Continuity mistake: The landing party beams down in daylight, but the window inside Mendez's office shows that it's night outside. In the next exterior shot, it's day again. Next shot back in the office: night. (00:00:30 - 00:05:20)
Continuity mistake: Kirk introduced Spock as "Lieutenant Commander" instead of "Commander".
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