I've heard of spacecraft designs that unloaded momentum wheels using gravity gradients, a little solar sail and even a small ion thrust (can't name the specific spacecraft). Cassini will probably unload its momentum wheels using thrusters although it might be big enough to use gravity gradients.

One of the Viking Landers was killed by operator error (not widely known). For years, Pioneer 12 was controlled by a single civil servant. Pioneer 12 died a natural death when its orbit decayed into the atmosphere of Venus (no operator error in this case). I'm reminded of a funny story I heard about the old Big Bird reconnaissance satellites. Big Bird pre-dated CCD cameras and had to use film spools in reentry vehicles to get its data badk to Earth. After a Big Bird orbit vehicle returned its last film spool, it was simply an empty and useless camera in Low Earth Orbit. Typically the orbit vehicle still had plenty of propellant left for orbit adjusts. The story is the USAF would hand the vehicle over to a 2nd Lieutenant and he was told to have fun with it. The 2nd Lieutenant would then fly the thing all over the place to keep the Soviets busy tracking it. When it was almost out of fuel, he'd dump it into the South Pacific. Imagine having your very own spacecraft to play with.

Gary