Japan's Hayabusa spacecraft, designed to collect a sample from an asteroid and return it to Earth, has lost the second of its three "reaction wheels".
Hayabusa launched in May 2003
These wheels help the probe maintain its "attitude", or orientation, in space without needlessly expending fuel by using thrusters to do the same job.
Hayabusa has now settled in a "home position" about 6.8km from its target, the asteroid Itokawa.
The Japanese Aerospace Exploration Agency (Jaxa) probe launched in 2003.
It is currently using a combination of its two chemical engines and the last remaining reaction wheel to maintain a stable attitude.
Teams are looking at how to conserve fuel following the unanticipated use of the engines and considering whether to make changes to the mission profile. Jaxa's Institute of Space and Astronautical Science (ISAS) said it will report its plans "as soon as the strategy is fixed".
ISAS said that fortunately, Hayabusa has nearly finished mapping asteroid Itokawa.
"I hope it doesn't stop them from doing what they want to do with the mission, because it has a very neat design," said one spacecraft expert.
If controllers stick to their original plan, the probe will make two brief touchdowns on the asteroid in November.
Each time it "lands", it will fire a metal pellet into the surface at 300m/s. After the firings, the probe will take off to collect the dust ejected by the impact.
It will also deploy a small probe called Minerva to the surface of Itokawa.
Hayabusa will begin its return voyage to Earth, with the samples, this December and is expected to arrive in June 2007.
The probe lost one reaction wheel on 31 July, forcing it into an alternative flight mode. The second wheel failed on 3 October.
Hayabusa was launched from Japan on 9 May 2003 aboard an M-V-5 rocket. Scientists hope that, by returning samples from an asteroid they will learn more about the raw materials that made up the early Solar System.