This is a matter of opinion, but personally, the most difficult part for me (assuming you mean emotionally difficult rather than challenging to read) was when the girl says goodbye to her brother. In their conversation, she tells her brother,
Maybe I’ll come to you in your dreams with my hair in braids and crying because the kitten in my arms is dead; maybe I’ll be the touch of a breeze that whispers to you as it goes by; maybe I’ll be one of those gold-winged larks you told me about, singing my silly head off to you; maybe, at times, I’ll be nothing you can see, but you will know I’m there beside you.
She wants her brother and her parents to remember her as she was in her life rather than “the other way,” which refers to the way her body will be affected when it is out in space. Though the entire story is quite sad, this part is the most filled with emotion and, for me, the most difficult part to get through.
What helps me get through that part is to remind myself that here on Earth, we have options. This type of situation in which only one option exists does not typically arise in real life. The setting of the story—a space ship on a mission to deliver medicine—is unrealistic if not impossible today. Though there are real life situations in which a person’s life has to be sacrificed for the greater good (such as war), there probably cannot be, and will never be, a situation where a young girl has to die in such a cold and calculated manner. On Earth, we have learned to come up with creative solutions to problems so that if a situation like this were to occur, we would find a way out of it. I like to believe that, as humans, we do everything we can to spare the lives of the innocent at any costs.