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In the Star Trek universe, people travel all across space searching for new forms of life with the intent of furthering the knowledge and perspective of humanity as a whole. In doing so, they typically pilot their spacecrafts at various iterations of “warp” speed, one of the fundamental propulsion methods of Star Trek’s science fiction canon. Warp speed involves traveling faster than the speed of light by using a matter/antimatter reactor to create plasma and sending this plasma through warp coils, creating a warp bubble which moves a craft into subspace, thus allowing it to exceed the speed of light. This happens within the ship’s warp engine.

By contrast, ships are occasionally piloted at “impulse power” or “impulse speed,” often as a result of the warp drive being temporarily disabled or damaged or simply when urgency is not of utmost importance. Impulse power refers to the momentum produced by a starship’s sublight impulse engines. A finite set of characteristics for these engines aren’t available, as they change throughout each installation in the franchise and vary by craft.

As Memory Alpha explains, “In Federation starships, the impulse drive was essentially an augmented fusion rocket, usually consisting of one or more fusion reactors, an accelerator-generator, a driver coil assembly, and a vectored thrust nozzle to direct the plasma exhaust. The fusion reaction generated a highly energized plasma. This plasma, (“electro-plasma”) could be employed for propulsion, or could be diverted through the EPS to the power transfer grid, via EPS conduits, so as to supply other systems. The accelerated plasma was passed through the driver coils, thereby generating a subspace field which improved the propulsive effect.”

Starship Operations furthers the technobabble, writing, “Impulse engines are used in sub-light speed flight, and to provide a secondary power supply for the ship’s onboard systems at all times. Impulse engines should not be confused with maneuvering thrusters, which are used when pinpoint accuracy is needed, such as inside Spacedock. The impulse drive is normally used within a solar system or within regions of space such as the Badlands, which are incompatible with the warp field. Each engine has four parts; the impulse reaction chamber, the accelerator, the driver coil, and the vectored exhaust director powered by a proton-antiproton fusion reaction. Cryogenic deuterium is stored in primary and auxiliary fuel tanks in both saucer and engineering sections of the ship. Once the reaction (or explosion) has occurred in the reaction chambers, the destination for the resulting high-energy plasma depends on whether the energy generated was needed to move the ship or power the systems.”

Impulse is largely a relative term and varies between installations due to the different technical specifications of the ship in question. For example, in The Next Generation (1987) episode “Suspicions,” a character calculates his shuttle’s distance from a star’s corona and determines that at ¾ impulse, he would reach it in three minutes. The claim calculates to ¼ impulse being approximately 1,852 km per second. Comparatively, The Star Trek Voyager Technical Manual calculates ¼ impulse for Voyager at approximately 18,665 km per second, nearly 10 times the speed of the aforementioned shuttle. Thus, “impulse” and its fractional variants aren’t so much fixed speeds as relations of the particular impulse engine’s maximum capabilities.

On a technical level, these fusion-powered drives exist in opposition to their antimatter-driven big brothers, providing the ship with a reliable redundant power plant capable of powering other ship-wide functions. They also expel exhaust like traditional rockets, making them more akin to real-world engines we’re used to seeing. Further, they activate an inertia dampening system which essentially reduces the mass of the ship, providing amplified acceleration than would otherwise be possible with relatively small, efficient engines.

Impulse engines are derived from real science and physics, where impulse is explained as “a quantity that describes the effect of a net force acting on an object (a kind of ‘moving force’).”

With all that, it is easy to understand why characters referencing impulse speeds and impulse power come with a bit of confusion. Since they are not fixed rates but dependent on the maximum thrust potential of individual vessels, they vary in intensity. However, the principles of impulse engine functions are quite different from warp engines, which remain Star Trek’s preferred propulsion method.