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Tomorrow: Japanese Asteroid Probe Hayabusa Limps Home

from Michael Khan, 12. June 2010, 22:50
+++ Update: The entry capsule has landed and shall be retrieved in the course of Monday. The Hayabusa craft dis-integrated in the atmosphere. More here, with pix +++

Talk about a chaotic mission. What could go wrong did go wrong, except for that final crippling damage that would have terminated the mission for good. But I bet nobody would have thought Hayabusa would make it.

Grafik von Hayabusa im Anflug auf Itokawa, Quelle: JAXA(Lesen Sie diesen Artikel hier auf Deutsch)

Nobody, that is, except the spacecraft control team in the  control centre in Sagamihara, not far from Tokyo. Somehow they managed to solve an endless stream of problems and coax a moribund spacecraft back from the brink, time and again, and they made it perform the manoeuvres required to return to the Earth.

Hayabusa (隼 in Japanese, or, written in Hiragana: はやぶさ), is the Japanese word for "peregrine falcon". Up to launch, it was also known as MUSES-C. The mission concept was quite straightforward: Fly to an Earth orbit crossing asteroid called 25143/Itokawa, after Hideo Itokawa (糸川 英夫), the father of the Japanese space programs, pick up some samples and bring them back to Earth.

Hayabusa was launched on May 9, 2003 with a Japanese Mu-V rocket from Kagoshima spaceport in Southern Japan. Although the probe's launch mass was only 510 kg, the rocket couldn't manage insertion into an escape trajectory. Instead, the launch orbit was a wide ellipse, and the spacecraft had to use its four ion engines to leave the Earth. Ion thrusters deliver a very small thrust but are very fuel-efficient. More than two years and one Earth swingby manoeuvre later, the spacecraft arrived at the asteroid in September 2005.

By then, it had already taken some hits. The October 2003 solar eruption, one of the most violent ever registered, had degraded its solar arrays. That was bad because the ion drive depends on electric power, and reduced power means lesser thrust or increased propellant consumption - or both. But there was still enough power to reach the asteroid. 

Two out of the three momentum wheels also ceased functioning. These are used to control the attitude, the spacecraft's orientation in space. Without wheels, every attitude change consumes propellant.

Once Hayabusa started orbiting the asteroid, things appeared to be looking up. The asteroid was mapped and photographed extensively. But then the situation went south. First, an attempt to deploy a tiny lander probe named MINERVA failed - inadvertently it was released when the spacecraft had just executed a thruster manoeuvre carrying it away from the asteroid rather than towards it. So MINERVA uselessly drifted off into space. 

Sample extraction was supposed to be an operation of the slam-bam-thank-you-ma'am kind. Asteroid surface temperatures are extreme, so it's not a good idea to hang around more than absolutely necessary. According to the mision plan, the spacecraft was to alight and perch on a tubelike structure and fire a small tantalum bullet into the regolith. The impacting bullet would whip up some dust that would travel up the tube and settle in the probe container, and then the spacecraft was to lift off immediately.

But that's not how it turned out. The dress rehearsal failed badly. Although the spacecraft wasn't even supposed to land at that time, it hit the surface and radio contact to the Earth was interrupted. In the following hours, it appeared the mission was lost - but then, suddenly, a signal from Hayabusa was captured. The spacecraft had managed to take off after all!

Grafik von Hayabusa auf der Oberfläche: Quelle: astronomie-heuteAnalysis of the telemetry data later showed that the spacecraft had spent 30 minutes on the surface and even come into close contact with the hot rocks and dust. Overheating apparently had led to some damage. A few days later, following a second, controlled landing attempt, the bipropellant system sprang a leak and subsequently losing all of its chemical propellant load.

Although the second landing was arguably more successful than the first, it remains unclear whether the sample extraction actually took place. In the weeks to come the spacecraft underwent a near-death experience resulting in amnesia, when parts of the onboard computer memory were erased. There are indications that the sample extraction mechanism may not have been triggered. But still, some surface dust may have ended up in the sample container - perhaps as much as one gram.

This definitely was the nadir of the entire venture. The bipropellant system: gone. Two of the momentum wheels: gone. This means that effectively the attitude control system was shot. The batteries were shorted and dead, most likely due to overheating. Two out of four ion thrusters were out of order, one appeared damaged and the only functioning one was the one with the highest number of operating hours, and therefore the highest likelihood of failure. When the chemical propellant vented, it caused a tumbling motion, so the high gain antenna no longer pointed towards the Earth and radio contact had been lost. The only positive thing was that the Xenon propellant tank for the ion drive was still intact. If that had gone too, it would have been curtains, then and there. 

Spacecraft engineers don't give up easily. As winter turned to spring in 2006, and Earth moved back into the viewing direction of Hayabusa's high gain antenna, the control team managed to re-establish command access and stop the tumbling. This required venting some of the precious Xenon propellant through small cold gas thrusters that had been added as a wise precaution for contingencies. There was no alternative, but the Xenon was badly needed for the return trajectory, so the team took care not to vent a drop too much.

