The Longest Day of “HAYABUSA”
November 27, 2005
Asscociate Executive Director, JAXA
We are sure that Hayabusa has accomplished the great feat to collect sample soils from a celestial body other than moon. I am heartily proud of those brilliant youths of this country. Below is a live coverage of “The Longest Day”.
That day, November 20th, I was studying the data from Hayabusa that flew as far as100km away in safe hold mode from ITOKAWA just like dreaming a nightmare. But Hayabusa team was not even allowed time to dream a nightmare. It took them a whole week by restless work to get it back to the original gate position.
At around 10:00 pm, Nov. 25th, Hayabusa started descending from 1km in altitude. The climax was coming for the 5th time. Each job done was not a joy any longer. They were not in that kind of mood any more. The goal for them was nothing, nothing but to get sample from the surface. The control room was filled up with their determination to fulfill the mission task. Each operation job sensitively swinging from joy to sorrow on each event until only yesterday was being just calmly carried on. No doubt, Japanese space exploration has entered the new stage just in a few days-----was my vivid feeling I had at that moment. Around 6:00 am, Hayabusa, under guidance by optical navigation system so far, was switched to vertical descent phase to maneuver Hayabusa sent down to “MUSES Sea” along with the direction of the gravity. Entering this phase, remote control from ground basically did not work any longer as scheduled. Hayabusa operates itself on its own. Well, this ability was originally given to the vehicle by human beings, though.
The team knew the spacecraft was heading for very close to the landing site of the target marker with 880,000 names that landed a few days earlier. If the second target marker was dropped as originally planned, the team thought Hayabusa might be confused with recognizing the two markers when it flashed the lamp. I heard the staffs murmuring, “Guidance to lateral direction might be possible even without target marker” from the confidence they gained from the previous rehearsals. The truth may be 880,000 voices called out, “We are here. Don’t worry. Come on down here.” though it sounds like too poetically.
For the above reason, the team decided not to drop the second marker. Thus around 6:52am, a signal to start a pretended release sequence of a target marker was issued. By then the target marker with 880,000 names had been already recognized by the camera onboard Hayabusa.
Around 6:53am, at altitude 35m, Hayabusa descending at the velocity 4.5cm/sec. stopped measurement by laser altimeter (light detection and ranging: LIDAR) and switched to Laser Range Finders (LRF). Project Manager, Junichiro Kawaguchi, and all other members paid attention to the altitudes by LRF read aloud by a staff.
The four beams were emitted from LRF toward four different directions onto the surface to measure the distance. Two beams only distinguish a line but three recognize a plane, i.e. able to make out how inclined the plane is on the surface of ITOKAWA. Four beams makes accuracy enhanced and also substitutional in case of trouble.
At the time point of “22m” called out by LRF staff, the lowest altitude is 17m and the highest 35m. “So much inclined---“ murmured by anyone around there. With angle of gradient sharper than 60 degrees, the probe autonomously aborts sampling sequence to move into ascent with safe hold mode. But now, it is not time yet to make a final decision.
7:00am, 14m in altitude, the spacecraft is hovering, adjusting its own attitude to the terrain of ITOKAWA by controlling attitude to keep its vertical axis (Z axis) of the probe perpendicular to the inclined slope of the surface. Communication between the control room and the probe is now switched to beacon mode from telemetry transmission. From now, carrier wave without many kinds of data, i.e. only Doppler data is going to be transmitted. Team leaders are breathlessly watching Doppler data, while hearing the altitudes called out by LRF staff.
At 7:04am, the mode of Laser Range Finder was switched from range finding mode to sampler control mode. For today's attempt, the team uses the parameter of fewer safeguards than before limited to only three in order to achieve continuing execution of the sequence as much as possible. This operation maneuverability actually applied to the practice is a valuable lesson learned from hard experiences for last few days almost as equal as the treasury heritage accumulated from scientific explorations for past decades. It’s a picture-perfect skillfulness of Kawaguchi team.
The three safeguards are: in the case of (1) LIDAR loses a sight of ITOKAWA (2) two beams out of four of LRF go out of order unable to make measurement (3) attitude of Hayabusa against the terrain goes beyond 60 degrees. One of the reasons of aborting sampling in previous rehearsals was Fan Beam sensors for obstacle checking, whose sensitively detected obstacles in spite of setting its detection range lowest. Therefore, the aborting system based on the Fan Beam was released of its activation from today’s operation. Although in fact, some obstacle was detected in today’s operation after attitude control was adjusted to the shapes of the surface. Nevertheless, the operation was not aborted. Not to repeat the same mistake-----it is an outstanding approach to the problem.
