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 unmannedspaceflight.com member Ishigame:
- Engines A/B/D can no longer stand alone.
- They won't use Engines A/B and C simultaneously. Engine C will be regarded as a back-up as long as possible.
- Combining two engines has never been tested on the ground but it has been working for one week [in space] (180 hours). It has been managed thanks to an emergency circuit.
- Combining two engines requires twice as much power/fuel as standards but Hayabusa has plenty resource. 5 kilograms 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.
- The acceleration will continue until mid March 2010. The situation doesn't allow premature conclusions [by which I think he means Hayabusa is not out of the woods]. If more troubles happen, e.g. Engines A/B stop before the end of the year, the team will have to consider another plan about returning in 2013.
Go Hayabusa, go!