The primary method to adjust TOPEX/Poseidon's orientation is through the use of reaction wheels like the one shown here. Inside the housing is a heavy metal disc and a drive motor which spins the disc. When the disc is accelerated around its axis, the spacecraft reacts by accelerating around that axis in the opposite direction.
TOPEX/Poseidon has four reaction wheels: three aligned along the three coordinate axes, and one aligned with a skewed axis to provide redundancy.
