Larry J wrote:
One of the biggest constraints on Orbiter life was probably propellant. They probably ran out of propellant to perform momentum wheel (reaction control wheel) unloads. Without the ability to unload momentum from the wheels, you can't maintain antenna pointing necessary for communications.

Alex writes:
If the two Viking Orbiters had been equipped with RWAs, then the above might be true. However, VO three-axis stablized attitude control involved only (12) nitrogen jets, an acquisition Sun sensor, a cruise Sun sensor, a Canopus star tracker, and an inertial reference unit consisting of 6 gyroscopes.

Alex R. Blackwell
University of Hawaii