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HAYABUSA’s Engine Turned Our Hopes into Reality Hitoshi Kuninaka Lead Developer of HAYABUSA’s Ion Engine

Three minutes that felt like eternity

Q. You witnessed HAYABUSA’s return to Earth at its drop point in Australia. What was the reaction at the site?

Light traces of HAYABUSA and its capsule
Light traces of HAYABUSA and its capsule

HAYABUSA’s capsule separated from the main body of the explorer before they re-entered the atmosphere, and the spacecraft burned and vanished, but the capsule was going to land in the desert in central Australia. Since this region is a military-controlled area, I was providing guidance as a Japanese safety management supervisor in a control room set up within a facility of the Royal Australian Air Force.
On the actual re-entry day, June 13, 2010, the sky was very clear. I had gotten there about two weeks before, and was preparing for HAYABUSA’s return, setting up video equipment and antennas to receive beacon signals from the capsule. But this day was the first time that we had had such a clear sky over the whole area. As HAYABUSA’s return approached, communications were coming into the control room from JAXA’s Ground Observation System and Direction Finding System teams, and the helicopter we had on stand-by. The DFS team estimated the landing point of the capsule based on beacon signals. The atmosphere in the control room was not hectic; I was working on a schedule based on HAYABUSA’s atmospheric re-entry at 22:51 JST. Because HAYABUSA’s onboard timer was turned on at 19:51 on the day the capsule separated, and everything from the separation of the heat shields to the deployment of the parachute was to happen automatically, we knew the time of atmospheric re-entry would not be changed even by one second.
However, I would be lying if I said I didn’t have any concerns. I was very confident about HAYABUSA’s orbit transfer using its ion engines, so I was pretty sure that the fireball would appear as scheduled. But I was concerned that we might not find the capsule in the desert. HAYABUSA had traveled for a very long time - seven years - which was way beyond its planned lifespan. Nobody could guarantee whether the parachute would open at the designated altitude, or whether the capsule’s transmitter would work properly and send the beacon signals. If the parachute opened under deceleration at a higher altitude than planned, a serious heat load from the atmosphere could damage the capsule. Or, if it opened at a lower altitude, the capsule could crash into the ground and break into pieces. Even if the parachute opened up as planned, it could be very difficult to find a capsule with a diameter of only 40 centimeters in that huge desert without the transmission of beacon signals. I was concerned about such things and felt uneasy.

Q. Was the beacon signal from the capsule transmitted?

HAYABUSA’s capsule landed in the Australian desert
HAYABUSA’s capsule landed in the Australian desert

The beacon signal was transmitted. But the three minutes from the time we saw the fireball in the sky to the time we received the signals felt like an eternity. I was patiently waiting in the control room when the beacon signal was suddenly detected, and I was so happy I shouted, "The beacon signals are emitted!!" At that point, we could move onto the next steps.
The DFS team started searching for the location of the capsule based on the beacon signal, and - surprisingly enough - it looked like the capsule had fallen exactly in the middle of the area we had chosen. I just thought "BINGO!!" I was convinced at that point that the capsule would be found. After that, a helicopter in the desert sky saw the white parachute, and the capsule was lit up with a searchlight. We didn’t see any major damage to the capsule at first glance, so we placed high expectations on its recovery, which was scheduled for the next day.
The heat shields also fell to the ground. These didn’t emit beacon signals, but they should still have been warm, so we used an infrared camera on a helicopter to search for them. Unfortunately we couldn’t find them with the infrared camera, and we ended up looking for them the next day, when the sun came up. That night, all I could think was, "I can’t wait to see the capsule with my own eyes."
And then, the next day, June 14, we recovered the capsule, and on June 15 the heat shields as well. When I saw the capsule in the desert, I thought it looked very clean. I couldn’t believe it had traveled through space for more than seven years and looked this clean, as if someone had just left a brand new capsule in the desert.

The ion engine set a world record for longest operation

Q. How does an ion engine work? What innovative technology was developed for HAYABUSA?

Asteroid explorer HAYABUSA
Asteroid explorer HAYABUSA

An ion engine gains propulsion force by turning xenon gas into plasma, and then uses electricity to accelerate the plasma and eject it at high speed. There are some variations depending on the method of making plasma. HAYABUSA’s engines are called microwave discharge ion engines, since they use microwaves to create plasma. We were the first in the world to put this unique system into practical use.
In addition, the ion engine is 10 times more energy efficient than a chemical engine, which is usually used for rocket launches, and at the same time has a longer lifespan. The lifespan required in space is at least 10,000 hours, which means it can keep operating continuously for more than a year. Once it’s launched into space, we can’t fix it, so a long lifespan without any breakdowns or need for maintenance is necessary. HAYABUSA’s ion engines achieved an accumulated operation of 40,000 hours - this is by far the longest an ion engine has operated in space.
The disadvantage of an ion engine is a very small propulsion force. On Earth, three of HAYABUSA’s engines operating together would have only enough propulsion force to lift two-and-a-half one-yen coins. But in space that’s enough, because there is no air resistance. With such low fuel consumption and long lifespan, the ion engine is perfect for deep-space exploration.

