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,

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:

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

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

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:

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.

Media Relations Office

Jet Propulsion Laboratory

California Institute of Technology

National Aeronautics and Space Administration

Pasadena, Calif. 91109.
Telephone (818) 354-5011

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