A mechanical device on a spaceship that is fired by a high current initiated by an on-board pyrotechnic switching unit (PSU). A pyro is used for such actions as separating a spacecraft and its rocket, releasing instrument covers, deploying a boom (swing arm) or parachutes, and jettisoning the aeroshell. More than 100 pyros may be used by a spacecraft during its ascent. The devices include ‘explosive’ bolts, pin pullers, and cable cutters.

A pyro is a simple, lightweight, and reliable device, but can only be used once and is dangerous when handled. New technologies are being developed to replace them.