Q: 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.

Q: 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)

TCM: Orbit corrections (Trajectory Correction Maneuvers) took a long time because they were performed using the ion engines.

Q: 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).

Q: 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.

Last image of Earth taken by Hayabusa

Ground control equipment in the research center (antenna control consoles)

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.

Q: 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)