Scientific instruments carried aboard spacecraft often have to be equipped with mechanisms to operate shutters, protective covers, filter wheels, aperture changers and devices that focus, scan and calibrate, just as spacecraft themselves may have to be equipped with mechanisms such as deployable booms, reaction wheels and gas valves for manœuvrability, and driven shafts to steer antennae and solar arrays. This chapter focusses on the design principles of the former, treating them as a special branch of mechanical engineering, although somewhat paradoxically the system designer's first duty is to avoid the use of mechanisms wherever possible to reduce complexity and the risk of end–of–life failure.

We define a mechanism as a ‘system of mutually adapted parts working together’. It may provide useful relative movement, as for a focussing device in a photographic instrument. In the absence of a human operator, it may include a drive motor to overcome friction, or perhaps to perform controlled amounts of useful work. For example, the instrument might be a rock sample drill on a planetary lander. High powered machines such as rocket–engine turbopumps are not considered. The development of robotics (Yoshikawa, [1990]) for manufacturing industry has been widespread in an era in which the remotely directed manipulator arm has been useful in space.

The electronics are usually digital, and have been largely dealt with in Chapter 4 already. (In a functioning space mechanism they can be so well integrated that the combined system is described by some authors as ‘mechatronics’.)