People who made the long journey possibleEngineers in charge of orbit planning,ground system design, and operations
A big difference between HAYABUSA and the probes before it is that its orbit had to actually be controlled every day. Those probes, including Sakigake and Suisei, which were bound for Halley's Comet, as well as Nozomi, which was bound for Mars, basically flew according to the laws of motion, and the engines were only used to correct the orbit a few times before they reached their destinations.
So, as long as no problems occurred, the work involved in operating older probes before they reached their destinations was relatively simple. All that was required was light work, such as receiving radio waves, receiving telemetry data indicating the state of the probes, making sure no problems were occurring, and performing required device checks before the probes reached their destinations.
However, HAYABUSA's ion engines were continuously used to supply propulsive force starting at the launch in 2003, well before the probe reached its destination, Itokawa. Firing the engines continuously changed the orbit. So, HAYABUSA involved way more work than past probes, such as constantly paying attention to the condition of the ion engines, measuring whether the probe was on the planned orbit, calculating the output of the ion engines and thrust direction based on the measurement results, and sending commands to HAYABUSA.
And that was just the beginning. When HAYABUSA touched down on the unexplored Itokawa, commands had to be created in a timely manner and rapidly sent to HAYABUSA to respond to constantly changing conditions. In addition, when communication was lost at the end of 2005, the software used for operations had to be quickly modified. Finally, during the operations to return the probe to Earth, because HAYABUSA had sustained extensive critical damage, new method after new method had to be considered.
NEC engineers thoroughly assisted in the HAYABUSA operations, which spanned seven years. HAYABUSA's dramatic return to Earth on June 13th was the result of their tireless daily operations that started with the launch on May 9th, 2003.
In the following interview, four engineers look back on the operations during HAYABUSA's seven-year journey, from devising the orbit plan to the return to Earth: Masatoshi Matsuoka, who was in charge of orbit operations, Masanori Sugiura, who was in charge of the operation-supporting facilities at Usuda station, and Sunao Kawada and Yosuke Nakamura, who were at the forefront of the daily operations.
The long days leading to Itokawa and the daily work of the operating engineers
- First, tell me about the HAYABUSA operations in detail.
Matsuoka: We set up teams that worked one-week shifts. First, a supervisor in charge of operations was included in each team. This was a JAXA engineer or scientist. Next, manufacturer engineers in charge of the probe orbit were added. In addition, an engineer in charge of directly sending commands to HAYABUSA, like Mr. Nakamura or Mr. Kawada, was included.
Finally, a student or other engineer called a support member was added to complete each team, which generally included five to six individuals, and which team looked after the probe changed once per week.
Nakamura: Mr. Kawada and I traded places once every two to three weeks as we participated in the HAYABUSA operations for seven years. Our job was to send commands. Every command was sent to HAYABUSA by one of us.
- By the way, how were commands sent? Was it really as easy as striking the Enter key?
Kawada: Yeah, nowadays, we enter commands by using the keyboard for the computer used to perform operations and then send them to HAYABUSA just by pressing the Enter key. Before, such operations were performed using a specially designed operation console that had buttons and switches. We started using keyboards like the ones we use now when the Mars probe Nozomi was launched in 1998.
- What was the one-week schedule like?
Matsuoka: Tuesday was the most important day, because that was the day we sent all the commands for the week. Wednesday was the day for orbit determination, when we stopped the ion engines and measured the orbit HAYABUSA was flying along. On Thursday, we devised the orbit plan for the next week. Based on the data measured on Wednesday, we calculated the thrust direction and output of the ion engines to use starting on the next Tuesday, proposed our plan at a meeting for determining the HAYABUSA operations, and obtained approval. We performed the operations themselves on Saturday and took Sunday off. The commands were created on Friday and then sent on Tuesday, and this schedule was repeated every week.
- How did you measure the orbit?
Sugiura: As the engineer in charge of the ground system software, I'll answer that. Orbit measurement is divided into distance measurement called ranging and Doppler measurement. Ranging involves sending a specific signal to a probe and then measuring how long it takes the probe to echo the same signal to investigate the distance. Doppler measurement is the measurement of the rate of change of the distance based on variation in the frequency of received radio waves. By measuring both the range and the range rate, we are able to determine the orbit the probe is flying along.
Matsuoka: I carried out Doppler measurement almost every day. In contrast, ranging was only performed once or twice a week. I determined the orbit based on the obtained data. Basically, my job was to create the next orbit plan based on the determined orbit. If using a chemical engine, all that’s required after injecting a probe into its orbit is keeping track of the probe state, but, because HAYABUSA’s ion engines were continuously used to supply propulsive force, its orbit changed every day, which meant that a weekly orbit plan was required. I had to diligently calculate the orbit for HAYABUSA every week.
