Cassini Passes Through Asteroid Belt
December 28, 2000

NASA's Cassini spacecraft, approaching Jupiter, has been operating trouble-free since its reaction-wheel system was reactivated for controlling the craft's orientation seven days ago.

"Everything has been working smoothly, so we're planning to send up commands today to resume all scientific observations," said Bob Mitchell, Cassini program manager at NASA's Jet Propulsion Laboratory, Pasadena, Calif.

Increased friction in one of the electrically powered reaction wheels, which are used to rotate the spacecraft, prompted Cassini on Dec. 17 to switch automatically to a different system, one that uses small, hydrazine-fueled thrusters. To conserve hydrazine for Cassini's primary mission at Saturn, managers suspended some scientific observations of Jupiter, beginning Dec. 19. Observations that require pointing the spacecraft, such as taking pictures, were put on hold. Those that do not require pointing, such as magnetic-field measurements, continued.

Cassini was put back on the reaction-wheel system for controlling its orientation on Dec. 21, after testing indicated that the above-normal friction had ended. However, the spacecraft has been kept in an attitude with its main antenna pointed to Earth, and observations that require pointing of scientific instruments have remained on hold while reaction-wheel operation is monitored.

Processing and analysis has continued on thousands of images and measurements taken by Cassini between early October and mid-December. Cassini's first color movie clip of Jupiter was released this week. It is available online from JPL at:

http://www.jpl.nasa.gov/pictures/jupiter .

and from the web site of the Cassini Imaging Science team at the University of Arizona, Tucson, at:

http://ciclops.lpl.arizona.edu/ .

Cassini has three reaction wheels mounted mutually perpendicular to each other and a fourth as a spare. 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.

A probable cause of the friction that temporarily increased the amount of force needed to turn reaction wheel number two is prolonged operation at relatively low speeds, which may have reduced lubrication in the bearings, mission engineers say. Running the wheel at higher speeds in tests later may have restored the distribution of the lubricant.

"That's our leading theory, but we may never know for sure," Mitchell said.

As a precaution, Cassini's flight team plans to develop operational procedures for the reaction wheels that will avoid low-speed operations for any significant amount of time, he said.

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://saturn.jpl.nasa.gov.

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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