Trivia: Director Marc Daniels employed an old trick to save Nona's topless bathing scene from NBC's censors. He deliberately shot twice the needed footage, including risque side shots exposing portions of bare breast. After the censors excised the "most naked" portions, what remained was precisely what Daniels had wanted to begin with: a bare-backed Nona bathing in the waterfall.
Trivia: Stock footage of the White Rabbit's footprints from "Shore Leave" were used for the mugato footprints.
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