- During orbit calculation, what sorts of difficulties did you encounter?
Matsuoka: I was often told to leave a margin for problems. I was told that, even though the orbit calculations were optimal, optimal wasn't good enough, and that, if a problem occurred, the orbit plan had to include a contingency plan for recovery. Even though I worked hard to optimize the orbit calculations as much as possible, I was told that that wasn't good enough, and had to come up with ways to incorporate a contingency plan.
- Which part of the journey to Itokawa left an impression?
Matsuoka: That would be the part after the Earth swing-by in May 2004. It was extremely hard to match HAYABUSA's orbit with that of Itokawa to rendezvous with the asteroid. One of the reaction wheels breaking in July before the probe reached Itokawa was especially painful. This made it impossible to use the ion engines to supply propulsive force at the planned times. I was told to somehow manage the problem by changing the orbit plan, and, based on a discussion with the ion engine engineer, we ran the remaining three ion engines at full power, and the probe eventually reached the asteroid. When Itokawa appeared, I thought "We made it!" I also figured, "We can finally take a break!" However, when the second reaction wheel broke in October, I ended up having to work on orbit planning every day for observation operations, and there was no break after all.
Kawada: During the touchdown operations in 2005, Mr. Nakamura and I traded places after every 12 hours of work to handle operations 24 hours a day.
Matsuoka: Naturally, I didn't get any breaks, either. (Interviewer: That was definitely no laughing matter.)
Nakamura: For some reason, whenever anything serious happened, I was always the one participating in operations, not Mr. Kawada. I was always on duty when the big events occurred. The first touchdown from November 19th to 20th happened right after I traded places with Mr. Kawada.
Kawada: As a matter of fact, I should have gone home and rested, but I couldn't help but worry, so I also stayed in the operations room and watched instead of going home.
Nakamura: The final landing operations started, and it seemed that the probe had landed, but no one was sure what was really happening. Everyone in the operations room was also trying to figure out what was going on, and no one could come up with the next command to send.
- This was when it was later discovered that the probe had landed on the asteroid for 30 minutes, right?
Nakamura: We eventually used the chemical engine to supply propulsive force for an emergency take off.
Matsuoka: At that time, we were in a high pressure situation: If the probe did not touch down for the second time by November 26th and depart from Itokawa at the beginning of December, the probe would not be able to return to Earth in 2007. How many times did we use the engines? Twice…?
Nakamura: Three times.
Matsuoka: Really? Well, I do remember being surprised by how far away from the asteroid the probe was getting.
- That was terrible, wasn't it? The probe got over 100 kilometers away from Itokawa, right? Because the arrival in September was the result of carefully and deliberately inching towards Itokawa, I thought that, if HAYABUSA is that far away, it will never be able to touch down on Itokawa again. That's why I was really surprised when the preparations for the second touchdown were finished in two days.
Matsuoka: I was told to get the probe back into its original position within two days. Because Itokawa was within view of HAYABUSA's camera, I went ahead and used the chemical engine to supply propulsive force at full power. Pretty bold, right? Such a bold action was possible mostly because the operations team had gotten used to operations near Itokawa.
The miraculous revival
- Did Mr. Nakamura send the commands during the second touchdown, too?
Nakamura: That's right. I was in charge at the time. Seems like I'm jinxed, right?
Sugiura: I was also in Sagamihara, preparing for communication system failures.
- At that point, it still wasn't time for Mr. Sugiura to play your part, right? Of course, things got difficult after that.
Sugiura: That's right. After the second successful touchdown on December 8th, 2005, we lost communication with the probe. I received an urgent phone call, and I was told to help find HAYABUSA.
I was also told to make rescue operation software, and I was up all night that day creating the software with another team member. Throughout December, I made countless minor adjustments to the software based on the operation results. Professor Kawaguchi said that we would definitely find the probe. I'm sure the professor understood that recovering a probe with which communication has been lost is difficult, but he seemed confident. He said various things from the beginning that were based on the assumption that we would be able to restore the probe operations.
- As someone who sent commands, what was it like when there was no response from the probe?
Kawada: We experienced a long-term lack of responses from our Mars probe Nozomi as well, so I guess you could say we were used to this problem. Of course, problems like that are never supposed to happen. At the time, we weren't too pessimistic. We knew HAYABUSA was on the antenna radar. It just wasn't responding.
