Hayabusa May Come Home After All

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As we reported last week, it seemed as if the Hayabusa asteroid explorer mission was dealt a fatal blow when the third of its four ion engines failed. But the Japanese Aerospace Exploration Agency (JAXA) announced yesterday that it may have come up with a solution to that problem to get Hayabusa back home by using components from two different inoperable thrusters in combination.

The Hayabusa mission has been rather plagued with problems throughout the entire mission. Its goal was to land on the asteroid Itokawa and return a sample to the Earth. It orbited the asteroid for three months in late 2005, and took near-infrared and X-ray spectral data. The landing, unfortunately, may or may not have gathered up a sample of the asteroid – though the container that the sample was supposed to go into may contain dust kicked up by the landing.

After taking off from the asteroid, communication was temporarily lost with the craft.  Communication was re-established with the craft, but the ion thrusters that propel it began to have problems, and as we reported last week, 3 of the 4 thrusters on Hayabusa were no longer operational. Thruster D, which has been the sole source of propulsion for the craft since February 2009, gave out due to a voltage spike. The remaining thruster, C, was shut down to avoid damage. Things were starting to look pretty grim for a mission that has overcome a lot of problems so far.

In an announcement made by JAXA yesterday, a solution has been proposed to use part of two different thrusters in combination to propel the craft. Ion engines work ionizing a neutral gas – in Hayabusa’s case, xenon – and forcing them out of an electrified grid on the back of the craft. When this is done, though, there is a negative charge imbalance in the craft which would attract the ions right back into the engines. To compensate, a neutralizer ejects electrons into the ionized gas that has been released, so that they don’t come back into the craft and the charge of the craft remains neutral.

To fix the current problem, the JAXA engineers have proposed using the ion propellant from thruster B and the neutralizer from thruster A, effectively combining them into one complete thruster. Thruster A was deemed “unstable” after launch, and remains unused.

Thruster C will remain shut off, and only be used in case of failure of this rigged-up system. Its neutralizer is operating poorly, and given that the cause of failure for thrusters B and D was due to a neutrializer problem, the mission engineers want to be cautious.

Combining the engines will consume twice as much fuel and power as they would consume alone, but Hayabusa apparently has plenty of both in spades. 5 kg of fuel will be required to gain 200 meters per second acceleration over 2000 hours, but Hayabusa still has 20 kilograms of fuel in reserve. This system wasn’t tested on the ground, but apparently has worked in space for over one week (180 hours).

This thrusting combination will continue to propel the craft (barring further problems) until March 2010. If any other problems crop up, though, the team will have to delay the return of Hayabusa until 2013. If you want to see a cool – though somewhat cheesy – video of the Hayabusa mission so far, there’s a 30 minute one available on the mission site here.

Source: JAXA, The Planetary Society Blog