Resuming Use of Reaction Wheels

December 21, 2000

NASA's Cassini spacecraft, approaching Jupiter, will tonight
resume use of three electrically powered reaction wheels for
controlling its orientation, because of encouraging test results
on that system, mission managers decided today.

Scientific observations, such as imaging, that require
pointing the spacecraft in specific directions remain suspended
in order to monitor the reaction-wheel system for a few days
while it keeps Cassini's main antenna steadily pointed at Earth.
Scientific studies that do not require pointing, such as
measurements of magnetic fields, are continuing.

An apparent drag on one of the wheels triggered an automatic
changeover on Dec. 17 to a different method of controlling the
spacecraft's attitude, one that uses small hydrazine-fueled
thrusters. Science studies that require pointing were suspended
Dec. 19 to conserve hydrazine for Cassini's main mission at

The craft's four reaction wheels -- three mounted mutually
perpendicular to each other and one spare -- were tested at high
speeds Dec. 19 and at slower speeds Dec. 20.

"The results were all normal," said 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."

The 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.

An increase in wheel number two's torque, or the amount of
force needed to turn it, triggered the automatic switch to
thrusters and was detected again at low rotation speed, but not
at higher speed, in an initial test on Dec. 17. The later tests
found no above-normal torque at either high or low speeds.

JPL engineers are speculating 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 said. "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

Cassini will pass Jupiter at a distance of 9.7 million
kilometers (6 million miles) on Dec. 30, gaining a boost from its
gravity that will allow the spacecraft to reach Saturn in July

Additional information about Cassini is available online at:

Cassini is a cooperative mission of NASA, the European Space Agency and the Italian Space Agency. JPL, a division of the California Institute of Technology in Pasadena, manages Cassini for NASA's Office of Space Science, Washington, D.C.

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