QUESTION:
How does the spacecraft know how to orient itself? 

ANSWER from FAQ:
When spacecraft engineers refer to attitude, they're discussing how
the spacecraft is oriented, or pointed, with respect to some
unmoving reference, like the stars. This is one of the jobs of the
Attitude and Articulation Control Subsystem, or AACS, which
performs a number of functions for the Spacecraft, including: 

1.Attitude Determination (i.e.- determining the orientation of
  the spacecraft in inertial space). The AACS uses data from
  several different sensors: the Star Scanner, the Sun
  Sensor, the Accelerometer, and the Spin Detector; 

2.Attitude Propagation (i.e.- keeping track of what the
  spacecraft orientation is in inertial space when the
  spacecraft hasn't just performed an Attitude Determination)
  using gyroscopes, the spin detector, and accelerometers; and 

3.Attitude Control (i.e.- changing the orientation, spin rate,
  or wobble of the spacecraft) using the various parts of the
  spacecraft that can change the spacecraft's attitude. These
  controls include the thrusters, nutation damper, and the
  400 Newton main engine. We can also move the booms
  that the Radioisotope Thermoelectric Generators (RTGs)
  are attached to, changing the RTG's position relative to the
  spacecraft spin axis. Watch how an ice skater uses his/her
  arms during a spin to get some idea how this might affect
  the spacecraft (though the skater turns much faster than the
  spacecraft!).