Revealing mistake: During the fist fight between the transported crew and Spock, the use of stunt doubles in the wide shot is quite obvious.
Revealing mistake: In the close up shots of the mirror Sulu, the poor make up job of the scar on his face is very evident.
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