This Side of Paradise - S1-E25
Continuity mistake: When Kirk and Spock successfully activate a subsonic transmitter, it cuts to a closeup of a spade's metal blade breaking the soil ground, but in the next wide shot Sulu and DeSalle are using two spades made entirely of wood. (00:44:45)
Chosen answer: If they're in orbit, they're being pulled along by the planet's gravity well, therefore, impulse engines would only be used for minor corrections and would be "on standby" while in orbit, but not active. (Like keeping your car idling without revving the engine and creating plumes of exhaust).
Captain Defenestrator
Thank you for the info.
Movie Nut