Okay so like, let's imagine, for hypothetical purposes, that once the Hail Mary was locked on its course back to earth, that course couldn't be changed.
So no matter how much Grace tried after he realized, he couldn't save Rocky. (Or maybe, if you want less angst, the Taumeboa outbreak didn't happen, or it was eating through Xenonite much, much slower, or smth, whatever.)
So Grace comes back after 4 entire years alone, in space, having only his memories for company, after the mission forced him to sacrifice his best friend he'd just had for months.
Grace, who, for those 4 years, has no one to watch him sleep. Who wakes up alone with his thoughts, his memories, and that same monotonous routine with no one to keep him company.
Grace who becomes severely depressed, doing nothing but sleeping all day, dissociating, to the point Mary has to repeatedly put him back in an induced coma simply to get him to eat. So he wouldn't starve to death.
Grace, who gets back to earth and is resentful of everyone. Not just those on Project Hail Mary, but everyone. He sacrificed everything he had for this planet that sent him off to die.
Grace who is entirely nonverbal when he gets back to Earth. He doesn't speak during the debriefing, just stares hollowly, taps documents when he needs to, often tapping in morse code because his vocal cords are so atrophied from not speaking to anyone besides Mary for 4 years straight, and even then, Mary isn't a person. She's not even AI. She reacted to his bodily functions and vitals. Not his voice. He realized that he didn't actually need to talk to her about a year into his journey back.
Grace, who refuses to look anyone who worked on Project Hail Mary in the eye. Refuses to even acknowledge Carl or Stratt's presence, brushing past them if they come near. Who clearly tunes anyone out the moment they start talking about how brave he was, or how it needed to be done.
Grace who refuses to look at the stars again, the things he was so close to, that remind him of Rocky and working on the Hail Mary and being free and those blissful months of not remembering that everyone on earth who he cared for during the project betrayed him, and he didn't have anything else.
Grace who desperately wants to go back to teaching, but his voice is gone, he can't look people in the eye, and he's constantly dissociating, so he knows he could never. He doesn't even bother trying.
Grace who spends of his day staring at the ocean, just sitting there, at the beach near his government-paid apartment that probably has cameras and mics all over it, but honestly, he doesn't care at this point. He doesn't have anyone to have over, nor does he have a voice to speak. This is probably just a precaution.
Grace who throws out everything that reminds him of the Pre-Launch project hail Mary, and keeps everything Rocky gave him in a glass case so that it doesn't get damaged by the atmosphere.
Grace, who cuts and dyes his hair and wears black clothes so he stops getting recognized in the street by kids who think he's an inspiration.
Grace who imitates Rocky in his speech mannerisms and grammatical structure, just to save a small piece of his old friend, to remember what he's lost.
Grace who prays, prays, that by some goddamn miracle, to every and any God, demon, angel, spirit, he can think of, that Rocky made it back to Erid safe. Who knows its improbable but not impossible, just with a very slim, 3.6% chance of Rocky making it home completely safe.
Grace, who is back on earth with nothing to show for it, who is just a hollow shell of the man who was kidnapped, sedated and betrayed to be launched into space.
