JAXA Japan Aerospace Exploration Agency

Q. What was the main challenge during HAYABUSA’s re-entry on its return to Earth?

A great obstacle was the extreme heat during re-entry. One of the mission’s objectives was to ensure the sample capsule’s safe return, and the entire sequence, from its separation from the spacecraft to retrieval on the ground, was a great technical challenge.
HAYABUSA’s re-entry capsule is pot-shaped, 40 cm in diameter, and 3 cm thick on average, consisting of the capsule body, a heat shield to protect the capsule from heat, and a parachute to soften its impact on the ground. The velocity at the capsule’s re-entry was about 12 km per second, and the capsule was exposed to heat of more than 10,000°C due to friction with the atmosphere. The surface temperature of the capsule was expected to reach 3,000°C, so the heat shield had to be extremely strong. We made it 3 cm thick, and used mainly carbon-fiber-reinforced plastic, which is used in aircraft fuselages and sports equipment.
After separation from HAYABUSA, the capsule re-entered the atmosphere at an altitude of 200 km. Then the heat shield separated at an altitude of about 5 km, and when the parachute opened, the capsule transmitted a beacon signal. In these operations, the critical elements were the center of mass and the angle of the capsule, as well as the timing of the parachute opening. In addition, to ensure a safe return, the sample capsule had to be designed to endure the drag and high heat during re-entry. But there is no environment on Earth to simulate both conditions at the same time, so, with only the heat resistance having been tested on the ground, the re-entry was essentially an unrehearsed attempt. Nonetheless everything went according to plan, and the capsule landed in the Australian desert.
Before the return, we had a serious concern: the parachute was designed to be triggered by a small explosion at an altitude of about 5 km, but there was a strong possibility that the explosives had degraded during the seven-year journey. HAYABUSA was originally scheduled to return in 2007, but there was a three-year delay due to various troubles, such as a fuel leak. So the spacecraft had stayed longer than expected in the ultra-vacuum, ultra-cold environment of space. If the explosives had become degraded, the parachute would not have opened and the capsule would have smashed into pieces on the ground. But fortunately, the explosives worked, and the parachute opened. I was really happy when we received the beacon signal - it was a sign that the parachute had in fact deployed. And not only that, the capsule landed in the center of the predicted landing area, and was easy to retrieve. In the end, the return of HAYABUSA went so smoothly - exactly according to plan - that it felt almost unreal.

Q. How did you feel when you saw the retrieved HAYABUSA sample capsule?

Interior of the retrieved capsule

There was no trace of burning seen inside the capsule, so my true first impression was that it was unexpectedly clean. There is no oxygen in space so there is no chance of oxidation, so it was no surprise that the inside of the capsule was undamaged. But still, I couldn’t help being surprised at how clean it was. As far as the heat shield was concerned, even though the front had scorch marks, the shield was intact and the entire surface had burned cleanly and evenly. I was also surprised at how much of the plastic tape that was applied on the heatshield stayed on, even though it was burned. This tape was attached to facilitate analysis - to study how hot the capsule had become after separation from HAYABUSA.
Likewise, I was deeply moved when I saw that the umbilical cable was still there, too. The cable connects the sample capsule and the spacecraft to supply electricity to the capsule’s heater and to transmit and receive information and commands. The cable was cut when the capsule separated from HAYABUSA, and then was enveloped in heat of over 10,000°C during re-entry, so I had expected that it would have melted away without a trace. However, to my great surprise, the cable was still there. I couldn’t believe it, and at the same time I thought it was a great memento of the HAYABUSA spacecraft.

Q. What is the status of the analysis of the returned sample capsule?

Interior of the sample collection container

The planned method of sample collection by HAYABUSA was to fire a metal "bullet" into the surface of the asteroid Itokawa and collect the particles stirred up by the impact. But in reality, the bullet was never released. Even so, I was still anticipating the possibility that HAYABUSA might have collected sand and dust stirred up by its landing on Itokawa. But an X-ray computed tomography scan has shown that there is nothing in the capsule bigger than one millimeter.
Further investigation was done using an optical microscope, and some small particles, approximately 10 micrometers in diameter, were found in a cylindrical container called the sample catcher. However, these particles may have already been in the container when it was launched from Earth. At present, our focus is to collect as many fine particles as possible from the sample capsule, and to prioritize for analysis those particles with a higher chance that they are not from Earth. We will also ask Japanese universities and scientists in the United States and Australia for help with detailed component analysis. If the fine particles turn out to be from Itokawa, it will be a world’s first - to bring samples that were collected by landing on a celestial body other than the Moon.

Q. Could you tell us about HAYABUSA’s achievements?

Light streak in the sky created by HAYABUSA and the capsule (courtesy: Yutaka Iijima)

I think that the greatest achievement is the acquisition of technology that enables a round-trip flight. The technical objectives of HAYABUSA were 1) to conduct interplanetary flight with a new ion-engine propulsion system; 2) to perform autonomous navigation and guidance; 3) to collect samples from an asteroid; 4) to perform an Earth swingby using ion engines; and 5) to ensure the sample capsule’s re-entry.
Although the result of the sample collection is not yet clear, I think that HAYABUSA’s round-trip flight would not have been possible without the technologies developed for all five of these objectives. Focusing on just one objective, such as sample collection, is impossible - you can’t achieve it in isolation. Besides, these technologies are all essential for future sample return and round-trip flights, as well as resource utilization of other celestial bodies at greater distances. I think acquiring these technologies and being able to pass them on to the next generation is quite a striking achievement.
Aided by occasional strokes of luck, our project team never gave up until the end, and was is the team’s abilities that made the return of HAYABUSA possible.

Q. Were you determined to bring HAYABUSA back?

Many people visited the HAYABUSA Capsule Exhibition

Of course. The ultimate goal of HAYABUSA was to return to Earth. When it arrived at the asteroid Itokawa in November 2005, some people said the mission was substantially achieved, but in my opinion it was only the halfway point. I was thinking that I must bring it back to Earth no matter what. So when we lost communication with HAYABUSA in December 2005, I really wanted to find and rescue it, no matter what that took.
HAYABUSA was supported by many people throughout the mission, but the public’s interest seemed to start growing around March, when we saw the light of its return. I think HAYABUSA’s return meant a lot to everyone.