Cassini spacecraft's orienting system problem apparently fixed

PASADENA, Calif. - Controllers decided to allow the Saturn-bound Cassini spacecraft to resume using a maneuvering system that developed a problem last weekend, NASA said.

Engineers, however, planned to use the reaction wheel system only to keep Cassini's main antenna steadily pointed at Earth for several days rather than immediately resume observations of Jupiter, which it will pass at a distance of 6 million miles on Dec. 30.

Tests of the reaction wheel system have produced normal results, Bob Mitchell, the Cassini program manager at Jet Propulsion Laboratory, said in a statement.

Mission officials could not be reached Friday to confirm that the system was put back in operation Thursday night as planned.

Cassini has been using three of its reaction wheels while keeping one as a spare.

The electrically powered wheels are spun in various combinations that cause the spacecraft to point its instruments in specific directions, such as toward Jupiter.

On Sunday, one of the wheels began to show apparent drag because it required extra power to make it turn. The spacecraft automatically switched from the reaction wheel system to a separate hydrazine thruster system to change its orientation.

Mission controllers spotted the problem and suspended observations of the Jupiter approach because the hydrazine supply is limited and must be conserved for Cassini's primary mission, a $3.4 billion four-year study of Saturn starting in July 2004.

Studies not requiring pointing, such as measuring magnetic fields, were not stopped.

JPL engineers speculated that a bit of material, perhaps from a motor magnet, got into a position that caused friction and then was either thrown out or ground up.

They also were considering the possibility that prolonged operation at low rotation speed reduced lubrication in the bearings and that tests at higher speeds restored the lubrication. If so, new restrictions on low-speed operation could be needed, JPL said.

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On the Net:

Cassini site: http://www.jpl.nasa.gov/cassini .

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