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Emily LakdawallaNovember 19, 2009

Hayabusa's still coming home: JAXA engineers come up with yet another creative solution

Ideally, you'd like to have a space mission that faces no problems. But in the real world, spacecraft run into trouble. Trouble has come time and again to JAXA's little Hayabusa asteroid sample return mission, yet the mission's engineers always come up with new and creative ways to solve problems.

Just last week, I reported on what seemed like a fatal blow to the mission, the failure of thruster D, one of only two still-functioning ion thrusters. With only one functioning thruster, it seemed unlikely that Hayabusa could perform the trajectory correction necessary to bring it back home.

They've solved the problem in a way I couldn't have imagined. To explain the solution, I need to explain a little bit about how ion propulsion works. In ion propulsion, a gas (usually xenon or some other noble gas) is ionized, stripped of one or more electrons, then accelerated electrically across a charged grid. This mechanism can accelerate the ions to very high speed, producing thrust more efficiently (per unit mass of propellant) than chemical thrusters, although ion engines can't match chemical thrusters for sheer thrusting power. But there's a charge balance problem here: if you ionized and then tossed away all those positively-charged atoms, you'd be building up a huge negative charge on your spacecraft, and pretty quickly your spacecraft would be attracting back all those positively charged ions (and I don't imagine an enormous negative charge would do great things for your electronics). So a key part of an ion engine is a neutralizer, which sits outside the ion engine and emits electrons that are gobbled up by the high-speed xenon cations, resulting in both propellant and spacecraft remaining neutrally charged.

The failure of thruster D was actually a failure of the neutralizer component. Thruster B failed for the same reason. Thruster C's neutralizer is on its last legs, but is still operable. Thruster A, however, has never been used; the engine was found to be "unstable" after launch. The solution that the engineers have come up with is that they are somehow able to use the neutralizer from thruster A to neutralize the ion beam from thruster B. With this configuration, and the continued operation of thruster C, they have determined that they can maintain the schedule that has them returning to Earth in June of 2010.

Here's some further notes from the press briefing, contributed by member Ishigame:

Go Hayabusa, go!

Hayabusa (MUSES-C)

JAXA / ISAS / Hitoshi Kuninaka

Hayabusa (MUSES-C)

Read more: Hayabusa (MUSES-C), mission status, explaining technology

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Emily Lakdawalla

Solar System Specialist for The Planetary Society
Read more articles by Emily Lakdawalla

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