QUESTION:
Why does Pathfinder spin on the way to Mars? Why does it spin at different
rates?

ANSWER from Rob Manning:
Early in the Mars Pathfinder design process we decided to use spin
stabilzation as the primary attitude stabilizing mechanisim. Like a giant
top, the spacecraft spins at 2 revolutions per minute during the 7 month
cruise phase of the mission. Of course, like a top, the spin dynamics causes
the orientation of the spacecraft to remain fixed in "inertial space"
indefinately provided you don't do anything to the spacecraft - like fire
thrusters. The other method of letting the spacecraft remain stationary
(like the Voyager and Mars Global Surveyor spacecraft) requires that the
attitude control system operate all the time to keep it from tumbling in
space. (So called, "three-axis" attitude control systems typically monitors
attitude using star and sun sensors and gyros and periodically fires
thrusters or turns reaction wheels or other torquing devices to keep things
like antennas pointed toward Earth and cameras pointed to targets.) There
are advantages and disadvantages to both approaches (the Galileo spacecraft
even does both! Half of it spins at up to 3 rpm and the other half remains
stationary!). The advantages of a "spinner" are that the attitude control
system can be turned off once the attitude of the spacecraft is where you
want it - but don't try taking long duration photographs of objects from
fixed cameras on the spacecraft; the images will be smeared!

Since Mars Pathfinder doesn't have cameras that can "see" outside of the
spacecraft during "cruise" to Mars, we weren't constrained to have the
spacecraft required to operate without some rotation. In fact the opposite
was true, we had to design it so that it could spin. First of all, the Delta
II rocket's upper stage had to be spun up to 70 rpm so that it could remain
stable during the time of the orbital injection burn which took us out of
Earth's orbit toward Mars. Since Mars Pathfinder was then bolted to the
upper stage, it also had to be designed (balanced) to handle being spun up
to 70 rpm then de-spun down to about 12 rpm just before it separated from
the upper stage. Secondly, near Mars, once the aeroshell (with the lander
and rover in it) separates from the cruise stage, the aeroshell must be
spinning at 2 rpm to stay stable during Martian atmospheric entry (remember,
that it is just a spinning bullet at this point, there is no active
control).

So why do we spin at 2 rpm? Actually 2 rpm is just right for the entry
process. If it rotated too fast, then the aeroshell would spend too much
time at its entry attitude and it would be aerodynamically lifted too much
(it would act like a skipping rock on a quiet lake), too slow and it could
start to tumble. During cruise it would be nice if the spacecraft could spin
faster, but 2 rpm is the fastest it can spin while also being able too
recognize stars with our star scanner (the stars would "blip" by too fast
for the scanner to see them!).

Now that the spacecraft has been "despun" down to 2 rpm, we will leave it
spinning at that rate all the way until we get to Mars. It will start to
slow down only after the parachute opens in the Martian atmosphere!

--Rob Manning, Mars Pathfinder Chief Flight Engineer