BY JIM WILSON
 NASA
managers are hopeful that they’ll be able to aim a camera
at Jupiter during Saturday’s fly-by.
Project managers
for NASA's Cassini believe they have identified the reason for
a potentially critical malfunction aboard the multibillion spacecraft,
which is now approaching Jupiter.
Last week,
drag on one of the reaction wheels that are part of the system
used to point scientific instruments caused a switch to backup
mode. Science studies that require pointing the spacecraft were
then suspended to conserve fuel for Cassini's main mission, a
rendezvous with Saturn.
Mission managers
at the Jet Propulsion Laboratory (JPL) in Pasadena, Calif., now
report that they have commanded the spacecraft to resume using
three of its electrically powered reaction wheels.
However, they
have not yet decided if they will attempt to use the reaction-wheel
system to point instruments toward Jupiter. This would require
sending commands that cause the wheels to rotate at different
speeds. Engineers fear that if the problem that caused the shift
to the backup system reappears, Cassini's main antenna might move
away from pointing at Earth. This would jeopardize the $3.4 billion
mission.
Reaction wheels
control the direction Cassini is facing, but not the direction
of its trajectory through space. When a motor accelerates a wheel,
the spacecraft rotates slowly in the opposite direction. Moving
the three wheels in various combinations can point the spacecraft
in any desired direction.
The problem
began on Dec. 17 when apparent drag on one of the reaction wheels
triggered an automatic changeover to small hydrazine-fueled thrusters.
Two days later, science studies that require pointing were suspended
to conserve hydrazine. Over the next several days, the craft's
four reaction wheels—three mounted mutually perpendicular
to each other and one spare—were tested.
“The
results were all normal,” says Bob Mitchell, Cassini program
manager at JPL. “If we had just seen results from these tests
and nothing earlier, we wouldn't have any concern. It's encouraging,
but we need to proceed cautiously.”
JPL engineers
speculate that a small bit of material, perhaps from one of the
motor's magnets, worked its way to a position that caused friction
in the motor. “If that's what happened, maybe centrifugal
force threw it out or the motor ground it up,” Mitchell says.
“It doesn't seem to be there now.”
Another possible
cause may be reduced lubrication in the bearings due to prolonged
operation at low rotation speed. If this is the cause, then the
higher speeds used in the tests appear to have restored the lubrication,
and new operating restrictions may need to be implemented about
low-speed operation.
Cassini will
pass Jupiter at a distance of 6 million miles on Saturday, Dec.
30, gaining a boost from its gravity that will allow the spacecraft
to reach Saturn in July 2004.
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