I am reading this paper from the NTRS, Passive stabilization of the Long Duration Exposure Facility (LDEF), about (not surprisingly) gravity-gradient stabilization of the LDEF satellite.
It looks like it says that gravity-gradient torque was used to stabilize the spacecraft in all three axes. It was my understanding that gravity-gradient torque tends to align the minor axis of inertia with the radius vector (perpendicular to the local horizon), giving only two axes of control. So how is 3-axis control possible?
The paper mentions that the third control torque is related to the difference between the two smaller axes of inertia, so it seems to be gravitational in nature. The stabilizing torque is also not magnetic or aerodynamic, since those torques are also discussed in the paper but only treated as perturbative. What causes the third component of torque? Is it related to the nonsphericity of the Earth?