Sugiura: Because Professor Kawaguchi told me there was no reason we wouldn't be able to find the probe, I feel that I was able to concentrate on the recovery operations without being distracted by too much worrying. I did my best not to think of the situation as an emergency.
Matsuoka: I knew that, as long as the probe was not destroyed by the low temperature, sunlight would strike the solar cells at some point, and then a response would definitely come.
Nakamura: As I mentioned before, there was a rule that I was the one on duty whenever anything important happened and, sure enough, I was on duty when communication was lost in December, as well as when it was restored in January. On January 23rd, 2006, a small radio wave peak appeared on the screen of the analyzer for received waves. HAYABUSA had come back to us.
Sugiura: When communication was restored, I was working in Usuda space center. The person in charge at Usuda station kept looking at the screen. Suddenly, a small radio wave peak that might have just been noise popped up. I still clearly remember someone saying "Is this HAYABUSA?" and the response that "This must be it."
(antenna control consoles)
Nakamura: To tell the truth, even though everyone else seemed confident, I still thought that recovering the probe was a miracle.
Matsuoka: To be honest, although I was sure that the probe would respond, I also suspected deep down inside that, since communication had been lost, then the mission was over. I think the fact that no one on the Sagamihara operations team gave up made all the difference. Their refusal to give up led to HAYABUSA's recovery.
Sugiura: Even though communication was restored, communication was only possible at first for a few minutes out of every 30 minute period, and that caused all kinds of trouble. At first, this was one-bit communication. HAYABUSA has a function that switches the carrier level between two levels, and we used this function for one-bit communication to gain an understanding of the probe's status and gradually correct it.
Matsuoka: As I mentioned earlier, our original plan was to have HAYABUSA depart from Itokawa early in December and return to Earth in 2007, but we missed our return timing during the period communication was lost, and we decided not to think about the return until after we found the probe. Until we found the probe, we wouldn't be able to tell which parts of it were broken and would therefore not be able to plan the returning orbit to Earth. Of course, I also knew that the hard days of work would start again once we found HAYABUSA, (laughs) but we eventually found the probe and restarted operations.
The joy of growing together and the final operations
- Specifically, how did the orbit planning for the return trip differ from that for the first leg of the trip?
Matsuoka: Because, in addition to the two reaction wheels breaking, there was a fuel leak at that time and we could no longer use the chemical engine to control the attitude, we had to drastically change our approach from that for the first leg of the trip. Specifically, because we had no choice but to rely on the ion engines, we couldn't very well stop them. For the first half of the trip, we optimized the orbit plan to reduce the used amount of xenon propellant as much as possible, but, for the return trip, we prioritized keeping the ion engines running, even if it meant somewhat poor efficiency. We planned the orbit based on the policy that it was okay for the orbit to be a little off as long as it was right at the end.
- However, in the end, HAYABUSA successfully re-entered Earth's atmosphere, and the re-entry capsule landed right in the Woomera Desert in Australia. How did you determine the last several orbital corrections (TCMs)?
Matsuoka: Starting in February 2010, we had many discussions to determine our final TCM strategy. The constraints imposed by the direction of the sun and orientation of the antenna were stricter than we'd expected, and we realized that we had to avoid changing HAYABUSA's attitude while we used the ion engines to supply propulsive force for the TCMs. It was extremely difficult to determine the TCMs that used the ion engines at the right times, in the right directions, and with the right output.
To explain our approach in layman's terms, accurately guiding HAYABUSA to Earth involved aligning the positions of Earth and HAYABUSA, but, for the TCMs, we were only able to change two parameters due to various constraints: the engine propulsion time and the thrust direction within a limited range. Therefore, our strategy was to align Earth and HAYABUSA by using only these two parameters. In other words, these parameters represented the re-entry time and the distance from the center of Earth at that time. We combined these two parameters to create the TCM orbit plan.
■Asteroid explorer HAYABUSA (MUSES-C) re-entry (from materials released on June 16th, 2010)
- On June 13th, the day HAYABUSA returned to Earth, were all of you up to?
Kawada: Mr. Nakamura and I were in the Sagamihara operations room, at the console for sending commands. Many commands had to be sent before and after detaching the re-entry capsule, so we worked together to send them.
Sugiura: As at the time of the touchdown, I was also in Sagamihara, preparing for communication system problems.
Matsuoka: I was in the operations room, analyzing orbit data received after separation of the re-entry capsule to calculate where the capsule would fall while communicating with a member of the Woomera capsule retrieval teams. When HAYABUSA finally disappeared behind the horizon and radio waves could no longer reach it, there was a burst of applause in the operations room, but I was too busy with my calculations to think about it. In actuality, the re-entry went exactly as planned, so my calculations ended up being unnecessary (laughs).
