|
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.
|
