Cassini Mission Status
21 Dec 2000
(Source: Jet Propulsion Laboratory)
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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 Saturn.
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 operation.
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 2004.
Additional information about Cassini is available online at http://www.jpl.nasa.gov/cassini.
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.
The European Space (ESA) this week released new information about the communication link for its Huygens probe carried on the Cassini orbiter. After beginning to orbit Saturn in July 2004, Cassini will release the European-built probe, and the probe will drop down through the thick atmosphere of Saturn's largest moon, Titan. As previously reported, ESA earlier this year identified a potential problem in the radio link through which the probe will send data to the orbiter, for relaying to Earth.
ESA convened an independent inquiry board to assess the status of the Huygens communication link and to recommend ways to assure the return of scientific data from the probe. The board's report is now available at http://sci.esa.int/home/huygens/index.cfm.