In Star Trek, what propulsion system moves a starship without entering warp speed?

In Star Trek, an impulse drive moves a starship without entering warp speed.

Impulse engines are conventional sublight propulsion systems used for maneuvering, combat, orbital operations, and travel within a star system. They are generally described as fusion-powered engines, although their exact operation varies across Star Trek series and technical references. A ship uses impulse power when warp travel would be unnecessary, unsafe, or impossible.

Impulse drive is different from warp drive. Warp propulsion allows faster-than-light travel by creating a warp field, while impulse propulsion keeps the vessel below the speed of light. A starship normally uses impulse engines to approach a planet, enter orbit, or conduct precise maneuvers.

The term can also refer to the broader propulsion system rather than only the visible engine units. Star Trek ships often have multiple impulse engines, commonly positioned at the rear of the engineering section or saucer. Their placement and design vary by class, but their sublight role remains consistent.