Q. Why did you equip HAYABUSA with four ion engines?

HAYABUSA’s ion engine system
HAYABUSA’s ion engine system

One reason was to have redundancy. HAYABUSA had four engines, but it was designed to use only three at a time - one was a spare. We also used a "cross circuit" design trick, to set up some extra redundancy for emergencies.
Each ion engine consists of an ion source and a neutralizer. The ion source is an ion generator that ejects positively charged xenon gas at high speed. However, if only positively charged ions are released, the explorer becomes negatively charged, and attracts those positive ions back toward itself - which means that it won’t accelerate. So to avoid becoming negatively charged, the explorer has to neutralize ions by using a neutralizer to release negatively-charged electrons. Each of the four engines had an ion source and a neutralizer, but we designed a power circuit that allowed "cross operation" - using a combination of ion source and neutralizer from different engines. For example, we could use the neutralizer from Ion Engine A and the ion source from Ion Engine B. This allowed us to set up various combinations of redundant systems - although we built this only for emergencies, and never thought we would actually have to use it.
A 500-kilogram-class explorer like HAYABUSA can generally generate about one kilowatt of electricity. So if three engines share this one kilowatt, we have the advantage of being able to alter the propulsion force. In other words, if only one engine is operating, we use 330 watts of electricity, and two engines use 660 watts. In addition, when we were testing the ion engines on the ground, it was easier to use a 330-watt ion engine than a one-kilowatt engine, as the test device was not that large. For all these reasons, we decided to equip HAYABUSA with four separate ion engines.

The cross circuit system saved HAYABUSA from its greatest danger

Q. The flight of HAYABUSA wasn’t smooth, as one of the ion engines malfunctioned soon after launch. What kind of crises did HAYABUSA overcome before its return to Earth?

Asteroid explorer HAYABUSA (Courtesy of Akihiro Ikeshita)
Asteroid explorer HAYABUSA (Courtesy of Akihiro Ikeshita)

First of all, right after launch in 2003, Ion Engine A’s performance became unstable and we had to stop using it. We didn’t have the confidence to operate that engine continuously, and we were just feeling our way through the operation each day, trying to make sure that Ion Engine A’s problems wouldn’t affect the other three engines’ ability to perform smoothly. In 2004, HAYABUSA performed an Earth swing-by and entered the extended elliptical orbit toward asteroid Itokawa. Everything was fine while the explorer was in the first part of the orbit, closer to the Sun, but as it traveled farther and farther from the Sun, it had less and less electric power. At one point, when we looked away for a moment, power was reduced, and the explorer was in extreme danger of losing power completely.
After that, two of its three attitude control systems broke down, but in November 2005 HAYABUSA still managed to land on Itokawa. However, after liftoff from Itokawa, while we were cheering at Ground Control, a fuel leak was found in a chemical engine used for attitude control. Due to this fuel leak, the explorer became confused and went missing. Communication was lost for about two months.
In March 2006, communication was somehow restored, but all the attitude control systems were still broken. We then worked out a new attitude control method using ion engine fuel, xenon gas and solar light pressure, and in 2007 HAYABUSA somehow started to travel toward Earth. At this point, though, Engine B was also having trouble, so HAYABUSA was heading toward Earth on only two engines, Ion Engines C and D.
Then, in November 2009, HAYABUSA’s biggest crisis occurred: Ion Engine D abruptly stopped. The performance of Ion Engine C, which had been operating in concert with Ion Engine D, also became reduced due to the long journey, and it too was also nearing the end of its lifespan. So I thought of using cross-circuit operation with components from two different engines, as I explained earlier. That way, HAYABUSA could use parts of two engines to produce the propulsion force of one properly functioning engine. Coincidentally, Ion Engine A stopped because its ion source was broken, and Engine B failed because its neutralizer didn’t work. So we combining Ion Engine A’s neutralizer and Ion Engine B’s ion source, and thus averted a crisis. Soon after, at the end of March 2010, HAYABUSA positioned itself into orbit to approach Earth, and prospects emerged for a June landing.

Q. When one of the ion engines stopped working in November 2009, there were people who thought there was no chance for a return to Earth. What did you think?

Curation Facility
Curation Facility

I had the cross circuit plan in mind, so I still had hope. We had designed this circuit for just such a purpose, but since it was meant only for emergency use, we hadn’t had a chance to actually use it. So when Ion Engine D died, I thought the time had come.
I was also overseeing both the explorer’s operation team and a team preparing for the capsule recovery. The recovery team had been preparing for several years, so if HAYABUSA’s mission had ended at this point, all their efforts would have been for nothing. In addition, we had built a curation facility [a facility to accept asteroid material samples] to handle samples brought back from Itokawa, so that would have gone to waste as well. I wanted HAYABUSA to somehow survive, all I was thinking about was how we could continue the project.

  
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