The spaceship physics demonstrated by Serenity, the Firefly-class transport, relied on its three types of engines. The primary engine, composing the rear of the ship, rapidly propelled Serenity through space to travel the distances between planets, leaving a signature firefly-like glow in its wake. It operated primarily by unleashing hot plasma, then igniting it for a quick burst of speed. This engine only needed to be fired once unless the crew wishes to change their heading; inertia kept the ship going because there was no significant friction in the void between planets. Serenity was drifting to its next destination when an explosion knocked the engine out of commission, leaving the ship dependent on inertia entirely.