Corrected entry: When the marines are in the dropship and ready to go to the planet below, the couplers release and the ship literally drops from the Sulaco which is in a gentle orbit. The dropship should have continued to move with the Sulaco in orbit.
XIII
14th Jan 2013
Correction: Assuming that the planet has the same gravity pull as our Earth, all that would be required for the dropship to appear to "drop down" would be for the Sulaco to give the it "a push" and/or the dropship to push back.
XIII
I think it's the dropship's own propulsion system which sends it plummeting at high speed towards the planet.