I've only found two articles about this: the
BBC's
Asteroid probe
runs into trouble and
New Scientist's
Japan's asteroid probe has steering trouble.
The probe has three reaction wheels, which function like
gyroscopes, that is, maintaining a particular orientation toward
the asteroid Itokawa and the Sun. Two of the three reaction wheels
have failed, one on July 31, the other on October 3. Without the
reaction wheels, that orientation must be maintained by expending
fuel, and of course, there's a finite amount of that on board.
Flight controllers are trying to calculate how long the fuel will
last.
The problem is that hydrazine fuel is
needed to push off the asteroid each time [the probe descends
toward the surface and touches down], and one rehearsal manoeuvre
is also planned for November. Hayabusa will spend mid October
scouting out touchdown sites. ...
[Controllers] could shorten the amount of time that Hayabusa spends
at the asteroid, thus reducing the amount of fuel that will be used
to keep the probe properly oriented.
Source: the
New Scientist article.