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Say I'm designing a probe or satellite. To control its attitude, I can add some gyros, or I can put thrusters on each corner, or I can add both and pick one in flight according to the situation. How do I decide which to use? How does it depend on the design lifetime, mass, expected frequency of attitude changes, and orbit of the probe?
It'd be great if you could support your explanation with an example or two.
For the record, there's actually a third possibility, Magnetorquers. Here's why you would use one vs the other.
Magnetorquers - Inexpensive, low maintenance, but don't work in all situations. Used by LEO spacecraft typically, and small in size. They work by pushing off of a magnetic field.
Reaction Wheels (Or Gyros) - The trick here is that you can't take out a momentum change using only reaction wheels. You either need to stop the spacecraft from moving, or else offload the momentum with another form of propulsion. These work great if you can reasonable expect to have the spacecraft with the same angular momentum eventually. They tend to break down over time, but can last for many years. The reason they can't is because basically they exchange momentum from the spacecraft with momentum in the wheel. You can only make a permanent change if you are pushing against something external, or pushing something towards or away from you, Reaction Wheels just change the momentum until the wheel stops spinning. They can change the orientation, just not the momentum. See this video for a good demonstration.
Thrusters - These work all the time, but use fuel. Thus, you are limited to how much you can use them based on the fuel load vs. the lifetime of the mission. Also their exhaust can be dangerous to your satellite, if it carries something sensitive - that is the case of the Hubble Space Telescope, where thrusters could contaminate the optics.
Bottom line is, you use what you can, and will cause the minimal impact to your spacecraft.
Further to the answer of @PearsonArtPhoto, it is possible to use a reaction wheel to make a permanent change in attitude. Consider a non-rotating satellite carrying a non-rotating gyro. If a motor is used to spin up the gyro, Newton's Third Law requires a reverse torque be exerted on the satellite, and it will start to rotate in the opposite direction. After a suitable period of time, the gyro is braked to a stop, at which point conservation of angular momentum dictates that the satellite and gyro are both non-rotating again, with the satellite pointing in the desired, new, direction.
If the satellite experiences any outside torque, then the reaction wheel can be used to store the new angular momentum, keeping the attitude of the satellite fixed. In this case, the gyro must be kept rotating constantly, until the angular momentum can be "dumped", or some opposite outside torque comes along...
