Cassini Spacecraft is Running Tests

December 20, 2000

The flight team of NASA's Cassini spacecraft is running tests on one of
the craft's maneuvering systems to understand a situation that caused the
craft to switch automatically to a second system.

Cassini had been using a trio of electrically driven reaction wheels for
turning itself in different directions to point science instruments. One
of those wheels was needing more than the normal amount of force to turn
it on Dec. 17, so Cassini's fault-protection switched to a maneuvering
system that uses small, hydrazine-burning thrusters.

To conserve hydrazine for Cassini's main mission at Saturn, science
studies of Jupiter have been suspended, beginning today, for at least a
few days while the situation is analyzed.

"We are responding cautiously while we test the systems," said Cassini
Program Manager Bob Mitchell at NASA's Jet Propulsion Laboratory,
Pasadena, Calif. "This might turn out to have no long-term consequences,
but we want to better understand what happened before we proceed with
using the wheels more."

The reaction wheels affect which direction Cassini is facing, but not the
direction of its trajectory through space. Cassini continues right on
course to pass Jupiter at a distance of about 9.7 million kilometers (6
million miles) on Dec. 30, and with a gravity boost from Jupiter, to reach
Saturn in July 2004.

Cassini began studying Jupiter in October in collaboration with NASA's
Galileo spacecraft, which has been orbiting Jupiter since 1995. Cassini
has already returned thousands of images and measurements of Jupiter and
its surrounding environment, and scientists are busy studying them.

Cassini has four reaction wheel assemblies. Three are mounted mutually
perpendicular to each other, and one is a spare. When an electric motor
accelerates a wheel, the spacecraft rotates slowly in the opposite
direction, obeying the physics law of each action having an opposite
reaction. Moving the three wheels in various combinations can point the
spacecraft in any desired direction.

"The spacecraft did what it was designed to do: It shut off the reaction
wheels and began using thrusters instead," Mitchell said.

In a diagnostic test on Dec. 18, reaction wheel number two still had
higher than normal torque, the amount of force needed to turn it, when it
was accelerating to a speed of 50 revolutions per minute, but it spun
freely at speeds between 50 and 300 rpm.

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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