The discovery of Astrium-126 in the 27th century almost went unnoticed, until a laboratory accident resulted in large amounts of direct-current electricity being applied to a block of Astrium. This caused the block to suddenly accelerate in the opposite direction of the electron flow. Subsequent experiments proved that this was not due to a Newtonian reaction, but some generation of force that still remains unknown. Regardless of the cause, it didn't take long for this property to be adapted into the reactionless propulsors used in every spacecraft since.
Astrium is naturally occurring, and scientists are working on trying to synthesize it, but to no avail. The amount of force produced is proportional to the mass of Astrium-126 and the amount of amperage passed through it, but it is the mass of Astrium that is the main limiter. The polarity of the current determines the direction of the force, but only in a single axis, determined by the crystal structure of the piece of Astrium, so the Astrium must be shaped with its structure in mind. Pulse-width modulation is used to control the maximum amount of force developed.