In the meantime, the return window had closed, but there was a second chance in April 2007, more than one and a half years after arrival at the asteroid. This would lead to an Earth encounter in June 2010, barring further disasters. Hayabusa gingerly set off, but everyone was aware that its chances were slim. Just one more breakdown, and it would be over. But then, for a change, there was a lucky break! They managed to re-start one of the two defective ion thrusters. That was very fortunate, because soon afterwards, one of the other thrusters, the one that had been half-working, failed. 

Animation of asteroid 25143/Itokawa, based on the data model for its shape generated on the basis of the Hayabusa imagery and laser altimeter measurements. This model can be downloaded from the JAXA web site. In the middle of the bottom side the "Muses Sea" is visible (the name is a pun on the original mission name MUSES-C). This area is free of large rocks and therefore was chosen as landing site. The maximum extent of the asteroid is around 550 meters. Source: Timo Prange, Michael Khan, ESA

So Hayabusa limped on, and in due course, guess what: there was yet another failure: With one of the two remaining ion thrusters, the ion accelerator failed, and with the other, the neutralizer. However, one needs both accelerator and neutralizer to operate an ion drive. The engineers found a workaround: they programmed the onboard software to combine the working accelerator of one thrsuter with the working neutralizer of the other.

So on the spacecraft went, but now, soon, its fearful trip will be done. Tomorrow, Sunday, June 13, 2010 around 16:00 CEST (14:00 UTC), very conveniently located just between the end of one world cup match (Algeria vs. Slovenia) and the start of another (Serbia vs. Ghana), the 17 kg entry capsule with its (hopefully not empty) sample container will enter the Earth atmosphere. During the past weeks Hayabusa has successfully manoeuvred itself into a trajectory that enters the atmosphere such that the capsule, which is deployed only 3 hours prior to entry, will land in the Australian desert near Woomera

Following the red-hot hypersonic phase the capsule will glide down to Earth under a parachute and plonk down softly in the sand. After recovery it will be airlifted to Japan and opened in a safe lab envioronment. If it is found to contain asteroid samples, this will have been the first sample return from the surface of any celestial body other than the Moon. A magnificent scientific and technological achievement and a true milestone of space research. But even if the sample return didn't work, this mission will enter the annals of spaceflight. The asteroid science phase is a great achievement in its own right.

And what about Hayabusa? After deploying the entry capsule it will no longer serve any function, and anyway, there is nothing one can do about it, so it will simply follow the entry capsule into the atmosphere and burn up ... 

My heartfelt congratulations to all project team members. おめでとう御座います!

And now for some editorializing!

The team has done a fantastic job. No doubt about that. But still, some things need to be pointed out. Traditionally, the Japanese space effort has been handled by competing agencies. Earth observation, communications satellites, development of the large H-series rocket and manned spaceflight have been the responsibility of NASDA, headquartered in Tsukuba. Conversely, ISAS in Sagamihara, managed jointly by the famous Toudai (the University of Tokyo) and the education ministry Monbusho, handled astronomy, interplanetary and space science missions. While NASDA enjoyed a large budget allowing it to tackle ambitious ventures, ISAS was always forced to operate on a shoestring. NASDA, ISAS and another agency have since been merged, forming the Japanese Aerospace Agency JAXA, but that move still doesn't address the underlying issue.

With space missions, as with all large technical projects, there is a minimum level of resources required to reach critical mass. If critical mass is not attained, the project is likely to experience a succession of defects and face a large risk of complete failure. This is axiomatic; it follows from the fact that one is forced to procure cheap (as opposed to good) components, the level of redundance is too low and one lacks the means to train people to do their jobs as well as they want to do them.

ISAS missions have been plagued by a history of crippling defects. Not just MUSES-C/Hayabusa, also PLANET-B/Nozomi (where the team did as good a job as Hayabusa's but ultimately just ran out of luck) and others. That's not because they don't have experienced, talented people there. I know people from ISAS. They're among the best in the world. The problem is that these projects didn't reach critical mass. Perhaps they were just a few millions short of a resounding mission success. If they'd had those missing resources, who knows? Hayabusa might have been running just as smoothly as shown in the youtube video below.  

Further Information

Live webcast from the Hayabusa control room in Sagamihara

Press Kit for Hayabusa's return

Web site on Hayabusa's return on jaxa.or.jp

JAXA press release: "Capsule reentry plan", June 12, 2010

Hayabusa project web site on jaxa.or.jp

Archive of all press releases on Hayabusa

Web article by E. Lakdawalla from the Planetary Society

Article on spaceflightnow.com with timelines, details and links

Here is a really cool movie with great music, showing how smoothly the mission could have gone, had everything worked according to plans. 

Scilogs post from March 9, 2010 on the maximum rotation rate of gravitationally bound asteroids



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