Operation -- Annihilate! - S1-E30
Dr. McCoy: [In an aside to Kirk] Please don't tell Spock I said he was the best first officer in the fleet.
Spock: Why, thank you, Doctor McCoy.
Capt. Kirk: You were so concerned about his Vulcan eyes, Doctor, you forgot about his Vulcan ears.
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