Status Report

Cassini Mission Status 20 Dec 2000

By SpaceRef Editor
December 20, 2000
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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 understand better 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.

SpaceRef staff editor.