From weak source of data from Doppler alone, Hayabusa has recovered reliable link with Goldstone station by high gain antenna (HGA) as the probe eventually turned to ascent from ITOKAWA. All the staffs’ attention is now being concentrated on a monitor. Computer sends a series of signal to activate sampling sequence including firing of a bullet upon detecting lengthwise or traverse transformation of the sampler horn by LRF. If signaled, “WCT” is to be shown up on a monitor, if not “TMT”.
All the eyes shot on a monitor in breathless silence. At 7:35am, the monitor screen was changing of its frames like revolving, the green colored alphabets of “WCT” clearly showed up at the bottom right. “We made it! WCT” joyfully cried out Prof. Tatsuaki Hashimoto, Sub-Manager of Hayabusa Project. The room was roaring. This very moment, Japanese planetary exploration was proved for reaching the very significant milestone. “We are not sure yet. If pyrotechnic bursts----and many others”, said Jun Kawaguchi, Project Manager. Sound worried but I knew his look was getting mild.
As I wanted to let the reporters know of the happy news for their severe coverage for last couple of weeks, I postured V-sign toward the fixed camera wired to the pressroom. Well, a little bit too un-matured for my age?
Two bullets were fired at an interval of 0.2 second to get as much sample as possible. LRF is designed to detect transformation of the sampler horn either of lengthwise or crosswise direction. This time it seems to have detected crosswise direction. Landing velocity was 10cm/sec. At that moment the sampler horn shrinks by about 10cm. From touch down to take off is only one second. The bullets were fired at 7:07am.
8:35am, communication link was switched to Usuda Deep Space Center where data was replayed. Shortly before 11:00am, a trouble was detected in the chemical thruster system. In fact, the sign of trouble was seen happening during the descent phase already, but was continued of its operation by switching to backup system. When thrusting was operated with switching over from backup to main system, the same trouble took place again. Dark clouds were creeping over the control room. The scene had been repeatedly seen over and over again. But Hayabusa was immortal. It was securely geared in to safe hold mode.
The thruster trouble was solved by valve control signaled from ground. Project Manager, Kawaguchi made a comment on this trouble, “We don’t know yet what happened at this moment. This trouble means that the spacecraft was not cruising quite smoothly in ordinary interplanetary space. The fact that it would never happen in ordinary space might be a proof that the vehicle might have landed on some celestial body. We take this trouble as a medal awarded evidence that the vehicle has surely landed on some celestial body.” Please be sensitive enough to perceive his unyielding spirit and optimism out of this remark. This is the very foundation to have accomplished the great feat this time.
The longest day of Hayabusa is over. Landing today was guided close to the site as attempted the other day. Landing point this time seems to be a little deviated from the last one, but the information learned from previous attempt helped a lot in accurate guiding of the probe. It was a victory of Hayabusa team who had prepared many ingenious tools for guidance control with two rehearsals and two touchdowns, which provided Hayabusa team with invaluable knowledge. After through the struggles from losing two reaction wheels, the team was operating the vehicle with error margin of centimeters in rehearsals but in today’s final operation it was executed within millimeters. Looking at those young operators doing their jobs as if a matter of routine works, I sensed for sure that these young groups are pioneering a new age. As Project Manager, Junichiro Kawaguchi said at the press conference, it is obviously clear that the key for successful operation was in the accuracy of guidance and navigation.
We will take about three days to get out of the safe mode as first priority, after which downloading of data will begin. If activation of pyrotechnic is verified by telemetry data to prove firing of bullets, and if the attitude of Hayabusa is proved perpendicular to the surface at the time of firing, the team will abort the next sampling and will command the spacecraft to take the flight back to the earth. Checking of fuels of its sufficiency is already finished. We are about to start challenging mountain climbing called “Asteroid Sample Return Mission” from the eighth post upward.
We will continue to report on Hayabusa from now on, meanwhile, I express my sincere gratitude to the readers of this website for your many pleasant encouragements given us so far. Your mail messages have been all transferred to the Hayabusa team members. How much your warm supports have empowered the team is beyond description. Thank you very much.
November 28, 2005