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