Q: 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.

Q: 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.

Q: 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.

Q: 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.

Q: 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.

Q: 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.

Q: 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.

Q: 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.