 Why doesn't Trella tell Quest about her mission?
Motwick was an irresponsible playboy whom Trella had known briefly on Earth, and Trella was glad to dispense with his company for the remaining three weeks before the spaceship blasted off. There were four crew members and three passengers aboard the ship's tiny personnel sphere, and Trella was thrown with Quest almost constantly. She told him only that she was a messenger, sent out to Ganymede to pick up some important papers and take them back to Earth. When it was found at last, he sent Trella, his most trusted secretary, to Ganymede to bring back to him the notebooks found there. Because of this, Trella was tempted to tell Quest the good news herself; but she decided against it.
 Trella's employer wants the mission kept confidential.
 Trella is afraid Quest won't love her if he finds out about her mission.  Trella is worried Quest will take his father's papers and leave her.  Trella was told specifically to stay away from Quest.  Trella's employer wants the mission kept confidential.