- I heard that that Professor Hashimoto from the ISAS was struggling to restore the camera and take photos of Earth. What was the last command sent to HAYABUSA?
Nakamura: Naturally, I ended up sending the last command. The last command was to send all the data recorded by the data recorder to the ground station. Communication was lost while executing the command, but the last image of Earth taken by HAYABUSA was retrieved.
Kawada: We actually wanted to follow HAYABUSA until the moment of re-entry. The last operations were performed by using the 34 meter parabolic antenna in Uchinoura, but communication was lost when HAYABUSA was hidden behind a mountain shadow. Someone said, "It’s a mountain."
Matsuoka: Normally, a command is sent to each satellite to stop radio wave communication and end operations, but, in HAYABUSA's case, the probe went outside the range of radio waves and disappeared. This is a rare pattern.
- What are your impressions after finishing seven years of operations?
Sugiura: After all the difficulties it overcame, I think HAYABUSA is a truly lucky probe.
Kawada: I think I was lucky too, since I was involved in the operations from the beginning to the end.
Nakamura: Me too. Being involved from the beginning to the end is something I will treasure in the years to come.
Matsuoka: I'm not sure I would call myself lucky… The going was tough sometimes, but HAYABUSA gave me a chance to experience growth as an engineer. I hope younger engineers also have a chance to learn from a similar experience.
On June 13th, the 34 meter parabolic antenna in Uchinoura performed the last communication with HAYABUSA. At 7:51 p.m. the re-entry capsule was successfully detached. After the capsule was detached, HAYABUSA gradually started to lose its attitude. A jet of raw xenon gas had been propelled from the ion engine neutralizer as a last resort to correct the attitude. At the same time, the wide-view navigation camera that had been off the whole time was turned on to start trying to take photographs of Earth. The probe, which was barely responding to commands, was rotated so that the camera faced Earth, and the last photograph was taken. At 10:28 p.m., while sending the last photograph, HAYABUSA, which was being observed from Uchinoura, disappeared behind the edge of a mountain on the horizon, and communication was lost. This is how the seven years of HAYABUSA operations came to an end.
At that time, there was a commotion in the operations room. Three bouquets of flowers appeared. They were for Sunao Kawada, Yousuke Nakamura, and the other engineer who had spent seven long years continuously sending commands to HAYABUSA. Everyone was surprised by their gift.
Kawada: I was really surprised.
Nakamura: I wasn't sure if it was okay for me to accept the gift.
And then there was applause, applause, and more applause. Normally, no one would pay much attention to engineers who send commands. However, their work had been essential for the probe operations. The flowers put the spotlight on them for a moment.
Sunao Kawada and Yousuke Nakamura receiving flowers
at the Sagamihara operations room
As the Sagamihara control room filled with applause, HAYABUSA was approaching the location from which it would re-enter Earth's atmosphere over the continent of Australia. At that moment, HAYABUSA saw Earth framed by a yawning night sky.
The lights below were those of the cities along the western Australian coast, and, somewhere in the dark region beyond the coast, the Woomera Desert, the final destination of HAYABUSA's long seven year journey was waiting.
"I'm home…" Perhaps the tiny bouquets of flowers for the engineers responsible for HAYABUSA's long journey were a last gift from the probe to convey its gratitude.
Engineer introductions (from the left in the photograph)
Space Field Services Department,
Public Infrastructure Systems Division
NEC Networks & System Integration Corporation
Since he joined the company in 1992, he has devoted his career to scientific satellite operations. Since the HAYABUSA operations ended, he has been working on the operations for the Venus Climate Orbiter Akatsuki.
Space Field Services Department,
Public Infrastructure Systems Division
NEC Networks & System Integration Corporation
Since he joined the company in 1993, he has devoted his career to scientific satellite operations. Since the HAYABUSA operations ended, he has been working on the operations for the Solar Physics Satellite Hinode.
Manager of 2nd Engineering Department,
Space Systems and Public Information Systems Division,
NEC Aerospace Systems, Ltd.
Ever since he started working with NEC, he has been in charge of designing ground systems for rockets and artificial satellites.
With HAYABUSA, he was in charge of designing the tracking station software for the deep space probe.
Assistant Manager of 3rd Engineering Department,
Space Systems and Public Information Systems Division,
NEC Aerospace Systems, Ltd.
He was in charge of planning the orbits for deep space probes (such as HAYABUSA and Kaguya). For HAYABUSA, he was in charge of the orbit plan from the start of development until the operations to return to Earth.
Researched and written by Shinya Matsuura: July 7th, 2010