> By using a specially designed sequence of manoeuvres, the control team realised they could redistribute the angular momentum stored onboard the satellite using two different reaction wheels spinning in opposing directions, causing the spacecraft to flip.
> "So at this point we knew we could control the build up of energy absorbed from the Sun, and christened this new manoeuvre the z-flip".
It’s a pretty cool thing that seems to be happening more and more frequently with spacecraft flight dynamics teams developing such a detailed model of all these forces that they can invent new “stable modes” like the one they used in this article, or the Kepler missions K2 mode where they found another useful mode instead of just ending the mission. It’s not as dramatic as something like a solar sailing mission, but I think it’s a pretty awesome technical achievement, like watching a trained stuntman drive a car on two wheels through a slalom course, like a form of highly skilled, kinetic, robot based gymnastics.
Over time external forces continue to add momentum to the system (from drag/solar radiation pressure). In order to maintain a desired pointing direction this means the wheels must increase in velocity to "absorb" this momentum.
Eventually you need to "dump" the momentum using another external force, such as thrusters, or in this case the same solar radiation pressure.
Goal is to orient spacecraft such that external forces can be used to spin down the various reaction wheels, thereby "dumping" momentum out of the wheels.
This is pretty common on all spacecraft, but gets complicated/innovative when you have to do it in an emergency or if you don't have a full set of reaction wheels.
Just turn it 180 every few days this way, so the torque from external influences is cancelled out.
Perhaps? The ISS uses these.
This is where thrusters are useful, because you can use the thrusters to give you a "source of force" while you spin the reaction wheels back down. I believe this is where the original parent's question is coming from - if you have no thrusters, how do you avoid saturation?
Presumably, they've worked out a way to accurately predict the external forces and gently operate against them while spinning the wheels down in a way that doesn't make the satellite spin faster.