Where No Man Has Gone Before - S1-E4
Trivia: This was the second pilot for the series, the first pilot being done with Jeffrey Hunter as Capt. Christopher Pike. This is also the first episode that has William Shatner as the famous Capt. James T. Kirk. The episode was made at the request of Lucille Ball, part owner of Desilu Studios, and friend to Gene Roddenberry, because she believed in the franchise.
Trivia: Throughout the series, planets are given a Class rating, most commonly "Class M", suitable for humans. The M is never defined in this series, or TNG, but in Star Trek: Enterprise, S1: E3 "Strange New World", T'Pol says the new planet is "Minshara Class", meaning suitable for humans. Therefore, the classifications for planets used by the Federation were probably determined by the Vulcans.
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