Action/reaction . . . remember? On Earth, if you want to move a piece of furniture, no problem: you just brace your feet against the ﬂoor and push. And a motor rotates a telescope by bracing against a part of the mount that is ﬁxed to the ground. But in space? – nothing to brace against!
To change the orientation of a body in space, a torque must somehow be applied around the body’s center of gravity. There are several ways of doing that, the most common being ejection of mass (gas jets, for example), as in a jet engine, and momentum transfer.
Gas jets are often used for ﬁne adjustments in satellite orientation or of space vehicles such as the Space Shuttle. For astronomical observatories, however, jets have several drawbacks: they have limited lifetimes (depending on the amount of gas that can be carried), and they are a potential source of pollution for the optics. A better solution is to apply momentum transfer. What is that? Remember the principle of the conservation of momentum: for a body in rotation, angular momentum is conserved. Applying this principle in space, a motorized ﬂywheel is mounted on the body of the spacecraft. By changing its speed, the spacecraft will turn around its center of gravity in the opposite direction, keeping its total angular momentum constant. Such ﬂywheels are called “reaction wheels.” Three are needed for orienting the spacecraft in all directions, with typically a fourth one being added for redundancy.
The procedure for pointing a telescope in a new direction is then as follows. The rotation speed of the ﬂywheel is suddenly stepped up or down. The telescope is set in motion, reaches a given speed, and continues to turn at that speed. On approaching the desired new orientation, the ﬂywheel speed is brought back to its original value, stopping the telescope precisely on target.
From a mechanical point of view, the advantage of space telescopes is that they turn with absolutely no friction: no shafts, no bearings, no jittery motion . . . This allows them to reach remarkable levels of pointing stability. A pointing stability of 0.1 arcsecond is trivial, and it is possible to do 10 times better. The HST, which holds the record in this domain, has a pointing stability of 7 milliarcseconds. At this accuracy, a riﬂe in a shooting gallery in New York City would hit the bull’s eye of a target in Chicago!