Japan Aerospace Exploration Agency (JAXA)
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's 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)
On January 23rd of 2006, the beacon, un-modulated radio signal was received during while the piecewise 'uplink sweep' plus 'commands transmission' 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's 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 provided.
|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|
Management and Integration Department,
Institute of Space and Astronautical Science (ISAS)
Public Affairs Department, JAXA
Tel: +81-3-6266-6413 to 6416