A Piece of the Action - S2-E17
Spock:Young man, this is likely to be quite hazardous. If I were you...
Kirk: Hold on, Spock. Out of the mouths of babes...
Kid: Who ya callin' a babe?
Kirk: I'm callin' you a babe.
Kid: You callin' me a babe?
Kirk: Yeah, I'm callin'... [Kid brandishes a knife, Kirk continues.] I'm callin' you a babe, but there's nothing personal. Sit down.
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