3 ms·

Why do the wheels need to be in constant motion? Is there a constant external torque acting on the spacecraft?

• lutusp 13y ago
  Assume that the spacecraft has a small rotational momentum that needs to be corrected. The reaction wheels begin to spin, and the wheels' rotational momentum corrects for the spacecraft's rotational momentum, causing a telescope to stop rotating and point accurately at a given target. Now, the only way to maintain the spacecraft's new orientation and lack of rotation is for the wheels to continue turning, forever. Such a correction might be needed simply to change the pointing of a spacecraft telescope from one target to another -- let's say, to depart one stationary angle, rotate 30 degrees, and come to a new stationary pointing angle. Such a change of pointing angle would in most cases require all three reaction wheels to change their speed of rotation, and all three wheels would have to continue spinning when the new position was reached. Remember that rotational momentum is conserved, there is essentially no friction on orbit, therefore the initial rotational momentum must be present in the spacecraft or its reaction wheels. And trace gases, micrometeorite hits, and even light pressure, over time contribute rotational momentum to the spacecraft.