.

There was anticipated another attitude disturbance, which may make the
communication lost again. And on February 6th, the Hayabusa project team decided
to start performing the Rhumb-Line attitude control using Xe gas available aboard.
A new software was uploaded on the day. The control was successfully done and
the spin axis has shifted about 2 degrees per day toward the Sun direction that is
almost similar to the Earth direction. (Fig. 1) On March 4th, the antenna between
the Sun direction and the Hayabusafs antenna axis was reduced down to 14 degrees
and the maneuver was successful. (Fig. 2, Fig. 3, Fig. 4)

The communication gradually has been improved and the telemetry data were
received via Low Gain Antenna on February 25th with the speed of 8 bps. And on
March 1st, a radio range measurement was correctly obtained. On March 4th, the
telemetry data were received with 32 bps via Medium Gain Antenna (MGA)-A.

Based on those range data obtained along with the Doppler measurement, the orbit
was determined / estimated successfully after three months hiatus. Hayabusa
currently is at 13,000 km leading the Itokawa toward its revolution direction from it.
And Hayabusa flies about 3 meters per second with respect to the Itokawa. Hayabusa
is at 190 million kilometers from the Sun, and is at 330 million kilometers from the
Earth.(Fig. 5, Fig. 6)

Flight Plan for the Hayabusa and Chance of Earth Return

There is still some possibility that substantial amount of fuel and oxidizer are left
leaked out on the spacecraft. And a special Baking operation is inevitable to exclude
any gas potential by raising the spacecraft temperature higher via the heaters aboard,
after the completion of the orbit determination as well as the emergency operation
software installation uploaded. The Baking operation, however, may cause another
fuel gas eruption risk that may easily tumble the spacecraft again, and the operation
needs to be done very carefully. It may take a few months. A similar Baking operation
will be performed next for the sample recovery capsule, and the capsule rid will be
closed with the sample collection catcher pushed into it. During the cruise back to
the Earth, the ion engines will be operated and the temperature goes higher.
In order that any gas should be baked out at any operation environment during the
cruise, a further Baking operation will follow with the appropriate number of ion
engines turned on. At highest, three ion engines will be driven at the same time.
This operation takes several months. The cruise with the ion engines on starts
early 2007, so that the spacecraft should go back to the Earth in June of 2010.

The Xenon gas remained aboard is estimated about 42 to 44 kg. As long as no further
gas eruption occurs, the existing gas amount suffices the cruise flight ahead to the
Earth. Note still ion engines, star tracker, attitude control computer and so on all have
not gone through the functional verification after this gas eruption accident, while
those must have been exposed to extreme low temperature between December
and January.

Public Release on the Resumption

While the radio communication resumed in January, the spacecraft status was under
a very serious/fragile/subtle condition with no information that accounts the public
for the status. The spacecraft has been exposed to the sudden gas eruption risk so
far. At the end of February toward early March, the status was obtained and the
information was gathered a lot these days. And today, the public release was issued
with not only the attitude but the orbit information as well.

For those who have anticipated so far, understanding the spacecraft status would
be appreciated and the project team would like to apologize for little public information



Fig. 1 Attitude Reorientation

Fig. 2 Precise Attitude Determined with Sun Sensor

Fig. 3 Attitude Control History

Fig.4 Attitude Direction in which no communication on Jan. 20th but identified on Jan. 23rd.
Red circle shows the spin direction area estimated for Jan. 23rd acquisition.

Fig. 5 Trajectory in Inertial Frame

Fig. 6 Trajectory in Sun-Earth Line Fixed Frame

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