Because The Cold Equations has a third person limited point of view, we can only infer how Marilyn is feeling by Barton's description of her actions. How was Marilyn feeling throughout the story and what pieces of evidence let you know this?
As the story begins, immediately after Marilyn is discovered, she is excited. While she may be a bit nervous at being found, she is overcome by excitement to see her older brother, which is the reason she stowed away in the first place. Barton describes her discovery: "The door opened and the stowaway stepped through it, smiling. 'All right — I give up. Now what?'" The fact that she is smiling and uses the words "I give up" - similar to a child playing a game - shows that she has no conception of the fate that awaits her. She continues smiling throughout their original conversation.
However, soon, Marilyn becomes aware that the situation is far more dire. When Barton calls in an emergency, her smile fades, and once she hears the commander say she should be jettisoned, Marilyn becomes shocked and angry. Her original reaction is described by Barton: "There was the sound of a sharply indrawn breath from the girl" and once Barton confirms the information, "She recoiled from him as though he had struck her, one hand half raised as though to fend him off and stark unwillingness to believe in her eyes." The shock eventually gives way to anger. She continues to insist that she "didn't do anything."
Next, the girl is resigned to her fate and more concerned about the reactions of her family. The story says that "girl waited, neither of them speaking" to show that she has given up trying to find an answer to this unsolvable problem. Barton notices that "some of the color had returned to her face and the lipstick no longer stood out so vividly red" insinuating that her normal color is returning from the shock and anger she had previously felt. Barton also comments that "her words trailed away, and he turned his attention to the viewscreen, not wanting to stare at her as she fought her way through the black horror of fear toward the calm gray of acceptance."
Her last request is to talk to her brother Gerry, and she and Barton must wait to see if he comes back to camp before she must absolutely leave the aircraft. Barton says "she straightened and looked toward the air lock with pale resolution" saying that she would leave as soon as she had the chance to speak to her brother. Once she speaks to Gerry, she gets visibly upset:
Then her own composure broke and the cold little hand gripped his shoulder convulsively.
“Don’t, Gerry — I only wanted to see you; I didn’t intend to hurt you. Please, Gerry, don’t feel like that—”
Something warm and wet splashed on his wrist, and he slid out of the chair to help her into it and swing the microphone down to her level.
We can see through Barton's description that she is crying and mourning the loss her family will feel in her impending death. Once it's time for her to leave the spacecraft, Barton doesn't help her to the hatch, saying she wouldn't have wanted it that way. She moves calmly and Barton notices "the pulse in her throat to betray the wild beating of her heart" showing the fear that she is experiencing.
Because of the narration, it is through Barton's descriptions that we get the most thorough picture of Marilyn's emotions throughout her ordeal.