QUESTION:
I know that Pioneers 10 and 11 and Voyagers 1 and 2 rotate,
and that Galileo has separate spinning and non-spinning
sections.  Does Cassini also spin most of the time?

ANSWER from Trina Ray on September 30, 1999:
Great question!  Cassini is not a spinning spacecraft - most of the
time.  We do spin for the occasional Fields and Particles experiment (or
calibration), or to exercise our control system while we are on our long
cruise to Saturn; but we do not spin for stability.  This kind of
spacecraft is called a 3-axis stabilized spacecraft.

To maintain attitude control (i.e., the position along three axes) we
have 4 sets of "thruster clusters" at the bottom of the spacecraft and 4
"reaction wheels".  We use one or the other at any given time.  If we are
on thrusters, then little puffs of hydrazine are used to push the
spacecraft back into the correct attitude once we reach a certain off-point
limit (called the deadband,  and usually set somewhere between 2 and 20
mrad).  If we are on the reaction wheels, then one or more of the massive
electrically powered wheels (mounted in three orthogonal axes aboard the
spacecraft) are spun in the opposite direction in proportion to how far off
point we are (a little off means a little spin to correct).  The reaction
wheel control of the spacecraft is much more stable and will be used for
all our pointing at Saturn that requires a steady craft (i.e., the
pictures), but it isn't very fast.  The thrusters are able to move the
spacecraft much faster and will be used for those experiments that require
the spacecraft to be turned quickly (Radio Science occultation's, RADAR
radiometry).

The spacecraft subsystem that controls all of this is called AACS, which
stands for Attitude and Articulation control; check out their website =
http://www.jpl.nasa.gov/cassini/Spacecraft/attitude.html.  By the way,
Voyager 1 and 2 are both 3-axis stabilized spacecraft, but the Pioneer's
and Galileo are spinning spacecraft.