Subj:	Cassini Spacecraft Maneuvering Problem Halts Jupiter Studies
Date:	12/21/00 6:21:50 AM Pacific Standard Time
From:    hblondel@tampabay.rr.com To:    hblondel@tampabay.rr.com (Newmill)

Dec 20, 2000 - 09:08 PM

Spacecraft Maneuvering Problem Halts Jupiter Studies

PASADENA, Calif. (AP) - NASA suspended the Cassini spacecraft's
observations of Jupiter on Wednesday because of a problem with a
maneuvering system.

Cassini had been sending back images and other scientific data about
Jupiter as it moves toward Saturn and its moon Titan on a $3.4 billion,
U.S.-European mission.

Observations were halted after one of the spacecraft's four "reaction
wheels" experienced problems, causing Cassini to switch to a different
maneuvering system.

Cassini's wheels can point the spacecraft in any desired direction by
taking advantage of the law of physics that each action has an opposite
reaction. When an electric motor spins one of Cassini's wheels, the
spacecraft rotates in the opposite direction.

The problem surfaced Sunday when the No. 2 reaction wheel began to need
extra force to turn, and the spacecraft reacted by automatically
switching from electricity to a hydrazine thrusting system to maneuver.

The hydrazine must be conserved for the primary mission at Saturn,
according to officials at Jet Propulsion Laboratory, which manages the
mission for the National Aeronautics and Space Administration and the
European and Italian space agencies.

Cassini program manager Bob Mitchell said the situation was improving
because the wheel functioned properly in a test Tuesday.

"If things go favorable for us from now on for the rest of the testing
that we're doing it's conceivable that we'd be back up and running in a
week to 10 days," he said.

Engineers don't know what caused the problem but speculation has
centered on the possibility that some type of material got into the
wheel mechanism and then either wore down or was spit out, Mitchell
said.

Since its 1997 launch, Cassini has flown by Earth once and Venus twice,
each time using gravity to gain speed and change direction as it heads
for Saturn. Its closest approach to Jupiter - at a distance of 6 million
miles - takes place Dec. 30.

Cassini is scheduled to arrive at Saturn on July 1, 2004.