Scott: Doctor, I've seen the captain feverish, sick, drunk, delirious, terrified, overjoyed, boiling mad... But up to now, I have never seen him red-faced with hysteria.
Kang: Kirk, my ship is disabled. I claim yours. You are now prisoners of the Klingon empire against which you have committed a wanton act of war!
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