When the space telescope is in orbit, where do these external forces come from? How much of a net torque is exerted by the solar wind? How long does it take for the reaction wheel angular momentum to build up to a level that is unacceptable, or is there some reason the wheels must be absolutely still? Also, once you start adding angular momentum to a wheel, isn't it hard to subtract exactly the same amount of momentum? (Hmmm... maybe that is why the wheels need to be normally still, so you can just apply a brake to stop a turn of the telescope).
The external forces (that cause a torque of the vehicle) our satellite “feels,” are mainly from atmospheric drag for the low Earth orbiters and the solar radiation pressure for the higher ones. Other torques may include magnetic fields, gravitational field gradients, solar wind particles, etc.
Also, once you start adding angular momentum to a wheel, isn't it hard to subtract exactly the same amount of momentum?
On some satellites, that is exactly what the torque rods are used for. To unload the wheel momentum.
(Hmmm... maybe that is why the wheels need to be normally still, so you can just apply a brake to stop a turn of the telescope
The wheels are used constantly to keep the satellite pointed properly. The onboard control laws perform the “dance” between the wheels, satellite, and torque rods. :-)
Here is a really nice write up on a reaction wheel strategy for the ESA’s Integral Gamma Ray Telescope: