JAXA issued a press release (in Japanese) on November 9 stating that one of Hayabusa's ion thrusters, thruster D, had stopped operating. Hayabusa launched with four ion thrusters, but D was one of only two that are still functioning. So the failure of thruster D is a serious problem.
I asked Japanese blogger "5thstar" to summarize for me what's being said on Japanese-language blogs and websites about this. He sent me this summary of the situation, based in part on the release and also in part on blog entries from space journalist Shin-ya Matsuura and Hayabusa team member Junya Terazono:
On Nov. 4, thruster D, one of the remaining two ion thrusters stopped operation due to the voltage of neutralizer has reached to its upper limit.
This means only one thruster, C, remains in action.
The original plan [for a return to Earth in June 2010 to drop off the sample capsule] before this incident was to use Thruster C and D for powered flight until March of next year. Several maneuvers had been planned between March and June for precise re-entry of the sample. This plan has to be changed.
Only one thruster may not provide enough propulsion for the currently planned trajectory. This imposes a severe decision whether the team may have to divert the orbit for even longer operations, or to increase the power of the thruster C to its limit.
In either case, chances for the successful attitude control and the orbit correction maneuver get slim.
The situation is very dire at best.
Every time Hayabusa has stumbled, the creativity of JAXA's engineers has revived hopes for its mission. Will they be able to pull off another save? Let's hope so.